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Underground ice in permafrost, Mackenzie Delta-Tuktoyaktuk Peninsula, N.W.T. Gell, William Alan 1976

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UNDERGROUND ICE IN PERMAFROST, MACKENZIE DELTA-TUKTOYAKTUK PENINSULA, N.W.I, WILLIAM ALAN CELL B . S c , L i v e r p o o l U n i v e r s i t y , 1971 M.A., U n i v e r s i t y of B r i t i s h Columbia, 1973 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department o £ Geography We accept t h i s t h e s i s as conforming to the required standard .111: (cT) William Alan Gell In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d tha t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f b o c ' ^ r f ̂  1 The U n i v e r s i t y o f B r i t i s h Co lumbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date >0 i t ABSTRACT . A s t u d y was made o f the petrology of a v a r i e t y of underground i c e types i n permafrost on t h s Tuktoyaktuk P e n i n s u l a and P e l l y I s l a n d , Mackenzie D e l t a , N.W.T. Ice bodies of a considerable range of ages occur, i n c l u d i n g s o m a deformed i n tha Wisconsin g l a c i a t i o n ; a l s o permafrost and i c e i s growing ab i n i t i o beneath r e c e n t l y drained lake bottoms. Tha spectrum of i c e body s i z e i s a l s o wide, extending from p o r e - s i z e d p a r t i - c l e s to beds 25 m t h i c k . The major o b j e c t i v e of the study was an understanding of the growth and deformation of such i c e bodies from a p e t r o l o g i c viewpoint. Thus s e v e r a l bodies of known, recent, age -were analyzed i n order to enumerate features t y p i c a l o f g r o w t h . T h i s was p o s s i b l e f o r i c i n g mounds, t e n s i o n cracks a n d a c t i v e l a y e r i c e which grew i n w i n t e r 1973-74. Growth c o n d i - t i o n s were i n f e r r e d i n terms of water supply, f r e e z i n g d i r e c t i o n s and r a t e s , s o l u t e r e j e c t i o n (bubble formation) and c r y s t a l s i z e , shape, l a t t i c e and dimensional o r i e n t a t i o n . On t h a basis o f t h i s knowledge of growth.features, o l d e r and l a r g e r i c e bodies w e r e s t u d i e d , a n d p o s t - s o l i d i f i c a t i o n c h a r a c t e r i s t i c s ware analyzed. Soma near-surface i c e g a v e evidence of thermomigration of bubbles, b u t t h e m a j o r changes i n f a b r i c w a r e due to th e r m a l l y and mech- a n i c a l l y i n d u c e d s t r e s s e s . I n t h e case of w e d g e i c e , p r o g r e s s i v e changes i n c r y s t a l s i z e , s h a p e , l a t t i c e a n d dimensional o r i e n t a t i o n w a r e recognized from t h a c e n t r e t o t h e boundary o f t h e w e d g e , due to r e c r y s t a l l i z a t i o n a n d grain' growth a s s o c i a t e d w i t h w e d g e development. Segregated i c e was s t u d i e d ia pingos and an i n v o l u t e d h i l l . A pingo core w i t h s t e e p l y - d i p p i n g beds showed l i t t l e evidence of flow w h i l e broader pingo with a greater pore ice content had undergone some f l o w i n the segregated i c e l a y e r s . A range of f a b r i c s was found i n the i n v o l u t e d h i l l , o p t i c a x i s o r i e n t a t i o n s becoming i n c r e a s i n g l y concentrated normal to compositional l a y e r i n g while dimensional o r i e n t a t i o n s tended towards p a r a l l e l i s m w i t h the l a y e r i n g i n a n t i c l i n e s i n the i c e . The i n f l u e n c e of bubbles on deformation i s pointed out i n that l a r g e r c r y s t a l s occur i n c l e a r i c e and thus have greater i n t r a c r y s t a l l i n e s l i p than i n bubbly i c e . Where a wedge penetrated such a f o l d , the f a b r i c changed along the f o l d limb i n a manner symmetrically r e l a t e d to the wedge. A d d i t i o n a l l y , s e v e r a l near-surface i c e s ware s t u d i e d and showed evidence of m u l t i p l e growth pe r i o d s , and m u l t i p l e f r e e z i n g d i r e c t i o n s , i n d i c a t i n g that the i c e grew i n enclosed water i n f r o z e n m a t e r i a l . Thus the complexity of f r e e z i n g and melting h i s t o r i e s may be recognized p e t r o - g r a p h i c a l l y w h i le i t i s not r e a d i l y apparent i n the f i e l d . i i t L I S T OF CONTENTS Chapter Page 1 I N T R O D U C T I O N : 1 2 BACKGROUND TO T H E P R E S E N T S T U D Y 1. . Permafrost i n the Outer Mackenzie D e l t a - Tuktoyaktuk P e n i n s u l a Area 6 2. Thermal C h a r a c t e r i s t i c s . . . . . . • . 8 3. Ground Ice Types . . . . . . . . . . 8 4. Previous Ground Ice Petrology S t u d i e s . . . . . . . . 9 5. Terminology. . . ' 12 6. Ice Growth: A Review. . . . . . . . . . . . . . . . . . 14 7. Post-Freezing Phenomena. . . . . . . . . . . . . . . 23 3 TECHNIQUES 1. I n t r o d u c t i o n . . . . . . . . . . . . . . . 32 2. F i e l d Techniques . . ^ . . . . . . . . . . . . . . . . 32 3. Laboratory Techniques. . . . . . . . . . . . . . . . . 35 4 R E S U L T S 1. Lake Ice 37 2. I c i n g Mound Ice. 43 3. Pingo Ice. ... . . . . . . . . . . . .... . . . . . 59 4. Involuted H i l l Ice . . . . . . . . . . . 94 5. Tension Crack Ice. . . . . . . . . ......... . . . . 13& • 6. Thermal C o n t r a c t i o n Cracks and Wedge Ice . . . . . . 151 7. R e t i c u l a t e V e i n Ice. .'• . - 181 8. A c t i v e Layer Ice 192 i v LIST OF CONTENTS (cont'd) Chapter . Page 4 9. Ice bodies w i t h m u l t i p l e f r e e z i n g h i s t o r i e s . . . . . 204 10. Aggradational Ice. . . . . . . . . . . . . . . . . . . 235 5 SUMMARY AND CONCLUSIONS. 245 LITERATURE CITED . . . . . . . . . . . . . . ... . . . . 2 5 1 APPENDICES 1. C l a s s i f i c a t i o n of underground i c e types 2. Glossary .. LIST OF TABLES Table Page I Shumskii's t e x t u r a l terminology . . . . . . . . . . •• • 13 I I C r y s t a l s i z e , Tuktoyaktuk Pingo . ... . . . . .' .. . . . 81 I I I C r y s t a l s i z e , Involuted h i l l core . . . . . . . . . • ; 99 TV C r y s t a l s i z e , Involuted h i l l , . adjacent to wedge . . 116 v i LIST OF FIGURES F i g u r e Page 1 The study area and sample s i t e s . . . . . . . 7 2 Thin s e c t i o n s of lake i c e 39 3 P e t r o f a b r i c s of lake i c e 42 4 F i e l d p o s i t i o n of L i v e r p o o l Bay i c i n g mound 44 5 Bubble c h a r a c t e r i s t i c s , Tuktoyaktuk i c i n g mound i c e . . 44 6 Bubble s t r a t i g r a p h y , Tuktoyaktuk i c i n g mound i c e . . . . 47 7 . C r y s t a l c h a r a c t e r i s t i c s , Tuktoyaktuk i c i n g mound. . . . . 47 8 P e t r o f a b r i c s , Tuktoyaktuk i c i n g mound .. . 51 9 Tuktoyaktuk i c i n g mound, f r a c t u r e i n f i l c r y s t a l s . . . . 5 3 10 L i v e r p o o l Bay i c i n g mound, c r y s t a l c h a r a c t e r i s t i c s . . . 53 11 Ice lenses i n peat, Pingo No. 11. . . . . . . . . . . . 63 12 C r y s t a l c h a r a c t e r i s t i c s , core, Pingo No. 11 . . . . . . 6 3 13 Fr a c t u r e s i n core i c e , Pingo No. 11 63 14 P e t r o f a b r i c s , Pingo No. 11. . . . . . . . . . . . . . . . . . . 67 15 S t r a t i g r a p h y and sample s i t e s , W h i t e f i s h Summit Pingo. . 69 16 Dimensional o r i e n t a t i o n , b a s a l c r y s t a l s , W h i t e f i s h Summit Pingo. . . . . . . . . . 69 17 P e t r o f a b r i c s , W h i t e f i s h Summit Pingo. . . . . . . . . . 74 18 Sediment banding and c r y s t a l c h a r a c t e r i s t i c s , Tuktoyaktuk Pingo . . . . . . . . . . 78 . 19 C r y s t a l dimensional o r i e n t a t i o n , Tuktoyaktuk Pingo. . . 81 20 P e t r o f a b r i c s , Tuktoyaktuk Pingo . . . . . . 82. 21 P e t r o f a b r i c s , Tuktoyaktuk Pingo 85 22 P e t r o f a b r i c s , Tuktoyaktuk Pingo . . . . . . .. 90 23 Summary p e t r o f a b r i c diagrams, Tuktoyaktuk Pingo . . . . 93 v i i LIST OF FIGURES (cont'd) Figure . Page 24 S t r a t i g r a p h y of i c e core, i n v o l u t e d h i l l . . . 97 25 Probable i c e type t r a n s i t i o n s , i n v o l u t e d h i l l . . . . . . 97 26, In f l u e n c e of i n c l u s i o n s on c r y s t a l s i z e . . 100 27 C r y s t a l dimensional o r i e n t a t i o n , .involuted h i l l i c e . \ . 103 28 P e t r o f a b r i c s of i c e core, i n v o l u t e d h i l l . . . . . . . . 106 29 C r y s t a l c h a r a c t e r i s t i c s , a n t i c l i n e i n i n v o l u t e d h i l l .- . 110 30 P e t r o f a b r i c diagrams, a n t i c l i n e i n i n v o l u t e d h i l l . . . . 1 1 1 31 Bed thickness around f o l d i n Figure 29 . . . . . . . . . 116 32 Wedge pe n e t r a t i n g i n v o l u t e d h i l l i c e . . . . . . . . . . 116 33 Change, i n massive i c e c r y s t a l s i z e adjacent to wedge. .• .120 34 P e t r o f a b r i c s of massive i c e and f r a c t u r e i c e adjacent to wedge. 124 35 G r a i n type d i s t r i b u t i o n s , core i c e , f o l d e d i c e and folded i c e w i t h wedge, i n v o l u t e d h i l l . . . . . . 129 36 Bubble banding i n . t e n s i o n crack i c e , Pingo No. 9 . . . . 138 37 C e n t r a l curved c r y s t a l s , t e n s i o n crack i c e . . . . . . . 138 38 Schematic ice-water i n t e r f a c e p r o f i l e s . . 138 39 P e t r o f a b r i c s , t e n s i o n crack i c e , Pingo No. 9 . . . . . . 143 40 Peninsula P o i n t t e n s i o n crack i c e . . . . . . . ... . ... 146 41 Bubble c h a r a c t e r i s t i c s , P e n i n s u l a P o i n t t e n s i o n crack. 14.6 42 P e t r o f a b r i c s , P e n i n s u l a P o i n t t e n s i o n crack. . . . . . . 148 43 C r y s t a l c h a r a c t e r i s t i c s , Peninsula P o i n t t e n s i o n crack. . . . . . . . . . . . . . . . . . . . . . 148 44 Thermal c o n t r a c t i o n cracks i n massive i c e . . . . . . . .. 154 : 45 Mode of i n f i i of f r a c t u r e s . .... . .... . . , . . . . 154 46 P e t r o f a b r i c s of i n f i l c r y s t a l s i n massive i c e . . . . . . 157 v i i i . LIST OF FIGURES (cont'd) Figure • . . . ' Page 47 P a r a l l e l and converging thermal c o n t r a c t i o n cracks i n massive i c e 160 48 Bubble bands and oblique f r a c t u r e s , wedge i c e , , P e l l y I s l a n d . . . . . . . . . . . . ... 162 49 Localized.melt-down and r e f r e e z i n g adjacent to la r g e wedge, P e l l y I s l a n d . . . . . . . . . . . . . 162 50 V e r t i c a l t h i n s e c t i o n , orthogonal to wedge a x i s , wedge cen t r e , P e l l y I s l a n d . . . . . . . . . . . . . . . 165 51 . Sketch of grains f o r p e t r o f a b r i c a n a l y s i s . . . . . .' . 165 52 P e t r o f a b r i c diagrams, centre of wedge, P e l l y I s l a n d . . 167 53 J u n c t i o n of wedge w i t h c l a y , P e l l y I s l a n d . . . ... . . 170 54 V e r t i c a l t h i n s e c t i o n , orthogonal to wedge a x i s , wedge cen t r e , P e l l y I s l a n d . . . . . . . . . . . . . . . 170 55 C r y s t a l dimensional o r i e n t a t i o n , wedge i c e , P e l l y I s l a n d . . . . . . . . . . . . . . . . . . . . . . . . . 172 56 Sketch of grains f o r p e t r o f a b r i c a n a l y s i s . .... . . . 172 57 P e t r o f a b r i c diagrams, edge of wedge, P e l l y I s l a n d . . . 173 58 . P e t r o f a b r i c s , j u n c t i o n of two wedges, P e l l y I s l a n d . . . 177 59 . R e t i c u l a t e i c e v e i n system . . . . . . 183 60 V e r t i c a l s e c t i o n , p a r a l l e l to plane of narrow v e r t i c a l v e i n 183 61 V e r t i c a l s e c t i o n , p a r a l l e l to plane of narrow v e i n . . . 183 62 V e r t i c a l s e c t i o n normal, to plane of v e r t i c a l v e i n . . . 186 63 P e t r o f a b r i c s of r e t i c u l a t e v e i n i c e . . . . . . . . . . 183 64 Sketch o f grains f or p e t r o f a b r i c a n a l y s i s . . . . . . . 186 65 V e r t i c a l s e c t i o n s normal to plans o f wide v e i n . . . . . 139 65 Columnar marginal c r y s t a l s normal to plane of v e i n . ... 191 67 Block slump o n coast exposing a c t i v e l a y e r i c e . . . . . 194 IX LIST OF FIGURES (cont'd) Figure Page 63 Peat and bubble p a t t e r n , v e r t i c a l s e c t i o n p a r a l l e l to wedge, a c t i v e l a y e r i c e . . . . . . . . . 194 69 P e t r o f a b r i c s , v e r t i c a l s e c t i o n , a c t i v e l a y e r i c e . . •'. .194 70 V e r t i c a l s e c t i o n , adjacent to s o i l , a c t i v e l a y e r i c e . . 197 71 . V e r t i c a l s e c t i o n , normal to s o i l , a c t i v e l a y e r i c e . . . 197 72. P e t r o f a b r i c s , c r y s t a l s i n v e r t i c a l s e c t i o n , normal to s o i l , a c t i v e l a y e r i c e . . . . . . . . ":. . . . . . . 197 73 A c t i v e l a y e r i c e , i n f l u e n c e of bubbles on c r y s t a l . growth . . . . . . . . . . •. 199 74 P e t r o f a b r i c s , a c t i v e layer i c e . . . . . . . . . .... . 199 75 A c t i v e l a y e r i c e , i n f l u e n c e of bubbles on f r a c t u r e propagation ; '. .199 . 76 A c t i v e l a y e r i c e , Tuktoyaktuk, r e l a t i o n s h i p of bubbles and c r y s t a l s . . . . . . . . . . . . . . . . . 202 . 77 H o r i z o n t a l s e c t i o n w i t h f r a c t u r e . . . ... . . . . ... . 207 78 P e t r o f a b r i c s of upper,, h o r i z o n t a l s e c t i o n . . . . . . . 207. 79 V e r t i c a l section, adjacent to s o i l , m u l t i d i r e c t i o n a l c r y s t a l dimensional o r i e n t a t i o n . 207 80 P e t r o f a b r i c s of v e r t i c a l s e c t i o n 207 81 V e r t i c a l s e c t i o n normal to s o i l .. . . ... . . .... . . 211 82 P e t r o f a b r i c s , v e r t i c a l s e c t i o n s . . . . . . . . . . . . 213 .83 Schematic diagram, m u l t i p l e growth periods . . '.• .. . . . 211 84 . P e t r o f a b r i c s , c r y s t a l s above and below t r u n c a t i o n zone . 216 . 85 • • .L'ensoid .ica body, c o a s t a l exposure . . . . 218 86 • Schematic diagram,- root and bubble shapes . . . . . . . . 218 37 . ' D e t a i l of bubble shapes' •. . .. .' . •. .', . . . . . . . . . . . . 213 '. 8-3 Bubble pattern.in. lens old b o d y . . . . . '. . . .' 213' • 89 V e r t i c a l s e c t i o n s , l e n s o i d body . . .... . . . . . . . 221 LIST OF FIGURES (cont'd) Figure ' Page 90 P e t r o f a b r i c s , l e n s o i d i c e body . . . . . . . . 221 91 F i e l d p o s i t i o n , pond i c e over wedge, P e l l y I s l a n d . . . . 224 92 Pond i c e body. . . . . . . . . . . . . . .224 93 I n c l u s i o n p a t t e r n , pond i c e 224 94 C r y s t a l p a t t e r n , pond i c e • •. • • 224 95 P e t r o f a b r i c s , pond i c e .' . . 229 96 Bubble and c l a y i n c l u s i o n p a t t e r n s , pond i c e . . . . . . . 231. 97 V e r t i c a l s e c t i o n , top of pond i c e . 231 98 Schematic diagram, peat accumulation, i n v o l u t e d h i l l . . . . . . . . . . . . . 237 99 . Schematic diagrams, i n c l u s i o n and c r y s t a l p a t t e r n s , aggradational i c e . . . . . . . . . . . . . . . 237 100 . P e t r o f a b r i c s , aggradational i c e , i n v o l u t e d h i l l . . . . . 240 101 P e t r o f a b r i c s , aggradational i c e , Tuktoyaktuk . . . . . . 244 ACKNOWLEDGEMENTS Field work was supported by the Geological Survey of Canada, The Polar Continental Shelf Project of the Department of Energy, Mines and Resources, and research grants (National Research Council of Canada, Department of Indian and Northern Affairs), to Dr. J.R. Mackay. The Inuvik Research Laboratory provided logistic help. The writer would like to thank Imperial Oil Ltd. for a fellowship grant (1971-74). Dr. J.R. Mackay is thanked for support and helpful discussions during the field and laboratory work and thesis preparation. Peter Lewis provided valuable field assistance in 1973. Committee members Drs. G.K.C. Clarke, J.V. Ross, H.O. Slaymaker and J.K. Stager gave helpful comments on the original thesis draft. Grateful appreciation is extended to Sherri Lee and Patricia Schreier who painstakingly typed the manuscript. Chapter 1 1 INTRODUCTION Permafrost i s a temperature condition of s o i l and rock materials where these materials have been maintained below 0*C for a minimum of two years. Two broad zones of permafrost are recognized, (a) continuous, (b) d i s c o n t i n - uous, (a) In the continuous zone permafrost i s present everywhere beneath . the surface, except below large water bodies. A temperature of -5°C e x i s t s at the southern boundary, at the depth of zero annual amplitude (about 15 m) and permafrost may reach 1000 m i n thickness, under B a f f i n Island f o r example, (b) In the discontinuous zone permafrost may be l o c a l l y absent; where present i t i s thinner than i n the continuous zone, and v a r i a b l e i n thickness. From the temperature conditions i t i s evident that water i n permafrost may be i n the s o l i d form, but t h i s i s not n e c e s s a r i l y the case as s u b s t a n t i a l amounts of l i q u i d water may be adsorbed on clays (Williams 1967), and a l s o s a l i n e permafrost has been reported. Where ground i c e occurs i t may be i n one of a wide range of forms; coatings on sediment p a r t i c l e s , pore cement, i n d i v i d u a l grains, veins, i n f i l s i n cracks, or massive beds. In the continuous permafrost zone a l l types may be a c t i v e l y growing, whereas i n the discontinuous zone i c e bodies are gen- e r a l l y smaller and i n a c t i v e . In p a r t i c u l a r , i c e wedges are i n a c t i v e i n the discontinuous zone (Brown and Pewe 1973). Ice may grow and melt annually i n the seasonally freezing and thawing layer, or a c t i v e l a y e r , above permafrost. Thus some forms are tr a n s i e n t . In the thick continuous permafrost zone, permafrost aggradation and associated ground i c e growth i s being monitored (Mackay 1973a) beneath recently drained lake bottoms adjacent to very old 2 permafrost and contained i c e . Some of these o l d bodies have been deformed b y Wisconsin i c e sheets .(Mackay, Rampton and F y l e s 1972). Thus i c e bodies i n permafrost are of widely ranging ages and h i s t o r i e s . The permafrost l i t e r a t u r e has abundant references to s u r f i c i a l forms u n d e r l a i n by i c e , f o r example a range of ice-cored mounds has been recognized (French 1971). However, there has been l e s s advance i n understanding mech- anisms of growth and deformation of ground i c e . Some aspects of the p e t r o l o g y of i c e wedges (Black 1953; Corte 1962a) and beds (Corte 1962a; Mackay and Stager 1966a) i n c l u d i n g i c e i n submarine permafrost (Mackay 1972a) have been s t u d i e d , but no comprehensive theory of growth and subsequent h i s t o r y of such i c e bodies from a p e t r o l o g i c viewpoint i s at hand. Valuable c o n t r i b u t i o n s to our knowledge of a range of ground i c e types have come from papers by Mackay (1966, 1971, 1973a). Heat conduction theory has been a p p l i e d to the f r e e z i n g of massive i c e bodies, and the r e s u l t s t e s t e d by d e t a i l e d f i e l d measurement of pingo growth (Mackay 1973a). The c r a c k i n g patterns of i c e wedges have been stu d i e d at s e v e r a l s i t e s over a p e r i o d of years (Mackay 1974a). A l s o a d i s c u s s i o n has. been presented of the development of r e t i c u l a t e i c e veins i n f i n e - g r a i n e d m a t e r i a l s (Mackay 1974b, 1975c). The above work has been based l a r g e l y on observations of exposed i c e . bodies and i c e grown under known c o n d i t i o n s . Such exposures and monitoring are rare and c r i t e r i a f o r r e c o g n i z i n g i c e type and growth and deformation . h i s t o r y from l i m i t e d samples are needed. In the past 13 years there have been two i n t e r n a t i o n a l conferences on permafrost, which included papers on ground i c e . The F i r s t I n t e r n a t i o n a l 3 C o n f e r e n c e on P e r m a f r o s t was h e l d i n 1963 and t h e p r o c e e d i n g s were p u b l i s h e d i n 1966, w h i c h c o n t a i n e d l i t t l e r e f e r e n c e t o the. p e t r o l o g y o f ground i c e a l t h o u g h i t was mentioned as an a i d i n i c e c l a s s i f i c a t i o n . The m o d e r a t o r ' s r e p o r t o f t h a t c o n f e r e n c e i n c l u d e d the f o l l o w i n g s t a t e m e n t s . c o n c e r n i n g ground i c e (p. 550): S h u m s k i i arid V t i u r i n p r e s e n t a c l a s s i f i c a t i o n based on g e n e t i c p r i n c i p l e s . U n f o r t u n a t e l y t h e o r i g i n s o f many ground i c e masses ar e n o t y e t w e l l enough known t o p l a c e them i n such a . c l a s s i f i c a - t i o n . T h e r e i s an immediate need f o r more d e t a i l e d knowledge o f th e p h y s i c a l p r o p e r t i e s and s t r u c t u r a l p e c u l i a r i t i e s o f t h e v a r i o u s t y p e s o f m a s s i v e ground i c e . . . A t p r e s e n t two t y p e s o f m a s s i v e ground i c e can be d i s t i n g u i s h e d : (1) P i n g o i c e - c h a r a c t e r i z e d by t r a n s l u c e n t , l a r g e - s i z e , s i m p l e shaped c r y s t a l s and by t h e o c c a s i o n a l s c a r c i t y or more o f t e n t h e c o m p l e t e absence o f i n t e r n a l s t r u c t u r e s . (2) Ice-wedge i c e - c o n s i s t i n g o f s m a l l s i z e c r y s t a l s and showing a d i s t i n c t v e r t i c a l t o i n c l i n e d f o l i a t i o n . A c o n s i d - e r a b l e amount o f m i n e r a l and o r g a n i c m a t e r i a l i s a l i g n e d w i t h t h e f o l i a t i o n . So f a r a l l o t h e r v a r i e t i e s o f m a s s i v e ground i c e must be lumped t o g e t h e r w i t h o n l y the c e r t a i n t y t h a t t h e r e a r e more t y p e s t o be d i s t i n g u i s h e d when more c o m p l e t e d e s c r i p t i o n s and q u a n t i t a t i v e d a t a a r e a v a i l a b l e . The p o s s i b l e T a b e r i c e i s i n t h i s group. The t h i c k t a b u l a r s h e e t s o f ground i c e c h a r a c t e r i z e d by h o r i z o n t a l l a y e r i n g w i t h f r e q u e n t d i r t y i c e l a y e r s r e p o r t e d f r o m t h e M a c k e n z i e D e l t a , n o r t h e a s t G r e e n l a n d , and v a r i o u s p l a c e s i n S i b e r i a may be a n o t h e r d i s t i n c t t y p e . . . , I n p r e s e n t i n g s u g g e s t i o n s f o r f u t u r e r e s e a r c h on m a s s i v e ground i c e , i t was s t a t e d t h a t ( p . 5 5 1 ) : ...Perhaps the g r e a t e s t p r e s e n t need i n t h e s t u d y of m a s s i v e ground i c e i s f o r q u a n t i t a t i v e i n f o r m a t i o n on the i c e b o d i e s and t h e i c e . i t s e l f . . . I n f o r m a t i o n s h o u l d a l s o be c o l l e c t e d on b u b b l e s i z e , shape, and d i s t r i b u t i o n . . . A f u r t h e r c o n c e n t r a t e d a p p l i c a t i o n , of. p e t r o f a b r i c s t o a l l t y p e s o f m a s s i v e ground i c e i s needed.. The above s t a t e m e n t s were made i n 1963, but by t h e time t h e Second I n t e r n a t i o n a l C o n f e r e n c e on P e r m a f r o s t t o o k p l a c e i n Y a k u t s k i n 19 73, l i t t l e s u ch work had been c a r r i e d o u t . I n a r e v i e w paper on t h e o r i g i n , c o m p o s i t i o n 4 and s t r u c t u r e of p e r e n n i a l frozen ground and ground i c e , Mackay and. B l a c k (1973) r e f e r r e d to the i n c l u s i o n c h a r a c t e r i s t i c s of i c e s , but the only mention of c-axis d i s t r i b u t i o n s was based on the e a r l y work of Black (1954). Thus i t appeared that no new studies on the petrology of ground i c e had been c a r r i e d out i n North America i n the years 1963-1973. . Again the recommenda- t i o n was made (Mackay and Black 1973, p. 190) tha t : A d e s c r i p t i v e c l a s s i f i c a t i o n and u n i f i e d terminology of a l l ground i c e types and of r e l a t e d morphologic forms should be adopted before the T h i r d I n t e r n a t i o n a l Permafrost. Conference meets. . This summarized the s t a t e of ground i c e p e t r o l o g y at that time. I n 1974, the Ad Hoc Study Group on Permafrost of the Committee on P o l a r Research, NRC-NAS, produced a survey of " P r i o r i t i e s f o r B a s i c Research on Permafrost." This included the f o l l o w i n g statement: S p e c i f i c s tudies of the. p e t r o l o g y , geochemistry, and p h y s i c a l c h a r a c t e r i s t i c s of ground i c e are r a r e and g r o s s l y i n s u f f i c i e n t to provide the understanding necessary f o r d e t e c t i n g , d e l i m i t i n g , and i d e n t i f y i n g ground i c e by i n d i r e c t means such as a i r b o r n e and s a t e l l i t e sensors and geophysical techniques. In f a c t , ground i c e as i t . appears i n cores has not been s u f f i c i e n t l y s t u d i e d to permit i d e n t i f i c a t i o n as to o r i g i n or to compare i t s chemistry, except r a r e l y , w i t h that of a d j o i n i n g waters (pp. 31-32). x^as pointed out that c r y s t a l c h a r a c t e r i s t i c s are important . r h e o l o g i c a l p r o p e r t i e s of i c e bodies and permafrost. From a point of view it.was s t a t e d that f o r ground i c e : No g e n e r a l l y u n i f i e d terminology e x i s t s , s i n c e each d i s c i p l i n e .and country uses i t s own w i t h only c e r t a i n words common to many. A standardized terminology should be developed (p. 31).. In terms of c l a s s i f i c a t i o n s of ground i c e , the committee, concluded t h a t : ' . A v a r i e t y of ground-ice c l a s s i f i c a t i o n s e x i s t ( s ) f o r s p e c i f i c '• purposes,' but there i s n e i t h e r a g e n e r a l l y accepted d e s c r i p t i v e I n a d d i t i o n i t i n determining t e r m i n o l o g i c a l 5 morphometric c l a s s i f i c a t i o n , nor a genetic c l a s s i f i c a t i o n . These should be developed and accompanied by a standardized system of symbols for f i e l d mapping and laboratory studies. Such a task can best be implemented by a cooperative i n t e r n a t i o n a l e f f o r t (p. 31). A. b r i e f review of c l a s s i f i c a t i o n s i s given i n Chapter 2 of this t h e s i s , the point being made here i s that the need for studies of the structure and petrology of ground i c e has been recognized, but very few papers have been produced i n North America. The major objective of this study i s art understanding of the growth and deformational c h a r a c t e r i s t i c s of ice bodies i n permafrost. In terms of ice growth, p e t r o l o g i c and pe t r o f a b r i c techniques are employed to determine. . the mode of water supply to the freezing front, the growth d i r e c t i o n s and growth rates, and mechanism of i n c l u s i o n incorporation and i t s influence on . c r y s t a l c h a r a c t e r i s t i c s . Post-freezing phenomena are in v e s t i g a t e d from a petro l o g i c viewpoint: thermomigration, flow and f r a c t u r e . No f i e l d measurements of flow and fracture are attempted. 6 Chapter 2 . BACKGROUND TO THE PRESENT STUDY 1. Permafrost i n the outer Mackenzie D e l t a - Tuktoyaktuk P e n i n s u l a Area The modern Mackenzie Delta i s g e n e r a l l y a low, f l a t a rea, but on the d i s t a l boundary are i s l a n d s of P l e i s t o c e n e age reaching 50 m i n • a l t i t u d e , due to g l a c i e r i c e t h r u s t i n g (Mackay 1971). Tuktoyaktuk Pen- i n s u l a , of P l e i s t o c e n e age, i s predominantly l o w - l y i n g but w i t h abundant p o s i t i v e r e l i e f features i n the form of i n v o l u t e d h i l l s , and pingos which . may reach up to 50 in. The p r i n c i p a l study area l i e s on the coast of the Beaufort Sea w i t h i n a 30 km radius of Tuktoyaktuk, and a secondary area i n c l u d e s P e l l y I s l a n d , one of the outer i s l a n d s beyond the modern D e l t a ( F i g . 1). The P l e i s t o c e n e c o a s t a l p l a i n i n the Tuktoyaktuk area has been c l a s s i f i e d as " U n d i f f e r e n t i a t e d c o a s t l a n d s " by Mackay (1963, p. 137). Parts of the coast are receding r a p i d l y , e s p e c i a l l y where abundant ground i c e occurs. The e n t i r e area i s i n continuous permafrost which may exceed 370 m (Jessop , 1970) but l o c a l l y depressions or through t a l i k s i n the permafrost occur below extensive water bodies which do not freeze to the bottom i n w i n t e r . Drainage of such lakes provides c o n d i t i o n s f o r permafrost aggradation. . Examples have been monitored and described by Mackay (1973a). I n one case c o a s t a l r e c e s s i o n caused lake drainage, permafrost and then pingo growth. Thus o l d permafrost and contained ground i c e i s being degraded and removed i n some areas, w h i l e new permafrost and ground i c e grow nearby. 7 P r i n c i p a l s t u d y a r e a and sample s i t e s Thermal C h a r a c t e r i s t i c s D e t a i l e d temperature data for the area are becoming a v a i l a b l e . The . mean annual s o i l surface temperature i s taken as -8" C (Mackay 1974c). However, v a r i a t i o n s i n surface temperature are important i n that thermal t e n s i o n and compression-occur a n n u a l l y , sometimes tensions are s u f f i c i e n t to cause f r a c t u r e and the growth of i c e wedges. A d d i t i o n a l l y the creep behaviour of i c e i s temperature dependent, and thermal gradients may cause thermomigration. Ground Ice Types Several i c e types have been observed i n the area; these have been enumerated by'Mackay (1972b, p. 4): open c a v i t y i c e , wedge i c e , v e i n . i c e , t e n s i o n crack i c e , closed c a v i t y i c e , e p i g e n e t i c segregated i c e , aggrada- t i o n a l i c e , s i l l i c e , pingo i c e and pore i c e . These i c e types have been studied by i n s p e c t i o n of slump face s , probing of i c e wedges, and d r i l l - h o l e a n a l y s i s . From exposures and d r i l l - h o l e records . i t i s evident that i c e u n d e r l i e ; a l l major topographic highs. Thus i c e i s important i n the geomorphic evo- l u t i o n of the area. However, exposures also, show that there i s no simple r e l a t i o n between surface form and the presence of i c e at depth; f o r example large i c e wedges may have no surface expression i n the form of troughs, and abundant i c e may u n d e r l i e l o w - l y i n g areas (Rampton and Walcott 1974). A l s o , mescscopic features of i c e bodies i n chance exposures may not r e a d i l y i n d i c a t e the growth or deformation mechanisms which have operated. Few workers have employed standard p e t r o l o g i c techniques on ground i c e ; these •methods are a p p l i e d here to a i d i n understanding i c e In permafrost. 9 Several c l a s s i f i c a t i o n s of underground i c e types have been prepared (Shumskii and V t i u r i n 1966; V t i u r i n a and V t i u r i n 1970; Mackay 1972c). The c l a s s i f i c a t i o n s are based on such f a c t o r s as place of development, o r i g i n of water, phase composition and m o d i f i c a t i o n of water ( V t i u r i n a and V t i u r i n 1970) or o r i g i n of water p r i o r to f r e e z i n g , p r i n c i p a l t r a n s f e r process and ground ice forms (Mackay 1972c). The petrology of the r e s u l t i n g i c e was not included i n such c l a s s i f i c a t i o n s ; the present study w i l l attempt to show the value of the approach. 4. Previous Ground Ice Petrology Studies The pioneers i n t h i s f i e l d wave Shumskii i n the U.S.S.R. and B l a c k i n A l a s k a , who c a r r i e d out t h e i r f i e l d work i n the l a t e 1940s. Black's work on i c e wedges was published i n 1963; more extensive r e s u l t s are a v a i l - able i n h i s Ph.D. t h e s i s (Black 1953) and an unpublished manuscript (1954). A wide range of i c e types"was studied by Shumskii whose research was t r a n s - l a t e d i n t o E n g l i s h i n 1954. The only l a t e r work outside the U.S.S.R. has been t h a t of Corte (1962a), based on Greenland s t u d i e s , M u l l e r (1963) and some Japanese r e p o r t s . Extensive work has been continued i n Prussia and V t i u r i n a and V t i u r i n (1970) have summarized recent r e s u l t s . The proceedings of the F i r s t and Second I n t e r n a t i o n a l Conferences on Permafrost c o n t a i n r e f e r e n c e to ground i c e p e t r o l o g y . Some of the major r e s u l t s of these s t u d i e s are discussed below. S h u m s k i i (1954) grouped underground ice types under the heading of " c o n g e l a t i o n i c e , " i n c l u d i n g p e r e n n i a l v e i n Lee (wedge i c e ) , segregation ice and i n j e c t i o n ice. The primary (growth) t e x t u r e of wedge i c e was 10 termed " a l l o t r i o m o r p h i c - g r a n u l a r " or."hypidiomorphic-granular"; a l s o i n - a c t i v e , or " f o s s i l " wedges were observed. The texture of segregation i c e was described by Shumskii (1964, p. 222) as "hypidiomorphic-granular" and " a l l o t r i m o r p h i c - g r a n u l a r " w i t h c r y s t a l s being elongated p a r a l l e l to the f r e e z i n g d i r e c t i o n , and normal to the l a y e r i n g ; a i r i n c l u s i o n s were a l s o observed, and i t was pointed out that there i s a range of forms from pore i c e to segregated i c e . " I n j e c t i o n . i c e " was considered by Shumskii to be important i n pingo growth. However, " i n j e c t i o n " i c e text u r e s and p e t r o - f a b r i c s were not analyzed i n d e t a i l , but st u d i e s showed very l a r g e g r a i n s w i t h random l a t t i c e o r i e n t a t i o n s . . These data and the i n c l u s i o n p a t t e r n s were taken to i n d i c a t e the f r e e z i n g of large masses of int r u d e d water. The three ground i c e types of wedge i c e , segregation i c e and i n j e c t i o n i c e were discussed by Shumskii; on.the other.hand, Black (1953) s t u d i e d only i c e wedges, but i n greater d e t a i l . His f i e l d areas were near F a i r - banks and Barrow, A l a s k a . Black defined some of the main f a b r i c s to be expected i n surface and buried wedges, and demonstrated that c o n t r a c t i o n and expansion of the ground caused flow and f r a c t u r e of i c e i n wedges. Deformation f a b r i c s were found to. be superimposed on growth f a b r i c s ; the o r i g i n of some f a b r i c s was not understood. Black pointed out the presence of i n c l u s i o n " f o l i a t i o n s " s u b p a r a l l e l to the wedge s i d e s , c o n t a i n i n g v e r t i c a l l y o r i e n t e d i n c l u s i o n s . C r y s t a l s i z e ranged from 0.1 ram to 100 mm; shapes were equidimensional, p r i s m a t i c or i r r e g u l a r ; and boundaries, s t r a i g h t to sutured. Seven types of c-axis d i s t r i b u t i o n were recognized, the three most widespread'being (1) v e r t i c a l , ( 2 ) normal to the wedge a x i s and h o r i z o n t a l , ( 3 ) normal to the wedge a x i s and i n c l i n e d to one or both s i d e s . Black recognized the importance of temperature g r a d i e n t s , the l a t e r a l s t r e s s system, and bas a l g l i d e i n f a b r i c development.' Corte (1962a) i n v e s t i g a t e d four patterned ground types at Thule, Greenland, and made c o r r e l a t i o n s among surface p a t t e r n , g r a i n s i z e and s t r u c t u r e of the a c t i v e l a y e r , and type and d i s t r i b u t i o n of ground i c e f o r the patterns i n v e s t i g a t e d . F a b r i c . a n a l y s i s , was performed on four ground i c e types: - i c e wedges, r e l i c t i c e , i c e mass and i c e l e n s , and on contacts of i c e wedges w i t h r e l i c t and mass i c e , and f a b r i c c r i t e r i a were found to b e . h e l p f u l i n d i s t i n g u i s h i n g i c e types. V t i u r i n a and V t i u r i n (1970) summarized Russian work on ground i c e development. They included d i s c u s s i o n of the growth of pore i c e , lens i c e , i n j e c t i o n i c e , wedge i c e and the b u r i a l of surface i c e such, as naleds and g l a c i e r i c e . ' L i t t l e reference was made to p e t r o l o g i c aspects of the i c e . types; no p e t r o f a b r i c diagrams were presented, but c r y s t a l shape r e c e i v e d some c o n s i d e r a t i o n . M i i l l e r (1963) stud i e d some c r y s t a l c h a r a c t e r i s t i c s of pingo i c e i n the Tuktoyaktuk area, and compared t h i s i c e w i t h wedge and g l a c i e r i c e . . C r y s t a l s i z e was measured and rubbings d i s p l a y e d c r y s t a l shape and con- tained bubbles. The bubbles were unusual i n that.they were p a r a l l e l i n a given c r y s t a l , but not i n adjacent c r y s t a l s . As no c-axis measurements were made i n that study, the planes of bubbles could not be r e l a t e d to c r y s t a l s t r u c t u r e ; however i t seems l i k e l y that they were i n the basal plane. C r y s t a l s i z e was ^ 1 0 mm mean diameter f o r pingo i c e , and <6 mm f o r wedge i c e . As pointed out by M i i l l e r c r y s t a l shapes d i f f e r e d markedly from those i n g l a c i e r i c e , e i t h e r a c t i v e or stagnant.. Some p e t r o f a b r i c a n a l y s i s . o f a segregated i c e body was c a r r i e d out by Mackay and Stager (1956a); Mackay (1972b, p.. 21) described a t h i n s e c t i o n of i c e from a d r i l l core from below the Beaufort Sea. 12 5. Terminology The few papers concerned w i t h the petrology of ground i c e have no co n s i s t e n t terminology f o r f a b r i c p r o p e r t i e s . Black (1953, p. 64) d i s - cusses the a p p l i c a t i o n of igneous and metamorphic. rock terms i n i c e wedges, and points out that t h i s should not be attempted as many such terms are genetic i n connotation. Elsewhere Black (1953, p. 43) l i s t s terms f o r types of dimensional l i n e a t i o n of c r y s t a l s : h o l o c r y s t a l l i n e , anhedral (zenomorphic), subhedral (hypautomorphic), euhedral (automorphic), e q u i - dimensional and elongated. In c o n t r a s t , Corte (1962a) d i d not employ such t e x t u r a l terminology, but used the terms " i r r e g u l a r " and "elongate" f o r c r y s t a l shape.. Shumskii (1964) developed a more complex nomenclature (Table 1), and a l s o gave a s e r i e s of terms, f o r f r o z e n s o i l t e x t u r e which i s discussed in a l a t e r s e c t i o n . Considering deformation textures., we f i n d that Black (1953) discussed the t r a n s f o r m a t i o n of growth " f a b r i c s " by ground expansion i n the summer causing h o r i z o n t a l s t r e s s e s . Previous cracks w i t h a i r bubbles and hoar i n f i l s become shear planes whereas cracks with c l e a r i c e are strong and shear takes place adjacent to them. I n i t i a l hoar c r y s t a l s are r e c r y s t a l - l i z e d and r e o r i e n t e d . Black pointed out the importance of temperature and s t r e s s i n determining the response of c r y s t a l s (p. 76): Rapid flow or shear at low temperatures seems to produce small 'rectangular grains., .but ac high temperatures l a r g e sutured grains seem to r e s u l t . ... o p t i c a x i s 1 ine.ations . . .. seem to be due to the response of i n d i v i d u a l c r y s t a l s to shear, i n which c r y s t a l s r o t a t e to permit g l i d i n g on the basal plane... ' TERM PAGE Euhedral 131 A11 o t: r i o m a r p 11 :i. c - 171 granular Hy p :i. d 1 duo r p h i e - 171 . g.cii ii'j l i i i ; P• ismati.e.- 160 ir.! i ar I n t e r s e r t a l 178-9 Folk LILt Lc 175,179 C a t a e l a s t i c 203,349 CRYSTAL CHARACTERISTICS C r y s t a l s bounded by r e g u l a r faces. I s o m e t r i c , anhadral, random c-ax.es. Golurnnar, i n a zone of geometric s e l e c t i o n ; a l s o c a l l e d c r y s t a l l i n e granular; intermediate between allotrio.norphic-gra.nular and pr isnia-t ic-granu l a r . P a r a l l e l - f i b r o u s o r iented growth; a l s o c a l l e d Panidiomorphic. - ).',ru.iHI I .'i r. Rejected i m p u r i t i e s arranged on g r a i n boundaries (term a l s o employed for frozen ground t e x t u r e ) . C r y s t a l s c o n t a i n i n g insoluble, s o l i d i m p u r i t i e s or f i n e a i r i n c l u s i o n s . Large primary c r y s t a l s remain among f i n e crushed granules. ICE TYPES Extruded i c e ; Perennial vein i c e . Extruded i c e ; Perennial v e i n i c e ; segregation i c e . Vein i c e , naiads, segregated ice and poss iI) 1 y i i I j ec ted ice... Congelation i c e . Congelation i c e . Perennial vein i c e . Table I. Shumskii's t e x t u r a l terminology. 1 4 Corte (1962a, p. 38) summarized h i s r e s u l t s on wedge i c e as: The smaller grains are those formed r e c e n t l y i n a thermal c o n t r a c t i o n crack, while those at.each s i d e (of the wedge) are o l d e r grains formed by r e c r y s t a l l i z a t i o n from small ones. S o i l t e x t u r e I n t e r r e l a t i o n s h i p s between i c e and sediment have been considered by Shumskii (1964) and L i n e l l and Kaplar (1966). Shumskii d i s t i n g u i s h e d textures of frozen ground from i c e t e x t u r e s : (1) i n t e r s e r t a l - r e f e r s to the i n s i t u f r e e z i n g of water without m i g r a t i o n - i c e grains f i l l the pores and are u s u a l l y smaller than the s k e l e t a l p a r t i c l e s (Shumskii 1964, p. 214); (2) p o i k i l i t i c (p. 215) describes s k e l e t a l p a r t i c l e s i n cluded i n . t h e l a r g e c r y s t a l s of the i c e cement which has grown along the pores. L i n e l l and Kaplar (1966) produced a c l a s s i f i c a t i o n system of f r o z e n s o i l s , and gave some d e s c r i p t i o n s on the basis of i c e content, i c e d i s t r i b u - ^ t i o n and i c e type ranging from i c e coatings on p a r t i c l e s to lenses. From the above d i s c u s s i o n i t i s evident that more work i s needed on the growth and deformation of ground i c e w i t h v a r y i n g i n c l u s i o n contents. Since the p u b l i c a t i o n of the above st u d i e s much experimental work on i c e has been performed, which i s reviewed i n the next s e c t i o n . Ice Growth: A Review (a) I n t r o d u c t i o n I t i s obvious that underground i c e bodies have grown under w i d e l y v a r y i n g c o n d i t i o n s of temperature, s o i l type, water supply, and time. P e t r o l o g i c data on s o l i d i f i c a t i o n features of such i c e are l a c k i n g . The i n t e n t i o n i n t h i s s e c t i o n i s to review work on the growth of i c e i n lakes 15 and under experimental c o n d i t i o n s . C o n s i d e r a t i o n i s given f i r s t l y to bulk growth of i c e from pure water; secondly to the r e d i s t r i b u t i o n of s o l u t e s at an advancing ice-water i n t e r f a c e ; and t h i r d l y to the r e j e c t i o n of s o l i d p a r t i c l e s at the i n t e r f a c e . The t h i r d case i s the c l o s e s t approach a v a i l - able i n the l i t e r a t u r e , to i c e growth i n a,porous medium. In the present study we are concerned with such . o p t i c a l l y recognizable f a c t o r s as g r a i n s i z e , g r a i n shape, dimensional o r i e n t a t i o n , , substructure and c r y s t a l l o - graphic o r i e n t a t i o n r e l a t i v e to the growth d i r e c t i o n ; these f a c t o r s are important i n the inference of growth d i r e c t i o n s , and a l s o determine the mechanical and other p r o p e r t i e s of a given i c e body. (b) The F r e e z i n g of Bulk Water Studies of lake i c e and i c e grown i n the l a b o r a t o r y (Perey and Pounder 1958) have shown that p o l y c r y s t a l l i n e aggregates d i s p l a y at l e a s t two t e x t u r a l l y d i s t i n c t zones: (a) a zone of competitive growth at the c o o l i n g surface ( l a k e - a i r i n t e r f a c e or l a b o r a t o r y c e l l w a l l ) ; and (b) a zone of elongated c r y s t a l s a l i g n e d p a r a l l e l to the heat flow d i r e c t i o n . A s i m i l a r banding occurs i n other m a t e r i a l s , such as. metal c a s t i n g s . I n the case of i c e the t e x t u r a l zonation i s accompanied by an increase i n l a t t i c e p r e f e r r e d o r i e n t a t i o n i n the columnar, zone. Numerous workers have i n v e s t i g a t e d c r y s t a l o r i e n t a t i o n s i n lake i c e and p r e f e r r e d o r i e n t a t i o n s were u s u a l l y found, although the o r i e n t a t i o n s reported by d i f f e r e n t authors, and i n some cases the o r i e n t a t i o n s i n d i f f e r e n t p a r t s of the same water body v a r i e d from c-axis h o r i z o n t a l to c-axis v e r t i c a l , to random. M i c h e l and Ramseier (1971) found v e r t i c a l , random and h o r i z o n t a l p r e f e r r e d o r i e n t a t i o n s , but v e r t i c a l columnar c r y s t a l s had h o r i z o n t a l c-axes. Lab- oratory c o n t r o l l e d studies were a l s o i n c o n c l u s i v e (Perey and Pounder 1958; IS Pounder 1963; Harrison and T i l l e r 1963) but a general tendency was found f o r c-axes normal to. the growth d i r e c t i o n . S e v e r a l t h e o r i e s have been proposed .to e x p l a i n the appearance of p r e f e r r e d o r i e n t a t i o n s . Most t h e o r i e s e x p l a i n some experimental and f i e l d ' r e s u l t s , . b u t not a l l . Ketcham and Hobbs (1957) studied the growth of two thousand g r a i n p a i r s and e s t a b l i s h e d that the c o n d i t i o n s f o r one g r a i n (A.) to encroach upon the other.(B) are that: (1 )' B must have i t s c-axis t i l t e d towards the l i n e formed by the i n t e r s e c t i o n of the g r a i n boundary between A and B and the ice-water i n t e r - face c a l l e d "Line L" , ( 2 ) the p r o j e c t i o n on the i c e - l i q u i d i n t e r f a c e of the c-axis of 3 must be perpendicular to l i n e L. • The major p o i n t s to be e x t r a c t e d here are that i n i c e grown, from the melt the i n i t i a l growth i s i n a zone o f randomly o r i e n t e d c r y s t a l s from \ which develops a zone of columnar c r y s t a l s elongated p a r a l l e l to the heat flow or f r e e z i n g d i r e c t i o n , but w i t h c - a x i s o r i e n t a t i o n s normal to that d i r e c t i o n . Thus i f these r e s u l t s can be a p p l i e d t o i c e growth i n perma- f r o s t we have u s e f u l c r i t e r i a f o r determining growth d i r e c t i o n s . However-, under n a t u r a l l y o c c u r r i n g c o n d i t i o n s , i c e growth does not u s u a l l y occur i n pure bulk water, and water, supply i s normally drawn through a porous medium. Thus we must consider these f a c t o r s and t h e i r i n f l u e n c e on growth, c o n d i t i o n s . (c) R e d i s t r i b u t i o n of Solutes The growth of i c e from aqueous s o l u t i o n s i s more complex than f o r pure water because of the r e d i s t r i b u t i o n of s o l u t e s that occurs during s o l i d i f i c a t i o n . ' Weeks and As sur (1.964) presented a theory f o r sea i c e based on the metals l i t e r a t u r e , but ground i c e . except i n unusual circum- stances., has impurity concent rae ioris f a r lower than i n sea i c e . S o l i d s o l u b i l i t y i n i c e i s very low (Glen 1974) and as i c e c r y s t a l s grow there i s r e j e c t i o n of s o l u t e at the i n t e r f a c e i n t o the l i q u i d . Since r e d i s t r i b u t i o n i s p r i m a r i l y by d i f f u s i o n , c o n c e n t r a t i o n gradients are e s t a b l i s h e d i n . t h e l i q u i d w i t h the highest s o l u t e concentrations at the i n t e r f a c e . Consequently an i n i t i a l l y planar i n t e r f a c e becomes unstable to changes i n shape. Any i c e p r o j e c t i o n s i n t o the zone of lower s o l u t e tend to grow and i n t e r d e n d r i t i c spaces are r i c h i n s o l u t e , thus i r r e g u l a r - i t i e s appear on the columnar c r y s t a l s . Although much of the s o l u t e content i s r e j e c t e d at. the i n t e r f a c e , some i n c o r p o r a t i o n occurs at zones of d i s - order such.as g r a i n boundaries and l a t t i c e . d e f e c t s . In the case of i n - s o l u b l e f o r e i g n p a r t i c l e s , d i s l o c a t i o n s are nucleated when p a r t i c l e s are grown i n t o the c r y s t a l . Where the c r y s t a l grows around the i m p u r i t y , the d i s l o c a t i o n s are propagated i n t o the growing c r y s t a l . When d e n d r i t i c growth occurs, the i n t e r d e n d r i t e spaces e v e n t u a l l y c l o s e , o f t e n w i t h a m i s o r i e n t a t i o n , and j o i n i n g occurs by d i s l o c a t i o n s . Other mechanisms of d i s l o c a t i o n formation, and t h e i r mechanical s i g n i f i c a n c e are discussed l a t e r . An important type of s o l u t e i n water i n the f i e l d s i t u a t i o n i s t h a t which forms bubbles on f r e e z i n g . . The presence of gas bubbles i n massive ground i c e bodies has been pointed out by Mackay (19.71) , i n i c e lenses by Gold (1957) and Penner (1961) but no d e t a i l e d d i s c u s s i o n of t h e i r character- i s t i c s has been given. - • Bubbles may ba c h a r a c t e r i z e d by t h e i r s i z e , shape, o r i e n t a t i o n , l a y e r i n g and changes in.those p r o p e r t i e s . But f i r s t l y we must consider • the n u c l e a t i o n of bubbles; t h i s i s approached through standard models of s o l u t e r e j e c t i o n at an advancing s o l i d - l i q u i d i n t e r f a c e (Pohl 1954). The . 18 d i s t r i b u t i o n c o e f f i c i e n t for a i r i n the ice-water system i s taken as 0.01, thus a strong c o n c e n t r a t i o n i s e s t a b l i s h e d ahead of the i n t e r f a c e i n an a i r saturated l i q u i d . N ucleation may occur on p a r t i c l e s , but i n e x p e r i - mental s t u d i e s , Maano (1967) and B a r i and H a l l e t t (1974, p. 503) showed that wetted p a r t i c l e s d i d not produce t h i s e f f e c t . Such would always be the case, i n tha f i e l d s i t u a t i o n ; th'are n u c l e a t i o n probably occurs at p o i n t s of high s o l u t e c o n c e n t r a t i o n . A f t e r n u c l e a t i o n , slow growth gives l a r g e s p h e r i c a l bubbles; Intermediate growth gives a c y l i n d r i c a l shape p a r a l l e l to the growth d i r e c t i o n ; i n r a p i d f r e e z i n g , bubbles are entrapped by ad- vancing c r y s t a l so a small s p h e r i c a l form i s r e t a i n e d (Chalmers 1959). . Very r a p i d f r e e z i n g gives i n s u f f i c i e n t time f o r bubble growth. The shapes are from theory; i n p r a c t i c e d i f f e r i n g shapes may occur c l o s e together,' as depicted by Vasconcellos and Baech (1975 p. 83, F i g . 3 ) , whose c o n t r o l l e d experiments were performed on the water-ice-CO^ system. In the f i e l d the s i t u a t i o n i s more complex, water supply through s o i l may vary, rates of. heat e x t r a c t i o n may vary, and freezing' may be m u l t i d i r e c t i o n a l f o r example i n the a c t i v e l a y e r . (d) Substructure Substructure i s here defined as o p t i c a l l y r e c o g n i z a b l e v a r i a t i o n s i n l a t t i c e p r o p e r t i e s w i t h i n a given c r y s t a l . U s u a l l y t h i s i s confined to • d i f f e r e n c e s i n l a t t i c e o r i e n t a t i o n . D i s l o c a t i o n formation and propagation due t o d e n d r i t i c growth and i n c o r p o r a t i o n o f f o r e i g n atoms has already been discussed. In a d d i t i o n , s m a l l a n g l e boundaries can o c c u r by the amalgamation o f d i s l o c a t i o n s by climb. Tha sub-boundaries so formed i n t e r s e c t the s o l i d - l i q u i d i n t e r f a c e . and a r e p r o p a g a t e d p a r a l l e l co t h e growth d i r e c t i o n , w i t h m i s o r i e n t a t i o n s 19 of s e v e r a l degrees. . Substructures may be produced during growth by mechanically, t h e r m a l l y , or c o m p o s i t i o n a l l y induced s t r e s s e s . Thermal g r a d i e n t s , e s p e c i a l l y non-uniform.gradients, can produce s t r e s s e s which w i l l gen- erate d i s l o c a t i o n s . As discussed above compositional changes occur w i t h v a r i a t i o n s i n f r e e z i n g r a t e . The r e s u l t a n t f l u c t u a t i o n s i n l a t t i c e constant can produce d i s l o c a t i o n s . The c o n c e n t r a t i o n of d i s l o c a t i o n s i s of fundamental importance to the deformation of i c e c r y s t a l s i n post- s o l i d i f i c a t i o n s t r e s s systems. (e) Laboratory Growth of Ice i n Sediments The growth of i c e i n porous media, e s p e c i a l l y s o i l s , has been i n - v e s t i g a t e d f o r over 45 years. E a r l y s t u d i e s . o f s o i l f r e e z i n g and asso- c i a t e d i c e growth were c a r r i e d out by Taber. (1930) and Beskow (1935). Taber pointed out that s e v e r a l f a c t o r s were i n v o l v e d i n i c e segregation: s i z e and shape of s o i l p a r t i c l e s , a v a i l a b i l i t y of water, s i z e and percen- tage of v o i d s , r a t e of c o o l i n g , and r e s i s t a n c e to heaving. A major c o n t r i b u t i o n was Taber's demonstration (p. 308) t h a t growing i c e c r y s t a l s are i n contact w i t h a water f i l m adsorbed on mineral p a r t i c l e s , and water flows through the f i l m to nourish the growing c r y s t a l s . The r e s u l t s of Taber and Beskow were supported by the xrork of Corte (1962b). The base of a w a t e r - f i l l e d box was subjected to f r e e z i n g temper- atures while the top was maintained above 0°C. Some p a r t i c l e s placed on the i c e surface rose w i t h the growing i c e , i n d i c a t i n g the presence of a water l a y e r between- the i c e and s o i l p a r t i c l e s . Such a process i s a n a l - agous to freeze-back from the top o f permafrost. I f the experimental' . 20 system i s - i n v e r t e d , the - mechanism of i c e lens growth i s represented. .In the case where a range of p a r t i c l e s i z e s i s present, s o r t i n g occurs, the i c e excluding those p a r t i c l e s which can migrate through the. pores. Higashi (1958) performed experimental s t u d i e s of f r o s t heaving, and r e l a t e d i c e segregation types to heaving. He c l a s s i f i e d three types of i c e segregation:-- (a) i c e filament l a y e r , (b) s i r l o i n - t y p e f r e e z i n g , (c) concrete-type f r e e z i n g . Types (a) and ( b ) , w i t h high i c e content, occurred under c o n d i t i o n s of slow f r e e z i n g boundary p e n e t r a t i o n . Concrete f r e e z i n g (c) d i d not show any degree of i c e segregation and occurred under . f a s t f r e e z i n g c o n d i t i o n s . Pe\iner (1961) grew i c e lenses i n s o i l w i t h c a r e f u l l y c o n t r o l l e d environment of temperature, pressure and water supply, and found t h a t the s t r u c t u r e of the lenses was not as uniform as a n t i c i p a t e d . Ice g r a i n s were elongated i n the d i r e c t i o n of heat flow, but o p t i c a x i s o r i e n t a t i o n i n adjacent c r y s t a l s was o f t e n markedly d i f f e r e n t . A l s o , c r y s t a l o r i e n t a t i o n was u s u a l l y d i f f e r e n t above and below the s o i l occluded i n the i c e l e n s . This was probably a r e s u l t of n o n - u n i d i r e c t i o n a l heat flow around the contained s o i l . Penner's sample was r e s t r i c t e d to only a few g r a i n s , so no s t a t i s t i c a l a n a l y s i s could be a p p l i e d . - Unpublished work by Kaplar and Goodby at CRREL (Kap l a r , personal communication, 1974) produced r e s u l t s s i m i l a r to those of Penner, but w i t h a stronger c o n c e n t r a t i o n of o p t i c axes p a r a l l e l to the heat flow d i r e c t i o n . ( f ) R e j e c t i o n of I n s o l u b l e P a r t i c l e s at the F r e e z i n g I n t e r f a c e The supply of water to i c e bodies such as lenses i s through a porous medium of sand or c l a y . Thus the' ice-water i n t e r f a c e i s more complicated 21 than i n the f r e e z i n g of bulk water. Mackay (1973a) has pointed out the major d i s t i n c t i o n between pore i c e and segregated i c e , which both form i n . porous media. In a d d i t i o n , experimental s t u d i e s (Uhlmann et a l . 1964; Hoekstra and M i l l e r 1967) have been performed on the i n t e r a c t i o n between i n s o l u b l e suspended p a r t i c l e s and the s o l i d - l i q u i d i n t e r f a c e . These are b r i e f l y reviewed. Uhlmann et a l . (1964) used ice-water and other transparent m a t e r i a l s at v a r y i n g f r e e z i n g r a t e s , and various suspensions, i n c l u d i n g s i l t , w i t h i r r e g u l a r - s h a p e d p a r t i c l e s , s i z e ranging from 1 micron to s e v e r a l hundred microns. At low growth v e l o c i t i e s , p a r t i c l e s were r e j e c t e d at the i n t e r f a c e , and pushed f o r s e v e r a l centimetres. With c l o s e l y spaced par- t i c l e s , as i n s o i l , the process continued, impinging p a r t i c l e s being pushed together. (In the f i e l d there i s a l i m i t to the space a v a i l a b l e f o r r e j e c t e d p a r t i c l e s . ) With i n c r e a s i n g f r e e z i n g r a t e , a c r i t i c a l v e l o c i t y was found at which the p a r t i c l e s ceased to be r e j e c t e d and were i n c o r p o r a t e d i n t o the i n t e r f a c e . There was, however, a dependence on p a r t i c l e s i z e . The c r i t i c a l v e l o c i t y f o r p a r t i c l e s l e s s than 15A i n diameter was inde- pendent of s i z e , whereas f o r l a r g e r p a r t i c l e s , the l a r g e r the s i z e the lower the c r i t i c a l v e l o c i t y f o r t r a p p i n g . Hoekstra and M i l l e r (1967) performed experiments on the ice-water- p a r t i c l e system, employing Pyrex-glass.spheres and s o f t - g l a s s c y l i n d e r s as f o r e i g n p a r t i c l e s . With an upward-moving f r e e z i n g i n t e r f a c e , the c r i t i c a l v e l o c i t y was i n v e r s e l y p r o p o r t i o n a l to the p a r t i c l e r a d i u s . They a l s o found that adding NaCl to the water caused reductions i n the' c r i t i c a l v e l o c i t y . ' • . • Uhlmann et a l . (1964). and Hoekstra and M i l l e r ' (1967) i n t e r p r e t the r e j e c t i o n of p a r t i c l e s as being due to an imbalance of surface t e n s i o n 22 forces between the p a r t i c l e , water and i c e . In order to s a t i s f y the energy requirements and keep the p a r t i c l e ahead of the i n t e r f a c e , ' l i q u i d r e p l e n i s h e d the i c e behind the p a r t i c l e s . I t was argued t h a t the p a r t i c l e - . i n t e r f a c e s e p a r a t i o n decreased w i t h increased f r e e z i n g . r a t e , and the c r i t i c a l v e l o c i t y corresponded to the po i n t at which f u r t h e r decrease of p a r t i c l e - i n t e r f a c e s e paration would lower the chemical p o t e n t i a l of the system, rendering the pushing c o n f i g u r a t i o n unstable and a l l o w i n g i n c o r - p o r a t i o n of the p a r t i c l e . Grooves or depressions on the i n t e r f a c e tended to trap the p a r t i c l e s at lower growth r a t e s . Thus i t i s to be expected t h a t g r a i n boundaries would trap - s o l i d p a r t i c l e s . Ketcham and Hobbs (1967) described an experiment whereby a piece of f i n e copper wire was placed i n a sample of p o l y c r y s t a l l i n e i c e and obser- v a t i o n s were made of the i c e surface as i t grew i n t o the water. I t was found that g r a i n boundaries moved away from the v i c i n i t y of the w i r e , which was a s c r i b e d to the wire forming a more e f f i c i e n t heat s i n k than the i c e , thus causing the grains adjacent to the w i r e to protrude f u r t h e r i n t o the water than the surrounding g r a i n s . Such v a r i a t i o n s i n temperature • . . across the i c e i n t e r f a c e due to i n c l u s i o n s would be expected to a f f e c t r e l a t i v e c r y s t a l growth. (g) Conclusion Few s t u d i e s have been performed on the petrology of underground i c e ; however, the r e s u l t s of the more d e t a i l e d s t u d i e s of i c e growth -in l a b o r a - t o r i e s and surface i c e types (sea, l a k e , r i v e r ) may be a p p l i e d to perma- f r o s t c o n d i t i o n s . Review of i c e growth' i n bulk water has shown that- f r e e z i n g d i r e c t i o n s may be i n f e r r e d i n terms of zonation of c r y s t a l s i z e and shape, bubble zones and bubble e l o n g a t i o n d i r e c t i o n . Bubble type 23 depends on f r e e z i n g r a t e , as w e l l as the amount o f d i s s o l v e d a i r i n t h e w a t e r . I n n a t u r a l l y o c c u r r i n g f i e l d s i t u a t i o n s f r e e z i n g r a t e s and s o l u t e c o n t e n t s a r e u s u a l l y unknown, thus r i g o r o u s a p p l i c a t i o n o f t h e o r y i s i m p o s s i b l e , but where b u l k w a t e r f r e e z e s the g e n e r a l p r i n c i p l e s may be e x p e c t e d to h o l d . Under p e r m a f r o s t c o n d i t i o n s : w a t e r s u p p l y i s g e n e r a l l y t h r o u g h a permeable medium. From the r e v i e w o f r e j e c t i o n o f i n s o l u b l e p a r - t i c l e s a t an i c e - w a t e r i n t e r f a c e , i t i s a p p a r e n t t h a t p r e f e r e n t i a l r e j e c t i o n o f f i n e - g r a i n e d p a r t i c l e s may o c c u r but d u r i n g r a p i d i n t e r f a c e a d v a n c e , a l l . p a r t i c l e s may be e n g u l f e d . Thus c r u d e e s t i m a t e s o f f r e e z i n g r a t e may be g a i n e d from sediment c o n t e n t , a l t h o u g h t h i s a l s o depends on.water a v a i l a - b i l i t y . A d d i t i o n a l l y d u r i n g f r e e z i n g , d i s l o c a t i o n s a r e i n c o r p o r a t e d i n t h e c r y s t a l g r o w t h mechanism, and a l s o a t i n c l u s i o n s . A r r a y s o f d i s l o c a t i o n s a r e o f fundamental" i m p o r t a n c e i n d e f o r m a t i o n . p r o c e s s e s . I t appears t h a t t h e major d i f f e r e n c e s i n i c e grown i n sediment from i c e grown i n b u l k w a t e r a r e c r y s t a l s i z e , d i s l o c a t i o n c o n t e n t and sediment i n c o r p o r a t i o n w h i c h a l l i n f l u - ence f l o w c h a r a c t e r i s t i c s . 7. P o s t - F r e e z i n g Phenomena (a) I n t r o d u c t i o n Some o f t h e . f e a t u r e s r e s u l t i n g f r o m f r e e z i n g a r e s u b j e c t t o s e v e r a l t h e r m a l l y and m e c h a n i c a l l y i n d u c e d phenomena, w h i c h must be r e c o g n i z e d . (b) T h e r m o r a i g r a t i o n F o r the moment l e t us c o n s i d e r the t h e r m a l f i e l d and i t s e f f e c t s . I n p e r m a f r o s t c o n d i t i o n s the upper 10 t o 20 m o f e a r t h m a t e r i a l s are. s u b j e c t to a p p r e c i a b l e annua.!' t e m p e r a t u r e v a r i a t i o n s . The r e s u l t i n g t emperature g r a d i e n t s may g i v e r i s e to t h a r m o m i g r a t i o n o' f i n c l u s i o n s . F o r example, 24 Stehle (1967) and K h e i s i n and Cherepanov (1969) have reported bubble migra- t i o n and the breakup of c y l i n d r i c a l bubbles i n t o s t r i n g s of s p h e r i c a l bubbles i n lake i c e . I n a d d i t i o n the mi g r a t i o n of s a l i n e i n c l u s i o n s i n i c e has been observed. The m i g r a t i o n of. such drop l e t s was observed i n experiments by Ha r r i s o n (1965) as f o l l o w s : - (a) droplet e l o n g a t i o n i n the m i g r a t i o n d i r e c - t i o n , (b) diagonal .migration -- due to mig r a t i o n p a r a l l e l to the c- a x i s r a t h e r than i n the heat flow d i r e c t i o n . I t i s evident that w h i l e a v e r t i c a l temper- ature gradient p r e v a i l s i n permafrost, m i g r a t i o n might occur p a r a l l e l to c-axes. Thus thermomigration i s a problem to be considered i n i c e p e t r o l o g y . (c) The Deformation of Ice (1) I n t r o d u c t i o n Under n a t u r a l permafrost c o n d i t i o n s i c e bodies are su b j e c t to v a r i o u s s t r e s s f i e l d s , e.g. thermally induced s t r e s s e s a s s o c i a t e d w i t h annual expan- s i o n and c o n t r a c t i o n of the upper ground l a y e r s ; a d d i t i o n a l l y some massive i c e bodies have t h i c k cores (Mackay 1973b) which have s u f f e r e d d i f f e r e n t i a l u p l i f t and may be expected to creep. .Thus a review i s made of the deformation mechanisms i n i c e , based on l a b o r a t o r y and g l a c i e r s t u d i e s , emphasis being g i v e n to pe t r o - graphic and p e t r o f a b r i c f e a t u r e s . The most common experimental technique i n the study of the deformation of i c e has i n v o l v e d e x e r t i n g compressive, t e n s i l e , or t o r s i o n a l s t r e s s e s on a c y l i n d r i c a l sample of p o l y c r y s t a l l i n e i c e . Frequently the experiments have been d i r e c t e d toward a q u a n t i t a t i v e a n a l y s i s of the flow, but o f t e n the r e s u l t s are i n t e r p r e t e d i n terms of i n t r a c r y s t a l l i n e and i n t e r c r y s t a l l i n e s l i d i n g , e t c . Some workers have 25 prepared t h i n s e c t i o n s from the i n i t i a l and deformed i c e i n order to examine changes i n c r y s t a l s i z e , shape, o r i e n t a t i o n , s u b s t r u c t u r e , and to r e l a t e these changes to. the s t r e s s f i e l d . • By t h i s method bulk r e l a t i o n s h i p s are • obtained, but d e t a i l e d knowledge of i n d i v i d u a l c r y s t a l s i s not always a v a i l - a b l e . S i m i l a r l y , - s t u d i e s on g l a c i e r - samples have been performed, but i n these cases the s t r e s s and s t r a i n f i e l d s are poorly known, thus i n t e r p r e t a - t i o n of f a b r i c diagrams i s d i f f i c u l t . . I n the present study no f i e l d measure- ments of s t r a i n were p o s s i b l e , but gross estimates are a v a i l a b l e from the t h e o r e t i c a l work of Lachenbruch (1962) and comparison w i t h g l a c i e r s t u d i e s . ( i i ) Deformation mechanisms i n i c e The major work i n t h i s f i e l d has been by Gold (1953, 1972), and by Kamb (1972) who performed long-term, high-temperature (-5°C to 0°C) e x p e r i - ments on p o l y c r y s t a l l i n e i c e , and presented photomicrographs and p e t r o f a b r i c diagrams r e p r e s e n t a t i v e of several, stages of the flow. E a r l i e r Steinemann (1954) and Shumskii (1958) deformed and annealed p o l y c r y s t a l l i n e i c e , and analyzed the r e c r y s t a l l i z a t i o n . - Kamb (1972) studi e d p e t r o f a b r i c and t e x t u r a l changes during flow, and showed that g r a i n s i z e and shape changed from 0.5 - 0.9 '.-nm equant, s t r a i g h t sided grains to coarser, h i g h l y i n t e r l o c k i n g shapes by a g r a i n boundary m i g r a t i o n mechanism.. The s i z e increase was enhanced nearer the melting p o i n t . S t r a i n shadows, frequent at a l i temperatures, were thought to i n d i c a t e k i n k i n g during t r a n s l a t i o n g l i d i n g on the b a s a l plane. The i n i t i a l p e t r o f a b r i c p a t t e r n of the i c e was approximately random, but o r i e n - t a t i o n s became s t r o n g l y p r e f e r r e d during ' r e c r y s t a l l i z a t i o n . Two maxima developed i n simple shear, the stronger maximum being normal to the shear 26 plana and the second i n c l i n e d at about 20° to the d i r e c t i o n of shear. Shumskii (1953).found a s i m i l a r p a t t e r n i n a shear experiment, and argued that the major maximum•comprised r e l a t i v e l y unstrained, grains grown d u r i n g deformation, but Karab (1972) found no such d i s t i n c t i o n . An important r e s u l t of Kamb's work was to show the dependence of p e t r o f a b r i c s on shear s t r e s s , by comparing p e t r o f a b r i c s of two samples deformed to the same t o t a l shear s t r a i n but at d i f f e r e n t s t r e s s e s . He went on to discuss the r e l a t i o n s h i p of g r a i n s i z e and shape to flow. The i n c r e a s e i n g r a i n boundary i r r e g u l a r i t y would be expected t o decrease the r o l e of g r a i n boundary s l i d i n g , and g r a i n coarsening should, increase the creep r a t e . I n t r a c r y s t a l l i n e p l a s t i c flow caused s t r a i n shadows, but these d i d not i n - crease w i t h t o t a l s t r a i n i n d i c a t i n g that u n d i s t o r t e d c r y s t a l l i n e m a t e r i a l was generated during r e c r y s t a l l i z a t i o n . I t was.found that the shape changes occurred much more r a p i d l y than p e t r o f a b r i c changes, a l s o t e x t u r e i s temper- a t u r e - s e n s i t i v e i n that the higher the temperature the coarser the g r a i n s i z e . Conversely, p e t r o f a b r i c s show no such s e n s i t i v i t y . I f an attempt i s made to r e l a t e t e x t u r a l changes to s t r e s s , .there is. the problem of temperature dependence, and the wide range of temperatures employed by d i f f e r e n t workers. But i t i s apparent from.Kamb's (1972) work that g r a i n boundary mi g r a t i o n occurs over.a wide range, of s t r e s s e s , i n c l u d i n g those below i kg cm"-. Although po.lygonization and primary r e c r y s t a l l i z a t i o n were reporte d by Shumskii ' (1958)•and Gold (1953) the two processes were not d i s t i n g u i s h e d p e t r o l o g i c a l l y , nor were the s t r e s s e s given. Kamb's (1972) r e s u l t s ' are h e l p f u l i n that, l a t t i c e o r i e n t a t i o n s were measured, which showed that s t r e s s e s "> 1 kg cm ~" are necessary f o r new c r y s t a l growth.. 27 (d) The Influence of Inreurities. on Deformation ( i ) I n t r o d u c t i o n Ice bodies i n permafrost may c o n t a i n i n c l u s i o n s i n the s o l i d , l i q u i d or gaseous s t a t e s . Therefore as some deformation mechanisms depend on unin t e r r u p t e d movement of boundaries and d i s l o c a t i o n s , f o r e i g n atoms or gross defects w i l l a f f e c t the deformation process. ( i i ) S o l i d I n c l u s i o n s The i n c l u s i o n s which are of primary i n t e r e s t i n the study of the petrology of underground i c e are sediment p a r t i c l e s , r a t h e r than s o l i d s o l u t i o n s , as only o p t i c a l methods were employed i n the present f i e l d study. Se v e r a l xwrkars have s t u d i e d the creep of frozen s o i l s and i c e c o n t a i n i n g dispersed sand (Goughnour and Anders land 1 9 6 8 ; Hooka et a l 1 9 7 2 ; Ladanyi 1972) but no t h i n s e c t i o n analyses ware performed. Thus the r e l a t i o n s h i p of i n c l u - sions to g r a i n c h a r a c t e r i s t i c s i s unknown. The e f f e c t of immobile second phases on g r a i n boundary motion i s de- termined by the boundary type and p o s i t i o n on the boundary. _High boundary curvature i n d i c a t e s the p o s s i b i l i t y of more r a p i d movement, and thus the i n c l u s i o n w i l l have- l i t t l e a f f e c t , whereas- p i n n i n g of boundaries of lower curvature i s l i k e l y . The in f l u e n c e of second phases at c r y s t a l . t r i p l e p o i n t s has not been considered i n the l i t e r a t u r e . ( i i i ) Gageous Inclusions'; A high c o n c e n t r a t i o n of gas bubbles occurs i n a l l n a t u r a l i c e s -. g l a c i e r , lake, saa and permafrost i c e - but the a v a i l a b l e l i t e r a t u r e contains no reference to f i e l d s t u d i e s of the in f l u e n c e of such bubbles on deformation. 28 A l s o , the experimental work on the e f f e c t of bubbles on the deformation of i c e i s f a r from c o n c l u s i v e . I t i s i n t e r e s t i n g to compare the r e s u l t s , of Steinemann (1958), Kamb (1972) and Kuon and Jonas (1973) i n terms of the i n f l u e n c e of a i r bubbles . on r e c r y s t a l l i z a t i o n and g r a i n growth. Kamb (1972, p. 233) has pointed out that Since the changes i n te x t u r e and f a b r i c here i n specimens c o n t a i n i n g abundant a i r bubbles ware comparable to the changes observed by Steinemann (1958) i n a i r - f r e e samples, i t f o l l o w s that the a i r bubbles do not s u b s t a n t i a l l y i n h i b i t the processes of r e c r y s t a l l i z a t i o n and g r a i n growth. On the other hand, Kuon and Jonas (1973) found a d i s t i n c t i n f l u e n c e of. the a i r bubbles i n r e t a r d i n g g r a i n boundary motion, and thus i n f l u e n c i n g the g r a i n growth process. The r e s u l t s of Kuon. and Jonas (1973) are i n agreement w i t h other r e s u l t s i n the ma t e r i a l s science l i t e r a t u r e ( G l e i t e r and Chalmers 1968) and i n the absence of more d e t a i l e d work on i c e , are accepted here. (e) The Fr a c t u r e of Ice I n t r o d u c t i o n F r a c t u r e occurs when the l a t t i c e loses cohesion. F r a c t u r e i n columnar grained i c e under compressive s t r e s s has been reviewed comprehensively by Gold (1972) but he d i d not t r e a t the thermal c o n t r a c t i o n mechanism which i s discussed below. Gold '(1961) discussed the c r y s t a l i o g r a p h i c dependence of cracks produced by thermal shock. 29 ( i i ) Thermal Stresses' The temperature of the upper few metres of permafrost v a r i e s sea- s o n a l l y . In p a r t i c u l a r , a steep v e r t i c a l temperature gradient becomes e s t a b l i s h e d i n winter. Thermal c o n t r a c t i o n i s l a r g e l y c o n s t r a i n e d , and t e n s i l e s t r a i n s a r i s e , which are a f u n c t i o n of the temperature d i f f e r e n c e between the ground surface and an "average" temperature, and the c o e f f i c i e n t of thermal c o n t r a c t i o n over that temperature range. There may be, however, a r a p i d d i s s i p a t i o n of thermal s t r a i n w i t h time. ( i i i ) Factors I n f l u e n c i n g F r a c t u r e Crack patterns developing under thermal shock i n i c e p l a t e s ware shown by Gold (1951) to be dependent on the c r y s t a l l o g r a p h i c o r i e n t a t i o n of the i c e w i t h respect to the shocked s u r f a c e . A preference was found f o r the surface trace of cracks to be. p a r a l l e l to the planes c o n t a i n i n g 'a' and • . 'c' d i r e c t i o n s . Abrupt changes i n crack d i r e c t i o n i n passing from one g r a i n to the next were observed. . In the f a i l u r e experiments of Gold (1972) and others the i c e samples have been f a i r l y pure or at l e a s t deaerated. By comparison, permafrost i c e u s u a l l y has a high.bubble, sediment and s o l u t e content. A l s o ground i c e may have a wall-developed c r y s t a l s ubstructure and v a r y i n g c r y s t a l s i z e and shape, compared w i t h the more uniform l a b o r a t o r y samples. F u r t h e r , a l l i c e bodies near the ground surface have complex c y c l i c s t r e s s h i s t o r i e s and may have been subject to s u b s t a n t i a l r e c r y s t a l l i z a t i o n , with an e f f e c t on 'subsequent f r a c t u r e c h a r a c t e r i s t i c s . 30 ( f ) •Conelu5 ion Mackay (19 71, 19 72b) has pointed out the wide range of c o n d i t i o n s to •which various ground ice types are subject. In the present review of post- s o l i d i f i c a t i o n phenomena we have considered the response of i c e to.the experimental i m p o s i t i o n of thermal gradients and mechanical l o a d i n g , and pointed out p e t r o l o g i c c r i t e r i a ' w h i c h may be a p p l i e d to the f i e l d s i t u a t i o n . 31 Chapter 3 TECHNIQUES 1. I n t r o d u c t i o n The major o b j e c t i v e of t h i s s t udy i s an understanding of mechanisms of growth and deformation of i c e bod ie s i n permafrost.in r e l a t i o n to t h e i r thermal, s t r e s s and gepmorphic environments. To t h i s end a sampling p l a n was e s t a b l i s h e d f o r exposed i c e bodies such that s t r a t i g r a p h i c and s t r u c - t u r a l r e l a t i o n s were known, and c r y s t a l and i n c l u s i o n c h a r a c t e r i s t i c s could be r e l a t e d to those data. F i e l d and l a b o r a t o r y techniques are discussed s e p a r a t e l y . 2. F i e l d Techniques Maps'of physiographic provinces were presented by Mackay (1963) and of sediment d i s t r i b u t i o n by Rampton (1972 a,b); the d i s t r i b u t i o n of i c e bodies i n . t h e f i e l d area has not been given and mapping i s not attempted Sampling was r e s t r i c t e d to those i c e bodies exposed on c o a s t a l s e c t i o n s or.subject to easy d r i l l i n g . . As an example, consider the case of an i n v o l u t e d h i l l , near Tuktoyaktuk. Here a. massive i c e core has been exposed f o r many years. I t i s apparent on 1935 a i r photographs ( a i r photo #A 5023-S7R). A sampling plan was devised to i n v e s t i g a t e : nere. Sampling (I) the r e l a t i o n s h i p among c r y s t a l and i n c l u s i o n c h a r a c t e r i s t i c s and depth i n core samples of r e l a t i v e l y undeformed i c e ; 32 (2) the r e l a t i o n s h i p of c r y s t a l and i n c l u s i o n c h a r a c t e r i s t i c s to f o l d symmetry where d i f f e r e n t i a l u p l i f t has occurred; (3) the i n f l u e n c e of wedge p e n e t r a t i o n i n a f o l d . A d d i t i o n a l l y core samples were taken from a 6 m deep p i t for compar- i s o n w i t h samples from the c l i f f exposures to i n v e s t i g a t e the e f f e c t on t e x t u r e and p e t r o f a b r i c s of load r e l e a s e and changed thermal regime due to c o a s t a l r e c e s s i o n . Elsewhere s i m i l a r p r i n c i p l e s were employed i n order to examine texture and p e t r o f a b r i c s w i t h reference to macroscopic symmetry, e.g. f o l i a t i o n s i n i c e wedges. For the purpose of understanding p o s t - s o l i d i f i c a t i o n changes i n a given i c a type, bodies of known age ( i . e . those which grew between the two f i e l d seasons of 1973 and 1974) were compared w i t h older bodies. This was p o s s i b l e f o r t e n s i o n crack i c e , where crack i c e i n the a c t i v e l a y e r of 1973 was sampled i n 1974. A l s o i c i n g mounds which grew a f t e r , f r e e z e b a c k of the 1973 a c t i v e l a y e r were sampled by Dr. J.R. Mackay; t h i s provided i n f o r m a t i o n concerning s o l i d i f i c a t i o n features and the response of. e a r l y c r y s t a l l a y e r s to heave. A c t i v e l a y e r i c e was a l s o studied.. In the case of i c e grown i n f r a c t u r e s the only i c e of known age was the t e n s i o n crack i c e , mentioned, above. No example of 1973-1974 wedge c r a c k . i c e was obtained, although recent thermal c o n t r a c t i o n crack i c e was found and the prograde f a b r i c s of wedges p e n e t r a t i n g sediment and massive i c e were compared. Thus i t was p o s s i b l e to compare i c e growth i n the two major f r a c t u r e types: tension cracks (mechanically induced) and ice.wedge cracks ( t h e r m a l l y induced). 33 Lake .ice'was-also studied i n order to i n v e s t i g a t e the p o s s i b i l i t y of buried lake i c e being mistaken f o r i c e grown i n s i t u . A SIPRE corer and a chain saw were employed f o r sampling; t h i s has been a standard method on other Ice types, e.g. g l a c i e r s , sea and lak e i c e . I t provided a r a p i d means of o b t a i n i n g good core lengths of 75 mm diameter and was used with a power u n i t when c o l d storage f a c i l i t i e s were nearby. Hand d r i l l i n g was c a r r i e d out i n an underground p i t and samples were s t o r e d there f o r t r a n s f e r by h e l i c o p t e r to the l a b o r a t o r y . Where c o r i n g was not p o s s i b l e , as around f o l d s on c l i f f s , a c h a i n saw was employed; t h i s method was p r e v i o u s l y used on ground i c e by Corte (1962a) .and on g l a c i e r s by Colbeck and Evans (1973). As i c e on 'the c l i f f s had s u f f e r e d changes i n loading and thermal c o n d i t i o n s due to c o a s t a l r e t r e a t , a channel was f i r s t cut i n t o the c l i f f and samples taken from the back of the channel and l o c a l v a r i a t i o n s i n c r y s t a l and i n c l u s i o n c h a r a c t e r - i s t i c s were sought, f o r example T y n d a l l f i g u r e s . A d d i t i o n a l l y samples of cores from a man-made p i t were compared, w i t h c l i f f samples. Sawn samples were t r a n s f e r r e d as q u i c k l y as p o s s i b l e , u s u a l l y <^ hour, i n f r e e z e r boxes to c o l d storage at -20°C. The e f f e c t of storage on c r y s t a l c h a r a c t e r i s t i c s has been s t u d i e d by Carte (1961b) who showed that t h i n s e c t i o n s could be stored at -20°C f o r months without major adjustment of g r a i n boundaries. Kamb (1972) stored, t h i n s e c t i o n s at a s i m i l a r temperature, and B a r i and H a l l e t t (1974) sug- gested storage at or below -20°C to avoid changes i n bubble c h a r a c t e r i s - t i c s . In the present study most core and block samples were analyzed w i t h i n a few days of sampling, but some were stored at <-20°C f o r about 8-10 months. Thick sections•were prepared and sandwiched between g l a s s • 34 s l i d e s with" a v a s e l i n e s e a l around the edge, to prevent s u b l i m a t i o n , and stored w i t h the samples; photographic s l i d e s taken before and a f t e r storage showed no reco g n i z a b l e change. Thus i t i s concluded that stored samples are reasonably r e p r e s e n t a t i v e of the f i e l d s i t u a t i o n . 3. Laboratory Techniques Th i n s e c t i o n p r e p a r a t i o n v a r i e d w i t h the i c e type, i n terms of i n c l u s i o n content and c r y s t a l s i z e . In the case of c l e a n , f i n e c r y s t a l l i n e i c e , a microtome was used, as '. . described by Langway (1958) and Mic h e l and Ramseier (1971). For c o a r s e l y c r y s t a l l i n e i c e , a more r a p i d method was employed, namely f r e e z i n g a smoothed t h i c k l y sawn s e c t i o n to a s l i d e , then t h i n n i n g w i t h a f l a t metal p l a t e , and sand- and emery-paper. This i s standard p r a c t i c e i n g l a c i e r s t u d i e s , and K r e i t n e r (1969) showed, i n a study of a u f e i s , that smoothing of sect i o n s w i t h a warm i r o n produced no change i n c r y s t a l c h a r a c t e r i s t i c s . In some i c e s there were bands of high sediment content, and the above two methods were i m p r a c t i c a l . In such cases gradual t h i n n i n g was p o s s i b l e u s i n g emery paper and carborundum; sediment p a r t i c l e s were removed w i t h a p o i n t . As found by Black (1953) and Corte (1962a) the best temperature f o r t h i n s e c t i o n p r e p a r a t i o n was about -10° C. As the study was f i e l d - b a s e d , only o p t i c a l methods were employed. T h i n s e c t i o n a n a l y s i s and u n i v e r s a l stage technique i s standard, and no 'discussion i s given here. Ice c r y s t a l c-axes ware measured and i n some cases a-axis o r i e n t a t i o n s were found by e t c h i n g , although t h i s was not widely employed. Where p o s s i b l e , at l e a s t 100 c-axes were measured and p l o t t e d on equal area, lower hemisphere p r o j e c t i o n s . In g e n e r a l , s c a t t e r , diagrams are given together with component diagrams based on such c r y s t a l c h a r a c t e r i s t i c s as s i z e , shape, su b s t r u c t u r e , i n c l u s i o n content and r e l a - t i o n s h i p to l a y e r i n g s . Most of the patterns show a high degree of pre- f e r r e d o r i e n t a t i o n and . contouring i s not always employed; but i n order to emphasize progressive changes i n f a b r i c some diagrams have, been contoured by Kamb's (1959) method. The method i n d i c a t e s the s t a t i s t i c a l s i g n i f i c a n t of o r i e n t a t i o n maxima. In the present work, contour i n t e r v a l s of 2«" are used, where <r i s the standard d e v i a t i o n from the expected d e n s i t y of a uniform p o p u l a t i o n . I n c l u s i o n l a y e r i n g s , f o l i a t i o n s , e t c . , are included to show symmetry r e l a t i o n s h i p s . C r y s t a l dimensional o r i e n t a t i o n s ' a r e given i n separate diagrams. 36 Chapter 4 RESULTS Permafrost c o n d i t i o n s i n the f i e l d area, the v a r i e t y of contained ground i c e types and t h e i r r e l a t i o n s h i p s to s u r f i c i a l form have been p o i n t e d out. I t i s the . i n t e n t i o n i n the present chapter t o d i s c u s s the growth and deformation of i c e bodies l a r g e l y from.a p e t r o l o g i c viewpoint. Thus Ice bodies of known ages ( < 1 year) were analyzed i n order to enumerate t y p - i c a l growth f e a t u r e s . Growth c o n d i t i o n s are d i s c u s s e d i n terms of water • supply, f r e e z i n g d i r e c t i o n s and r a t e s , s o l u t e r e j e c t i o n (bubble formation) and c r y s t a l s i z e , shape and l a t t i c e and dimensional o r i e n t a t i o n . . With t h i s knowledge as a foundation, c o n s i d e r a t i o n i s given to o l d e r i c e bodies, and p o s t - s o l i d i f i c a t i o n features are.discussed i n r e l a t i o n to deformation asso- c i a t e d w i t h , f o r example, growing i c e wedges. A summary diagram, based on Mackay rs (1972b) c l a s s i f i c a t i o n of under-' ground i c e i s given i n Appendix 1. The diagram i s an attempt at showing i n t e r r e l a t i o n s h i p s among i c e types, surface form, and water source and t r a n s f e r mechanisms. E x t r a .categories are i n c l u d e d to i n d i c a t e i n c l u s i o n , t e x t u r a l . a n d p e t r o f a b r i c features s i g n i f y i n g growth and l a t e r c o n d i t i o n s . Chapter O u t l i n e An examination of lake i c e introduces the d i s c u s s i o n of i c e types. The i c e grew during winter 1973-74 and represents growth i n bulk water. This i s followed by c o n s i d e r a t i o n of s e v e r a l topographic r i s e s of v a r i o u s s i z e s , u n d e r l a i n by i c e w i t h d i f f e r e n t i n c l u s i o n contents. Such i c e bodies, and a l s o frozen sediment, may be f r a c t u r e d . . Thus a d i s c u s s i o n i s given of mechanically- and thermally-induced f r a c t u r e s . T h i s Is followed by i n v e s - 37 t i g a tion.- of other i c e types which d i f f e r i n form and i n r e l a t i o n to surface, expression. The importance of m u l t i p l e growth and melt periods are p o i n t e d . out. . Lake Ice I n t r o d u c t i o n The p o s s i b l e existence of a v a r i e t y of surface i c e s , which became buried and preserved w i t h i n permafrost, has been pointed out (Mackay 1972a, p. 5) although they are thought to be uncommon i n North America. The case against a buried o r i g i n of massive., i c e bodies has been given by Mackay (1971, 1973b) i n terms of i c e body t h i c k n e s s , topographic p o s i t i o n , s t r a t i g r a p h i c p o s i t i o n , water q u a l i t y , i c e f a b r i c s and bubble p a t t e r n s . I t i s to be expected that small i c e bodies may have o r i g i n a t e d from b u r i e d snow banks, lake i c e or sea i c e where coasts are a c t i v e l y slumping. I t i s the i n t e n t i o n i n t h i s s e c t i o n to examine non-buried lake i c e , as an a i d i n d i s t i n g u i s h i n g such i c e from underground i c e grown i n s i t u . F i e l d C h a r a c t e r i s t i c s A s i t e was chosen on a small shallow lake near Tuktoyaktuk. The c o r i n g p o s i t i o n was nearshore but not l i a b l e to slumping.. A core 0.68 m long was obtained,- i n c l u d i n g dark i c e at the top and the contact w i t h sediment and organic matter at the base. Ice C h a r a c t e r i s t i c s The upper 0.08 a comprised dark i c e and the remainder had. long.' v e r t i c a l bubbles. .So disturbance of the p a t t e r n was observed,' e i t h e r ' i n terms of growth c o n d i t i o n s or p o s t - s o l i d i f i c a t i o n s t r u c t u r e s . 33 Bubble C h a r a c t e r i s t i c s In the upper 0.08 m i s a high co n c e n t r a t i o n of apparently randomly s c a t t e r e d bubbles which are s p h e r i c a l and <^0.5 mm diameter together w i t h some f i n e v e r t i c a l bubbles. At 0.08 m depth the bubble, p a t t e r n changes; l a r g e , 15 mm long, 3 mm diameter v e r t i c a l l y elongated bubbles appear, superimposed on the upper p a t t e r n . A d d i t i o n a l l y there are v e r t i c a l t r a i n s of small ( < 1 mm) s p h e r i c a l bubbles separated by 2-3 mm. This p a t t e r n continues to the.basal contact w i t h sediment. C r y s t a l C h a r a c t e r i s t i c s " • . Figure 2 (a)-(e) demonstrates the change i n c r y s t a l s i z e and shape w i t h depth. The h o r i z o n t a l s e c t i o n s ( F i g . 2 ( a ) , ( b ) , ( c ) ) are at depths 0.015 m, 0.3 m and 0.5 m. In the upper s e c t i o n c r y s t a l s are g e n e r a l l y r e l a t i v e l y small,.6 mm x 3. mm; there i s an increase i n s i z e downwards to 30 mm x 20 mm at 0.5 m depth ( i n h o r i z o n t a l s e c t i o n s ) . C r y s t a l shape i s anhedral and s e r r a t e d , g i v i n g an i n t e r d i g i t a t i n g but not complexly i n t e r - l o c k i n g p a t t e r n , as found by Ragle (1963). V e r t i c a l s e c t i o n s show the v e r t i c a l dimensional o r i e n t a t i o n t y p i c a l of the f r e e z i n g of bulk water ( F i g . 2 ( d ) , ( e ) ) . A marked zone of wedging out of competing c r y s t a l s e x i s t s i n the upper 100 mm, and at 200 mm depth c r y s t a l s have widened to >10 mm and may exceed 200 mm i n length.. There are no l a t e r a l i r r e g u l a r i - t i e s i n c r y s t a l shape. This p a t t e r n continues to the base of the core, and i n d i v i d u a l c r y s t a l s were t r a c e a b l e f o r over 0.35 m. There i s no pronounced substructure throughout. The r e l a t i o n s h i p between bubbles and c r y s t a l c h a r a c t e r i s t i c s i s such that i n the upper zone of high bubble c o n c e n t r a t i o n , bubbles .are both 39 Figure 2. Thin sections of lake i c e . Crossed p o l a r i z e r s T o p 40 41 •• i n t e r g r a n u l a r and. i n t r a g r a n u l a r ; at depth the l a r g e r elongated bubbles are f r e q u e n t l y on v e r t i c a l boundaries. At 0.5 m elongate bubbles are not. always v e r t i c a l although groups'are p a r a l l e l w i t h i n i n d i v i d u a l c r y s t a l s , which i n d i c a t e s l a t t i c e c o n t r o l . ..' L a t t i c e o r i e n t a t i o n s are shown In F i g u r e 3. A h o r i z o n t a l c - a x i s p r e f e r r e d o r i e n t a t i o n . i s evident throughout, but w i t h a p r o g r e s s i v e decrease i n spread about the h o r i z o n t a l w i t h depth. The small number of c-axes p l o t t e d f o r the p e t r o f a b r i c diagrams of deep i c e i s due to the small number of long c r y s t a l s . The c o n c e n t r a t i o n of c-axes represents a high s e l e c t i v i t y of l a t t i c e o r i e n t a t i o n s . I n t e r p r e t a t i o n The i c e i s known to have grown over one winter (1973-74). The body was not observed during growth, nor were any major surface s t r u c t u r e s v i s i b l e at the time of sampling owing to presence.of a small snow cover. The bubble and c r y s t a l c h a r a c t e r i s t i c s are t y p i c a l o f . l a k e i c e ' f r o m other areas (Knight 1952a; Lyons and S t o i b e r 1952; Ragle 1953).. There i s evidence f o r only one period and d i r e c t i o n of growth; downward from the top, as a competitive zone of c r y s t a l growth occurs there, w i t h a p r o g r e s s i v e downward increase i n l a t e r a l s i z e of v e r t i c a l l y elongated c r y s t a l s . . C-axis o r i e n t a t i o n s • a r e h o r i z o n t a l throughout i n d i c a t i n g b a s a l plane growth; the range-of v a r i a b i l i t y around the h o r i z o n t a l decreases w i t h depth. Bubble . patterns are. a l s o i n d i c a t i v e of v e r t i c a l growth.. There i s no evidence f o r i n t e r r u p t i o n s ' of the growth, or l a t e r f r a c t u r e . I t i s i n t e r e s t i n g to com- pare' t h i s i c e w i t h the i c i n g mound i c e , discussed elsewhere, i n which heave, caused a m o d i f i c a t i o n of c r y s t a l c h a r a c t e r i s t i c s i n . t h e upper ice,'and a a 42 F i g u r e 3..' (a)., (b) , (c) are from p r o g r e s s i v e l y deep v e r t i c a l s e c t i o n s ; ( d ) , ( e ) , ( f ) are from p r o g r e s s i v e l y deep h o r i z o n t a l s e c t i o n s ; (d) i s from above (a) ; (e) i s from above ( b ) ; ( f ) i s from above ( c ) . .Diagrams i n plane of'samples . ' - ' . • ' . ' • • ' . '43 .. f r a c t u r e became i n f i l l e d with, i c e of d i f f e r i n g , features from the primary- growth. On such evidence i c i n g mound i c e and lake i c e could be d i s t i n - guished i f the l a t t e r became b u r i e d . A l s o .lake i c e i s q u i t e d i s t i n c t from lens i c e and wedge i c e i n terms.o'f i n c l u s i o n s , c r y s t a l s i z e and shape, and l a t t i c e o r i e n t a t i o n s . F u r t h e r , i f lake i c e became buried by slumping the o v e r l y i n g m a t e r i a l would be d i s s i m i l a r i n sedimentary features and l a t e r f r e e z i n g texture from non-slumped or otherwise undisturbed m a t e r i a l which froze i n s i t u . Thus on s u c h : s t r a t i g r a p h i c and p e t r o l o g i c c r i t e r i a , i n a d d i t i o n to those enumerated by Mackay (1971, 1973b), i t should be p o s s i b l e to d i s t i n g u i s h buried i c e from i c e grown underground.. I c i n g Mound Ice • • -• I n t r o d u c t i o n ' An i c i n g i s a mass of f r e s h water i c e which has fr o z e n at or near the ground s u r f a c e , from s p r i n g or r i v e r water. Where the water passes through f r o z e n ground, the f o r c i n g mechanism i s a r t e s i a n pressure. The water does not always reach the ground s u r f a c e ; some may spread l a t e r a l l y i n t o or between sediment horizons, thus u p l i f t i n g the overburden to form an 'icing mound. .Although i c i n g s are f a i r l y frequent i n permafrost (and non- permafrost) areas (Carey 1973) they have not been stud i e d i n d e t a i l i n North America. Few have bean reported in.th e f i e l d , area (Mackay 1975b) b u t . i n view of t h e i r s u r f i c i a l nature, p o s s i b l e e xtent, geomorphic form and water source, i t i s important•to be able to d i s t i n g u i s h the i c e type from, say,, pingo i c e which grows by '-a d i f f e r e n t mechanism. Gra d a t i o n a l forms e x i s t ' between i c i n g mounds and' those of segregated ice.' Two such •' mounds ( F i g . . 4) grew i n winter 19 73-1974 on the. Tuktoyaktuk peninsula • Figure 4 . F i e l d p o s i t i o n (Photo by Dr. J.R. Mackay) Liverp o o l Bay i c i n g mound 45 and provided an opportunity to inspect i c e bodies of known age before major p o s t " s o l i d i f i c a t i o n changes could occur. (a) Tuktoyaktuk I c i n g Mound • F i e l d C h a r a c t e r i s t i c s A s m a l l i c i n g mound (3 m high) grew over the w i n t e r 1973-1974 . on the si d e of a pingo (Mackay 1973a, F i g . 18, Pingo No. 13; 1975b) 20 km east of Tuktoyaktuk.. The mound was not present i n August 1973, and was f i r s t observed in. J u l y 1974, thus i t s maximum age i s . known. D r i l l i n g on the lake bottom has shown that a r t e s i a n pressures have developed i n sub-permafrost ground water by pore water, e x p u l s i o n during.permafrost aggradation i n sands (Mackay 1972b). The i c i n g mound grew from water moving up a t e n s i o n c r a c k from depth and being i n j e c t e d i n t o the a c t i v e l a y e r . A crack was s t i l l v i s i b l e , w i t h water f l o w i n g In J u l y 1974 and flow continued i n t o l a t e August 1974, and was observed i n March and August 1975 (Mackay, personal cbmmunica- . t i o n ) . Ice C h a r a c t e r i s t i c s A 0.5 m t h i c k sample wasytaken (by J.R. Macka;/) from the upper part of the mound, to in c l u d e the. contact w i t h the a c t i v e l a y e r . • S t r u c t u r e s i n . the sample were s l i g h t ' f o l d i n g of the compositional l a y e r i n g due to heaving, and. a l a t e r f r a c t u r e . The compositional l a y e r i n g was determined by bubble content (no sediment or organic matter being present) ;..the bubbles occur in. d i s t i n c t bands p a r a l l e l to" the mound surface. Bubble s i z e s and shapes were uniform w i t h i n - •• a given band, but varied' from band to .band ( F i g . 5). A - d e t a i l e d bubble s t r a t i g r a p h y i s given i n Figure 6.' Near the contact w i t h the organic matter, the i c e had a milky appearance due to the high, con- tent of very small a i r bubbles followed below by a bubble-free zone,., then'bubbles ,. a p a t t e r n which- c o n t i n u e d : t o depth; • , •''.,-'• C r y s t a l C h a r a c t e r i s t i c s . . • C r y s t a l s i z e v a r i e s down the sample. Adjacent to the organic matter i s a zone of small c r y s t a l s i n d i c a t i n g a c h i l l zone ( F i g . 7 ( a ) ) . Where bubble l a y e r i n g appears, small c r y s t a l s (2.0 x 1.0 mm) occur . below bubbles. Below t h i s depth, g r a i n s i z e becomes .more consistent,, c r y s t a l s being very elongated, > 80 mm and widening from 2 mm t o as much as 8 mm i n the depth range 80 mm to 160 mm. . C r y s t a l s become longer than the t h i n s e c t i o n (80 mm) and average 5 mm i n width t o a depth of 0.4 m, then widen to 10 to 15 mm at the base ( F i g . 7 ( b ) ) . C r y s t a l shape v a r i e s w i t h depth. I n the•upper zone, c r y s t a l s , are anhedral, some having s l i g h t l y s e r r a t e d boundaries.. Many t e r - minate a b r u p t l y , being wedged out by adjacent c r y s t a l s . At the s m a l l bubble bands, g r a i n shape becomes more i n t e r l o c k i n g . Below, i n the zone of elongate c r y s t a l s , shapes are anhedral, w i t h some s e r r a t i o n s i n the upper part which become more g e n t l y curved at depth. The gently.curved boundaries continue to depth. The only major shape changes are at.some bubble bands, where one c r y s t a l grows l a t e r a l l y at the expense of i t s neighbour. . . • • ' . " . • ' • ' ' . . • ' • • . . .1 ' ; Substructure, in-the form'of V a r i a t i o n , i n e x t i n c t i o n angle w i t h i n a c r y s t a l , i s confined to the upper- part of the sample', which. froze most r a p i d l y . A l s o , i t has s u f f e r e d the most- .heaving 'arid Figure 6. Figure 7 . Bubble Band Sequence Depths i n metres. (a) C r y s t a l c h a r a c t e r i s t i c s Upper part of mound, 10 mm g r i d . ' 1 V e r t i c a l s e c t i o n P 0 05 J J4W !00 c 0-J5-;.';; (lit 0 2 0 - • 0 0 cu fiUi PA! !| 1/ • .:<) 0 - 3 0 - ; . ^ 0 * n. • 0 of* 0-35- - ' J - 00(7 | L L , ; 0 0 000? 0 - 4 0 - V;5 [ * .-• fry.' Top 0 - 4 5 - J • o .050- (b) C r y s t a l c h a r a c t e r i s t i c s Lower part of mound, 10 mm g r i d . V e r t i c a l s e c t i o n Top 48 .'folding'-.of l a y e r s and g r a i n boundaries are most i r r e g u l a r . Dimen- s i o n a l o r i e n t a t i o n i s c o n s i s t e n t throughout the body, being ortho- gonal to the bubble banding, and p a r a l l e l to the f r e e z i n g d i r e c t i o n . The r e l a t i o n s h i p of. bubbles to c r y s t a l c h a r a c t e r i s t i c s i s as f o l l o w s : • • (a) In the upper, milky zone, bubbles are f r e q u e n t l y near g r a i n boundaries. (b) Bubbles .are c o n s i s t e n t i n s i z e and shape w i t h i n given bands throughout the s e c t i o n , not a l l are a s s o c i a t e d w i t h a g r a i n boundary p o s i t i o n . Thus a widespread n u c l e a t i n g event occurred, r e g a r d l e s s of t e x t u r e . Such an event could be s u p e r s a t u r a t i o n of gas at an essen- t i a l l y planar i n t e r f a c e . Thus bubbles occur i n both i n t e r c r y s t a l l i n e and i n t r a c r y s t a l l i n e p o s i t i o n s . . (c) . Further from the c h i l l zone, the f r e e z i n g r a t e decreased and bubble nucleation. occurred p r e f e r r e d l y . a t g r a i n boundaries. At such s i t e s there i s increased gas c o n c e n t r a t i o n i n the l i q u i d where two i n t e r f a c e s are advancing. F i l a m e n t - l i k e bubbles l i e i n the boundaries, then widen downwards i n t o bulbous shapes, as more gas i s e x p e l l e d at the r e s t of the interface.. Gas moves along the c o n c e n t r a t i o n g r a d i e n t , thus the bubbles become fewer but l a r g e r . Some such bubbles have s m a l l bulbous zones on the f i l a m e n t s , or the f i l a m e n t s are detached, or are - i n 2 parts ( F i g . 5). These features i n d i c a t e l o c a l v a r i a t i o n s i n supply of gas to i n d i v i d u a l bubbles, which are surrounded -by "normal" bubbles. S i m i l a r bubbles have been observed i n experimental ice- growth (Carte 1961a), I f more "abnormal" bubbles occurred, i t might be i n d i c a t i v e of p o s t - s o l i d i f i c a t i o n break up ( K h e i s i n and Cherepanov 1969), but t h i s process seems l e s s l i k e l y . The inverse - form of bubble, w i t h the filament at the base was never observed, so t h a t wa have a u s e f u l 'way-up' i n d i c a t o r f o r f r e e z i n g d i r e c t i o n . (d) A l o c a l e f f e e t . o c c u r s where'many c r y s t a l s , terminate at a • h o r i z o n t a l band of small bubbles and a l a r g e r number of c r y s t a l s grow below, i n d i c a t i n g that c r y s t a l s d i d not grow between bubbles, but ". n u c l e a t i o n occurred on the d i s t a l s ide of the band. The usual p a t t e r n of type ( c ) , above, returns below. I t i s p o s s i b l e that bubble n u c l e a t i o n occurs w i t h i n the l i q u i d or at an upward advancing i n t e r f a c e i f i c e i s also.growing upward due to f r e e z i n g at the base of the i n t r u s i o n . Such bubbles could become detached and r i s e through the l i q u i d to become attached to the upper i n t e r f a c e . No samples, ware obtained from the base of the i c i n g mound. In the intermediate zone where some bubbles are i n t r a c r y s t a l - . li n e , , formation may have been enhanced by small gas bubbles' i n sus- pension produced during t u r b u l e n t flow up the f r a c t u r e , , or by . submic ros cop i c f o r e i g n p a r t i c l e s which have d i f f e r e n t s u rface energies and roughnesses from c r y s t a l s u r f a c e s . Howaver, growth i s p a r a l l e l to the basal plana, and Knight (1971) found that 0 , the e q u i l i b r i u m contact, angle, tends to zero f o r a i r bubbles i n water c o n t a c t i n g tha basal plane, so. n u c l e a t i o n would be r e l a t i v e l y easy. • . I t has been noted .above•that f i l a m e n t - l i k e growth was sometimes i n t e r r u p t e d l o c a l l y before l a r g e r , - c y l i n d r i c a l bubbles developed. This i s I n t e r p r e t e d , as i n d i c a t i n g a t r a n s i e n t s t a t e i n which bubble diameter increases but c o n d i t i o n s are very c r i t i c a l in-.terms of gas . c o n c e n t r a t i o n i n the water and s o l i d i f i c a t i o n r a t e . A change i n s o l i d i f i c a t i o n . r a t e may cause e i t h e r a diameter increase or a growth stoppage. When the,diameter increases a steady s t a t e may be. a t t a i n e d , which gives c y l i n d r i c a l bubbles p a r a l l e l to the f r e e z i n g d i r e c t i o n . In comparison w i t h the n o n - r e p r o d u c i b i l i t y of experimental bubble growth, i t i s s u r p r i s i n g that bubble c h a r a c t e r i s t i c s are so constant i n a given l a y e r i n the i c i n g mound. Vasconcellos and.Beech (1975) discussed the development of bubbles i n the ice/water/C02 system and . demonstrated that three adjacent bubbles grew: (a) f o r the most p a r t i n the t r a n s i e n t s t a t e , w i t h the diameter i n c r e a s i n g , (b) i n i t i a l l y t r a n s i e n t , then steady s t a t e , (c) i n steady s t a t e a l l the time. Thus i n the same c e l l , the s o l i d i f i c a t i o n r a t e v a r i e d w i t h p o s i t i o n , r a t e (a) <_ r a t e (b) -Crate ( c ) . In comparison bubble bands i n the i c i n g mound i c e d i s p l a y l i t t l e l a t e r a l v a r i a b i l i t y i n bubble s i z e and shape. P e t r o f a b r i c diagrams were prepared f o r . a s e r i e s of v e r t i c a l . • samples ( F i g . , 8 ) . A s t r o n g l y developed g i r d l e i s e v i d e n t , showing c-axes to be p a r a l l e l to the compositional, l a y e r i n g . T his i s charac- t e r i s t i c of. r a p i d i c e growth i n t o bulk water. C r y s t a l s at the contact of i c e w i t h organic matter were not measured due to-'.the d i f f i c u l t y of making t h i n s e c t i o n s of i c e con- t a i n i n g s o l i d i n c l u s i o n s and the. small c r y s t a l s i z e t y p i c a l of the c h i l l zone.. However, the uppermost measured c r y s t a l s show a wider g i r d l e ( F i g . 8(b)) than succeeding lower s e c t i o n s ( F i g . 8 ( c ) ) . Figure 8. (a) Tuktoyaktuk mound, V e r t i c a l S e c t i o n . 80 c r y s t a l (b) Upper 20 c r y s t a l s (c) Lower 20 c r y s t a l s (d) 70 f r a c t u r e i r i . f i 1. c r y s t a l s (e) L i v e r p o o l Bay.mound, V e r t i c a l S e c t i o n , 50 c r y s t a l s Diagrams i n plane of samples c '= compositional l a y e r i n g f = f r a c t u r e surfa.ce This i s to be expected.on.the basis of s e l e c t i v e growth of c r y s t a l s .'' w i t h b a s a l planes p a r a l l e l to.the f r e e z i n g d i r e c t i o n . . F r a c t u r e Zone The above p a t t e r n was dis t u r b e d i n the centre of the sample, where the bubble l a y e r i n g was i n t e r r u p t e d by a zone of i r r e g u l a r l y shaped bubbles of various s i z e s . This zone was obiique to the general l a y e r i n g and veered p a r a l l e l to that l a y e r i n g near the top of the sample. The upper s e c t i o n s show a te x t u r e s i m i l a r t o t h a t of the.previous s e r i e s , but an abrupt change occurs i n the f r a c t u r e zone ( F i g . 9 ). Bubble shape v a r i e s c o n s i d e r a b l y , but w i t h a general. trend away from the surfaces of the f r a c t u r e zone. T h r e a d - l i k e . bubbles are. short (2-3 mm), narrow.(<1 mm), and i n t e r s p e r s e d w i t h s p h e r i c a l (1 mm) bubbles. C r y s t a l s i z e v a r i e s w i d e l y , from <3 mm '.' to >25 mm long a x i s , i n c o n t r a s t to the previous elongate p a t t e r n . C r y s t a l shapes are anhedral w i t h much more complex shapes than i n the banded i c e , having s e r r a t i o n s , cusps, and intergrowths. Sub- . s t r u c t u r e ( i . e . e x t i n c t i o n v a r i a t i o n ) i s not developed, but a c e l l u - l a r m i c r o s t r u c t u r e p a r a l l e l t o - b a s a l planes, as i n sea i c e occurs. There i s a tendency toward a. dimensional o r i e n t a t i o n t r e n d i n g i n t o the zone ( F i g . 9). P e t r o f a b r i c diagrams f o r the zone are shown i n F i g . 8(d), the . co n t r a s t w i t h F i g . 8(a) being evident. There i s no h o r i z o n t a l g i r d l e p a t t e r n , i n s t e a d c-axes are more, dispersed r e l a t i v e to the f r e e z i n g d i r e c t i o n s . Figure 9. Tuktoyaktuk i c i n g mound, Fract u r e i n f i l c r y s t a l s , V e r t i c a l S e c t i o n , 80 ram across Figure 10a. L i v e r p o o l Bay i c i n g mound, V e r t i c a l S e c t i o n , contact w i t h o v e r l y i n g s o i l . Note c h i l l zone elongated c r y s t a l s . S e c t i o n 80 mm square. Figure 1 0 b . L i v e r p o o l Bay i c i n g mound, Lower, v e r t i c a l s e c t i o n . 10 mm g r i d . 1 > A l l s e c t i o n s under c r o s s e d p o l a r i z e r s 54 I n t e r p r e t a t i o n . The i c i n g mound r e s u l t e d from w a t e r under a r t e s i a n p r e s s u r e a t depth moving up a t e n s i o n c r a c k and. i n s e r t i n g , i t s e l f i n t o the. a c t i v e l a y e r on the s i d e o f the p i n g o p r o b a b l y i n l a t e autumn o r e a r l y w i n t e r . The a c t i v e l a y e r was c o l d , and c o p i o u s n u c l e a t i o n p r o d u c e d a c h i l l zone, from.which grew v e r y e l o n g a t e c r y s t a l s . A l t e r n a t i n g b u b b l y and b u b b l e - f r e e l a y e r s o c c u r r e g u l a r l y , b u b b l e shape b e i n g . cons i s t e n t i n o r i e n t a t i o n , . s i z e and shape w i t h i n i n d i v i d u a l bands. F i l a m e n t s o c c u r on t h e upper ends o f e l o n g a t e b u b b l e s , a u s e f u l way-up c r i t e r i o n . C - a x i s p r e f e r r e d o r i e n t a t i o n i s o r t h o g o n a l t o • t h e g r o w t h d i r e c t i o n . As g r o w t h c o n t i n u e d , . u p d o m i u g o c c u r r e d , f o l d i n g the u p p e r l a y e r s . . A l a t e r f r a c t u r e became i n f i l l e d w i t h i c e w i t h m a r k e d l y d i f f e r i n g t e x t u r e and p e t r o f a b r i c s . The t e x t u r e and p e t r o f a b r i c s o f the f r a c t u r e zone a r e i n t e r p r e t e d as i n d i c a t i n g the f r a c t u r e o f the r e g u l a r p a t t e r n , a n d l a t e r i n f i l l i n g . Some growth t o o k p l a c e i n l a t t i c e c o n t i n u i t y w i t h p r e v i o u s l y e x i s t i n g c r y s t a l s , but n u c l e a t i o n o f new c r y s t a l s a l s o o c c u r r e d ( F i g . 9 ) . Tha e l o n g a t e c r y s t a l s a r e o b l i q u e t o t h e p a t t e r n : o f the s u r r o u n d i n g i c e , . . and r e f l e c t m u l t i d i r e c t i o n a l growth i n t o a c a v i t y . D i m e n s i o n a l o r i e n - t a t i o n i s . . l o c a l l y p a r a l l e l to' f r e e z i n g ' d i r e c t i o n s . A c e l l u l a r ' sub- s t r u c t u r e i n t h i s i c e s u g g e s t s . a h i g h e r c h e m i c a l c o n t e n t . I t i s i n t e r e s t i n g t o compare c r y s t a l g r o w t h i n t h e main mass o f tha mound w i t h t h a t i n the f r a c t u r e . . In' the. main mass c r y s t a l s a r e v e r y e l o n g a t e p a r a l l e l t o the f r e e z i n g d i r e c t i o n w h i c h changed v e r y l i t t l e . I n c o m p a r i s o n the t h e r m a l g r a d i e n t v a r i e d a r o u n d the 55 fracture.; "Thus f o r a columnar g r a i n to s u r v i v e over any great distance n e c e s s i t a t e s considerable c u r v a t u r e , due to the p r o g r e s s i v e change i n the favoured growth d i r e c t i o n . As the g r a i n ' s c r y s t a l l o - graphic o r i e n t a t i o n remains constant, the curvature must be generated •by l a t e r a l branching. At some stage t h i s may become more d i f f i c u l t than the i n i t i a t i o n and growth of a new g r a i n oriented.more s u i t a b l y f o r continued growth. . Kence the v a r i a t i o n i n l a t t i c e o r i e n t a t i o n i n the p e t r o f a b r i c diagrams. The petrology of such mounds has not been di s c u s s e d elsewhere i n the l i t e r a t u r e . I t i s apparent that they d i f f e r markedly from segregated i c e i n the cases described. The mounds may be temporary, depending on the overburden thickness and water supply. (b) L i v e r p o o l Bay I c i n g Mound F i e l d C h a r a c t e r i s t i c s . A second i c i n g mound was found by Dr.. J.R. Mackay i n J u l y 1974. I t occurred on the si d e of a pingo (Mackay 19/3a, F i g . 19, Pingo No. 15) near L i v e r p o o l Bay, Tuktoyaktuk P e n i n s u l a . The mound was 2.3 m high. The mechanism of growth i s b e l i e v e d to be the same as that of the previous example; water was bubbl i n g up i n a pool i n J u l y 1974 (Mackay, personal communication).. The growth p e r i o d i s known to have been during or a f t e r the freeze-back of the a c t i v e l a y e r i n the wi n t e r 1973-74, as the s i t e was surveyed i n summer 1973, and i c e growth took place at the base of the a c t i v e l a y e r . A sample of the upper p o r t i o n of the mound i n c l u d i n g p a r t of the overburden was c o l l e c t e d by Dr. J.R. Mackay, 56 Ice C h a r a c t e r i s t i c s ' ,'.'•'• The contact with the overburden-was.abrupt but hot planar. This represented the I n i t i a l contact - i . e . there had been no melt-through of the a c t i v e l a y e r at the time of sampling.' There were a l s o some t h i n ( < 5 mm) i c e lenses up to 50 mm long in. the overburden, which was mainly organic matter w i t h some sediment. I n c l u s i o n s of t h i s m a t e r i a l occurred .in the- top 50 mm of the i c e , i n p a r t i c l e s from 1 mm to 10 mm i n diameter, decreasing i n concen- t r a t i o n downwards. The very few bubbles which occurred i n the top 50 mm tended to be small (< 1 mm), s p h e r i c a l i n t r a i n s , or s l i g h t l y elongated. Below and throughout the whole sample was,a p a t t e r n of '- very i r r e g u l a r s u b - v e r t i c a l l y elongated bubbles, g e n e r a l l y <L_ 25 mm long, but a few reached 75 mm.': No s t r u c t u r e s such as f r a c t u r e s were observed'.' A s e r i e s of v e r t i c a l t h i n s e c t i o n s 'was prepared. The over- burden at the contact has a low, i c e content, and the contact w i t h the i c e i s abrupt and i r r e g u l a r , at two s c a l e s , i . e . sinuous at hand, specimen sc a l e ( F i g . 10(a)) and at microscopic s c a l e . Sediment \ content, i n the i c e tends to decrease downward from the contact. The bubble p a t t e r n d i f f e r s markedly from that In the p r e v i o u s l y discussed mound, there being no w e l l developed l a y e r i n g . There, i s a gen e r a l increase i n bubble c o n c e n t r a t i o n downwards. Few bubbles occur a t : the contact, and below'the' p a t t e r n i s e s s e n t i a l l y random, with l o c a l l y higher c o n c e n t r a t i o n s . ..Shapes range from s p h e r i c a l (^ 1 mm), to elongate i r r e g u l a r (8 mm long, 3 mm wide) - these bubble charac- t e r i s t i c s , continue downwards.' A c e l l u l a r s u b s t r u c t u r e increases i n c o n c e n t r a t i o n with depth i n the form of a s e r i e s of l o c a l l y p a r a l l e l c e l l s , which are probably zones of higher chemical content p a r a l l e l to the basal plane. • • C r y s t a l C h a r a c t e r i s t i c s D i f f e r e n c e s from the previous i c i n g mound, continue i n tex- t u r a l p r o p e r t i e s . Grain s i z e ranges from <1 mm at the contact ( F i g . 10(a)) through an intermediate zone (5 mm x 2 mm) to an elongate zone (> 55 mm long by 20 to 30 mm wide). T h i s elongate c r y s t a l . z o n a extends f o r another 30 mm, some c r y s t a l s exceeding 80 mm i n length ( F i g . 10(b)). Smaller c r y s t a l s (10 mm x 5 mm) occur i n t e r s p e r s e d or i n groups among the l a r g e r . . • .' . C r y s t a l shape i s anhedral throughout the sample. In tha competitive growth zone at the overburden contact, boundaries l a c k strong curvatures or s a r r a t i o n s . The zone of intermediate c r y s t a l s contains both v e r t i c a l and h o r i z o n t a l s a r r a t i o n s u n r e l a t e d to i n c l u s i o n s . Tha elongated c r y s t a l s have i r r e g u l a r . b o u n d a r i e s ( F i g . 10(b)). Intergrowth i s demonstrated by r e p e t i t i o n of e x t i n c - t i o n angle i n nearby c r y s t a l segments and a l s o by s a r r a t i o n s which are mainly h o r i z o n t a l , but. may have secondary promontories., Dimen- s i o n a l o r i e n t a t i o n i n the two se c t i o n s (Fig.. 1.0a,b) i s -markedly, v e r t i c a l , i . e . p a r a l l e l t o the f r e e z i n g d i r e c t i o n . . Substructure- i n the form of d i f f e r i n g e x t i n c t i o n bands occurs i n tha large columnar, c r y s t a l s , defined by i r r e g u l a r sub-boundaries. This type of • .subs t rue ture decreases with depth,- to be replaced- by a • c e l l u l a r s u b s t r u c t u r e , small pockats ( C 1 mm) p a r a l l e l i n a given 58 c r y s t a l , i n d i c a t i v e of s a l i n e i n c l u s i o n s . Superimposed on t h i s is-.' a tendency to varying e x t i n c t i o n p o s i t i o n . . No r e l a t i o n s h i p of bubbles to c r y s t a l c h a r a c t e r i s t i c s e x i s t s i n the upper i c e , n e i t h e r s p h e r i c a l nor elongated bubbles being p r e f e r r e d l y s i t e d i n c r y s t a l s or on boundaries. F a r t h e r from the c o n t a c t , small elongate bubbles are a l i g n e d on sub-boundaries and boundaries, w h i l e large i r r e g u l a r bubbles have no apparent r e l a t i o n - s h i p to t e x t u r e . Sediment i s mainly at or near to boundaries. Two h o r i z o n t a l s e c t i o n s were prepared from the top of the i c e body, one. i n the competitive zone and the second 25 mm below. . C r y s t a l s i z e changed from 2 mm x 1 mm-to 8 mm x 5 mm i n t h i s d i s t a n c e . C r y s t a l shape at the top was anhedral w i t h most boundaries being e s s e n t i a l l y s t r a i g h t but w i t h minor s e r r a t i o n s l o c a l l y . . This changed below to more s e r r a t e d , complex shapes i n the l a r g e r c r y s t a l s . I n .the upper c r y s t a l s , no.subs t r u e t u r e i s apparent, but i n the lower s e c t i o n low angle boundaries occur, meeting boundaries i n s e r r a t i o n grooves. Dimensional o r i e n t a t i o n i s nowhere w e l l developed. Most bubbles occur on g r a i n boundaries, but the c o n c e n t r a t i o n decreases downward,- i n the large c r y s t a l zone. P e t r o f a b r i c diagrams 'were prepared only f o r the v e r t i c a l s e c t i o n s ( F i g . 8 ( e ) ) . The h o r i z o n t a l g i r d l e i s c h a r a c t e r i s t i c of'.., r a p i d i c e growth i n bulk water, ra t h e r t h an i n a porous medium. I n t e r p r e t a t i o n . . The o v e r a l l form and the gross p a t t e r n of c r y s t a l s i z e and shape are s i m i l a r i n both i c i n g mounds. I n d e t a i l the second mound 59 l a c k s a w e l l developed, bubble l a y e r p a t t e r n , bubble shapes are i r r e g - u l a r , g r a i n boundaries have more s e r r a t i o n s , and the c e l l u l a r sub- s t r u c t u r e i s apparent i n the whole body, compared w i t h i t s occurrence i n f r a c t u r e i c e only i n the previous mound. I t appears that the melt had higher s o l u t e content which produced the s e r r a t e d p a t t e r n and c e l l u l a r s u b s t r u c t u r e - This a l s o c o n t r i b u t e d to the complex . bubble shapes. Topographic Expression and Ice C h a r a c t e r i s t i c s The i c i n g mounds, were not observed i n the f i e l d by the author, but d e t a i l e d d e s c r i p t i o n s were s u p p l i e d w i t h the samples by Dr. J.R. Mackay (personal communication 1974, 1975). - - From these d e s c r i p t i o n s and those of other authors (Shumskii 1964) i t i s apparent that Such mounds may range w i d e l y . i n l a t e r a l extent and height. Growth may continue as long as water i s a v a i l a b l e , and f r a c t u r e s are common. The mounds thus resemble small pingos. The i c e c h a r a c t e r i s t i c s enumerated above demonstrate c l e a r l y the d i f f e r e n c e from pingo i c e (discussed i n s e c t i o n 3 ). The e v i - dence from i c e petrology i s that i c i n g mound i c e i n the above cases . ; i s t y p i c a l of t h e • f r e e z i n g of bulk water, r a t h e r than of segregated i c e . •' '. 3. Pingo Ice I n t r o d u c t i o n Although very l i m i t e d i n t h e i r s p a t i a l d i s t r i b u t i o n , pingos are. dominant geomorphic features and have long a t t r a c t e d a t t e n t i o n . S e v e r a l 60 t h a o r i e s of o r i g i n have been proposed ( P o r s i l d 1938; M i i l l e r 1953; Shumskii 1964;-Mackay 1962, 1972b, 1972s, 1973a, 1975b;Mackay and Stager 1966b) " and a d e t a i l e d understanding of many a s s o c i a t e d phenomena i s now at hand. In h i s 1952 paper Mackay a p p l i e d heat conduction theory to the f r e e z i n g of a lake b a s i n . i n permafrost with boundary c o n d i t i o n s a p p l i c a b l e to the Mackenzie D e l t a - Tuktoyaktuk Peninsula, area. In a d d i t i o n , theory and , lab o r a t o r y experimental knowledge of i c e l e n s i n g c o n d i t i o n s was employed . to e x p l a i n the v a r i a b l e i c e contents i n exposed pingo cores. This i n i t i a l theory of pingo growth has been tested by d e t a i l e d surveys of a c t i v e l y . growing pingos, and has been modified to i n c l u d e a r t e s i a n . p r e s s u r e s at the base of permafrost, p u l s a t i n g growth, t e n s i o n crack p a t t e r n s and a t e n t a - t i v e l i n k between growth r a t e and c l i m a t i c parameters (Mackay 1973a, 1975b). Despite these developments, there has been no concomitant advance in.our understanding.of the p e t r o l o g i c a l aspects of i c e w i t h i n , t h e cores of . pingos. As reviewed p r e v i o u s l y , almost no l a b o r a t o r y c o n t r o l l e d work has been performed on i c e growth i n sediment, from a c r y s t a l l o g r a p h i c view- p o i n t . In p a r t i c u l a r the i n f l u e n c e s of pore water pressure and i n c l u s i o n s have not been i n v e s t i g a t e d . . In terms of f i e l d study of core i c e , no reports of p e t r o l o g i c a n a l y s i s have appeared, s i n c e 1965. M i i l l e r . (1953) compared pingos i n Greenland and.the Mackenzie D e l t a area and inc l u d e d some d i s c u s s i o n of c r y s t a l s i z e and shape, but no p e t r o f a b r i c diagrams were presented. • . . A f i e l d . i n s p e c t i o n of exposed pingo' i c e i n a cave on Richards I s l a n d was carried out by, Mackay and Stager (1955b) who.found that: The .ice was usually.bubble free. Although c r y s t a l sizes v a r i e d . . . from s i t e to. s i t e few were less than o n e - t h i r d of an i n c h across and many were 1 inch to 2 inches I n • diameter, 8 inches' being the-' largest dimension noted-. worm-like bubble tubes,, as much as 61 0.1 inc h i n diameter and 5 inches long, were sometimes found along i n t e r c r y s t a l faces. An examination of many tens of .bubbles, from s e v e r a l l o c a l i t i e s , showed a preference f o r two a x i a l o r i e n t a t i o n s : the f i r s t was toward the pingo c e n t r e , the second toward the outer base (p. 367). A d d i t i o n a l l y an u n s p e c i f i e d number of c r y s t a l o p t i c axes was measured i n a sample from one i c e l a y e r , and .. .• an .estimated 80 per cent of the o p t i c axes w e r e . h o r i z o n t a l . and l a y p a r a l l e l to the i c e - c l a y contact; that . i s , the axes pointed toward the geometric centre of the pingo (p. 367). Thus i t i s apparent that i n t h i s case both the o p t i c axes and elongate bubbles x^ere ge n e r a l l y . o r t h o g o n a l to the adjacent sediment, bands, d e s p i t e the dip of the l a y e r i n g , and i t seems reasonable to conclude t h a t the l i n e a t i o n s represent the f r e e z i n g d i r e c t i o n . Shumskii (1964) a l s o considered pingo i c e , and as i s found elsewhere i n the Russian l i t e r a t u r e (Sumghin 1940) he r e f e r r e d to i n j e c t i o n ' o f water at the f r e e z i n g f r o n t to cause r a p i d f r e e z i n g and u p l i f t . Mackay (1973a, p. 1000) discounted i n j e c t i o n ice. as a major f a c t o r i n pingo growth but pointed out t h a t . i t may occur temporarily. Thus one aspect of the present study i s to determine the mode of growth. . In our d i s c u s s i o n of i c i n g mound i c e the c h a r a c t e r i s t i c s of i c e grown from water i n t r u d e d beneath a t h i n overburden have been enumerated. Owing to the l a c k of. l a b o r a t o r y and f i e l d data on segregation i c e some of the f o l l o w i n g d i s c u s s i o n on growth of pingo cores i n terms of segregation or i n j e c t i o n and the i n f l u e n c e of heaving and overburden pressure on growth features must be considered s p e c u l a t i v e . 62 In the present work, three pingos were s t u d i e d : (a) a small pingo, one of a s u i t e s t u d i e d by Mackay (1973a, F i g . 14, Pingo No. 11); (b) W h i t e f i s h Summit•Pingo; . (c) a hollowed-out pingo i n Tuktoyaktuk. (a) Pingo'No. 11 (69° 23 'Nv 133° 30 'W) A drained lake near Tuktoyaktuk contains three l a r g e growing pingos and one s m a l l non-growing pingo (Mackay 1973a, F i g . 14, Pingo No. 11). The small pingo ceased to grow as i t was centred near the edge of the former lake and permafrost aggradation' cut o f f growth. I t thus provides an example of. e a r l y pingo growth which has not been d i s t u r b e d g r e a t l y by l a t e r heaving,.although general lake-bottom heave i s o c c u r r i n g . A l s o i t s approximate age i s known, so p o s t - s o l i d i f i c a t i o n changes can be dated. F i e l d C h a r a c t e r i s t i c s • The pingo i s 1.5 m high with.a w e l l developed v e g e t a t i o n cover. Two v e r t i c a l cores were- removed, one from the summit and one from the s i d e , each core being about 3.1 m long. 1.2 m of i c e - f r e e peat • o v e r l i e s 0.8m of a l t e r n a t i n g peat and i c e l a y e r s grading i n t o a pure i c e core w i t h a few peat I n c l u s i o n s at the top. Coarse sediment u n d e r l i e s the core. Ice C h a r a c t e r i s t i c s Ice l a y e r s w i t h i n the peat are lens-shaped, the t h i c k e s t are . . 20. mm and taper l a t e r a l l y . Ice-peat boundaries are i r r e g u l a r and peat i n c l u s i o n s up to .3 mm occur w i t h i n the i c e ( F i g . 11). Otherwise' Figura 11. Figure 12. Pingo No. 11, V e r t i c a l s e c t i o n . Ice lenses i n peat. 10 mm g r i d . ' Crossed p o l a r i z e r s Pingo No. 11, V e r t i c a l s e c t i o n . C r y s t a l s i n i c e core. 10 mm g r i d . ' 1 Crossed p o l a r i z e r s F i g ure 13. Fractures i n core i c e , Pingo No. 11. (a) H o r i z o n t a l s e c t i o n , ( b ) V a r t i c a l s e c t i o n . / the i c e i s c l e a r except f o r c y l i n d r i c a l bubbles and s t r i n g s of s p h e r i c a l bubbles., arranged v e r t i c a l l y w i t h diameters up to 1 mm and lengths of 3 mm. C r y s t a l C h a r a c t e r i s t i c s ... I n d i v i d u a l c r y s t a l s cross the i c e l a y e r s , and have grown to . 2 cm v e r t i c a l l y . a n d l a t e r a l l y . Peat i n c l u s i o n s are contained i n s i n g l e c r y s t a l s , thus the peat d i d not encourage f u r t h e r c r y s t a l ' growth. Elongate bubbles occur both i n c r y s t a l s and on boundaries. Grain boundaries, are g e n e r a l l y s t r a i g h t to gently curved, and v e r t i c a l , s e r r a t i o n s mark the wedging out of a c r y s t a l by i t s neigh- bours. . No.pronounced- substructure occurs i n the c r y s t a l s . W i t h i n the i c e core, peat i n c l u s i o n s become fewer, and s m a l l e r w i t h depth. Here, peat a f f e c t s c r y s t a l shape, boundaries t r e n d i n g h o r i z o n t a l l y below peat pockets, but v e r t i c a l l y where peat i s absent, which i n d i c a t e s s e l e c t i v e growth at the i m p u r i t y , due to , d i f f e r e n t i a l heat flow. Bubbles occur i n v e r t i c a l t r a i n s , d e creasing downwards i n s i z e from 2 mm. They occur on or c l o s e to c r y s t a l boundaries. C r y s t a l s are very.elongated, being greater than 160 mm long and.widen downwards to 30-40 mm wide. Shape i s anhedral w i t h curved and serrated boundaries. These boundaries have a general trend on which are superimposed, d e n d r i t i c shapes ( F i g . 12). This p a t t e r n continues f o r 0.3 m depth, where c r y s t a l s are s t i l l elongate and narrow, but more complexly intergrown. A l s o some grains d i s p l a y a l t e r n a t i n g e x t i n c t i o n . From a depth of 2.3 to 2.8 m. the i c e i s " e s s e n t i a l l y i n c l u s i o n - f r e e . C r y s t a l s are anhedral w i t h boundary shapes ranging from simple curvature to h i g h l y s e r r a t e d . These major s e r r a t i o n s occur on each s i d e of a c r y s t a l at a given depth, but become more frequent and l e s s pronounced downwards where s e v e r a l c r y s t a l s are wedged but. H o r i z o n t a l s e c t i o n s show c r y s t a l s to be anhedral i n that, plane a l s o . These more r e g u l a r c r y s t a l s con- tinue' downward f o r 0.2 m where more complex shapes occur, c r y s t a l s are intergrown and c o n t a i n bands d i f f e r i n g i n e x t i n c t i o n angle by s e v e r a l degrees. These r e s u l t from branches of a c r y s t a l growing together along a m i s f i t boundary". The second core, from the s i d e of the pingo, had the same general c h a r a c t e r i s t i c s but w i t h some n e a r l y v e r t i c a l f r a c t u r e s ' between depths of 2.05 and 2.33 m. At-2.33-m occurs a 20 mm t h i c k peat l a y e r . The lower p e a t - i c e i n t e r f a c e i s . g r a d a t i o n a l , and v e r t i c a l , d i s c o n t i n u o u s . t r a i n s of peat fragments descend f o r 120 mm. Bubbles are few:; those which do occur are mainly w i t h i n the peat, and at the bottom of the i c e core, where a mass of f i n e bubbles gives the i c e a milky appearance. Fra c t u r e s are approximately v e r t i c a l and are i n d i c a t e d by f l a t t e n e d v o i d s , unlike.any bubble ( F i g . 13). I n h o r i z o n t a l s e c t i o n s the f r a c t u r e s meet at r i g h t angles. There i s no change i n f r a c t u r e s at c r y s t a l boundaries but two. f r a c t u r e s o f t e n meet at such a boundary. In v e r t i c a l s e c t i o n s f r a c t u r e s are seen to be sinuous, merge and b i f u r c a t e and to terminate upwards or downwards. They do not reach to the ground surface and have not been subject to l a t e r a l o f f s e t or new c r y s t a l groxvth; however a. small . c r y s t a l grows at a j u n c t i o n of two cracks (Fig.'1 3 ) . • A change i n texture occurs'at a h o r i z o n t a l peat l a y e r . Small c r y s t a l s occur i n the peat, but l a r g e c r y s t a l s grow immediately at the lower p e a t - i c e i n t e r f a c e . Such growth of large c r y s t a l s i s u n l i k e l y to. be new n u c l e a t i o n , and i n the absence of evidence of melt-back of e a r l i e r c r y s t a l s , i t seems l i k e l y that these are h o r i - z o n t a l extensions of c r y s t a l s . f r o m beyond the peat layer. which i s known to be l a t e r a l l y d i scontinuous. These c r y s t a l s grow competi- '• t i v e l y and at the base of the core (3.06 m) there i s only'one c r y s t a l i n a t h i n s e c t i o n . .This c r y s t a l has a w e l l developed.substructure at the base, associated w i t h a high bubble content. The i n c l u s i o n s have given r i s e to t r a i n s of d i s l o c a t i o n s , and a l t e r n a t i n g e x t i n c t i o n C r y s t a l dimensional o r i e n t a t i o n Is dominantly v e r t i c a l through- out, except i n upper lenses where c r y s t a l long axes are c o n t r o l l e d by lens shape. C-axis o r i e n t a t i o n s (Fig-. 14) i n these lenses show a .. c o n c e n t r a t i o n i n the h o r i z o n t a l , and a d i f f u s e v e r t i c a l grouping . ( F i g . 14(a)) .. No other c r y s t a l c h a r a c t e r i s t i c s c o r r e l a t e w i t h the d i f f e r i n g l a t t i c e o r i e n t a t i o n s . In the i c e core ( F i g . 1 4 ( b ) - ( f ) ) , c-axes.tend to l i e i n a h o r i z o n t a l plane., which contains p o i n t maxima, c r y s t a l s i n other o r i e n t a t i o n s being wedged out. Thus growth has been mainly i n the b a s a l plane. T h i s . p a t t e r n i s i n t e r r u p t e d i n the discontinuous peat, l a y e r s , where small c r y s t a l s show more d i s - • persed c-axes, but the c - a x i s h o r i z o n t a l p a t t e r n i s found immediately below'the peaty l a y e r . I n t e r p r e t a t i o n The pingo began to grow between 1950 and "1957 (minimum date from w i l l o w s , Mackay 1973a, p. .987) i n a " r e s i d u a l pond" i n a drained F i g u r e 14. Pingo No. 11 (a) v e r t i c a l s e c t i o n , c r y s t a l s i n lenses i n peat, core (t>) , (c) , (d) s u c c e s s i v e l y deep v e r t i c a l s e c t i o n s i n core (e) H o r i z o n t a l s e c t i o n , lower i c e ; ( f ) v e r t i c a l s e c t i o n , c r y s t a l s i n peaty l a y e r , core 2. Diagrams i n plane of samples • • Take. . -Early, growth was. i n the form of lenses w i t h i n peat, below which, c r y s t a l c h a r a c t e r i s t i c s suggest growth i n bulk water. The v e r t i c a l columnar, shape with h o r i z o n t a l i r r e g u l a r i t i e s and h o r i z o n t a l c-axes are t y p i c a l of much growth. . There i s no evidence for' f r e e z i n g upward • from the base of t h e , i c e body, so i n t h i s case water was not. i n j e c t e d i n t o already f r o z e n m a t e r i a l , r a t h e r bulk water e x i s t e d t e m p o r a r i l y at the f r e e z i n g f r o n t . •' .-" (b) W h i t e f i s h Summit Pingo (69°23'N, 133°33'W). During June 1973 t h i s 16 m high c o a s t a l . p i n g o was s u b j e c t , t o wave a t t a c k which exposed the i c e core. Samples were taken as shown i n P i g . 15: a s e r i e s i n the upper i c e layer and a second s e r i e s approximately ver- t i c a l l y through the core. Slumping q u i c k l y , b u r i e d the i c e core.. F i e l d C h a r a c t e r i s t i c s .' The exposed s t r a t i g r a p h y comprised, from the top down: (a) 3.5 m of stoney c l a y , which i s widespread i n . t h e area . (B.ampton 1972b) . This i s s t r u c t u r e l e s s i n terms of both primary d e p o s i t ! o n a l s t r u c t u r e s and features produced by: ; f r e e z i n g . No r e t i c u l a t e i c e v e i n s were observed i n 1973. An i c e wedge, 1 m long and 50 mm wide at- the shoulder, penetrated the top of the pingo; - • . , ' • (b) 0.35 m of f i n e .sand d i s p l a y s laminations 3 mm to 50 mm . ; • • t h i c k , w i t h pockets of iron's t a i n e d sand; •' •(c) i c e core, 3 m thick;'. (d) pore i c e ( i n sand) of unknown t h i c k n e s s . F i g u r e 16. Dimensional o r i e n t a t i o n , basal, c r y s t a l s , W h i t e f i s h .Summit core. Diagrams in v e r t i c a l plane c o n t a i n i n g maximum dip of l a y e r i n g 70 • Ice C h a r a c t e r i s t i c s The pure i c e par t of the core was 3. m t h i c k , and u n d e r l a i n by f i n e sand c o n t a i n i n g some.pore i c e . Tt i s p o s s i b l e that, a f u r t h e r i c e l a y e r , u n d e r l i e s the. pore i c e , but was not exposed, and d r i l l i n g was not attempted through the frozen sand. However, the. steepness of the compositional l a y e r i n g and i c e - i c y sediment contact suggests excess i c e growth at depth. The only compositional l a y e r i n g i s determined by bubble content, i n terms of the presence or absence of bubbles and t h e i r s i z e and shape. The lay e r s are approximately p a r a l l e l to the upper surface, of the core and to the i n t e r f a c e w i t h the underlying sediment-rich i c e . Very l i t t l e sediment occurs i n the upper banded ice.. Bubble C h a r a c t e r i s t i c s .-. Few bubbles occur i n the upper part of the core. They are appar- ently, randomly p o s i t i o n e d , and s p h e r i c a l , 1 mm i n diameter or s l i g h t l y elongated p a r a l l e l to the banding. Minor f r a c t u r e s a s s o c i a t e d w i t h the u p l i f t of the core have t h e i r p o s i t i o n s i n d i c a t e d by bubbles^and voids i n s u b v e r t i c a l t r a i n s . A d d i t i o n a l l y a 90 mm zone of melt f i g u r e s was observed i n the i c e at the slump su r f a c e , and p a r a l l e l to that surface. The high concen- t r a t i o n of f i g u r e s w i t h i n c r y s t a l s contrasted s t r o n g l y w i t h the adjacent bubble-poor i c e where the few.bubbles were m a i n l y on c r y s t a l boundaries. Many figures' were l i n k e d by i n t e r - c r y s t a l l i n e . threads, i n d i c a t i n g - a m a l t i n g ' o r i g i n . '..-..•• One- meter above the pore-ice begin zones of higher bubble content. A zone of large bubbles, o v e r l i e s a zone of small bubbles, the. boundary, being abrupt.' Bubbles i n the upper zone are more widely, separate and vary i n shape: ( i ) elongate bubbles are orthogonal to the banding, but are not simple c y l i n d e r s . Upper ends are o f t e n p o i n t e d , i n c o n t r a s t to the lower ends. Bulbous and in v e r t e d U shapes are common. These e l o n - gate, bubbles range up to 15.mm i n length. The r e t e n t i o n of. these bubble shapes suggests that no strong deformation^of the i c e has taken p l a c e ; ( i i ) s p h e r i c a l shapes are r a r e , occur i n groups, and are l e s s , than 1 mm i n diameter; ( i i i ) f l a t t e n e d f i g u r e s occur, u s u a l l y l e s s than 2 mm i n diameter. These are confined to the slump s u r f a c e , which suggests they are melt f i g u r e s , although s i m i l a r f i g u r e s were reported by M u l l e r (1963) i n deeper i c e . Bubble s i z e i n the small bubble zone i s r e s t r i c t e d to 3 mm, s p h e r i c a l bubbles do not exceed 1 mm diameter. Closer to the contact .with the sediment-rich i c e , bubble s i z e g e n e r a l l y decreases. Worm bubbles decrease to 5 mm.in l e n g t h , 0.5 mm diameter, s p h e r i c a l are le s s than 0.3 mm. C r y s t a l C h a r a c t e r i s t i c s C r y s t a l s i z e i s s t r o n g l y r e l a t e d to bubble content. As bubble • 2 content increases w i t h depth, so c r y s t a l s i z e decreases from 630 T 40 mm '' 4. 2 at the top of the core to 120 i 20 mm at the base. L o c a l l y bubble bands occur in.the upper i c e , w i t h a s s o c i a t e d small c r y s t a l s . The ge n e r a l , r e l a t i o n s h i p of c r y s t a l s i z e and bubble content and the presence of . sediment-rich i c e at depth i n d i c a t e s an increase in. f r e e z i n g r a t e w i t h depth, r e l a t i v e to rate of water supply. Considering. c r y s t a l shape, i t i s found that small c r y s t a l s tend toward an equigranular shape w i t h no strong curvatures or embayments, wh i l e large c r y s t a l s . a r e more i r r e g u l a r with deep embayments and m u l t i p l e curvatures. S t r a i g h t compromise boundaries are r a r e . S t r a i n ' shadows occur throughout the i c e body, but i n l e s s than 307o of the c r y s t a l s . . C r y s t a l dimensional o r i e n t a t i o n i s orthogonal to the l a y e r i n g at the base of the i c e body ( F i g . 16) and becomes more n e a r l y . p a r a l l e l to tha l a y e r i n near the top. Bubble p o s i t i o n s r e l a t i v e to c r y s t a l s are such that the m a j o r i t y occur on c r y s t a l boundaries, although near tha slump surface a zone, of melt fi g u r e s , occurs p a r a l l e l to that surface, the included f i g u r e s being p a r a l l e l i n an i n d i v i d u a l c r y s t a l . This feature and the presence of threads l i n k i n g some f i g u r e s i n d i c a t e a m e l t i n g o r i g i n . The p r e f e r r e d dimensional o r i e n t a t i o n of elongate bubbles at the base of the i c e body i s p a r a l l e l to that of c r y s t a l s , , and orthogonal t o the compositional l a y e r i n g . This .indicates the heat flow d i r e c t i o n during growth, and that no changa i n the patterns has occurred s i n c e growth, i . e . no major flow has occurred to produce a c r y s t a l dimensional o r i e n t a t i o n p a r a l l e l , to the l a y e r i n g , as occurred i n the i n v o l u t e d h i l l ice,- • Many minor f r a c t u r e s are recognized i n the i c e core, thasa are both i n t e r g r a n u l a r and i n t r a g r a n u l a r . No new c r y s t a l growth i s presant, but voids occur which are f r e q u e n t l y f l a t and orthogonal to the f r a c t u r e s u r f a c e . 73 •Petrofabric diagrams f o r samples around the top of the core and i n a v e r t i c a l s e r i e s are shown i n Figure 17(a)-(o). Because of great v a r i a b i l i t y , each t h i n s e c t i o n i s given s e p a r a t e l y : (a)-(g) are from' the upper l a y e r , (h).-(j) from 1.5 m depth, (k)-(m) from 3.0 m depth, and (n ) , (o) from 4.5 m.. The tendency i s f o r c-axis o r i e n t a t i o n s to be more concentrated w i t h depth i n t o a g i r d l e p a r a l l e l to the compositional l a y e r i n g . , The c-axis p a t t e r n i s not t y p i c a l of segregated i c e i n exper- i m e n t a l l y grown lenses (Penner 1961, Ka p l a r , personal communication 1974) or i n other large pingos (see Tuktoyaktuk pingo, next s e c t i o n ) or i n v o l u t e d h i l l i c e . The g i r d l e , patterns occur i n the zones of bubbly i c e which . have smaller but elongate c r y s t a l s , r a t h e r than columnar c r y s t a l s as was the case i n i c i n g mounds, and Pingo No. 11. I n t e r p r e t a t i o n The compositional l a y e r i n g throughout the i c e body was p a r a l l e l to the f r e e z i n g f r o n t at the time of growth. C r y s t a l s and bubbles near the base of the i c e core have a dimensional p r e f e r r e d o r i e n t a t i o n orthog- onal to the l a y e r i n g which i n d i c a t e s that no major flow has occurred, a c o n c l u s i o n which i s supported by the l a t t i c e o r i e n t a t i o n s which have c-axes p a r a l l e l to the l a y e r i n g . Higher up the i c e body the dimensional o r i e n t a t i o n s are l e s s w e l l pronounced and c-axas are more d i s p e r s e d , which' c o n t r a s t s w i t h the i n v o l u t e d h i l l , , where more u p l i f t has occurred, and basal planes are p a r a l l e l to the compositional l a y e r i n g . - In the e a r l y growth stage, f r e e z i n g xvas slow, as i n d i c a t e d by the low bubble content and large c r y s t a l s i z e . An increase i n f r e e z i n g r a t e i s i n d i c a t e d by successively:.' i c e c o n t a i n i n g large bubbles orthogonal to; the banding F i g u r e 1 7 . ' ' 75 Figure 17 (cont 1d) I F i g u r e 17. (a)-(g) sections i n upper i c e l a y e r ; ( h ) - ( j ) s e c t i o n s from 1.5 m depth; (k)-(m) sec t i o n s from 3.0 m depth; (n) j (o) sections from 4.5 m depth.. • c = compositional l a y e r i n g ' 77 w i t h smaller c r y s t a l s than the bubble-poor ice,, then i c e c o n t a i n i n g small, bubbles, then pore i c e . L a t t i c e o r i e n t a t i o n s provide a d d i t i o n a l evidence, the lower i c e c o n t a i n i n g c r y s t a l s with c-axes p a r a l l e l to the banding. Such an increase i n f r e e z i n g r a t e could be produced by u p l i f t of the lake : bottom and exposure to c o l d a i r temperatures. (c) Tuktoyaktuk Pingo This i s lower and broader than W h i t e f i s h Summit Pingo, and appears o l d e r , judging by the surrounding polygon p a t t e r n . I t . i s one of a group of three i n the Tuktoyaktuk hamlet area. The pingo,has been excavated to expose the core which comprises segregated i c e and pore i c e . The s t r a - tigraphy i n the core was discussed by Rampton and Mackay (1971) and.is summarized here. Pond s i l t contains i c e lenses and peat l a y e r s , and i s penetrated by i c e wedges. Below tha s i l t i s sandy g r a v e l which o v e r l i e s the pingo core. Nowhere do wedges penetrate the pingo i c e , which comprises a l t e r n a t i n g l a y e r s of i c e and sandy i c e . Rampton and Mackay (1971) r e f e r , to normal f a u l t i n g which occurred during u p l i f t of the pingo - s i m i l a r f a u l t s have been reported i n other pingos (Mackay and Stager 1956b). I n Tuktoyaktuk pingo the f a u l t can be traced on aach face of the c e l l a r , and the pingo core i s upthrown r e l a t i v e to the g r a v e l overburden, on a f a u l t plane d i p p i n g ca. 70°. Ice C h a r a c t e r i s t i c s The core c o n t r a s t s g r e a t l y with that of W h i t e f i s h Summit Pingo. Bubbles are very r a r e , and the compositional banding i s determined by sediment content ( F i g . 18(a)., (b)) C l a y . p e l l e t s up to 4 mm i n diameter  7 9 occur i n discontinuous l a y e r s , more f r e q u e n t l y sediment bands are of sand grade which continue l a t e r a l l y for many metres, i n d i c a t i n g the r e g u l a r i t y of the system. The bands are t y p i c a l l y up to. 10 mm t h i c k , separated by 20-50 mm of sediment-poor i c e . These la y e r s are not p l anar but have l o c a l i r r e g u l a r i t i e s with v e r t i c a l symmetry planes. C r y s t a l C h a r a c t e r i s t i c s C r y s t a l shape c h a r a c t e r i s t i c s were s t u d i e d i n v e r t i c a l and hor- i z o n t a l s e c t i o n s , i . e . orthogonal and p a r a l l e l to c o m p o s i t i o nal l a y e r i n g , and were found tb vary w i t h c r y s t a l s i z e and p o s i t i o n r e l a t i v e to sediment bands. .Large c r y s t a l s are anhedral w i t h sinuous mutual c o n t a c t s ; boundaries w i t h s m a l l e r c r y s t a l s are s t r o n g l y embayed, i n d i v i d u a l segments being s l i g h t l y curved or s t r a i g h t . Mutual boundaries of s m a l l c r y s t a l s are ; u s u a l l y s t r a i g h t and give polygonal shapes. The small c r y s t a l s have no pronounced substructure but embay large c r y s t a l s along t h e i r sub-boundaries. Near sediment bands shapes of a l l g r a i n s i z e s change such that boundaries approach the bands at r i g h t angles. C r y s t a l s i z e s are tabulated i n Table 2, o m i t t i n g c r y s t a l s w i t h i n sediment bands. Average s i z e s are f a i r l y c o n s i s t e n t throughout, f o r 9 9 both s e c t i o n o r i e n t a t i o n s , ranging from 47 mm- to 79 mm-. However there 9 e x i s t s a recognizable range i n s i z e w i t h i n a given s e c t i o n , from > 100 mm- • 2 to <.10 mm . Size f a l l s a r u r t h e r order of magnitude i n sediment l a y e r s . 30 Substructure i s confined to l a r g e r grains which have been embayed by small c r y s t a l s l a c k i n g .substructure.. This suggests the s u b s t r u c t u r e developed before formation of the small c r y s t a l s . P e t r o f a b r i c a n a l y s i s shows that the.small and large grains do not have markedly d i f f e r e n t l a t t i c e o r i e n t a t i o n s , i n d i c a t i n g that the small grains may.have formed by p o l y g o n i z a t i o n of large s t r a i n e d grains.. Dimensional o r i e n t a t i o n diagrams do not e x h i b i t s i n g l e maxima ( F i g . 19). V e r t i c a l t h i n s e c t i o n s c o n t a i n v e r t i c a l c o n c e n t r a t i o n s i n c r y s t a l s away from sediment bands and h o r i z o n t a l concentrations, adjacent t o sediment. H o r i z o n t a l s e c t i o n s are dominated by long axes p a r a l l e l to the s t r i k e of sediment bands. . Thus sediment content p l a y s a major r o l e i n determining dimensional o r i e n t a t i o n . Optic a x i s o r i e n t a t i o n s are shown i n Figures 20, 21; F i g u r e 20 r e p r e s e n t s . s e c t i o n s p a r a l l e l to the compositional l a y e r i n g and Figure 21 represents orthogonal s e c t i o n s . Figure 20(a) and (b) a r e from adjacent, t h i n s e c t i o n s , (a) i s 25 mm above ( b ) ; (a) shows a more d i f f u s e p a t t e r n than ( b ) , but there i s a tendency toward a c o n c e n t r a t i o n approximately orthogonal to the l a y e r i n g . Component diagrams have been prepared on the b a s i s of presence or absence of sub-boundaries ( F i g . 2 0 ( c ) , ( d ) ) and c r y s t a l s i z e ( F i g . 2 0 ( e ) , ( f ) ) . The diagrams are e s s e n t i a l l y s i m i l a r , a l l are d i f f u s e s i n g l e maxima, but tha c r y s t a l s w i t h sub-boundaries are s l i g h t l y more concentrated than those without sub-boundaries and the l a r g e c r y s t a l s are less s c a t t e r e d than the s m a l l . Sub-boundaries i n d i c a t e basal.plane s l i p and the small c r y s t a l s represent break-up of l a r g e r g r a i n s . Figure 19. C r y s t a l dimensional o r i e n t a t i o n , Tuktoyaktuk Pingo, (a)-(c) h o r i z o n t a l sections; (d)-(f)' v e r t i c a l sections. ., TABLE I I C r y s t a l S i z e , Tuktoyaktuk Pingo 2 S e c t i o n O r i e n t a t i o n C r y s t a l S i z e , mm V e r t i c a l 100 cr ys t a l s 53 119 cr y s t a l s 63 114 cr y s t a l s 79 97 cr ys t a l s .'• 47 105 cr ys t a l s 55 114 c r •ystals. 67 . 82 Figure.20. Figure 20 (cont'd) 83 C F i g u r e 20. Tuktoyaktuk Pingo. (a),(b) h o r i z o n t a l s e c t i o n s (c) c r y s t a l s without sub-boundaries (d) c r y s t a l s w i t h sub-boundaries (e) l a r g e c r y s t a l s ( f ) s m a l l c r y s t a l s (g) h o r i z o n t a l s e c t i o n (h) s m all c r y s t a l s . ( i ) l a r g e c r y s t a l s ( j ) c r y s t a l s without sub-boundaries (k) c r y s t a l s w i t h sub-boundaries ( 1 ) c r y s t a l s w i t h dimensional o r i e n t a t i o n normal to l a y e r i n g , (m) c r y s t a l s w i t h dimensional o r i e n t a t i o n at 4 5 . to l a y e r i n g (n) . c r y s t a l s w i t h dimensional o r i e n t a t i o n p a r a l l e l to l a y e r i n g . c = compositional l a y e r i n g Figure 2 1 . 86 Figure 21 (cont'd)• . 1_. I 87 Figure 21 (cont'd) gure 21. Tuktoyaktuk Pingo P e t r o f a b r i c s . ( a ) , ( b ) , ( c ) V e r t i c a l s e c t i o n s (d) 73 c r y s t a l s w i t h at l e a s t one s t r a i g h t side Ce) 67 c r y s t a l s w i t h no s t r a i g h t sides • ( f ) 86 c r y s t a l s away from sediment bands (g) 64 c r y s t a l s adjacent to sediment, bands Ch) 73 c r y s t a l s w i t h sub-boundaries . ( i ) 78 c r y s t a l s without sub-boundaries ( j ) 58 c r y s t a l s w i t h long axes greater than 10 mm (k) 25 c r y s t a l s w i t h long axes l e s s than 6 mm CD 44 c r y s t a l s w i t h l e s s than 6 sides (m) 72 c r y s t a l s w i t h more than 6 sides Cn) 33 c r y s t a l s w i t h 6 sides Co) 31 c r y s t a l s w i t h 40" dimensional o r i e n t a t i o n CP) 37 c r y s t a l s w i t h 90° dimensional o r i e n t a t i o n (q) 24 c r y s t a l s w i t h 0° dimensional o r i e n t a t i o n Cr) 59 or i c r y s t a l s Lentation w i t h other than 40° , 90° , 0° dimensional c = compositional l a y e r i n g 89 . -Figure . 20(g) represents another .section approximately p a r a l l e l to the compositional l a y e r i n g (sketched i n F i g . 18), and Fi g u r e 20(h)-. (n) are component diagrams. Again there occurs a major c-a x i s concen- t r a t i o n orthogonal to the l a y e r i n g , but w i t h minor maxima a l s o . In the component diagrams, Figure 20(k) i n d i c a t e s that c r y s t a l s with sub-boundaries have a stronger c o n c e n t r a t i o n orthogonal to the l a y e r i n g than other c r y s - t a l s , w h i c h suggests these c r y s t a l s are p r e f e r r e d l y o r i e n t e d f o r b a s a l g l i d e . F i g u r e 21(a)-(c) represent a v e r t i c a l t h i n s e c t i o n , and. component diagrams are shown i n Figure 2 1 ( d ) - ( r ) . The general p a t t e r n i s f o r a. maximum at 60° to the l a y e r i n g , contained i n a g i r d l e orthogonal t o the l a y e r i n g . The component diagrams show no major .difference, although the co n c e n t r a t i o n maximum Is more pronounced i n c r y s t a l s w i t h sediment i n t h e i r g r a i n boundaries ( F i g . 21(g)) and the g i r d l e p a t t e r n i s b e t t e r developed, i n . o t h e r c r y s t a l s ( F i g . 2 1 ( f ) ) . In terms of c r y s t a l shape, F i g u r e 21(d) shows c r y s t a l s w i t h at .least'one s t r a i g h t s i d e ; the. p a t t e r n does not d i f f e r s u b s t a n t i a l l y from Figure 21(e). which represents c r y s t a l s having a l l sides curved. The r e l a t i o n s h i p between number of sides i n . c r y s t a l s and t h e i r c - a x i s . o r i e n t a t i o n was a l s o i n v e s t i g a t e d . . The r e s u l t i n g diagrams f o r n <6,. n> 5 and n = 6, where n = number' of. s i d e s , are shown i n Fig u r e 2 1 ( o ) - ( r ) ) . Figure 22(a) and (b) represent c r y s t a l s i n a v e r t i c a l s e c t i o n adjacent to that of Figure 21. The c-axis p a t t e r n d i f f e r s sub- s t a n t i a l l y from Figure 21, many c-axes being c l o s e to the compositional l a y e r i n g . This i s a l s o evident in Figure 22(g) and (h) which represent v e r t i c a l s e c t i o n s , Hera the compositional l a y e r i n g i s l o c a l l y v a r i a b l e i n t hickness and o r i e n t a t i o n of which, an approximation i s shown i n the Figure 22. Figure 22. Tuktoyaktuk Pingo.. (a),(b) v e r t i c a l s e c t i o n s (c) 59 c r y s t a l s w i t h sub-boundaries (d) 39 c r y s t a l s without sub-boundaries (e) . "'32 small c r y s t a l s ( f ) 29 large c r y s t a l s ., (g) v e r t i c a l s e c t i o n , 76 c r y s t a l s : (h) v e r t i c a l s e c t i o n , 78 c r y s t a l s c = composit i o r i a l . layering. 92 f i g u r e s . Thus there i s considerable v a r i a b i l i t y of o p t i c a x i s o r i e n t a t i o n s , i n adjacent s e c t i o n s , and patterns cannot be r e l a t e d s y s t e m a t i c a l l y to c r e n u l a t i o n s i n compositional l a y e r i n g s . I n t e r p r e t a t i o n • Tuktoyaktuk pingo core i s c h a r a c t e r i z e d by a l t e r n a t i n g l a y e r s of pore i c e and segregated i c e ; bubbles are almost completely absent. Thus growth c o n d i t i o n s d i f f e r e d s u b s t a n t i a l l y from those of W h i t e f i s h Summit Pingo, and Tuktoyaktuk Pingo i s a r e s u l t of both segregated and pore i c e growth. The l a y e r s of. pore i c e are up to 25 mm t h i c k and the segregated, i c e l a y e r s reach 100 mm. Layers are t r a c e a b l e l a t e r a l l y f o r s e v e r a l meters, and,are f a i r l y constant i n t h i c k n e s s . Despite the o v e r a l l symmetry of the mesdscopic fea t u r e s of the c o re, p e t r o f a b r i c diagrams show a range of p a t t e r n s from s i n g l e maxima orthogonal to the l a y e r i n g ( F i g . 20(b)) to g i r d l e s p a r a l l e l , to the l a y e r i n g ( F i g . 22 ( g ) , ( h ) ) . These v a r i a t i o n s occur over short l a t e r a l and v e r t i c a l d i s t a n c e s ; o f t e n the t h i n s e c t i o n s are from the same specimen, so there i s no pos- s i b i l i t y of mistake i n tha o r i e n t a t i o n of the s e c t i o n s . L i t t l e heave has occurred compared w i t h the i n v o l u t e d h i l l and there i s no evidence of. s u b s t a n t i a l flow i n tha body, although b a s a l plane s l i p and deformation band development have occurred, i n d i c a t i n g that any flow was concentrated i n i c e l a y e r s r a t h e r than i n pore i c e . However, t h i s does hot e x p l a i n the l o c a l v a r i a b i l i t y i n p e t r o f a b r i c s . . In terms of o r i g i n a l growth con- d i t i o n s , the a l t e r n a t i n g segregated and pore i c e l a y e r s i n d i c a t e v a r i a t i o n s , i n water supply and pore water pressure.-' Figure 23. Summary p e t r o f a b r i c diagrams, Tuktoyaktuk Pingo (a) Summary diagram of Figures ,20,. 21. (b) Summary diagram of Figure 22(a), ( b ) , ( g ) , ( h ) . Conclus ion The three pingos discussed represent three d i f f e r e n t stages i n -. Mackay's. (1973a) c l a s s i f i c a t i o n . Pingo No. 11 i s i n d i c a t i v e of a tempo- r a r y , e a r l y growth stage i n bulk water, and d i s p l a y s some s i m i l a r i t y to an i c i n g mound. A change i n growth co n d i t i o n s was recognized i n W h i t e f i s h Summit Pingo, r e l a t e d to u p l i f t of the lake bottom, w i t h growth of segre- gated i c e . In Tuktoyaktuk Pingo a f u r t h e r stage was shown by a l t e r n a t i n g segregated and pore i c e . In a d d i t i o n to d i f f e r e n c e s i n s i z e , shape and i n c l u s i o n patterns i n the three pingos, i t was found that there were r e l a t e d c r y s t a l c h a r a c t e r i s t i c s , although not a l l p e t r o f a b r i c diagrams could be exp l a i n e d . Involuted H i l l Ice . .. I n t r o d u c t i o n • The term -"Involuted h i l l " was a p p l i e d by Mackay (19S3,. p. 138) to extensive ice-cored h i l l s w i t h f l a t tops. A notable f e a t u r e i s the pre- sence of steep ridges which are fr e q u e n t l y l a t e r a l , or cross the tops. . The h i l l s are abundant near Tuktoyaktuk. • F i e l d C h a r a c t e r i s t i c s The surface form of the h i l l s has been discussed by Mackay (1963, 1973b) and a g r a v i t y p r o f i l e of one such h i l l was presented by Rampton and Walcott (1974). The i n t e r n a l s t r u c t u r e has been recorded from c o a s t a l and.Inland slumps (Mackay 1973b; Rampton and Mackay 1971) and a G e o l o g i c a l Survey of Canada d r i l l i n g p r o j e c t ( S c o t t , personal communication 1974). C h a r a c t e r i s t i c a l l y 1-10 m of stoney c l a y c o n t a i n i n g a r e t i c u l a t e i c e - v e i n system o v e r l i e s an i c e core, w i t h sand at depth. Observations of s e v e r a l h i l l s show that, surface ridges are u n d e r l a i n by r i s e s i n the. i c e core. Generally the r e t i c u l a t e i c e veins are orthogonal and p a r a l l e l to the upper surface of the massive i c e . Mackay (1974b) argues that the i c e veins formed i n the c l a y during downward f r e e z i n g , and the massive i c e core grew by a. segregation process and that sand below the cores s u p p l i e d the water necessary f o r core growth. The cores may reach 25 m i n t h i c k n e s s , but d r i l l i n g i n a number of h i l l s has shown t h i n discontinuous g r a v e l , sand and c l a y l a y e r s . , In t h i s study xje discuss only one h i l l , 5 km south-, west of Tuktoyaktuk. The presence of compositional l a y e r i n g i n the core was pointed but by Mackay (1963) to c o n s i s t of a l t e r n a t i n g : l a y e r s of c l e a r and bubbly i c e , and o c c a s i o n a l sediment-rich i c e . The l a y e r i n g at the top i s approximately p a r a l l e l to the upper i c e s u r f a c e , and becomes h o r i z o n t a l at depth. Ver- t i c a l f r a c t u r e s . a r e present i n the massive i c e , and some i c e wedges pene- t r a t e through the stoney c l a y overburden i n t o the core. Co a s t a l e r o s i o n has removed a major s e c t i o n of the h i l l ; t o t a l r e t r e a t i n the period s i n c e 1935 a i r photography has heeni> 240 m. This r e t r e a t has produced steep i c e c l i f f s , and has added to the creep process. Sampling was c a r r i e d out at. s i t e s where creep was minimal: (a) an a r t i f i c i a l p i t on the landward side of the h i l l ; (b) exposures away from c l i f f s . The presence of a n t i c l i n a l f o l d s i n the l a y e r i n g of the i c e core has been pointed out; l o c a l l y these f o l d s are. penetrated by i c e wedges., 9 6 The sampling plan was as f o l l o w s : ' ( i ) by c o r i n g front -'the exposed top of , the massive i c e , ( i i ) by c o r i n g from a p i t , ( i i i ) by sampling round a f o l d beneath a surface r i d g e , ( i v ) by sampling along the limb of- an a n t i c l i n e adjacent to a wedge. The. i n t e n t i o n was to demonstrate (a) any changes i n s t r u c t u r e , p e t r o f a b r i c s and t e x t u r e through the h i l l . , i n d i c a t i v e of growth mechanisms and subsequent deformation a s s o c i a t e d w i t h heave and g r a v i t y creep, (b) 1 the f o l d i n g mechanism beneath " i n v o l u t i o n s " (c) the i n f l u e n c e of wedge growth on the p e t r o f a b r i c s and. texture i n such a f o l d , (d) the c h a r a c t e r i s t i c s of thermal c o n t r a c t i o n cracks i n massive ice,, and t h e i r , mode of i n f i l . ' . (a) V e r t i c a l i c e cores I n t r o d u c t i o n A SIPRE corer was used to o b t a i n v e r t i c a l cores ( i ) at the. top of the h i l l , ( i i ) a t . s e a - l e v e l . Thus the p r o f i l e s obtained have a h o r i z o n t a l o f f s e t . The c o r i n g s i t e s were chosen.near exposed c l i f f s where l i t t l e f o l d i n g was observed, thus the core i s thought to represent r e l a t i v e l y , undisturbed i c e . Exposures on various parts of the h i l l d i s p l a y the r e t i c u l a t e v e i n i c e system w i t h i n the stoney c l a y overburden, the veins are approximately normal and p a r a l l e l to. the c o n t a c t . w i t h the u n d e r l y i n g ice.. Those, n e a r l y orthogonal to the contact dominate. Ice C h a r a c t e r i s t i c s A 3.3 m s e c t i o n of the upper core i s shown s c h e m a t i c a l l y i n F i g u r e 24. The a l t e r n a t i n g bubbly/non-bubbly l a y e r i n g and sediment bands are apparent. Bubbles w i t h i n a given, band vary i n s i z e and shape, ranging up 97 Figure 25. Probable downward t r a n s i t i o n s from one i c e type to the. next: A = c l e a r i c e , .Bj_ •= small bubbles, B£ = medium bubbles, S3 = l a r g e bubbles, C = c l a y , D = sand. 98 to 10 mm.long. The l a y e r s are shown as h o r i z o n t a l , although a s l i g h t . dip was present.. I t i s evident chat the upper i c e has been'^subject to c o n s i d e r a b l e u p l i f t , at l e a s t 15 m (the thickness of the i c e core) and l o c a l l y d i f f e r e n t i a l u p l i f t ( f o l d i n g ) . A d d i t i o n a l l y i t i s to be expected that some creep under, g r a v i t y has occurred, and v a r y i n g thermal g r a d i e n t s have.been imposed. Thus some m o d i f i c a t i o n of the o r i g i n a l growth forms of the bubbles may have occurred. From F i g u r e 24 we see that elongated bubbles are orthogonal to the c o n t a i n i n g l a y e r s d e s p i t e the dip of those l a y e r s . Thus they have not been rot a t e d p a r a l l e l to the l a y e r i n g , or to the f o l d a x i a l s u r face. Sediment occurs as c l a y p e l l e t s and t h i n sand l a y e r s ; the two types occur s e p a r a t e l y . These bands are much narrower and f r e q u e n t l y l e s s extensive l a t e r a l l y (where exposed)' than the bubble bands. Banding P a t t e r n . . The sequence of l a y e r i n g i n terms of bubble and sediment content i s i n v e s t i g a t e d by recording, the frequency of t r a n s i t i o n s from one type of l a y e r t o the next and p r e p a r i n g a downward t r a n s i t i o n p r o b a b i l i t y m a trix. Probable t r a n s i t i o n s are shown i n F igure 25, suggesting that i n some cases s e v e r a l p o s s i b i l i t i e s are almost e q u a l l y l i k e l y , e.g. from medium bubbles to e i t h e r c l e a r i c e or large bubbles or c l a y , whereas i n other cases one t r a n s i t i o n i s more probable, e.g. c l a y to c l e a r i c e . Some probable sequences are c l e a r . i c e to c l a y to c l e a r i c e , c l e a r i c e to s m a l l bubbles to c l a y to c l e a r i c e , e t c . In terms of f r e e z i n g c o n d i t i o n s i t . i s apparent that there, i s no simple p a t t e r n , i n l a y e r i n g s and thus no • r e c o g n i z a b l e p a t t e r n i n sediment or gas i n c l u s i o n pr r e j e c t i o n f o r the given sample. Generally the i n c l u s i o n of sediment i n d i c a t e s lower pore water pressure. t • C r y s t a l C h a r a c t e r i s t i c s Tha compositional l a y e r i n g s of c l e a r i c e , bubbly i c e and sediment- r i c h , i c e each have r e l a t e d c r y s t a l s i z e s . The l a r g e s t c r y s t a l s occur i n the c l e a r i c e , intermediate s i z e s i n the. bubbly i c e , and the s m a l l e s t i n the i c y sediment. Average s i z e s are given i n Table 3 f o r s e v e r a l depths. Table III Depth C r y s t a l s i z e (m) (mm2) Ice type 0 3 6 9 12 '..' 84 .200 : 39 3 9 3 C l e a r i c e C l e a r i c e Bubbly i c e Sediment-rich i c e C l e a r i c e C r y s t a l s i z a i n i n c l u s i o n zpne3 i s . c o n t r o l l e d by tha d i s t a n c e between i n c l u s i o n s ( F i g . 26). This p a t t e r n i s repeated throughout the . thickness of the i c e and i n d i c a t e s the i n f l u e n c e of i n c l u s i o n s on. g r a i n boundary migration. Small i n c l u s i o n s are concentrated on g r a i n boundaries, i n d i c a t i n g that dragging of i n c l u s i o n s has occurred. 1 0 0 c F i g u r e 26. Influence of i n c l u s i o n s on c r y s t a l s i z e ; (a),(b) i n f l u e n c e of sediment (sand), v e r t i c a l s e c t i o n , ( c ) , ( d ) i n f l u e n c e of bubbles, v e r t i c a l s e c t i o n , 10 mm g r i d . 1 0 1 C r y s t a l shape i s r e l a t e d to i n c l u s i o n type. In the case of c l e a r i c e there are no gross i n c l u s i o n s and no r e t a r d a t i o n of g r a i n growth has occurred, thus c r y s t a l s are l a r g e , anhedral and o f t e n i n t e r l o c k e d . S e r r a t i o n s are rare at the.boundaries of large c r y s t a l s , r a t h e r the i r r e g u l a r i t i e s are on a centimetre s c a l e . The s m a l l e r c r y s t a l s i z e i n ' bubbly i c e i s linked, to a d i f f e r i n g c r y s t a l shape;, intergrowths are ab- sent, boundaries are more gently curved except where i n f l u e n c e d by bubbles, and i n many cases are approximately s t r a i g h t . S i m i l a r l y i n . s e d i m e n t - r i c h i c e the i n c l u s i o n s a f f e c t shapes;' c r y s t a l s i z e i s s m a l l e r , boundaries are e s s e n t i a l l y s t r a i g h t . f r o t a one i n c l u s i o n to the next. Zones are not always separated by abrupt j u n c t i o n s , f r e q u e n t l y one zone merges i n t o the next. However, w e l l defined sediment or bubble bands occur and the shape change i s abrupt. In a d d i t i o n , to g r a i n boundary m i g r a t i o n and the i n f l u e n c e of i n - c l u s i o n s , there are other f a c t o r s r e l a t e d to c r y s t a l s i z e and shape. These are the presence of sub-boundaries, polygonized subgrains, and-new grains formed during r e c r y s t a l l i z a t i o n . Sub-boundaries d e l i m i t zones of, c r y s t a l s w i t h s l i g h t l y d i f f e r i n g l a t t i c e o r i e n t a t i o n s and may thus i n t e r - s ect c r y s t a l boundaries. I n the lower s e c t i o n of the core the l a r g e c r y s t a l s may have s e v e r a l sub-boundaries whereas they are absent from s m a l l g r a i n s . In the l a r g e c r y s t a l s the sub-boundaries are orthogonal to the p r e f e r r e d dimensional- o r i e n t a t i o n and are p a r a l l e l .to c-axes. • ' C r y s t a l dimensional o r i e n t a t i o n i s w a i l developed p a r a l l e l . t o the compositional l a y e r i n g i n the upper part of the core. The p a t t e r n becomes l e s s pronounced with depth, but i s l o c a l l y . s t r o n g where sediment bands influence, the p a t t e r n . Frequency d i s t r i b u t i o n diagrams of dimensional o r i e n t a t i o n f o r v e r t i c a l sections p a r a l l e l to the dip of the l a y e r i n g are shown i n Figure 27(a)-(e). The r e l a t i o n s h i p of bubbles to texture a l s o v a r i e s w i t h depth.. At t:he' top .bubbles are p r e f e r r e d l y located on boundaries,. although not n e c e s s a r i l y at i r r e g u l a r i t i e s . The l a r g e r the bubble.the greater the e f f e c t on t e x t u r e . There appears to be a minimum size, f o r a bubble to have c o n t r o l , and many small bubbles are contained w i t h i n c r y s t a l s . The l a r g e r the bubble the greater i t s e f f e c t on g r a i n boundary m i g r a t i o n . Boundaries may be temporarily retarded by, then break away from, : or drag small bubbles. Larger bubbles cause greater i r r e g u l a r i t i e s i n boundary shape. Bubbles are l e s s frequent on sub-boundaries, although i n the . l a r g e r c r y s t a l s , the l a r g e r bubbles may be so s i t u a t e d . . Bubbles tend t o be absent from sediment bands, as i s the case under growth c o n d i t i o n s . Sediment occurs as lay e r s of c l a y p e l l e t s and i c y sand. These l a y e r s are of l e s s e r v e r t i c a l and l a t e r a l extent than the c l e a r and bubbly bands, but, depending on the sediment"concentration, they have a marked e f f e c t on t e x t u r e . The zones of higher sediment content provide d i s t i n c t t e x t u r a l breaks; the o v e r l y i n g i c e , whether clear, or bubbly ( F i g , 26), contains r e l a t i v e l y l a r ge c r y s t a l s which terminate at the sediment, w i t h g r a i n .boundaries orthogonal to the l a y e r i n g . C r y s t a l s , i n the sediment bands, were not r e a d i l y observed by the t h i n s e c t i o n t e c h nique due to the d i f f i c u l t y of p r e p a r a t i o n of s e c t i o n s of d i r t y i c e , but s i z e i s . very l i m i t e d • Where i t i s not concentrated- i n bands, sand tends to l i e on g r a i n boundaries, but not n e c e s s a r i l y at i r r e g u l a r i t i e s i n Figure 27 103 Figure 27. C r y s t a l dimensional o r i e n t a t i o n , i n v o l u t e d h i l l . . (a)-(e) v e r t i c a l core ( f ) - ( i ) a n t i c l i n e ( j ) - ( p ) a n t i c l i n e penetrated by wedge A l l diagrams a r e . v e r t i c a l , p a r a l l e l to dip of compositional l a y e r i n g .105 those boundaries. . Clay p e l l e t s , are l e s s t e x t u r a l l y c o n t r o l l e d , and often''-, occur w i t h i n c r y s t a l s . Where sediment bands are c l e a r cut there are sharp changes i n c r y s t a l s i z e , from small i n the sediment to very- large i n the i n c l u s i o n f r e e i c e ( F i g . 26). .-'•'''••-. The.record of l a t t i c e o r i e n t a t i o n s i s incomplete as pa r t s of the core were l o s t i n t r a n s i t . The a v a i l a b l e , record i s summarized i n F i g u r e 28. I t i s evident that strong.concentrations occur at some depths; elsewhere the diagrams are more d i f f u s e , but there i s an o v e r a l l tendency f o r c-axes to be orthogonal to the . l a y e r i n g . Concentrations are g r e a t e s t at the top of the core where some f o l d i n g has occurred, and a l s o i n . sec t i o n s c o n t a i n i n g the strongest dimensional p r e f e r r e d o r i e n t a t i o n s p a r a l l e l to the l a y e r i n g . An example of the l a t t e r i s where sediment bands occur, Figure 28(e);. here flow has been concentrated i n the i c e \ w i t h l e s s sediment i n c l u s i o n s , w i t h b a s a l planes becoming p a r a l l e l to. the l a y e r i n g . Towards the base of the core the most r e c e n t l y grown i c e has more d i f f u s e d i s t r i b u t i o n diagrams, but the major c o n c e n t r a t i o n i s evident i n a d d i t i o n to minor groupings and g i r d l e s . F i g u r e 2 8 ( j ) shows the c h a r a c t e r i s t i c s of c r y s t a l s outside the major c o n c e n t r a t i o n in.. Figure 2 8 ( i ) . In. the upper part of the s e c t i o n such c r y s t a l s are grouped r a t h e r than evenly d i s t r i b u t e d ; i n the lower part the c r y s t a l s are s m a l l e r , separated and surrounded by l a r g e r c r y s t a l s w i t h c-axes i n the major co n c e n t r a t i o n . I t i s evident that some c r y s t a l s w i t h c-axes outside the maximum are l a r g e , but the m a j o r i t y are small and are probably being consumed by t h e i r • neighbours. ' • • .• . .'.'•'•. Figure 28 107 F i g u r e 28 (cont'd) ( j ) v e r t i c a l sec- t i o n i n d i c a t - i n g c r y s t a l s outside the maximum. Crys t a l s outs ide maximum. Fig u r e 28. P e t r o f a b r i c s of i c e core, i n v o l u t e d h i l l . (a)-(g) s u c c e s s i v e l y deep s e c t i o n s , ' . . 0-14 m; i ( h ) , ( i ) h o r i z o n t a l and v e r t i c a l s e c t i o n s , depth 8m. ' . (cont'd) c = compositional layering.. 108 S t r u c t u r a l Features: Fractures F r a c t u r e s occur throughout, and are t y p i c a l l y approximately v e r t i c a l , and pass through the l a y e r i n g sequence; r a r e l y i s there any o f f s e t t i n g . -. Microscopic Features of F r a c t u r e s In t h i n s e c t i o n f r a c t u r e s appear as narrow planar features marked by f l a t t e n e d gas i n c l u s i o n s , and sediment. The gas i n c l u s i o n s occur on . f r a c t u r e surfaces passing through bubbly and bubble-free i c e . Bubbles adjacent to.the f r a c t u r e s are not deformed more than others. The paths of. the f r a c t u r e s . r e l a t i v e to t e xture are such that they are both i n t e r g r a n - u l a r and i n t r a g r a n u l a r . No l o c a l d e v i a t i o n s occur; nor has there been any new c r y s t a l growth on the f r a c t u r e s u r f a c e s . I n t e r p r e t a t i o n I t i s apparent that while there are c o n t r a s t s i n p r o p e r t i e s from, l a y e r to l a y e r i n the v e r t i c a l i c e core, there i s no major change w i t h . . depth of c h a r a c t e r i s t i c s of a given l a y e r type. C-axis. o r i e n t a t i o n s are g e n e r a l l y orthogonal to the l a y e r i n g throughout, although more d i s p e r s e d patterns occur i n bubble bands. Weak c-axis maxima orthogonal to the l a y e r i n g are probably produced during the f r e e z i n g process; t h i s i s t r u e a l s o of some pingos. However i t i s evident that the i c e has been u p l i f t e d by heaving, and that creep under the weight of i c e and overburden has occurred. The l a t t i c e p r e f e r r e d o r i e n t a t i o n has been accentuated espec- i a l l y i n the i n c l u s i o n f r e e i c e which now contains l a r g e r c r y s t a l s . Dimensional o r i e n t a t i o n i s p a r a l l e l to the l a y e r i n g , whereas i n the growth of ice. i n free water the o r i e n t a t i o n i s p a r a l l e l to the heat flow d i r e c t i o n , and thus orthogonal to any compositional l a y e r i n g . A l s o i n the l i m i t e d work on textures i n segregated i c e , c r y s t a l s tended to be columnar and • orthogonal to the plane of the.lens (Penner 1951; Kaplar, personal com- munication 1974). Thus the p a t t e r n observed here i n d i c a t e s flow p a r a l l e l to tha l a y e r i n g . D i f f e r e n t i a l flow may.have occurred on l a y e r s of d i f - • f e r e n t i n c l u s i o n content. (b) A n t i c l i n e s beneath " I n v o l u t i o n s " I n t r o d u c t i o n Superimposed on the broad p a t t e r n of tha topographic highs are r i d g e s which may be p e r i p h e r a l or may cross tops of h i l l s . C o a s t a l exposures r e v e a l the underlying s t r u c t u r e to be a n t i c l i n e s ' i n the i c e , the overburden being t h i n n e s t over f o l d c r e s t s , which undulate l o c a l l y . Some ridges c o n t a i n i c e wedges, w i t h a s s o c i a t e d surface troughs, but i n i t i a l l y we consider a f o l d where wedges are absent, then proceed to. i n v e s t i g a t e , the i n f l u e n c e of wedge growth., on such a f o l d . The charac- t e r i s t i c , banding determined by bubble and sediment content continues i n t o the fold3, w i t h l i t t l e v a r i a t i o n i n band t h i c k n e s s being observed over f o l d s . Thicknesses vary- from 50 mm to 1 m, w i t h o c c a s i o n a l discontinuous sediment l a y e r s 10 mm chick. The. sample s i t e s ( F i g . 30) f o r the f o l d comprised a v e r t i c a l s e r i e s of samples through the a x i a l plane, a series'around the f o l d c l o s u r e on a bubble-free band, and. samples of the contacts of the. discontinuous sediment band's with adjacent i c e . . 110 I l l / ' F i gure 30. A n t i c l i n e i n i n v o l u t e d h i l l , c-axes Figure 30 (cont'd) . ( f ) ~ ( j ) Sections on f o l d a x i a l plane, i n v o l u t e d h i l l . • c = c o m p o s i t i o n a l l a y e r i n g 113 Ice C h a r a c t e r i s t i c s • ."• The compositional l a y e r i n g i s determined by bubble and sediment' content. W i t h i n a given band bubble shape and s i z e vary but w i t h a general increase i n s i z e downwards. E l l i p s o i d a l , f l a t t e n e d and i r r e g u l a r bubbles range up to 4 mm,, w i t h long.axes p a r a l l e l i n g the dip of the. banding; s p h e r i c a l bubbles are s m a l l e r . Where sediment bands occur, bubbles, are few, and there are no"bubbles f o r 30 mm beneath the sediment. ; C r y s t a l C h a r a c t e r i s t i c s . ( F i g . 29) Bubbles u s u a l l y occur on grain, boundaries, the l a r g e r .ones espec- i a l l y at sharp i r r e g u l a r i t i e s i n the boundaries, or l e s s frequently- on sub-boundaries. Smaller bubbles are randomly s c a t t e r e d i n r e l a t i o n to te x t u r e . C r y s t a l s i z e v a r i e s w i t h p o s i t i o n r e l a t i v e to sediment and bubble bands. In bands of high bubble content, c r y s t a l long axes average 10 mm, and range up.to 25 mm. I n c l u s i o n - f r e e zones c o n t a i n c r y s t a l s up to 50 mm long; w i t h i n sediment r i c h bands, maximum dimensions are r e s t r i c t e d to. < 5 mm. C r y s t a l shape v a r i e s w i t h c r y s t a l s i z e ( F i g . 29).. Larger c r y s t a l s are u s u a l l y anhedral, i r r e g u l a r and i n e q u i g r a n u l a r . Boundaries are curved to cuspate, strong embayments occur where g r a i n boundaries and sub-bound- a r i e s i n t e r s e c t . Large c r y s t a l s may be embayed by. each other or s m a l l , s t r a i n - f r e e c r y s t a l s . Strong boundary curvatures other than embayments. occur.at bubbles, i n d i c a t i n g a n.influence on g r a i n boundary motion. Small c r y s t a l s are anhedral but many mutual boundaries are . s t r a i g h t . 'These are more r e g u l a r and mora n e a r l y equigranular than l a r g e c r y s t a l s . 114 S d r a i n shadows occur r a r e l y i n small c r y s t a l s , but f r e q u e n t l y i n . l a r g e r c r y s t a l s , and are p a r a l l e l to the c-axis o r i e n t a t i o n and orthogonal . to the sediment or bubble l a y e r i n g . C r y s t a l dimensional o r i e n t a t i o n . ( F i g . 27) i s g e n e r a l l y p a r a l l e l to the com p o s i t i o n a l l a y e r i n g , e s p e c i a l l y f o r large c r y s t a l s ; smaller c r y s t a l s aire more ne a r l y . e q u i d i m e n s i o n a l . Sediment i s g e n e r a l l y of medium to f i n d sand grade, o c c u r r i n g i n d i s c r e t e , discontinuous bands. A l s o some i s disp e r s e d i n c r y s t a l s and boundaries, w i t h no p r e f e r r e d t e x t u r a l p o s i t i o n . Clay p e l l e t s are ob- served s c a t t e r e d i n l a y e r s , these are i r r e g u l a r i n shape, and up to 3 mm diameter. Dense sediment bands cause t e x t u r a l changes - i c e i n such l a y e r s comprises small c r y s t a l s . . Zones of small, c r y s t a l s occur below such sediment bands. Where sediment grains are more separate, c r y s t a l s from above the la y e r penetrate through, but w i t h s l i g h t changes i n dimensional o r i e n t a t i o n . . The c-axes of. a series, of v e r t i c a l samples from c l e a r and bubbly i c e over a v e r t i c a l distance of 4 m i n the f i e l d , and samples from f o l d limbs were analyzed. From these were prepared component diagrams ( F i g . 30), based .on t e x t u r a l c h a r a c t e r i s t i c s and r e l a t i o n to s t r u c t u r e s . A l l diagrams are e s s e n t i a l l y i d e n t i c a l , i n the form of a x i a l symmetry, the a x i s being orthogonal to the compositional banding, around the f o l d . I n t e r p r e t a t i o n : I t . i s evident that a n t i c l i n a l , f o l d s u n d e r l i e zones of thi n n e r over- burden, but i t i s not c l e a r how the thickness p a t t e r n arose. The stoney c l a y m a t e r i a l i s widespread i n the area and o v e r l i e s most massive i c e bodies d r i l l e d so f a r (Mackay 1973b). Rampton (1972b) has described . the m a t e r i a l as a reworked t i l l which has been subject to slumping and . . mudflow a c t i v i t y , thus l a t e r a l v a r i a t i o n s i n thickness are to be expected.- A d d i t i o n a l l y Mackay (personal communication 1975). p o i n t s out that there is. i n c r e a s i n g evidence f o r a s e v e r a l metre deep thaw i n the area, which c o u l d ' be r e s p o n s i b l e for removing m a t e r i a l on h i l l s i d e s . The r e t i c u l a t e v e i n i c e p a t t e r n over the massive i c e shows l i t t l e evidence of creep, but over- l y i n g m a t e r i a l may have moved downs lope. Whatever i t s o r i g i n , the v a r i a - ' ... t i o n i n overburden thickness i s r e l a t e d to the upfolds i n the i c e . ' The o r i g i n a l compositional l a y e r i n g has not been g r e a t l y a f f e c t e d by the f o l d i n g process; bed thickness i n the upper layers, i s . g r e a t e s t over '• the f o l d c r e s t , but lower down the s e c t i o n bed t h i c k n e s s becomes more uniform around the f o l d ( F i g . 31). . In r e l a t i v e l y undeformed i c e , bubble e l o n g a t i o n i s p a r a l l e l to the temperature gradient during growth, whereas i n t h i s i c e bubbles tend toward p a r a l l e l i s m w i t h the dip of the l a y e r i n g . Assuming the bubbles were o r i g i n a l l y orthogonal to the l a y e r i n g , d i f f e r e n t i a l flow occurred during f o l d i n g . A l s o f l a t bubbles are e s s e n t i a l l y p a r a l l e l to the f o l i a - . t i o n . These f l a t bubble surfaces are p a r a l l e l to the b a s a l plane of the c o n t a i n i n g c r y s t a l s . A f u r t h e r t e x t u r a l f e a t u r e i s the p o s i t i o n of bubbles r e l a t i v e to g r a i n boundaries. Large c r y s t a l s were s t r a i n e d and p o l y g o n i z a t i o n has occurred. A d d i t i o n a l l y r e c r y s t a l l i z a t i o n has produced a.strong c-axis maximum f a b r i c . 116 Figure 31. Bed thickness around f o l d i n Figure 29, Fig u r e 32. Wedge p e n e t r a t i n g a n t i - c l i n e i n i n v o l u t e d h i l l . Note upturning of banding of massive i c e . Table IV C r y s t a l s i z e i n i n v o l u t e d h i l l i c e adjacent to wedge Distance from wedge (m) C r y s t a l s i z e (mm ) 5.0 735 3.0 563 2.0 361 1.0 303 0.25 68 0.1 26 117 (c) . A n t i c l i n e Penetrated by Ice Wedge I n t r o d u c t i o n ' . . Thus f a r we have considered r e l a t i v e l y undisturbed core i c e , and i c e folded by d i f f e r e n t i a l u p l i f t ; wa now. consider such a f o l d penetrated by a wedge. Tha wedge has grown where the c l a y overburden•is t h i n (1.5 m). The time of i n i t i a l c r a c k i n g i s unknown, but from the s i z e of the wedge (approximately 3 m across) i t has been growing f o r a few thousand y e a r s . This i s a rough estimate, as tha p r o b a b i l i t y , o f c r a c k i n g v a r i e s w i t h wedge s i z e (Mackay 1974a). However, near t h i s wedge there has been long-term peat accumulation i n d i c a t e d by a wedge w i t h at l e a s t 4 growth periods (Mackay 19 74a, F i g . 18).. . This s i t e was - tha depression between two i n v o l - u t i o n r i d g e s , and wedge growth occurred during peat accumulation. Thus co n d i t i o n s s u i t a b l e f o r wedga growth have p r e v a i l e d for. the time taken to accumulate at l e a s t 2 m of peat, namely about 5000 years. Although both wedges d i d not n e c e s s a r i l y grow at the same time or r a t e , the l a r g e r xjadga on tha r i d g e may w e l l have grown f i r s t as i t was on a r i d g e (otherwise the smaller wedge would have penetrated massive' i c e ) , thus c l e a r of snow, and, subject to r a p i d c o o l i n g which aids i n the c r a c k i n g process. .Recent f r a c - t u r i n g , was detected p e t r o g r a p h i c a l l y . The compositional l a y e r i n g i n tha i c e could be traced from i t s r e l a t i v e l y undeformad s t a t e up to the wedge contact, where i t became upturned and. penetrated by c r a c k s , s u b - p a r a l l e l to the. wedga. As i n other exposures, the compositional l a y e r i n g was determined by bubble and.sediment content w i t h . l a r g e bubbles above but u s u a l l y not immediately below sediment bands. • .' 118 The sampling plan f o r these' s t r u c t u r e s was designed.to t r a c e any t e x t u r a l and f a b r i c changes w i t h distance from the wedge, c h a r a c t e r i s t i c s of f a u l t s and j o i n t s , and any v a r i a t i o n s i n wedge i c e . Banding C h a r a c t e r i s t i c s Bands of d i f f e r i n g composition have d i f f e r e n t t h i c k n e s s e s , but a l l bands are e s s e n t i a l l y p a r a l l e l and uniform i n . t h i c k n e s s on f o l d , limbs. No major t h i c k e n i n g occurs adjacent to the wedge, but a t t i t u d e changes, the c h a r a c t e r i s t i c upturning being shown i n F i g u r e 32. A s e r i e s of f r a c t u r e s occurs p a r a l l e l to the wedge between which segregated i c e can s t i l l be seen. Banding a l s o occurs i n the wedge, i n . t h e form of. v e r t i c a l t o s t e e p l y d i p p i n g bubble f o l i a t i o n s . Ice C h a r a c t e r i s t i c s . As was found elsewhere i n the i c e body, bubbles occur above sediment bands but very r a r e l y immediately below. Various s i z e s and shapes of .bubble occur w i t h i n , the bands: (a) Above sediment l a y e r s the bubbles are o f t e n f l a t , and i n the ba s a l plane of the c o n t a i n i n g c r y s t a l ; (b) s p h e r i c a l bubbles up to 1 mm diameter are o f t e n surrounded by " s a t e l - l i t e " bubbles 0.13 mm i n diameter; (c) elongate bubbles 2-3 mm long;- (d) i r r e g u l a r bubbles, e s p e c i a l l y where connected by threads along g r a i n ' boundaries. In general, s p h e r i c a l bubbles occur i n groups,.but elongate and i r r e g u l a r bubbles show no z o n a t i o n w i t h i n a given band. At 0.25 m from the wedge the compositional l a y e r i n g i s d i s t u r b e d by f r a c t u r e s a s s o c i a t e d w i t h , and p a r a l l e l t o , the wedge. F r a c t u r e 119 s e p a r a t i o n decreases to 10-20 mm adjacent .to • the wedge and the f r a c t u r e s are up to 15 mm wide. The i n f i l s comprise bubbly i c e , .but segregated i c e i s s t i l l evident between them. Bubbles i n the f r a c t u r e zones are <C 0.5 mm, g e n e r a l l y s p h e r i c a l , and v e r y numerousj g i v i n g the i c e a cloudy appearance. Laye r i n g of bubbles p a r a l l e l to the f r a c t u r e w a l l s i s evident. Where the f r a c t u r e s o f f s e t sediment bands, elongate, bubbles up-to 10 mm long t r e n d p a r a l l e l to the f r a c t u r e . Despite the f r a c t u r i n g , the o r i g i n a l l a y e r i n g i s decipherable up to the wedge, and contrasts, w i t h the wedge i c e . The wedge i c e i s ..charac- t e r i z e d by s t e e p l y d i p p i n g bands c o n t a i n i n g bubbles and some f i n e g r a i n e d sediment. The bubbles, are (a) s p h e r i c a l , approximately 0.3 mm; (b) el o n g - ated, up to 4 mm, p a r a l l e l to f r a c t u r e s ; (c) i r r e g u l a r , 0.05 mm. C r y s t a l C h a r a c t e r i s t i c s C r y s t a l s i z e i n the segregated i c e changes s y s t e m a t i c a l l y w i t h d i s t a n c e from the wedge as shown i n Table 4 and F i g u r e 33. The.reduction i n g r a i n s i z e i s considered to be due to flow and p o l y g o h i z a t i o n of l a r g e r grains, and growth of new grains o r i e n t e d favourably to accommodate, the j s t r e s s exerted by the growing wedge. A s i m i l a r v a r i a t i o n , i n c r y s t a l s i z e was found by Corte (1962a). A d d i t i o n a l l y there i s a v a r i a t i o n i n s i z e r e l a t e d to sediment content; c r y s t a l s w i t h i n sediment bands are <^0.5 mm' i n diameter, w h i l e adjacent to such bands c r y s t a l s are 5 ™ and increase to '3.5 mm away from the sediment. C r y s t a l shape a l s o v a r i e s w i t h distance'.from the wedge, and w i t h ' the presence of sediment. 'Three metres from the wedge, c r y s t a l s are - 120 F i g u r e 33. Change i n c r y s t a l s i z e i n massive i c e adjacent to wedge. V e r t i c a l s e c t i o n s orthogonal to wedge. (a) sample s i t e s , (b) sample 3.0 m. from wedge ... continued.  122 ine q u i g r a n u l a r and i n t e r l o c k i n g , w i t h curved, s e r r a t e d and sometimes cuspate boundaries. The degree c f i r r e g u l a r i t y decreases, toward the wedge; w h i l e some l a r g e r c r y s t a l s r e t a i n some s t r o n g l y curved s i d e s , the smaller c r y s t a l s have s t r a i g h t mutual boundaries. This suggests polygon- . i z a t i o n gave r i s e to a r e d u c t i o n i n c r y s t a l s i z e , and t h a t new c r y s t a l s . grew. Fractures cross t h i s i c e i n an i n t e r g r a n u l a r and i n t r a g r a n u l a r f a s h i o n and f r a c t u r e i n f i l s c o n t r a s t w i t h the above p a t t e r n ; s m all c r y s - t a l s ( <" 2 mm) form a competitive zone at the f r a c t u r e boundary, from which grow elongated c r y s t a l s •(•< 7.mm) orthogonal to the f r a c t u r e , w h i l e i n the centre some c r y s t a l s are p a r a l l e l to the seam. L o c a l l y the g e n e r a l l y s t r a i g h t f r a c t u r e sides show strong i r r e g u l a r i t i e s and i t i s apparent that f r a c t u r e c r y s t a l s have invaded the surrounding segregated i c e , by a g r a i n boundary m i g r a t i o n mechanism. .. " . . Between f r a c t u r e s the segregated i c e comprises s t r a i g h t - s i d e d c r y s t a l s , <C 15 mm diameter.. The segregated i c e contains o f f s e t s on which no new c r y s t a l growth has occurred; o f f s e t s up to 5 mm have been observed. The adjacent wedge i c e d i f f e r s 1 i n that c r y s t a l s are elongated p a r a l l e l t o : the bubble zones and reach 25 mm i n length. The r e l a t i o n s h i p of bubbles, to texture i s such that s p h e r i c a l bubbles . l i e on boundaries, on sub-boundaries and x^ i t h i n c r y s t a l s . F l a t bubbles, occur only w i t h i n c r y s t a l s , i n d i c a t i n g a l a t t i c e c o n t r o l ; a l s o , no bubbles <C 1 mm were f l a t , so a minimum s i z e i s necessary. Some l o c a l . i r r e g u - l a r i t i e s i n boundaries are a s s o c i a t e d w i t h bubbles. In the f r a c t u r e zones, bubbles are <̂  1 mm i n diameter and s p h e r i c a l x ^ i t h i n boundaries of competi- t i v e growth c r y s t a l s ; near the c e n t r a l seam bubbles are elongated p a r a l l e l to the seam, and cross g r a i n boundaries. "• I 2 3 P e t r o f a b r i c diagrams were prepared for samples progressively near to the wedge. At 5.0 m from the wedge a c-axis maximum occurs orthogonal to the compositional layering, which i s s i m i l a r to that for the f o l d which has suffered nb wedge penetration (section .(b)). ,.At 3.0 m from the wedge, a single maximum l i e s at approximately 60° to the lay e r i n g ( F i g . 34(a)), and becomes rotated into the lay e r i n g at,1.0 m from the.wedge ( F i g . 34(c)). Thus the c-axis patterns become progressively more s i m i l a r to those of the wedge, indicating, the influence of wedge growth. Although the composition- a l layering of the segregated i c e remains, the pe t r o f a b r i c s are such that the contained c r y s t a l s respond to the growth stress of the wedge i n a . . . s i m i l a r manner to the outer wedge c r y s t a l s . Superimposed on t h i s pattern are fractures i n which the texture i s s i m i l a r to that of fractures discussed i n section (a), above. S i m i l a r l y the l a t t i c e orientations (Fig. 34(e)-(h)) compare with recent fracture i n f i l s . discussed elsewhere (p. 155 f f . ) , i n d i c a t i n g that pos.t-solidif i c a - '.; tion- changes have been l i m i t e d . In addition there occur older fractures which have been deformed and the contained c r y s t a l s have l a t t i c e - o r i e n t a - tions nearer to the wedge pattern. Figure 35 gives grain type, d i s t r i b u t i o n s , i . e . histograms of numbers of sides to grains i n t h i n sections from the core, folded i c e and folded Ice with wedge. There i s an obvious tendency for grains to have 5 to 6 sides i n the core Ice and folded i c e . In the ice adjacent to the wedge . a change i s evident - grains tend to be 6 to 7 sided, and there are more •-. grains with > 10 sides. This may be associated with .the progressive f a b r i c change i n that new c r y s t a l growth has occurred on old grain boundaries and that a previously s i n g l e boundary has become mu l t i p l e . 124 F i g u r e 34. F o l d penetrated by wedge, i n v o l u t e d h i l l . (a) . Sample 3.0 m from wedge (b) sample 2.0 m from wedge (c) sample 1.0 m from wedge . (u) sample adjacent to wedge ' (e) c r y s t a l s i n f r a c t u r e near wedge ( f ) c r y s t a l s i n f r a c t u r e near wedge (g) x 20 c r y s t a l s i n bubbly i c e . 20 c r y s t a l s i n c l e a r i c e (h) 25 c r y s t a l s . a t edge of f r a c t u r e . ( i ) 50.elongated c r y s t a l s i n recent f r a c t u r e . ( j ) 18 c r y s t a l s i n o l d f r a c t u r e (k) x 10 sm a l l c r y s t a l s i n f r a c t u r e . 4 0 . c r y s t a l s i n c l e a r i c e (I) x 23 c r y s t a l s adjacent to sediment . 37 c r y s t a l s away from sediment, (m) 50 small c r y s t a l s (n) x 50 small c r y s t a l s . 50 large c r y s t a l s (o) 47 c r y s t a l s between two f r a c t u r e s : (p) . 96 c r y s t a l s i n f r a c t u r e s (q) 109 c r y s t a l s away from f r a c t u r e . c = c o m p o s i t i o n a l l a y e r i n g f = f r a c t u r e w . b . = w e d g e b o u n d a r y 125 Sample p o s i t i o n s i 126, F i g u r e 34.'  128 Figure 34 (cont'd)  130 30 20 10 Figure 3 5 (cont'd) h 30 20 10 2 4 6 8 10 12 14 . 2 4 6 8 10 12 14 30 30 20 20' 10 10 £2. t „ . „ „i. 2 4 6 8 10 12 14 2 4 6 8 10 12 14 30 30̂ 20 20 10 10 1 ,.l— i 2 4 6 8 10 12 14 2 4 6 8 10 12 ,14 m 30 20 10 Figure 35 (cont'd) n 30 20 131 10 o 30 20 10 2 4 6 8 10 12 14 2 4 6 8 10 12. 14 P 3 0 20 10 r • i  ^ 2 4 6 8 10 12 14 2 4 6 8 10 12 14 q 3 0 20 10 3 0 20 10 2 4 6 8 10 12 14 132 S 30 t 30 20 2 0 10 10 6 8 10 12 14 2 4 6 8 10 12 14 u 30 20 10 V 30 2 0̂ 10 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16 F i g u r e 35. Grain type d i s t r i b u t i o n s f o r t h i n s e c t i o n s from core i c e , folded i c e and fo l d e d i c e penetrated by wedge, i n v o l u t e d h i l l . •(a) - ( f ) core, i c e , s u c c e s s i v e l y deep samples ( g ) - ( l ) a n t i c l i n e • (m)-(v).. limb of a n t i c l i n e , samples p r o g r e s s i v e l y near wedge. C r y s t a l dimensional o r i e n t a t i o n s are shown i n Fi g u r e 27. The max- imum moves from p a r a l l e l i s m w i t h the compositional l a y e r i n g at 5.0 m from the wedge ( F i g . 27(j)). to approximate p a r a l l e l i s m w i t h the wedge contact at that contact ( F i g . 2 7 ( n ) - ( p ) ) . . In the l a t t e r case a secondary maximum occurs orthogonal, to the. f i r s t ( F i g . 27(n), (o)) , r e p r e s e n t i n g the columnar c r y s t a l s i n recent f r a c t u r e s . E a r l y dimensional o r i e n t a t i o n s a s s o c i a t e d w i t h growth c o n d i t i o n s i n the segregated i c e have become o b l i t e r a t e d . I n t e r p r e t a t i o n .' Progressive changes i n t e x t u r a l and p e t r o f a b r i c c h a r a c t e r i s t i c s w i t h d istance from the wedge are recognized. Comparisons among the unde- formad banded i c e ( a ) , folded banded i c e (b) , and the present samples i n d i c a t e the i n f l u e n c e of the wedge,. Many.crystal features are symmet- r i c a l l y r e l a t e d to the wedge. L a t t i c e o r i e n t a t i o n s i n the, banded, i c e change from p a t t e r n s t y p i c a l .' of the folded i c e without a wedge i n t o patterns s i m i l a r to. wedge i c e , .along a distance of 5.0 m. The sequence of f r a c t u r e s i n d i c a t e s the t r a n s - formation of growth f a b r i c s due to wedge growth, .-. Adjacent to the wedge, c r y s t a l dimensional o r i e n t a t i o n changes from p a r a l l e l i s m w i t h the compositional l a y e r i n g of the segregated i c e to par- a l l e l i s m w i t h the wedge. C r y s t a l s i z e decreases towards the wedge, due to p o l y g o n i z a t i o n of l a r g e r g r a i n s , and'growth of new g r a i n s . • "-' . 134 Comparison of the r e s u l t s of secti o n s (b) and (c) i n d i c a t e . t h e i n f l u e n c e of wedge "growth on the i c e . We know that the wedge i s growing a c t i v e l y as recent cracks have been recognized p e t r o g r a p h i c a l l y . Thus, the c h a r a c t e r i s t i c g r a i n s i z e s and shapes and p r e f e r r e d o r i e n t a t i o n s have been produced p r i m a r i l y by sy n t e c t o n i c p l a s t i c deformation i n the form of d i s l o c a t i o n g l i d e , p o l y g o n i z a t i o n by d i s l o c a t i o n c l i m b , and r e c r y s t a l l i z a - t i o n . I t i s evident that r e c r y s t a l l i z a t i o n has occurred, as marked changes i n c r y s t a l l o g r a p h i c o r i e n t a t i o n have occurred. These could not be produced s o l e l y by p o l y g o n i z a t i o n of e a r l y . g r a i n s , as subgrains would have t h e i r o r i e n t a t i o n s c l o s e to those of the o r i g i n a l . However, d i s l o c a t i o n g l i d e and climb are a l s o o c c u r r i n g . The decrease i n c r y s t a l s i z e toward the wedge i s i n d i c a t i v e of p o l y g o n i z a t i o n causing r e d u c t i o n of the primary g r a i n s , and a l s o the growth of new c r y s t a l s , i . e . r e c r y s t a l l i z a t i o n , to give p r e f e r r e d dimensional o r i e n t a t i o n s r e l a t e d to the wedge. I t i s evident that wedge growth has l e d to the establishment of h o r i z o n t a l compression i n the frozen ground. Lachenbruch (1962) discussed the zone,of s t r e s s r e l i e f around a thermal c o n t r a c t i o n crack a f t e r f r a c - t u r e . The h o r i z o n t a l s t r e s s component normal to the crack w a l l , vanishes at the crack w a l l s , but increases a s y m p t o t i c a l l y t o tha p r e c r a c k i n g value ..at large h o r i z o n t a l distance from the crack. In the. present study wa are also, concerned w i t h "compression caused by expansion of permafrost i n summer. This was not t r e a t e d by Lachenbruch, but I t i s to be expected that maximum s t r a s s w i l l occur adjacent to tha. wedge, and s t r e s s w i l l f a l l w i t h d i s t a n c e from the wedge. 135 The only previous mention of the i n f l u e n c e of a wedge on adjacent i c e was by Corte (1962a) who found that the change i n f a b r i c i n the surround- ing i c e was confined to 30 cm from a small ( 1 m wide) wedge. I n the present study m o d i f i c a t i o n of f a b r i c was recorded up to 3 m from a l a r g e (3 m wide') wedge. Corte did not comment on any upturning adjacent to the wedge, but Pa"we" (1962) reported the e f f e c t up to 3 m from wedges. I n a d d i t i o n to, the e f f e c t on the surrounding m a t e r i a l , i c e i n a wedge i s i t s e l f deformed. Black (1953) argued that h o r i z o n t a l compression produced shear planes adjacent and p a r a l l e l to wedge s i d e s . Thus i t i s d i f f i c u l t to s p e c i f y the s t r e s s f i e l d adjacent to the wedge. I f wa assume u n i a x i a l compression, the theory of Kamb (1959) p r e d i c t s a c-a x i s maximum around the unique s t r e s s a x i s , although i n experimental work Kamb (1972) found an incomplete s m a l l - c i r c l e g i r d l e around the compression a x i s , i n i c e at 0°C. Kamb (1972) a l s o deformed i c e i n simple shear (-5° to 0°C), which r e c r y s - t a l l i z e d to give a two maximum f a b r i c , one maximum at the pole of the shear - plane and the other at 20° from the shear d i r e c t i o n . When a compressive s t r e s s was superimposed across the shear plane the two maxima combined i n a s m a l l - c i r c l e g i r d l e around the compression a x i s . In the present study the f a b r i c s were s i n g l e maxima but not centred on the s t r e s s a x i s (assuming compression normal to the wadge.axial p l a n e ) . But the maximum i s p a r a l l e l t o the maximum i n the wadge i c e , and thus p a r a l l e l to the pole to the wedge boundary and compositional l a y e r i n g i n the wedge. Thus i t may be th a t shear has occurred p a r a l l e l to the wedge boundary. 136 Tension Crack Ice I n t r o d u c t i o n Tension crack i c e grows' i n cracks r e s u l t i n g from mechanical rupture of the ground a s s o c i a t e d w i t h the growth of segregated or i n t r u s i v e i c e (Mackay 1972b, p. 8) and i s best observed on pingos. Exposure to depth of the cracks i s r a r e , but probing shows some of them to be s e v e r a l metres deep. Open cracks have been observed f r e q u e n t l y i n w i n t e r and s p r i n g when there i s no surface water flow, and i t has been argued that i n f i l i s from s u r f a c e water. Thus there i s no evidence f o r s y n t e c t o n i c c r y s t a l growth as may occur i n rock veins (Raybould 1975). However, t e n s i o n cracks may open year-round and i n summer : i n f i l might be more r a p i d , i f water i s a v a i l a b l e . Crack patterns are u s u a l l y dominated by a master crack, w i t h other cracks r a d i a t i n g from the pingo. The cracks are u s u a l l y v e r t i c a l , and planar. Tension crack i c e was c o l l e c t e d from two s i t e s : (a) Pingo Number 9 (Mackay 1973a, F i g . 15), (b) P e n i n s u l a P o i n t Pingo, near Tuktoyaktuk ( F i g . 1). The aims of the present i n v e s t i g a t i o n were: - ( i ) to study one season's growth of, t e n s i o n crack i c e ; ( i i ) to compare new growth w i t h older i c e ; to. show changes over time; ( i i i ) to compare tension crack i c e w i t h wedge i c e . 137 (a) Pingo Number 9 Tension Crack F i e l d C h a r a c t e r i s t i c s , This pingo i s growing i n a lake which drained s h o r t l y before 1950, and i t s growth has been monitored s i n c e 1970. Tension cracks are evident across the pingo (Mackay 1973a,'Fig.•15). Bench marks on each.side of the crack were surveyed i n June 1973 and again i n June 1974 and the mean annual growth of t e n s i o n crack i c e was 100 mm (Mackay, personal communication). There was no observable movement i n a d i r e c t i o n p a r a l l e l to the. crack. Ice samples were removed from w i t h i n the previous season's a c t i v e l a y e r , before thaw-down i n J u l y 1974, thus the maximum age of the i c e i s known. Cr a c k i n g occurred after.complete freeze back of the a c t i v e l a y e r d u r i n g w i n t e r 1973.-74. A crack may open year round (Mackay, personal communication 1975), thus, i t may have occurred any time a f t e r freeze back of the a c t i v e l a y e r and before spring.melt. Water has not been known to move up the t e n s i o n crack from depth, so the source i s thought to be s u r f a c e snow melt. The i c e grew immediately below the ground s u r f a c e , on a f r a c t u r e s u r f a c e i n f r o z e n s o i l . . Thus n u c l e a t i o n was not on c r y s t a l s i n the f r a c t u r e w a l l , although such may have occurred at greater depth, where the crack probably propagated through e a r l i e r t e n s i o n crack i c e . The l a t e r a l contacts between the i c e and adjacent a c t i v e l a y e r are abrupt, but' l o c a l l y i r r e g u l a r . A l s o soma small pockets of i c a occur i n tha a c t i v e l a y e r , p a r a l l e l to the crack. A prominent banding occurs i n tha i c e , determined by,bubble content ( F i g . 35). These bands a r e ' p a r a l l e l to one another and to the plane of the crack, but w i t h l e s s e r i r r e g u l a r i t i e s than, the i c e - s o i l contact. The crack was c l o s e d f o r most of i t s l e n g t h , but d i s p l a y e d l o c a l open zones which c o u l d be probed to 1 metre... 133 (1973). 139 Ice C h a r a c t e r i s t i c s Banding'was- determined s o l e l y by gas content; the l a c k of s o i l i n c l u s i o n s suggests tha a c t i v e l a y e r was s t i l l f r o z e n during melt-water flow. . Band type and thickness were approximately.symmetrical about the crack c e n t r e . On each.side of tha crack occurred a 10-20 mm t h i c k zone of mainly c l e a r i c e i n contact w i t h tha s o i l . Adjacent to these were 1-2 mm bands of small s p h e r i c a l bubbles, tha bands being p l a n a r and continuous l a t e r a l l y and v e r t i c a l l y , but s l i g h t l y o f f s e t a t r a r e sediment i n c l u s i o n s . Next i n sequence came 20-30 mm bands of bubbly i c e , bubbles being <( 1 mm and decreasing i n c o n c e n t r a t i o n towards "the c e n t r e of the crack., Next there occurred abrupt changes to very high bubble content,, again decreasing toward the crack centre across bands 10-15 mm t h i c k . Narrow bands (4-5 mm) repeated tha c o n c e n t r a t i o n p a t t e r n , f o l l o w e d by c l e a r i c e to the centre of tha crack. In t h i s body, growth occurred a t a v e r - t i c a l i n t e r f a c e , y e t i t i s evident that many bubbles have been r e t a i n e d i n tha i c a , and d i d not f l o a t up under buoyancy.- This may ba because the i c e grew r a p i d l y and surrounded bubbles w h i l e they were s t i l l too s m a l l to f r e e themselves from the i n t e r f a c i a l t e n s i o n , o r because growth was not t a k i n g place i n t o a. " p o o l " of water, but i n a t h i n f i l m on tha s u r f a c e . I t i s i n t e r e s t i n g to compare bubbles i n t h i s i c e w i t h those i n the Tuktoyaktuk i c i n g mound, which grew at approximately the-same time. I n the i c i n g mound a steady water supply was a v a i l a b l e and f r e e z i n g r a t a - gr a d u a l l y f a l l , thus large bubbles grew, elongated i n the f r e e z i n g d i r e c - t i o n whereas i n the tension crack i c e water supply was probably i n t e r - m i t t e n t , and f r e e z i n g was r a p i d so that only s m a l l bubbles grew. .140 C r y s t a l Charactar i s t i c s C r y s t a l s i z e v a r i e d across, the crack. Adjacent to the s o i l occurred 2 a zone of small c r y s t a l s , less than 0.1 mm , from which grew a zona of l a r g e r c r y s t a l s elongated orthogonal to the banding. Soma reached 5 mm x 2 mm and extended i n t o the f i r s t bubbly band, w h i l e most c r y s t a l s terminated at the band contact', and gave way to new c r y s t a l growth (^0.1 mm ) with, l a r g e r c r y s t a l s extending from the .competitive zone. F u r t h e r c r y s t a l s grew from t h i s zone, reaching ^ 10 mm x 3 mm; some were truncated, but others widenad at that surface. In a d d i t i o n to the widening.of p r e - e x i s t i n g c r y s t a l s new c r y s t a l s grew, t h i s being the bubbly zone discussed above. C r y s t a l s became more elongated as bubble- c o n c e n t r a t i o n decreased, a p a t t e r n which was repeated toward the crack c e n t r e , the c e n t r a l clear, zone c o m p r i s i n large, c r y s t a l s , ( 7 an long by 5 mm wide. - C r y s t a l shape v a r i e d w i t h s i z e . In competitive growth zones,, shapes . were g e n e r a l l y anhedral, but some s t r a i g h t compromise boundaries occurred. Elongated c r y s t a l s tended to be gently curved r a t h e r than s e r r a t e d . In-, many cases the outer boundary of elongate c r y s t a l s was s t r a i g h t i n h o r i z o n - t a l and v e r t i c a l sections,, thus p a r a l l e l to the crack. T h i s r e f l e c t s v a r i a t i o n s i n supply of melt water. A temporary c e s s a t i o n of water supply was followed by s l i g h t melting on resumption of flow. Rapid c o o l i n g would give copious n u c l e a t i o n at the i n t e r f a c e , although l o c a l l y growth would occur i n l a t t i c e c o n t i n u i t y w i t h p r e - e x i s t i n g c r y s t a l s . At the centre of the i c e body, there were departures from the trend of dimensional o r i e n t a - t i o n orthogonal to the plane of the crack. L o c a l zones of curved c r y s t a l s are shown i n Figure 37,' These i n d i c a t e m u l t i - d i r e c t i o n a l c r y s t a l growth i n t o enclosed space, rather.than curved growth dua to incremental d i l a t i o n 141 of the f r a c t u r e . In tha l a t t e r circumstance curvature would be as shown i n Figure 3 8 ( c ) , i . e . no converging p a t t e r n i s e v i d e n t . A l s o such curva- ture r e q u i r e s displacement non-normal to the f r a c t u r e w a l l s . There i s no evidence f o r t h i s from d e t a i l e d bench mark surveys. I t i s i n t e r e s t i n g to.consider the form of the ice-water i n t e r f a c e during the l a t t e r part of f r e e z i n g , i t s e f f e c t on c r y s t a l c h a r a c t e r i s t i c s . , and on s u s c e p t i b i l i t y to l a t e r c r a c k i n g . From the above, i t i s apparent that due to s l i g h t l a t e r a l v a r i a t i o n s i n crack w i d t h some p a r t s impinged before others. Where a pool of l i q u i d was l e f t i t i s argued that the i n t e r f a c e advanced as i n Figure 38(a), r a t h e r than as the more rounded form shown i n F i g u r e 33(b). I n t h i s instance ( F i g . 38(a)) there i s an almost i n v a r i a n t d i r e c t i o n of maximum thermal gradient a t a l l points;' thus any g r a i n favourably o r i e n t e d f o r growth i s able to grow at optimum speed and "wedge out" l e s s favourably o r i e n t e d g r a i n s . I n c o n t r a s t , i n the case of a more-rounded i n t e r f a c e ( F i g . 3 8 ( b ) ) , the d i r e c t i o n of. maximum thermal gradient changes c o n t i n u a l l y and thus no one g r a i n i s favourably o r i e n t e d f o r a long p e r i o d , and more grains s u r v i v e to the centre. S i m i l a r r e s u l t s have been found i n metals (Savage and Aronson 1955). Once f r e e z i n g i s complete,, there i s a general decrease i n temperature of the body, and we must consider the response to c o n t r a c t i o n of each p a r t . During the i n f i l of the t e n s i o n crack,, c r y s t a l s grow from each s i d e and r e j e c t s o l u t e which p i l e s up between the two i n t e r f a c e s . A f t e r impingement of the g r a i n s these segregates may p e r s i s t as g r a i n boundary f i l m s , below 0°C. Thus c o n t r a c t i o n s t r e s s e s may r i s e to high l e v e l s w h i l e tha g r a i n boundary contact area i s s m a l l . A l s o , the l a r g e r tha s o l i d i f y i n g g r a i n s i z a , tha. s m a l l e r tha area of grain-boundary contact f o r a given l i q u i d content (Smith 1953). Thus " 142. coarser grained s e c t i o n s are more s u s c e p t i b l e to c o n t r a c t i o n c r a c k i n g , and . e s p e c i a l l y where there i s a steep angle of g r a i n abutment ( F i g . 3 8 ( a ) ) . I t i s a l s o noted that i n cast metals i t has bean observed (Leas 1946) t h a t . f i n e - g r a i n e d m a t e r i a l s are f a r more r e s i s t a n t to cracking, than coarser m a t e r i a l s , due to t h e i r greater a b i l i t y to accommodate the c o n t r a c t i o n : s t r a i n s . Thus from the d e s c r i p t i o n of c r y s t a l features i t i s c l e a r why c e r t a i n areas of the i n f i l l e d f r a c t u r e may open before o t h e r s . Optic a x i s o r i e n t a t i o n s are shown i n F i g u r e 39(a). The o v e r a l l p a t t e r n i s a g i r d l e p r e f e r r e d o r i e n t a t i o n i n a plane p a r a l l e l to the banding. Thus ba s a l planes are orthogonal to banding. Component diagrams i n d i c a t e that small c r y s t a l s i n competitive zones show d i f f u s e g i r d l e p a t t e r n s ( F i g . 39(b)); elongate c r y s t a l s show stronger g i r d l e c o n c e n t r a t i o n s (Fig.. 3 9 ( c ) ) , thus a stronger p r e f e r r e d o r i e n t a t i o n developed as growth proceeded. I t i s noted that i n comparison, c-axas of c r y s t a l s i n veins i n d i c a t e growth i n o p t i c a l c o n t i n u i t y w i t h w a l l c r y s t a l s , although c-axes p a r a l l e l to long axes are f r e q u e n t l y found. I n t e r p r e t a t i o n The pingo i n which the t e n s i o n crack o c c u r s . i s a c t i v e l y growing and has been under observation f o r s e v e r a l years (Mackay 1973a). Tension cracks are r e c o g n i z a b l e on a i r photographs. As part of a study of pingo growth, bench marks have been i n s t a l l e d on the pingo, i n c l u d i n g one on each sIda of the crack at the top, D e t a i l e d surveys show th a t a s e p a r a t i o n of 100 ram occurred between June 1973 and 1974. No r e l a t i v e v e r t i c a l or l a t e r a l displacement of the. benchmarks was recorded. The time of f r a c t u r e i s unknown, but was a f t e r complete freeze-back of the a c t i v e layer.. 143 F i g u r e 39. Tension Crack, Pingo No. 11. (a) V e r t i c a l s e c t i o n , orthogonal to crack, 100 c r y s t a l s (b) 20 c r y s t a l s adjacent to organic s o i l . . (c) 20 columnar c r y s t a l s at crack c e n t r e . 144 I t i s not known whether the crack opened 100 rain i n one event, or whether gradual opening occurred a f t e r the i n i t i a l f r a c t u r e . Neither f i e l d not p e t r o l o g i c data provide s u i t a b l e i n d i c a t o r s . The Ice s t u d i e d grew . i n the a c t i v e l a y e r ; surface water, probably from snow melt, drained i n t o the crack and f r o z e with copious n u c l e a t i o n on the crack w a l l . C r y s t a l s w i t h c-axis. p r e f e r r e d o r i e n t a t i o n s i n a g i r d l e p a r a l l e l to the plana of the crack grew from the c h i l l zone. These c r y s t a l s a l s o had a strong dimensional o r i e n t a t i o n orthogonal to the crack w a l l . Thus c r y s t a l growth occurred i n the basal plane... By comparison w i t h the i c i n g mound i c e , which a l s o grew by a basal plana mechanism, the t e n s i o n crack c r y s t a l s are more numerous, and s m a l l e r . A l s o i n the t e n s i o n crack there are more l a y e r s of naw growth. This i n d i c a t e s v a r i a t i o n s i n supply of water. Supply of. surface water to the crack may have been i n t e r r u p t e d f r e q u e n t l y . I f f l o w ceased t e m p o r a r i l y , then recommenced, c r y s t a l growth c o u l d occur (a) i n l a t t i c e c o n t i n u i t y w i t h p r e v i o u s l y e x i s t i n g c r y s t a l s , (b) by new nuclea- t i o n , (c) by growth on vapour c r y s t a l s . Evidence f o r (a) occurs i n the d i s c u s s i o n of t e x t u r a l c r i t e r i a ; (b) and (c) cannot be d i s t i n g u i s h e d . Where tha c r y s t a l s growing from each s i d e of tha crack meet, a c e n t r a l . seam occurs, w i t h l o c a l openings. T e x t u r a l c r i t e r i a may be used to d i s - t i n g u i s h betwaan openings which never c l o s e d , and new " f r a c t u r e s " . L o c a l l y there were found lens-shapad zones in.which c r y s t a l s had dimensional o r i e n - t a t i o n s orthogonal to that opening, a l l around the. adge. I n some cases . c r y s t a l s could be almost p a r a l l e l to the crack, whereas i f there had been c l o s u r e and a naw opening occurred, such curved c r y s t a l s would not neces- s a r i l y occur. (b) P e n i n s u l a P o i n t Pingo Tension Crack F i e l d C h a r a c t e r i s t i c s T his i s a pingo near Tuktoyaktuk ( F i g , 1) which has been s u b j e c t to c o a s t a l e r o s i o n ; only h a l f the pingo remained i n 1935 a i r photographs but l i t t l e f u r t h e r e r o s i o n has occurred s i n c e then. . I t i s thought u n l i k e l y that the pingo has been growing r e c e n t l y , thus the pingo core and t e n s i o n , crack i c e are o l d compared w i t h Pingo No. 9 and t h i s t h e r e f o r e provides an opportunity to look f o r m o d i f i c a t i o n of growth f e a t u r e s i n the i c e . No major i c e core has been observed during the p e r i o d of exposure of the s e c t i o n , although 2 m t h i c k i c e . l a y e r s have.been reported o c c a s i o n a l l y . In J u l y 1973 slumping exposed t e n s i o n crack i c e ( F i g . 40) h a l f way up the pingo i n sands; a contact of' t e n s i o n crack i c e and core i c e was not exposed. The surrounding sands are f i n e to medium grained; sedimentary.structures have not been g r e a t l y d i s t u r b e d during f r e e z i n g ; plane beds c o n t a i n some organic matter, and r i p p l e marks a l s o occur. These beds are not disturbed'- adjacent to the i c e , which i n d i c a t e s a t e n s i o n crack o r i g i n without sub- s t a n t i a l l a t e r a l s t r e s s caused by growth and summer expansion as occurs i n i c e wedges. A.mineral s t a i n e d l a y e r 75 mm wide occurs adjacent to the i c e on one s i d e . Ice C h a r a c t e r i s t i c s .. The i c e body was, approximately 160 mm wide with.an abrupt contact .'with the surrounding sand. The compositional l a y e r i n g was determined by gas content; sediment content was low. The l a y e r i n g ( F i g . 41) was q u i t e d i f f e r e n t from that of Pingo No. 9. There was no symmetry t o the banding, 146 147 there being three bands of d i f f e r i n g widths and contained bubble types p a r a l l e l to the plane of the i c e body. Bubble C h a r a c t e r i s t i c s (a) A 50 am v i d e zone of e l l i p s o i d a l , elongata and i r r e g u l a r bubbles w i t h a h o r i z o n t a l l i n a a t i o n , approximately orthogonal to the cr a c k , and 2-5 am long. They are not volumes of r e v o l u t i o n and have thus s u f f e r e d p o s t - s o l i d i f i c a t i o n m o d i f i c a t i o n . Many curve upwards 20°- 30° adjacent t o zone ( b ) ; (b) Tha c e n t r a l zone comprises 45 mm of vary elongate bubbles w i t h a sharp bend (40°) near the j u n c t i o n w i t h zone (a) and po i n t e d a t the other end. These bubbles are up to 18 mm long and ara separated by more c l e a r i c e than those i n zone ( a ) , but w i t h a few small (2 mm) s p h e r i c a l bubbles i n t r a i n s , suggasting break-up of l a r g e r bubbles ( K h e i s i n and Cherepanov 1969); (c) The j u n c t i o n between (b) and (c) i s abrupt and co n t a i n s a d u s t i n g of sand. Zone (c) i s w h i t i s h due to the high c o n c e n t r a t i o n of small bubbles, and some l a r g e r , up to 3 mm. At the outer ice-sand contact a r a some I r r a g u l a r , elongated (4 mm) bubbles orthogonal to the c o n t a c t . C r y s t a l C h a r a c t a r i s t i c s C r y s t a l s i z e v a r i e s throughout the i c e body ( F i g . 43) but shape i s le s s v a r i a b l e ; zones are considered w i t h r e f e r e n c e to bubble zones: (a) C r y s t a l s are 20 .Tim x 10 mm and anhedral, i r r e g u l a r i n shape. Elongate bubbles are p a r a l l e l to c r y s t a l long axes and u s u a l l y i n g r a i n boundaries. 143 Figure 42. P e n i n s u l a Poinc Pingo Tension Crack. (a) V e r t i c a l s e c t i o n orthogonal to crack plane, 96 c r y s t a l s . (b) Right s i d e of ( a ) , 44 c r y s t a l s . (c) L e f t side of ( a ) , 52 c r y s t a l s . (d) V e r t i c a l s e c t i o n orthogonal to crack plane, 100 c r y s t a l s . t . c . = t e n s i o n crack plane Figure 43. C r y s t a l c h a r a c t e r i s t i c s . (compare w i t h bubble and sediment i n Figure 41) orthogonal to crack. 10 mm g r i d . 1 1 V e r t i c a l s e c t i o n 149 (b) C r y s t a l s a r e 20 mn x 10 .mn and elongated p a r a l l e l t o the bubbles, i . e . a t 30° to the h o r i z o n t a l . Shanes a r e anhedral and i r r e g u l a r . Many elongate bubbles l i e i n g r a i n boundaries, others cross boundaries and change shape at the boundary. Tapering bubbles always l i e on g r a i n boundaries; conversely small s p h e r i c a l bubbles are i n t r a g r a n u l a r . ( c ) C r y s t a l s i z e i s smaller than zones (a) and ( b ) , g e n e r a l l y 15 mm x 5 mm, w i t h some 3 mm x 2 mm. O r i e n t a t i o n i s h o r i z o n t a l , and shapes are very i r r e g u l a r and s e r r a t e d . Sub-boundaries occur w i t h s t r a i n e x t i n c - t i o n . Larger bubbles tend to occur at g r a i n boundaries. Thus there i s a strong r e l a t i o n s h i p between c r y s t a l c h a r a c t e r i s t i c s and bubbles i n a l l i c e zones. A d d i t i o n a l l y zone (c) contains s m a l l amounts of sediment which again are concentrated on g r a i n boundaries. In comparing the c r y s t a l and i n c l u s i o n c h a r a c t e r i s t i c s of t h i s i c e with those of the p r e v i o u s l y discussed tension crack i t i s evident that the primary growth features have been modified; c o m p e t i t i v e growth zones have disappeared, c r y s t a l shapes are considerably m o d i f i e d and bubbles have moved r e l a t i v e to g r a i n boundaries. P e t r o f a b r i c diagrams are given i n Figure 4 2 ( a ) - ( d ) , f o r v e r t i c a l t h i n s e c t i o n s orthogonal t o the plane of the crack. The p a t t e r n comprises a broad maximum i n the p l a n e of the i c e body and a t about 4.5 ° to the h o r i - z o n t a l superimposed on a minor g i r d l e . Some c r y s t a l s i n the eas t e r n s i d e o f tha body nave more d i s p e r s e d z -axis o .tier, t a t ions but t h e s e c r y s t a l s have no apparent d i f f e r e n c e s from the o t h e c s t n t h i n s e c t i o n . I t i s r e - c a l l e d that i n the t e n s i o n crack -'a ?iago f.'o, 9 a p t i c axes gave a v e r t i c a l g i r d l e which became more narrow w i t h d i s t a n c e f r o m tha i c e - s o 11 i n t e r f a c e 150 and represented growth c o n d i t i o n s where basal planes were p a r a l l e l to the f r e e z i n g d i r e c t i o n . I n t e r p r e t a t i o n The tension crack i c e discussed here Is of c o n s i d e r a b l y g r e a t e r aga than that i n Pingo No. 9 and the pingo i n which i t i s l o c a t e d has s u f f e r e d long term c o a s t a l e r o s i o n . The sampling s i t e has been s u b j e c t to v a r y i n g temperature gradients and s t r e s s systarns as unloading occurred during ero- s i o n . I t i s evident from c r y s t a l and bubble c h a r a c t e r i s t i c s t h a t o r i g i n a l s i z e s and shapes have been modified considerably. Bubble and c r y s t a l char- a c t e r i s t i c s would have bean r e l a t e d symmetrically to the f r e e z i n g d i r e c t i o n during i n i t i a l growth, as has bean observed i n s e v e r a l r e c e n t l y grown i c e bodies (e.g. Tension Crack Ice, Pingo No. 9; I c i n g Mound I c e ) ; bubbles would be volumes of r e v o l u t i o n elongated i n the f r e e z i n g d i r e c t i o n and columnar c r y s t a l s would be p a r a l l e l to that d i r e c t i o n . In the P e n i n s u l a P o i n t Pingo, bubbles have i r r e g u l a r shapes, the i r r e g u l a r i t i e s being r e - l a t e d f r e q u e n t l y to c r y s t a l g r a i n boundaries; a l s o bubbles have broken up i n t o s t r i n g s or groups. Such features ara r e a d i l y e x p l a i n e d i n terms of changing thermal c o n d i t i o n s , but the o p t i c a x i s d i s t r i b u t i o n diagrams, which ara homogeneous f o r a l l parts of the body, ara unusual. The i n f l u e n c e of sediment on c r y s t a l s i z e i s evident from Figure 43; crapping of sediment on g r a i n boundaries has retarded g r a i n boundary m i g r a t i o n i n the r i g h t hand s i d e of the f i g u r e r e l a t i v e to the l e f t hand s i d e . 151 6 • T hernial Cone met ion Cracks and Wad^a I c e Introduct i o n l e a wedge i c e , formed by t n a i n f i l l i n g of thermal c o n t r a c t i o n c r a c k s , i s widespread i n the Tuktoyaktuk and P e l l y I s l a n d areas, as i n other a r c t i c r e gions. Mackay (1974a) has discussed cracking of wedge i c e on Garry I s l a n d , but there i s no published work on i c e wedge pe t r o l o g y i n the area. Elsewhere Black (1953, 1954, 1953) and Corta (1952a) have reported on the form and c r y s t a l c h a r a c t e r i s t i c s of i c e wadgas i n A l a s k a and Greenland, r e s p e c t i v e l y . In the Soviet Union, s i m i l a r work has bean c a r r i e d out by Shumskii (1954). Cracking i n A n t a r c t i c a has been s t u d i e d by B l a c k (1973). Wedge Growth Mechanism l e a wadgas grow from winter thermal c o n t r a c t i o n cracks which become i n f i l l e d by hoar f r o s t , snow and surface water. In h i s t h e o r e t i c a l work Lachenbruch (1952) argued that a r a p i d temperature drop superimposed on g e n e r a l l y low temperatures i s r e s p o n s i b l e f o r c r a c k i n g . However, G r e c h i s h - chev (1970) considered Lachenbruch's work as only a f i r s t approximation, as i t was based on a l i n e a r dependence of thermal expansion and c o n t r a c t i o n on temperature. F u r t h e r , Grechishchev p o i n t e d out t h a t the moisture content i n the s o i l has a considerable a f f e c t on thermal c h a r a c t e r i s t i c s . Ha argued thac the water c o n t e n t of the a c t i v e layer d e c r e a s e s downward and so the t h e r m a l c o n d u c t i v i t y - / a r i a s . Thus Ln the f i r s t h a l f o f the c o l d p e r i o d tuns i o n o c c u r 3 i n the Lower one-ch i r d o f chs a c t i v e l a y e r w h i l e the top i s i n l o m p r a s s i o n . 152 I n tha present study, no measurements ware made of temperatures, s t r e s s e s , crack frequency or depth, as such would r e q u i r e a much longer period of study. The F e l l y I s l a n d s i t a , discussed below, i s near Garry I s l a n d where Mackay (1974a) has made a long term study of ground tempera- tures and c r a c k i n g patterns i n low and high centred polygon wadges. Once f r a c t u r a has occurred, soma i n f i l l i n g proceeds by hoar c r y s t a l growth and snow melt before warming of the ground closes tha c r a c k s ; i t i s l i k e l y that only a m i n o r i t y of c l o s u r e i s due to i n f i l by c r y s t a l growth (Black 1953). Expansion of tha ground i n summer causes h o r i z o n t a l s t r e s s on tha whole i c e wedge. No q u a n t i t a t i v e astimate of the s t r e s s e s i n v o l v e d has been found i n the l i t e r a t u r e , although 31ack (1953, p. 72) s t a t e s that h o r i z o n t a l s t r e s s e s are produced "... w e l l above the l i m i t s of ... shear of Ic a . " A l s o a rough estimate of h o r i z o n t a l compression can be obtained from Lachenbruch (1952, p. 23) as summer expansion approximates w i n t e r c o n t r a c - t i o n . Thus s t r e s s e s of s e v e r a l bars are o p e r a t i v e , and temperatures a r e high, causing m o d i f i c a t i o n of growth features. I t i s the purpose i n t h i s s e c t i o n to discu s s p e t r o l o g i c aspects of the mode of f r a c t u r e i n ground i c e , f r a c t u r a i n f i l , tha r e l a t i o n s h i p of succeeding f r a c t u r e s t o e a r l i e r ones, tha prograda f a b r i c o f a growing wadge, and the i n f l u e n c e of a growing wedge where i t penetrates massive i c a . (a) F r a c t u r a propagation i n re i.ation to parmafrost features S i n g l e f r a c t u r e s i n sediment are a i r t i c _ i I t to f i n d , and t h i n s e c t i o n p r e p a r a t i o n noses manv p rob lams, tn.us i n d i v i d u a l thermal Ly induced f r a c - t u r e s i n massive ground i c e o n l y are c o n s i d e r e d . The i c e body i s an 153 i n v o l u t e d h i l l near Tuktoyaktuk, the core of which has bean exposed by c o a s t a l e r o s i o n . Some knowledge o f i n c l u s i o n and c r y s t a l c h a r a c t e r i s t i c s of the massive i c e i s necessary i n order to understand f r a c t u r e propaga- t i o n . A d e t a i l e d d i s c u s s i o n i s g i v e n elsewhere (pp. 95-151) but a summary i s included hare. The massive i c e has a c h a r a c t e r i s t i c a l l y l a r g e g r a i n s i z e , ranging from 15 mm- i n bu b b l y l a y e r s to 4 600 mm i n bubble-free zones. Bubbles i n the massive i c e occur i n wide bands, range up to 3 mm i n diameter and are located both on g r a i n boundaries and w i t h i n c r y s t a l s . As such they represent major defects i n tha s t r u c t u r e and might be ex- pected to i n f l u e n c e f r a c t u r i n g . Bubbles c o n t r o l c r y s t a l s i z a i n the massive i c e , s m aller c r y s t a l s o c c u r r i n g i n bubbly zones. Thus there e x i s t s a gre a t e r g r a i n boundary area, also g r a i n boundaries by d e f i n i t i o n saparata m a t e r i a l of d i f f e r i n g l a t t i c e o r i a n t a t i o n s . These g r a i n boundaries ara zones of atomic d i s o r d e r , and f r e q u e n t l y c o n t a i n sediment and segregated s o l u t a s . A l l these f a c t o r s tend to a l t a r the response of the i c a body to s t r e s s . The c-axes of tha massive i c e c r y s t a l s are approximately v e r t i c a l and i n the f r a c t u r e plana, thus the basal planes ara orthogonal to tha f r a c t u r e s u r f a c e . Thus, p r i o r to f r a c t u r e , the massive ground i c e has markedly d i f f e r e n t c r y s t a l c h a r a c t e r i s t i c s and bubble pattern from t e a samples usad i n l a b o r a t o r y experiments on f r a c t u r e . T y p i c a l f r a c t u r e s are shown i n F i g u r e I t i s evident t h a t cracks have p r o p a g a t e d t h r o u g h c o a r s e - g r a i n e d i c e ?.ni tended t o be trans g r a n u l a r r a t h e r than L nte r g r a n u i a r . Mo t a j o r cn.an|as i n f r a c t u r e o r i e n t a t i o n o c c u r at g r a i n b o u n d a r i e s , thus s l i g h t changes In L a t t i c e o r i e n t a t i o n e x a r t no 154 Figure 45. Types of f r a c t u r e i n f i l . (a) open f r a c t u r e , (b) no new c r y s t a l growth, (c) new growth on one s i d e , (d) new c r y s t a l growth on both s i d e s , (e) new growth on parts or both s i d e s . 155 major c o n t r o l , i n c o n t r a s t to the r e s u l t s of Gold (1961) on thermal shock. However, bands of d i f f e r i n g l a t t i c e o r i e n t a t i o n i n c r y s t a l s are a p p r o x i - mately v e r t i c a l and thus may have aided the f r a c t u r e process. The t r a n s g r a n u l a r cracks c o n t r a s t w i t h those of Anderson and Weeks (1958) f o r a sea-ice beam which f a i l e d i n t e n s i o n by f r a c t u r e along the b a s a l planes of c r y s t a l s . However, i n sea i c e the b a s a l planes are a l s o the s i t e s of b r i n e pockets which act as s t r e s s c o n c e n t r a t o r s . Such gross l i q u i d i n c l u s i o n s have not been observed i n ground i c e but gas bubbles occur; these are not g e n e r a l l y l o c a t e d p a r a l l e l t o b a s a l .planes. The i n f l u e n c e of bubbles i s not c l e a r , as any bubbles i n the f r a c t u r e path are o b l i t e r a t e d during l a t e r i n f i l of c r a c k s . . The spaed of f r a c t u r e propagation i s unknown, but from the p e t r o l o g i c evidence i t appears that f r a c t u r e has been r a p i d , such that t e x t u r e has exerted l i t t l e i n f l u e n c e . (b) I n f i l of f r a c t u r e The exact widths of f r a c t u r e s are unknown, as some c o n t r a c t i o n of the cracks may occur before i n f i l ( B l a c k 1953) and soma flow has occurred p r i o r to sampling. However, an astimate can be obtained from bubble zones p a r a l l a l to tha f r a c t u r e saam, and from boundaries between o r i g i n a l f r a c - tured grains and the i n f i l c r y s t a l s . These zones are up to 3 mm wide. As i s evident from F i g u r e 44, i n some cases the "massive" i c e c r y s t a l s have grown across to the f r a c t u r e seam; i n other' cases, a group of new c r y s t a l s • grew. The question a r i s e s of why both these cases occur. P e t r o f a b r i c diagrams of f r a c t u r e d c r y s t a l s show that there i s no s i g n i f i c a n t d i f f e r e n c e 156 batwaen c r y s t a l s i n the two cases. An i n d i v i d u a l c r y s t a l which i s f r a c - tured ( F i g . 45(a)) may be subject to one of s e v e r a l growth, pa t t e r n s : i n (b) new growth occurs on n e i t h e r s i d e ; i n (c) naw growth occurs on one s i d e ; i n (d) naw growth occurs on both s i d a s ; i n (e) new growth. occurs on parts of both s i d e s . Whatever the growth type, the seam i s c e n t r a l , so n u c l e a t i o n and growth rates i n a l i cases are approximately equal; •otherwise o f f s e t s would occur i n the seam. The f r a c t u r e s are o r i e n t e d such'.that "massive i c e " c r y s t a l b a s a l planes are approximately orthogonal to the crack s u r f a c e . The b a s a l plane, i s the plane of most r a p i d growth o f ' i c e ( H i i l i g 1958), thus n u c l e a t i o n and growth of new c r y s t a l s i s r a p i d . A l s o the i n f i l c r y s t a l s have a wider ranga of c-a x i s o r i e n t a t i o n s , which has.had l i t t l e e f f e c t on the i n f i l process ( F i g . 46). Figure 46(a) represents a v e r t i c a l s e c t i o n orthogonal to a f r a c t u r e showing c-axes of massive i c e c r y s t a l s to be v e r t i c a l i n the f r a c t u r e plane, w h i l e i n f i l c r y s t a l s form a v e r t i c a l g i r d l e normal to the f r a c t u r e . F i g u r e 46(b), (c) show c-axes of massive i c e c r y s t a l s , i n a h o r i z o n t a l s e c t i o n , and F i g . 46(d) i n d i c a t e s t h a t I n f i l c r y s t a l s give a v e r t i c a l g i r d l e . A f u r t h e r v e r t i c a l s e c t i o n i s represented i n F i g . 46(e) and i n f i l c r y s t a l s give a s i m i l a r p a t t e r n to F i g . 46(a). The p e t r o f a b r i c p a t t e r n f o r these f r a c t u r e i n f i l c r y s t a l s thus d i f f e r s markedly.from t h a t of tha recent tension crack ( F i g . 39) where a broad v e r t i c a l g i r d l e p a r a l l e l to the crack was found. However, i t i s noted t h a t In tha t e n s i o n crack copious n u c l e a t i o n occurred on the s o i l , r a t h e r than on f r a c t u r e i c e or hoar, and such c r y s t a l s had a much l e s s p r e f e r r e d o r i e n t a t i o n than the columnar c r y s t a l s which developed from the c h i l l zone. In the case of the thermal f r a c t u r e a much smaller space f o r c r y s t a l growth i s a v a i l a b l e , and a w a l l developed columnar zone has not developed. The i n f l u e n c e of hoar 157 •I Figure 46. F r a c t u r e s . i n "massive i c e " . ,. (a) V e r t i c a l s e c t i o n orthogonal to crack; x 18 massive i c e c r y s t a l s ; •62 i n f i l c r y s t a l s . (b) (c) H o r i z o n t a l s e c t i o n s o r t h o - gonal to crack, massive i c e c r y s t a l s . (d) 30 i n f i l c r y s t a l s i n s e c t i o n s ( b ) , ( c ) . (e) V e r t i c a l s e c t i o n orthogonal to l o c a l l y d i p p i n g f r a c t u r e . 40 i n f i l c r y s t a l s . . f = f r a c t u r e 158 c r y s t a l growth on i n f i l c r y s t a l s i s not known. I t i s i n t e r e s t i n g to note t h a t both tension crack and thermal f r a c t u r e i n f i l s d i f f e r from rock v e i n i n f i l s where"c-axes tend to be normal to the v e i n . The i n f i l texture d i f f e r s markedly from the "massive i c e . " C r y s t a l s i z e i s obviously l i m i t e d by the space a v a i l a b l e f o r growth, and c r y s t a l shape tends toward.columnar, p a r a l l e l to the growth d i r e c t i o n , and orthog-' onal to the f r a c t u r e s u r face. No w e l l developed zone of c o m p e t i t i v e growth occurs. Mutual boundaries are s t r a i g h t or gently curved, l o c a l l y w i t h small, gas, bubbles. No intergrowths occur at the c e n t r a l seam i n the i n i t i a l growth p e r i o d . Sub-boundaries were not observed i n the newly grown i n f i l c r y s t a l s . Subsequent changes i n t h i s o v e r a l l p a t t e r n are discussed l a t e r . A. small, amount of i c e growth occurs on the s u r f a c e of the f r a c t u r e d massive i c e before bubbles form. This i n d i c a t e s a b u i l d up of d i s s o l v e d gas at the s o l i d - l i q u i d i n t e r f a c e . i n the i n i t i a l f r e e z i n g . The bubbles occur on both sides of the f r a c t u r e over large areas thus i n d i c a t i n g a widespread event. Black (1953) discussed the response of f r a c t u r e i n f i l s t o compressive stress.. Cracks, w i t h a i r bubbles and hoar i n f i l s became shear, planes whereas cracks w i t h . c l e a r i c e ware stronger and shear took place adjacent to them. S i m i l a r l y , t e n s i l e s t r e s s e s would be expected to produce d i f f e r i n g responses on d i f f e r e n t f r a c t u r e i n f i l s . I t must be remembered that at d i f f e r e n t .depths the ground i s subject to d i f f e r e n t s t r e s s systems at the same time. .. Thus f r a c t u r e s which are i n i t i a t e d i n a zone of t e n s i l e s t r e s s may propa- gate i n t o compressed zones (Lachenbruch 1962). Where a " s t r a t i g r a p h y " of f r a c t u r e s was observed, i t was seen t h a t small c r y s t a l s , i n older f r a c t u r e s had embayed .the "massive i c e " c r y s t a l s . . 159 Thus the o r i g i n a l f r a c t u r e surface was no longer approximately p l a n a r , r e p r e s e n t i n g the adjustment of c r y s t a l s to the s t r e s s system. (c) Subsequent Fr a c t u r e s I t i s evident from the above d i s c u s s i o n that an i n f i l l e d f r a c t u r e presents markedly d i f f e r e n t texture and p e t r o f a b r i c s from the o r i g i n a l massive i c e . The smaller i n f i l c r y s t a l s have a g r e a t e r s p e c i f i c g r a i n boundary area, p a r t i a l l y i n a v e r t i c a l seam on which are abundant gas bubbles. A l s o a greater range of c - a x i s o r i e n t a t i o n s occurs, i n c l u d i n g some, c r y s t a l s w i t h v e r t i c a l b a s a l planes. However, i t i s apparent from F i g u r e 47 that where r e f r a c t u r i n g has occurred the cracks do not f o l l o w . the same plane. Series of f r a c t u r e s are observed (between which massive i c e may' s t i l l be recognized); some c r o s s , and i n other cases a crack may trend i n t o a previous one. In general there i s no apparent c o n t r o l by e a r l i e r f r a c t u r e s , i . e . the texture and presence of c e n t r a l seams of f r a c - t ures c o n t a i n i n g bubbles had l i t t l e e f f e c t on subsequent f r a c t u r e s . This may be due to the f a c t o r , pointed out. In the d i s c u s s i o n of Tension Crack Ice , t hat f i n e r grained m a t e r i a l s a remore r e s i s t a n t to cracking! than coarse grained due to t h e i r greater a b i l i t y to accommodate c o n t r a c t i o n . s t r a i n s . (d) The Prograde F a b r i c of Wedges A s i t e on P e l l y I s l a n d ( F i g . 1) w i t h l a r g e - s c a l e wedges was s t u d i e d i n order to I n v e s t i g a t e changes i n texture and p e t r o f a b r i c s across wedges-, and the i n t e r s e c t i o n of two wedges. Figure 47. P a r a l l e l and c o n t r a c t i o n cracks i n s e c t i o n . 10 mm g r i d . converging thermal massive i c e . H o r i z o n t a l Crossed p o l a r i z e r s 161 . . F i e l d C h a r a c t e r i s t i c s . Tha Northwest coast of P e l l y I s l a n d has a l o w - l y i n g a r e a of polygon f l a t s i n l a c u s t r i n e c l a y s , p r e s e n t l y undar a c t i v e c o a s t a l r e c e s s i o n . . Wedges are r e a d i l y observed on a low c l i f f , , and polygon troughs and. r i d g e s are w e l l developed. Many, wedges are greater than 2 m across and some are over 3m.. The.upper.surfaces•of most c o a s t a l wadgas and surrounding peat and c l a y have been subject to melt down and p e r i o d s . o f freeze-back. Thermo- k a r s t and thermal e r o s i o n was g r e a t e s t over and adjacent to wadges, the hollows having been i n f i l l e d subsequently by peat, c l a y and pond i c e . I n one casa t h i s l e f t a l a r g e wedge i n an i n a c t i v e state, and l e d to new wedge growth adjacent and approximately p a r a l l e l to the f i r s t . Tha wedges have the c h a r a c t e r i s t i c fan-shaped f o l i a t i o n (Black 1953) determined by bubble and sediment content, w i t h a general decrease, i n bubbles from the centre, outwards. Larga c l a y i n c l u s i o n s are found at the contact w i t h surrounding m a t e r i a l . In a d d i t i o n t o the fan-shaped f o l i a - t i o n s , o b lique f r a c t u r a s cross tha wadges ( F i g . 48). Samples were taken from a l a r g e wedge, 3.3 m wide w i t h an overburden of peat and c l a y 0.4-0.5 m. deep. Malt-down had occurred below the present a c t i v e l a y e r ( F i g . 49) as i n d i c a t e d by a c h e m i c a l l y s t a i n e d l a y e r i n the c l a y which a d j o i n s the lower shoulder of the wedge. This i s not a t w o - t i e r wedge,, but g r e a t e s t m e l t i n g occurred, at the boundary of the wedge, above which occurs a-body of pond i c e which f r o z e o m n i d i r e c t i o n a l l y . The upper wedge -surface has a r e l i e f of 0.1 m. Exposure of i c e was l i m i t e d to a depth of T to 1.5 m below the wedge top due to slumping, thus no samples could be removed from greater depths. . Four samples were taken .across the wadge from the centre to the boundary. Figure 48. Bubble bands and oblique fractures in.wedge, (schematic) A . L r-rrTTT ^ ,,'','II'-'V-' " I ' l l 1 1 1 "/"/ ^ 1 ••'',''///A////'V Figure Melt-down adjacent to large wedge, A.L. = present a c t i v e l a y e r ; = "pond i c e " ; — — = base of thawed zona. 163 Ice C h a r a c t e r i s t i c s , wedge centra • ' The bubble f o l i a t i o n of the c e n t r a l p o r t i o n of the wedge presents a. complex p a t t e r n . I t i s evident that cracking.does not always occur cen- t r a l l y , as l a t e r o b lique cracks o f f s e t e a r l i e r f o l i a t i o n s (by a few mm). Bubbles, u s u a l l y elongated v e r t i c a l l y , average 10. mm i n le n g t h and .< 1. mm diameter; some s p h e r i c a l bubbles ( < 1 mm) are i n t e r s p e r s e d w i t h i r r e g u l a r ones. W i t h i n a given f o l i a t i o n , bubble shape i s f a i r l y constant, these f o l i a t i o n s are < 3 mm wide and separated by c l e a r e r i c e . Peat and sediment . i n c l u s i o n s are s c a t t e r e d along the t r a c e s . o f o l d f r a c t u r e s ; l a r g e r i n c l u - sions may have been broken up and o f f s e t by subsequent o b l i q u e f r a c t u r e s . More recent f r a c t u r e s have larger,. sub-planar c o n c e n t r a t i o n s . I n c l u s i o n s thus occur as: (a) i n d i v i d u a l fragments of 1-2 mm; (b) pl a n a r zones con- tinuous l a t e r a l l y f o r 80 mm; (c) pods extending 2-3 mm out from the f r a c t u r e surfaces.. These i n c l u s i o n s provide no i n f o r m a t i o n concerning crack w i d t h s , as p a r t i c l e m i g r a t i o n , and flow of i c e have occurred s i n c e the f r a c t u r e s were i n f i l l e d . W i t h i n a given f r a c t u r e there i s no apparent r e l a t i o n s h i p between sediment or organic matter and bubble i n c l u s i o n s , i . e . the bubble contents above and below.organic matter are s i m i l a r . Ice C h a r a c t e r i s t i c s , wedge boundary The i c e at the wedge boundary d i f f e r s from t h a t described above. The f r a c t u r e p a t t e r n i s l e s s complex, the bubble f o l i a t i o n s and the i n t e r - vening " c l e a r bands" are wider, reaching up to 20 mm. Bubbles are mainly v e r t i c a l l y elongated, 10 mm long, •with fewer s p h e r i c a l bubbles than i n the centre of the wedge. Thus, w h i l e the f o l i a t i o n s have become r o t a t e d . t h e bubbles remain e s s e n t i a l l y v e r t i c a l . The f r a c t u r e s s u b - p a r a l l e l to the 164 wedge boundary have c l a y i n c l u s i o n s up to 3 mm wide and are more continuous than those i n the centre.of the wedge. U s u a l l y the f r a c t u r e s w i t h h i g h . c l a y content have few bubbles. Later f r a c t u r e s are ob l i q u e to the. e a r l i e r f r a c t u r e s and to the wedge boundary, and have low sediment content. Wedge Centre Bubble C h a r a c t e r i s t i c s . - Maximum i n f o r m a t i o n was obtained from v e r t i c a l s e c t i o n s orthogonal to the trend of the wedge. Bubbles occur i n markedly d i f f e r e n t d e n s i t i e s w i t h i n i n d i v i d u a l bands, which range i n dip from 40° to v e r t i c a l , but not a l l bands s t r i k e p a r a l l e l to the wedge. Bubble shapes are (a) elongate, (b) s p h e r i c a l , (c) i r r e g u l a r . (a) Elongata bubbles are o r i e n t e d approximately v e r t i c a l l y , whather the c o n t a i n i n g f o l i a t i o n i s v e r t i c a l or o b l i q u a . I n d i v i d u a l ..bubbles are <3 mm long and < 0.5 mm diameter, some having s l i g h t c u r v a t u r e , but.most are r e g u l a r c y l i n d e r s . (b) S p h e r i c a l bubbles are < l l mm i n diameter and occur i n d i v i d u a l l y , i n groups, or w i t h i n bands of mainly elongate bubbles. (c) I r r e g u l a r l y shaped bubbles are s i m i l a r In d i s t r i b u t i o n to the s p h e r i c a l , but may reach 3 mm i n length. Organic matter occurs mainly i n small pockets in. discontinuous, t r a i n s approximately p a r a l l e l to bubble bands. C r y s t a l C h a r a c t e r i s t i c s . - C r y s t a l s i z e v a r i e s markedly throughout the s e c t i o n ( F i g . 50), the average s i z e being .4 mm x 3 mm and. the range from <1 mm x <1 mm to 42 mm x 8 mm, tha l a t t e r b e i n g . v e r t i c a l l y elongated. C r y s t a l s are anhedral w i t h s t r a i g h t or s l i g h t l y 'curved boundaries. Soma 165 F i g u r e 51. Sketch of grains F i g u r e 50 for p e t r o f a b r i c a n a l y s i s , shown i n Figure 52. 166 of the l a r g e r c r y s t a l s have more i r r e g u l a r boundaries due to the presence of small c r y s t a l s • a l o n g t h e i r margins. There are d i f f e r i n g t e x t u r a i zones, areas where grains are equigranular w i t h no complex intergrowths c o n t r a s t - . i n g w i t h areas where grains are markedly d i s s i m i l a r i n shape, s i z e and s u b s t r u c t u r e . Dimensional o r i e n t a t i o n ( F i g . 55(a)) i s predominantly v e r - t i c a l throughout, but l o c a l l y more s t r o n g l y developed, and some recent f r a c t u r e i n f i l c r y s t a l s have h o r i z o n t a l axes. S p h e r i c a l bubbles tend to be on or near c r y s t a l boundaries r a t h e r than i n the centres of c r y s t a l s . No major change i n shape of bubbles occurs at g r a i n boundaries. Elongated bubbles r a r e l y occur w i t h i n a s i n g l e c r y s t a l , most cross boundaries or terminate upwards at boundaries. The l a t t e r suggests that m i g r a t i o n i s c o n t r o l l e d by the g r a i n boundary. Most i n c l u s i o n s of. organic matter are at g r a i n boundaries. A photograph of one t h i n s e c t i o n Is given i n F i g . 50, and a sketch of grains f o r p e t r o f a b r i c a n a l y s i s I n F i g . 51.. The general p e t r o f a b r i c p a t t e r n f o r the sample i s a broad s u b h o r i z o n t a l g i r d l e ( F i g . 5 2 ( a ) , ( b ) ) , but l o c a l concentrations e x i s t , i n d i c a t i n g the complex i n f l u e n c e of m u l t i p l e f r a c t u r e s . C r y s t a l growth i n f r a c t u r e s i s expected to be i n i t i a l l y productive of a v e r t i c a l g i r d l e . n o r m a l to the crack, unless c r y s t a l s grow as extensions of cracked c r y s t a l s , where a h o r i z o n t a l g i r d l e p a t t e r n would be expected. Subsequent periods of thermally - induced s t r e s s cause flow and. r e o r i e n t a t i o n . L a t e r f r a c t u r e s may a l s o i n t e r r u p t the p a t t e r n . Component p e t r o f a b r i c diagrams ( F i g . 52(c)-(1)) have been prepared on the b a s i s of c r y s t a l s i z e , number of sides per c r y s t a l and bubble content. There' are no s i g n i f i c a n t l y d i f f e r e n t patterns f o r each set of diagrams. Thus the broad h o r i z o n t a l g i r d l e i s homogeneous throughout the s e c t i o n . 167 F i g u r e 52. (a),(b) 140 c r y s t a l s , v e r t i c a l s e c t i o n orthogonal to wedge a x i s , at wedge centre, .(c) 57 c r y s t a l s w i t h v e r t i c a l dimensional', • o r i e n t a t i o n , (d) 83 other c r y s t a l s , (e) 40 c r y s t a l s w i t h "> 6 s i d e s , ( f ) 40 c r y s t a l s w i t h 5 sides., ... continued. Diagrams i n plane of seccions a.p. = a x i a l plana of wedge Contours at i n t e r v a l s ' 1, 2,.. 4, 6, 8 cr (Cont'd) (g) 60 c r y s t a l s with < 5 s i d e s , (h) 24 c r y s t a l s , >5 mm long axes, ( i ) 52 c r y s t a l s 3-4 mm axes, ( j ) 58 c r y s t a l s <2 mm axes (k) 69 c r y s t a l s c o n t a i n i n g hubbies (1) 71 c r y s t a l s without bubbles. Diagrams i n plane of s e c t i o n s a.p. = a x i a l plane of wedge 169 Wedge Boundary The contact of the wedge wi t h the surrounding c l a y i s i r r e g u l a r and cl a y blocks are contained w i t h i n the i c e . These blocks have rounded .edges and are d i s s i m i l a r to c l a y i n a lens ice-clay, system. The fr a c t u r e , p a t t e r n i s simpler than at the wedge centre; most f r a c t u r e s trend p a r a l l e l to the side of the wedge and are oft e n traceable c o n t i n u o u s l y through the s e c t i o n , although some are s l i g h t l y o f f s e t by l a t e r , o b l i q u e f r a c t u r e s ( F i g . 53). Many.earlier f r a c t u r e s have 2-5 mm t h i c k sediment i n c l u s i o n l a y e r s , w h i l e others are l e s s continuous. These e a r l y f r a c t u r e s have fewer bubbles but more sediment than l a t e r f r a c t u r e s . Bubble C h a r a c t e r i s t i c s . - Bubbles i n the o l d e r f r a c t u r e s have been subject to more thermomigration and f r a c t u r i n g . As the younger f r a c t u r e s tend to occur i n the c e n t r a l p o r t i o n , bubbles are b e t t e r preserved there than at the s i d e s . Owing to these changes, the c h a r a c t e r i s t i c wedge " f o l i a t i o n " i s l e s s e a s i l y traceable near the wedge boundary, e s p e c i a l l y where c l a y i n c l u s i o n s , and l e s s commonly organic matter, are present. •" Where bubbles occur at the s i d e s , they are more i r r e g u l a r than i n the wedge centre. S p h e r i c a l bubbles are le s s abundant than a t the wedge ce n t r e , 0.5 mm i n diameter, and tend to surround elongate bubble zones. These probably represent the i n t e r a c t i o n of bubbles d u r i n g wedge growth. U s u a l l y t h e greatest i r r e g u l a r i t y appears on the s i d e of the bubble nearest the. wedge boundary. The l a t e r " f o l i a t i o n s " are bubble-bands 3 to 5 mm wide. c o n t a i n i n g more re g u l a r v e r t i c a l l y elongated bubbles, whatever the o r i e n - . •tation of tha .band. , . C r y s t a l C h a r a c t e r i s t i c s . - C r y s t a l s i z e v a r i e s throughout the s e c t i o n , but i s g e n e r a l l y l a r g e r . a t the wedge boundary than i n the wedge centre, Figure 53. J u n c t i o n of wedge with c l a y . Note bubble t r a i n s p a r a l l e l to j u n c t i o n , and tha oblique f r a c t u r e . 10 mm g r i d . V e r t i c a l s e c t i o n perpendicular to a x i s of wedge. Plane p o l a r i z e d l i g h t . F igure 54. V e r t i c a l s e c t i o n , wedge boundary, orthogonal to wedge a x i s . 10 mm g r i d . Crossed p o l a r i z e r s . . 171 average s i z e being 7 x 3 mm. Away from the edge of the wedge, c r y s t a l s reach 15 x lOymm, but small c r y s t a l zones a l s o occur ( F i g . 54; a l s o compare F i g . 50). C r y s t a l shape i s anhedral but w i t h the s m a l l e r c r y s t a l s tending to have some s t r a i g h t s i d e s . The most pronounced g r a i n boundary i r r e g u l a r i - t i e s are v e r t i c a l l y upward in d e n t a t i o n s at the base of c r y s t a l s , a s s o c i a t e d w i t h bubbles. 1 Upward m i g r a t i o n i s u n l i k e l y as m i g r a t i o n would, tend to be downward f o r most of the year to the warmer r e g i o n . G r a i n boundary mi g r a t i o n can occur i n any d i r e c t i o n , thus i t i s more l i k e l y t hat bubbles have l o c a l l y retarded that process. Some of the l a r g e r g r a i n s (> 10 x 5 mm are cut by l a t e r f o l i a t i o n s , thus an increase i n c r y s t a l s i z e occurred before the l a t e s t f r a c t u r e s . Some bubble i r r e g u l a r i t i e s o ccurred a f t e r the l a t e s t f r a c t u r e s , but the time of f r a c t u r i n g i s unknown. Dimensional o r i e n t a t i o n s ( F i g . 55(b)) are g e n e r a l l y , v e r t i c a l or pa r a l l e l - c . o the l o c a l f r a c t u r e , the o l d e r f r a c t u r e s dominating, but. t h i s p a t t e r n i s complicated where new f r a c t u r e s occur. Substructure occurs, mainly i n the l a r g e r g r a i n s , as s l i g h t l y d i f f e r i n g e x t i n c t i o n across c r y s t a l s , not as d i s t i n c t bands. There i s thus an o v e r a l l r e l a t i o n s h i p of t e x t u r e and f o l i a t i o n . In the zone of older f r a c t u r e s , small c r y s t a l s are e f f e c t i v e l y bounded by f r a c t u r e s . L a t e r f r a c t u r e s cross these o l d e r g r a i n s . In. comparison w i t h s e c t i o n s from the wedge centre, a higher p r o p o r t i o n of bubbles occurs w i t h i n c r y s t a l s , thus w h i l e bubble m i g r a t i o n and g r a i n boundary adjustment have occurred, bubbles have not a l l been trapped on boundaries. 172 Fi g u r e 55. Dimensional o r i e n t a t i o n , v e r t i c a l s e c t i o n s . .(a) Centre of wedge, s e c t i o n orthogonal to wedge a x i s ; (b) Boundary of wedge, s e c t i o n orthogonal to wedge axis.; (c) J u n c t i o n of two wedges. Fig u r e 56. Sketch of grains f o r p e t r o f a b r i c a n a l y s i s , shown i n F i g . 57. 173 Figure 5 7. (a),(b) 120 c r y s t a l s , v e r t i c a l . s e c t i o n orthogonal to x^edge a x i s , at x^edge boundary; (c) 43 c r y s t a l s <10 mm"; (d) 42 c r y s t a l s 10-20 mm2.; . "•' (e) 30 c r y s t a l s >20mra 2. Diagrams i n planes of s e c t i o n s a.p. ..= a x i a l plane of wedge w.b. '= xvedge. boundary contour i n t e r v a l s 2, 4, 6, 8, 10, 12, 14 cr 174 P e t r o f a b r i c diagrams are given i n F i g . 57, f o r the s e c t i o n sketched i n F i g . 55. The p a t t e r n i s seen immediately to be more concentrated than, the previous case, d i s p l a y i n g a strong point c o n c e n t r a t i o n orthogonal to the plane of the major f o l i a t i o n , w i t h a minor, g i r d l e orthogonal to the compositional l a y e r i n g . The f a b r i c , has thus become r e o r i e n t e d i n t o higher symmetry than that of the wedge centre. Component diagrams ware prepared on the b a s i s of c r y s t a l s i z e , p o s i t i o n r e l a t i v e to f r a c t u r e s , and dimen- s i o n a l o r i e n t a t i o n . . The three diagrams ( F i g . 57(c)-(e)) d i f f e r e n t i a t i n g c r y s t a l s i z e show that c r y s t a l s <^10 mm- tend toward a g i r d l e p a t t e r n which was c h a r a c t e r i s t i c of the wedge c e n t r e , w h i l e the l a r g e r the g r a i n s , the nearer the p a t t e r n approaches a p o i n t c o n c e n t r a t i o n . There i s no s i g n i - f i c a n t d i f f e r e n c e among diagrams based on dimensional o r i e n t a t i o n . H o r i z o n t a l Sections As a check on t e x t u r a l v a r i a t i o n across the s e c t i o n , h o r i z o n t a l t h i n . se c t i o n s were compared. The f r a c t u r e p a t t e r n i s more pronounced i n the . wedge'centre, most f r a c t u r e s are p a r a l l e l to the wedge trend,.but some i n t e r s e c t . Bubble i n c l u s i o n s are most s t r o n g l y concentrated i n younger f r a c t u r e s which separate zones of c l e a r e r i c e and of randomly s c a t t e r e d bubbles; few d i s c r e t e bands occur near the wedge edge. Organic matter i s much l e s s common than bubbles i n the c e n t r a l • s e c t i o n -- i n c l u s i o n s are r e s t r i c t e d to small p a r t s of a few f r a c t u r e s , o c c u r r i n g i n the form of s treaks and blobs. At the wedge boundary, organic matter i s i n the form of l i n e a r i n c l u s i o n s only. Average c r y s t a l s i z e v a r i e s from 3 x 2 mm i n the: wedge centre to 6 x 5 mm i n the outer s e c t i o n . C r y s t a l shape i n the centre i s g e n e r a l l y subhedral, curved faces being on the s i d e away from recent f r a c t u r e s . Most complex shapes are found near those f r a c t u r e s i n c r y s t a l s which have dimensional o r i e n t a t i o n s orthogonal to.the c r a c k s , whereas those f u r t h e r away are more n e a r l y equidimensional. I n the S e c t i o n near the wedge boundary, shapes are mostly anhedral but w i t h no complex intergrowths or s e r r a t i o n s . Major boundary i r r e g u l a r i t i e s are a s s o c i a t e d w i t h bubbles, i n d i c a t i n g r e l a t i v e bubble-boundary m i g r a t i o n . Substructure i s not w e l l developed i n any s e c t i o n ; i t occurs i n l a r g e r c r y s t a l s i n the wedge centre and more fr e q u e n t l y at the wedge boundary. The only pre- f e r r e d dimensional o r i e n t a t i o n i s . orthogonal to recent f r a c t u r e s , i n d i c a t i n g space i n f i l l i n g ; l a t e r periods of s t r a i n modify such a p a t t e r n . In the c e n t r a l s e c t i o n bubbles and texture are r e l a t e d i n two ways:. . (a) recent f r a c t u r e s are.marked by continuous l i n e s of bubbles In t h e i r c e n t r e s , c r y s t a l s from each s i d e of the wedge meeting at the seam, and (b) away from recent f r a c t u r e s , bubbles tend to be i n g r a i n boundaries, probably r e s u l t i n g from t r a p p i n g during r e c r y s t a l l i z a t i o n . Near the wedge boundary, bubble bands are l e s s d i s t i n c t , and the l a r g e r g r a i n s have grown past the o l d bands, bubbles o c c u r r i n g w i t h i n c r y s t a l s and at boundaries., Organic matter always occurs at boundaries. J u n c t i o n of Two Wedges In a d d i t i o n to the f a b r i c s of s i n g l e wedges, a j u n c t i o n of two o r t h o g o n a l l y i n t e r s e c t i n g wedges was s t u d i e d . No. major d i f f e r e n c e s from . . s i n g l e wedges were found. C r y s t a l s i z e -ranged from 2 mm x 1 mm.tb 22 mm x 8 mm, averaging 7 mm x 5 mm. No prominent banding was d i s t i n g u i s h e d on the b a s i s of g r a i n s i z e , except where a recent, f r a c t u r e was i n d i c a t e d by a small c r y s t a l zone. Shape i s g e n e r a l l y a n h e d r a l , although some c r y s t a l s have one of more s t r a i g h t s i d e s , and dimensional o r i e n t a t i o n i s . v e r t i c a l . p a r a l l e l to bubble f o l i a t i o n , and more pronounced than i n s i n g l e wedges. The p e t r o f a b r i c diagram ( F i g . 58) shows a broad h o r i z o n t a l g i r d l e which i s broader and weaker than the p a t t e r n i n s i n g l e wedges, but contains two orthogonal maxima, normal to the two wedges. From the l i m i t e d work done here i t i s not p o s s i b l e to suggest r e l a t i o n s h i p s of i n d i v i d u a l maxima to each wedge. D i s c u s s i o n of Wedge Ice The number of samples described here i s l i m i t e d compared w i t h the work of Black (1953), but w i t h i n a given wedge systematic changes were recognized. In a s i n g l e wedge grain, s i z e increased outward from the . centre and c r y s t a l s became dimensiorially o r i e n t e d p a r a l l e l to the compo- s i t i o n a l l a y e r i n g . Towards the sides of the wedge o p t i c axis- o r i e n t a t i o n s form a strong po i n t maximum orthogonal to the l a y e r i n g due to r e c r y s t a l - . l i z a t i o n . The lower symmetry of the f a b r i c diagrams of the wedge centre i s due to the presence of m u l t i p l e oblique f r a c t u r e s and a s s o c i a t e d new c r y s t a l growth. With i n c r e a s i n g d i s t a n c e from the centre there i s l e s s disturbance and f a b r i c s adjust to the imposed s t r e s s system, producing a strong p o i n t maximum. While the major s t r e s s Is h o r i z o n t a l , wedges r e t a i n a wedge shape and b a s a l planes of c r y s t a l s are p a r a l l e l to the bubble l a y e r i n g . Towards the wedge boundary the l a y e r s increase i n d i p , but non- s p h e r i c a l bubbles w i t h i n the layer s r e t a i n v e r t i c a l o r i e n t a t i o n s , due i n part to the s t r e s s system and p a r t i a l l y to the v e r t i c a l temperature gra- d i e n t . The sample from the j u n c t i o n of two wedges shows the i n f l u e n c e of both wedges. F i g u r e 58. (a),(b) 100 c r y s t a l s at j u n c t i o n of two orthogonal i c e wedges. V e r t i c a l s e c t i o n a.p. = a x i a l planes of wadges contour i n t e r v a l s 2, 4, 6 cr In comparison w i t h Black's (1953) r e s u l t s , the f a b r i c s . f o u n d . h e r e . are f a i r l y simple. C-axis maxima are g e n e r a l l y orthogonal t o c o m p o s i t i o n a l l a y e r i n g s whereas Black found t h i s p a t t e r n plus a range of o t h e r s . However Black studied many more wedges which included wedges i n v a r i o u s s t a t e s of a c t i v i t y , i n c l u d i n g b u r i e d wedges. Buried, wedges had equigranular c r y s t a l s - (Black. 1953, p. 65) which i s evidence of g r a i n growth. . In the t h e o r e t i c a l -work of Lachenbruch (1962) and Gfechishchev (1970) the wedges were considered to be f a i r l y uniform. I n the present study, the. v a r i a b i l i t y i n compositional and c r y s t a l c h a r a c t e r i s t i c s has been recog- n i z e d . Compositional l a y e r s d i f f e r in. o r i e n t a t i o n throughout the wedge and the contained i c e s respond i n d i f f e r e n t manners. The l a y e r s are d e f i n e d by bubbles and other gross defects which act as s t r e s s c o n c e n t r a t o r s . Thus t h e o r e t i c a l models of c r a c k i n g are not r i g o r o u s l y a p p l i c a b l e . A d d i t i o n a l l y the presence of g r a i n boundaries, xvhich are zones of atomic d i s o r d e r , may i n f l u e n c e crack propagation.. The general i n c r e a s e i n g r a i n s i z e toward the edge of the wedge and a s s o c i a t e d change i n o p t i c a x i s l i n e a t i o n s toward a p o i n t c o n c e n t r a t i o n orthogonal to the . l o c a l f o l i a t i o n i s due to b a s a l s l i p and g r a i n growth occurs i n s u i t a b l y o r i e n t e d c r y s t a l s , w h i l e other, c r y s t a l s are consumed i n the boundary m i g r a t i o n mechanism. I n c l u s i o n s may a f f e c t the boundary adjustments, and bubbles are s u b j e c t to h o r i z o n t a l s t r e s s e s and a v e r t i c a l temperature gradient which produces t h e i r v e r t i c a l e l o n g a t i o n . . Shumskii (1954, p. 202) reported that at the upper end of wedges ... c r y s t a l s are columnar. The s t r a t i f i c a t i o n becomes q u i t e i n d i s t i n c t ... and the number of mineral i n c l u s i o n s decreases c o n s i d e r a b l y . • 1 7 9 T h i s suggests that the upper i c e was pool i c e , f r o z e n i n the wedge trough. Such m i s i n t e r p r e t a t i o n s are r e a d i l y made i n l i g h t of the f a c t that thermal e r o s i o n of i c e wedges may occur, as discussed by Mackay (1974d). Refrozen • i n f i l s of such channels have been observed, and subsequent f r a c t u r e s cross these i n f i l s (Mackay, personal communication 1975). (e) The Influence of Wedge Growth on Massive Ice In a d d i t i o n to s i t e s such as P e l l y I s l a n d , where wedge systems have developed i n sediments and organic matter, wedges have a l s o grown i n segregated i c e bodies. The. l a t t e r type i s l e s s frequent and c h a r a c t e r i z e d by l a r g e r polygons. As recent f r a c t u r e s were i d e n t i f i e d p e t r o g r a p h i c a l l y , the wedge, appears to be growing, and thus a c t i v e l y s t r e s s i n g the surrounding i c e . The compositional l a y e r i n g of the massive i c e i s deformed adjacent t o. the wedge, shown i n Figure 32. P e t r o l o g i c a n a l y s i s shows a decrease i n ... g r a i n s i z e toward the wedge, and a change i n p e t r o f a b r i c s from that t y p i c a l of f o l d e d massive i c e toward that of a wedge. I n a d d i t i o n recent f r a c t u r e s showed t h e i r c h a r a c t e r i s t i c f e a t u r e s . By comparison w i t h f o l d e d i c e which d i d not c o n t a i n a wedge, the i n f l u e n c e of the wedge i s apparent. ( f ) Comparison of Tension Crack Ice and Wadge Ice Wedge i c e i s a r e s u l t of the i n f i l of f r a c t u r e s produced by thermal c o n t r a c t i o n of the ground, whereas t e n s i o n crack i c e i n f i l s f r a c t u r e s pro- duced by mechanical rupture of the ground a s s o c i a t e d w i t h the growth of excess i c e at depth, as i n the case of pingos. However, t e n s i o n crack Ice i s a l s o subject to thermally-inducad s t r a i n s . Wedges c h a r a c t e r i s t i c a l l y form a; polygonal p a t t e r n (although there may be no surface expression on sl o p e s , due to s o i l creep) whereas t e n s i o n cracks are best observed on pingos, but may extend on to adjacent lake f l a t s , and have been traced f o r 2 km (Mackay 1973a, p. 992, F i g . 18). Thus the two i c e types may be d i s t i n g u i s h e d f r e q u e n t l y by surface e x p r e s s i o n . Where exposure to depth occurs the r e l a t i o n s h i p of the ice" body to. surround i n g m a t e r i a l d i f f e r s ; no upturning occurs at the t e n s i o n crack boundary ( F i g . 40), whereas sediment and i c e banding are deformed adjacent t o wedges ( F i g . 32). Where such rare s e c t i o n s are not a v a i l a b l e , the p e t r o l o g i c c h a r a c t e r - i s t i c s of the i c e are u s e f u l ; however, no t e n s i o n crack i c e from the lake f l a t s was st u d i e d . In comparison w i t h wedge I c e , one season's growth of t e n s i o n crack i c e may be much greater ( i n t h i s study 100 mm) and has mul- t i p l e bubble bands and c r y s t a l l a y e r s . c o n t a i n i n g l a r g e r c r y s t a l s than wedge i c e . A l s o l a t t i c e p r e f e r r e d o r i e n t a t i o n s are more concentrated i n the . g i r d l e p a t t e r n . In the case of the o l d t e n s i o n crack i c e there are again major d i s s i m i l a r i t i e s from wedge i c e in. terms of banding, bubbles, c r y s t a l s i z e , shape and o r i e n t a t i o n . •. 131 R e t i c u l a t e Vein Ice I n t r o d u c t i o n F i n e - g r a i n e d sediments, such as g l a c i a l t i l l s , lake and marine c l a y s and mudflow deposits are of widespread d i s t r i b u t i o n i n the f i e l d area. W i t h i n such sediments, r e t i c u l a t e i c e veins have been recognized forming a three-dimensional p a t t e r n (Mackay 1974b). The veins tend to occur i n tha upper 10 m of exposures and are f r e q u e n t l y u n d e r l a i n by massive segre- gated ice.. The primary veins may be e i t h e r v e r t i c a l or h o r i z o n t a l (Mackay 1975c) and range up to 5 m i n length and 0.3 m i n w i d t h . Ice w i t h i n the veins i s o f t e n i n c l u s i o n f r e e . S e v e r a l t h e o r i e s of v e i n growth have bean proposed (Popov 1967; D a n i l o v 1969; Katasonov 1967; Mackay 1974b, 1975c; McRoberts and. Nixon 1975). The t h e o r i e s of PopOv, Danilov and Katasonov have been discounted, by Mackay (1974b) who presented a theory which e x p l a i n s the near s u r f a c e p o s i t i o n of the v e i n systems and the lack of excess water on thaw; the v e r t i c a l and h o r i z o n t a l o r i e n t a t i o n s are due to shrinkage c r a c k s and the water i s d e r i v e d from the adjacent " f r o z e n " c l a y b l o c k s , a f t e r downward p e n e t r a t i o n of the f r e e z i n g ' f r o n t . McRoberts and Nixon (1975) gave a d i s c u s s i o n of Mackay's (1974b) paper.in terms of h y d r a u l i c f r a c t u r i n g . In r e p l y Mackay (1975c) p o i n t e d out that h o r i z o n t a l h y d r a u l i c f r a c t u r i n g was unlikely., and that v e r t i c a l veins formed by such a mechanism could probably, be d i s t i n g u i s h e d by f u r t h e r study of i c e v e i n p a t t e r n s , i c e p e t r o f a b r i c s and water chemistry. . 182 I n " t h i s study r e t i c u l a t e v e i n i c e was obtained from'above the massive segregated core of an i n v o l u t e d h i l l . Emphasis i s given to c o n s i d e r a t i o n s of i c e p e t r o f a b r i c s as an a i d i n understanding growth and p o s t - s o l i d i f i c a - t i o n f e a t u r e s . F i e l d C h a r a c t e r i s t i c s The s i t e chosen f o r sampling i s an upper, a c t i v e slump face, i n a c o a s t a l exposure where s e v e r a l periods of slumping have occurred. A v a r - i a b l e t h i c k n e s s of stoney c l a y (1-10 m) o v e r l i e s a massive segregated i c e . core. A t t h i s s i t e the r e t i c u l a t e v e i n i c e p a t t e r n i s dominated by v e r t i c a l veins ( F i g . 59) which reach 0.25 m i n thickness but are u s u a l l y 10 t o 100 mm t h i c k , and widen downwards.' They are t r a c e a b l e v e r t i c a l l y f o r s e v e r a l metres,, to the top of the present slump, and terminate downwards i n massive segregated, i c e ; f r e q u e n t l y they become t h i n n e r j u s t above the i c e core. H o r i z o n t a l veins a l s o occur but are l e s s continuous; the enclosed c l a y , b l o c k s are up to 1 m x 0.3 m x 0.3 m. The topography of the h i l l and the upper surface of the u n d e r l y i n g massive segregated i c e i s u n d u l a t i n g . The veins are perpendicular and p a r a l l e l to the upper massive.ice s u r f a c e , except where s l i g h t downs lope creep has occurred. Thus the system has been subject to heaving during l a t e r massive i c e growth. Ice C h a r a c t e r i s t i c s The contact of the v e i n i c e and surrounding c l a y i s i r r e g u l a r , but abrupt, and a mineral f i l m i s observed between i c e and c l a y . I n c l u s i o n s , occur i n the form of bubbles and small clay, b l o c k s ; no s t r u c t u r e s are apparent. Samples were taken from both v e r t i c a l and h o r i z o n t a l v e i n s , and, t h i n s e c t i o n s p a r a l l e l t o and orthogonal t o the v e i n trends are d i s c u s s e d . Figure 61. V e r t i c a l s e c t i o n , p a r a l l e l to v e i n plana. G r i d 10 am..1 1 Crossed p o l a r i z e r s 184 ( i ) ' Narrow V e r t i c a l . Veins Inclusion C h a r a c t e r i s t i c s . Bubbles are few, spherical and small ( <1 mm diameter) or e l l i p s o i d a l . (3-4 mm long). Positions are apparently not r e l a t e d to the vein boundary or to clay i n c l u s i o n s . The clay blocks are i r r e g u l a r with conchoidal.faces s i m i l a r to those reported for h o r i z o n t a l lenses by Penner (1961), and range i n s i z e up' to 30 mm on a side. No pattern of the blocks r e l a t i v e to the v e i n o r i e n t a - t i o n was recognized. .' C r y s t a l C h a r a c t e r i s t i c s C r y s t a l s i z e i s large ( F i g . 60) ranging from 4 x 5 mm to 20 x 30 mm, and shapes are anhedral, inequigranular. Boundaries vary widely; i n a given c r y s t a l , 2. or 3 sides may be e s s e n t i a l l y s t r a i g h t while the others are very highly curved and i n t e r l o c k i n g , e s p e c i a l l y along embayments. These embayments develop along sub-boundaries, which are we l l developed in many grains. While many sub-boundaries, are p a r a l l e l i n a.given c r y s t a l , others radiate from a point, often an i n c l u s i o n . There i s no well developed dimensional o r i e n t a t i o n . .'" Sediment in c l u s i o n s have an obvious c o n t r o l on texture. Most p a r t i - c les are: . (a) on boundaries where they help to p i n boundary migration; (b) on sub-boundaries which r e s u l t from d i s l o c a t i o n production at the in c l u s i o n ; or (c) at. the junction of sub-boundaries and boundaries where embayment has been arrested. L o c a l l y very complex boundaries are associated with.a group of i n c l u s i o n s . The few.bubbles are generally on grain boundar- ies . ' ' ' • : ' ' . 135 F i g u r e 61 shows a t h i n s e c t i o n v e r t i c a l l y below t h a t - p r e v i o u s l y d i s - cussed. This shows the l o c a l v a r i a b i l i t y i n c r y s t a l s i z e . I n some cases i t i s apparent that one c r y s t a l has been subdivided by s t r a i g h t sub-bound- . a r i e s , by a p o l y g o n i z a t i o n mechanism. These s u b - c r y s t a l s have c l o s e e x t i n c t i o n p o s i t i o n s . ( i i ) Wide V e r t i c a l Veins I n c l u s i o n C h a r a c t e r i s t i c s • - Generally the wider veins d i f f e r i n i n c l u s i o n c h a r a c t e r i s t i c s from the narrow v e i n s . Bubbles occur i n the c e n t r a l zone, but not near the contact w i t h the c l a y . U s u a l l y the bubbles are v e r y f i n e , s p h e r i c a l , and occur i n groups and networks. Tha absence of bubbles i n tha outer zone suggests slow f r e e z i n g , and r e j e c t i o n of s o l u t e , as i s a l s o seen i n the m i n eral f i l m s on the c l a y b l o c k s . Clay occurs as f i n e l y dispersed p a r t i c l e s , . and i r r e g u l a r blocks 10 mm a c r o s s , a l l away from the contact. C r y s t a l C h a r a c t e r i s t i c s . • C r y s t a l s i z e and shape determine two zones: c l o s e to the v e i n edge c r y s t a l s are elongate, y 10 mm x 6 mm, and i n tha c e ntre of the v e i n are large c r y s t a l s ^ 30 mm x >̂ 20 mm ( F i g . 62).. The elongate c r y s t a l s are anhedral w i t h curved to s e r r a t e d boundaries w i t h s e r r a t i o n s normal to' tha. dimensional o r i e n t a t i o n which i s orthogonal to tha c l a y c o n t a c t . The l a r g e r c e n t r a l c r y s t a l s are anhedral, boundaries are curved or have s m a l l s a r r a - t i o n s . These c r y s t a l s are more n e a r l y e q u i g ranular and more i n t e r l o c k i n g 136 Figure 62. V e r t i c a l s e c t i o n normal to v e i n plane. Note h o r i z o n t a l columnar c r y s t a l s on l e f t - h a n d s i d e , adjacent' to c l a y . G r i d 10 mm.Crossed p o l a r i z e r s Figure 64. Sketch of gra i n s for p e t r o f a b r i c a n a l y s i s , F i g u r e 63(b). Shaded grains have c-axes outside the maximum. V e r t i c a l s e c t i o n , p a r a l l e l to v e i n plane. 187 i n nature. Substructure occurs i n the c e n t r a l c r y s t a l s but i s r a r e i n the marginal, elongated c r y s t a l s . A l s o the r e l a t i o n s h i p of c r y s t a l s and i n c l u s i o n s d i f f e r s i n the two zones.. Most of the sediment i s i n t r a c r y s t a l l i n e i n the centre of the v e i n , but i n t e r c r y s t a l l i n e i n the zone of elongated c r y s t a l s . The networks of bubbles are not everywhere r e l a t e d t o present g r a i n boundaries; the bubbles g e n e r a l l y l i e v e r t i c a l l y below the boundaries, suggesting r e l a t i v e downward motion of bubbles. P e t r o f a b r i c diagrams are shown i n Fi g u r e 6 3 ( a ) - ( f ) f o r samples p a r a l l e and orthogonal to lens trends. F i g u r e 63(a),(b) represent the c-axes of c r y s t a l s i n t h i n s e c t i o n s p a r a l l e l to v e r t i c a l v e ins ( F i g . 60, 61, 64). Here the o p t i c a x i s p a t t e r n tends toward a maximum orthogonal to the plane of the v e i n , as found by Mackay (T974b, p. 231). C r y s t a l s , o u t s i d e the maximum are shown by shading i n Fi g u r e 64; they, do not d i f f e r i n t e x t u r a l c h a r a c t e r i s t i c s , but o f t e n occur i n groups. . Figures 6 3 ( c ) - ( f ) are f o r c r y s t a l s i n s e c t i o n s orthogonal to the v e i n t r e n d , ( F i g . 6 5 ( a ) , ( b ) ) . From the previous diagrams, a h o r i z o n t a l p o i n t c o n c e n t r a t i o n orthogonal to the v e i n would be expected. This i s not the case, and i s p a r t i a l l y e x p l a i n e d by the f a c t that the-previous s e c t i o n s were from the v e i n centre, whereas the l a t e r s e c t i o n s i n c l u d e c r y s t a l s at the edge of the v e i n . F i g u r e 63(d) shows the r e l a t i v e p a t t e r n s f o r c e n t r a l , and marginal c r y s t a l s of Figure 63(c). The c e n t r a l c r y s t a l s tend toward a h o r i z o n t a l g i r d l e and the marginal c r y s t a l s a v e r t i c a l g i r d l e . However, t h i s . i s based on a small number of samples., and i s not repeated i n the other samples, e s p e c i a l l y the wide v e i n s , i n f a c t i n Fi g u r e 63(e) the marginal c r y s t a l s are on a h o r i z o n t a l • g i r d l e . F i g u r e 63(f) comprises only 188 Fi g u r e 63. (a),(b) v e r t i c a l s e c t i o n s , . p a r a l l e l plane o f - r e t i c u l a t e v e i n '.- (40, 100 c r y s t a l s ) ,' (c) v e r t i c a l s e c t i o n , normal to v e i n plane (42 c r y s t a l s ) , (d) x 16 c r y s t a l s i n v e i n c e n t r e , o 26 c r y s t a l s adjacent to c l a y , ( e) 3 ( f ) v e r t i c a l s e c t i o n normal t o v e i n , x c r y s t a l s on boundary. ' v .= v e i n plane' ''.-•••.••  190 marginal c r y s t a l s (those i n F i g . 66) and tends to a v e r t i c a l g i r d l e p a t t e r n . T h i s complexity of f a b r i c diagrams i s r e l a t e d to s e v e r a l f a c t o r s ; . d i r e c t i o n s of heat flow and water supply i n the i n i t i a l and l a t e r growth p e r i o d s , and adjustments during heave caused by the growth of the u n d e r l y i n g massive segregated i c e . ' 1 I n t e r p r e t a t i o n . Lens i c e i n c l a y s has been discussed by Penner (1961); the c r y s t a l s were large and extended across the l e n s . T h i s has a l s o been observed by the present author i n small lenses i n c l a y above Tuktoyaktuk pingo core. I n these cases the i c e bodies were perpe n d i c u l a r to the heat flow d i r e c t i o n s , i . e . normal lens growth, i n c o n t r a s t to r e t i c u l a t e v e i n growth where v e i n s occur p a r a l l e l and perpendicular to the " f r e e z i n g f r o n t . " A l s o i n the r e t i c - u l a t e p a t t e r n , lenses are more wid e l y separated and the enclosed c l a y con- t a i n s no smaller lenses and i s o v e r - c o n s o l i d a t e d . I t i s considered (Mackay 1974b, p. 235) that veins continue t o grow on t h e i r outer surfaces by water m i g r a t i o n from the adjacent c l a y b l o c k s , w e l l above the lower permafrost s u r f a c e . C o n s i d e r i n g the narrow v e i n s , c e n t r a l c r y s t a l s have t h e i r c-axes orthogonal to the v e i n trend, and are s i m i l a r to c r y s t a l s i n lenses,. whereas marginal c r y s t a l s , have t h e i r b a s a l planes orthogonal to the v e i n . .Thus the c e n t r a l c r y s t a l s are t y p i c a l of lens growth; the marginal c r y s t a l s d i f f e r i n c r y s t a l s i z e , shape, dimensional and l a t t i c e o r i e n t a t i o n s , and r e l a t i o n s h i p to i n c l u s i o n s and may represent d i s t i n c t growth c o n d i t i o n s . They may thus, represent the l a t e r stage of growth from water m i g r a t i n g from the adjacent clay,. as described by Mackay (1974b). However, i n the case of the wider veins c r y s t a l c h a r a c t e r i s t i c s d i f f e r again. The c e n t r a l c r y s t a l s have t h e , features of i c e growth i n bulk water w h i l e the marginal c r y s t a l s are s i m i l a r to those' i n the narrow v e i n s . 191 Figure 66. Columnar marginal c r y s t a l s i n v e r t i c a l s e c t i o n normal to plane of wide v e i n . 192 A c t i v e Layer Ice I n t r o d u c t i o n . . The a c t i v e l a y e r i s the zone of m a t e r i a l above permafrost which thaws and freezes a n n u a l l y . While the thawing occurs u n i d i r e c t i o n a l l y , down- wards, there i s evidence that . freeze-back may occur both downwards from the s u r f a c e , and upwards from the top of permafrost. D e t a i l e d temperature measurements by Lachenbruch et a l . (19S2) i n d i c a t e the complexity of the thermal regime of the a c t i v e l a y e r and the m u l t i - d i r e c t i o n a l nature of freezeback. In any given area the a c t i v e l a y e r t h i c k n e s s may vary c o n s i d - era b l y w i t h s o i l and v e g e t a t i o n type, as discussed by Mackay (1975d). F u r t h e r , changes i n a c t i v e l a y e r c h a r a c t e r i s t i c s can occur due to change i n c l i m a t i c parameters, surface cover, or by sedimentation. Where sedimen- t a t i o n , say, takes p l a c e , the base of the a c t i v e l a y e r , and.any i n c l u d e d i c e , becomes in c o r p o r a t e d i n t o permafrost as the permafrost t a b l e aggrades during re-establishment of thermal e q u i l i b r i u m . I t i s the i n t e n t i o n i n t h i s s e c t i o n to examine the c h a r a c t e r i s t i c s of i c e grown i n the previous season's thawed l a y e r as an a i d to under- standing the thermal regime of a c t i v e l a y e r s , and- to enumerate some.fea- tures of i c e s so formed as a b a s i s f o r d i s c u s s i o n of a g g r a d a t i o n a l i c e . Ice i n the a c t i v e l a y e r i s discussed from two s i t e s , an area o f , tundra polygons subject to c o a s t a l r e t r e a t , and a second s i t e i n high centred polygons near Tuktoyaktuk. 193 (a) Ice i n the A c t i v e Layer Adjacent to Wedges. . I n t r o d u c t i o n Extensive marine, u n d e r c u t t i n g of the polygon area l e d to block c o l l a p s e along i c e wedge boundaries which exposed small i c e bodies i n the : a c t i v e l a y e r of the adjacent o r g a n i c - r i c h s o i l ( F i g . 67). T h i s i c e had apparently grown s i n c e the previous.summer. F i e l d C h a r a c t e r i s t i c s The i c e bodies were 0.1 m t h i c k on the exposures nearest to the wedge, and tapered away from the wedge, under an overburden of 0.25 m of organic s o i l ; the bodies extended f o r up to s e v e r a l metres p a r a l l e l t o the trend of the wedge.. A l t e r n a t i n g 1-2 mm bubbly and non-bubbly l a y e r s were v i s i b l e i n the f i e l d , l o c a l i r r e g u l a r i t i e s occurred i n the l a y e r s , but elongate bubbles were g e n e r a l l y orthogonal to the l a y e r i n g . S e c t i o n P a r a l l e l to Wedge Trend Bubble C h a r a c t e r i s t i c s . - The contact of the i c e w i t h adjacent organic matter i s abrupt and few v e g e t a t i o n a l i n c l u s i o n s occur; bubbles : comprise the major i n c l u s i o n type. In a v e r t i c a l t h i n s e c t i o n p a r a l l e l to the wedge the bubbles occur g e n e r a l l y i n s u b - h o r i z o n t a l l a y e r s w i t h i n which s i z e and shape are c o n s i s t e n t , but there a l s o - o c c u r s h o r t e r (10-20 mm) narrow (3-4 mm) curved (convex, upward) layers of. peat p a r t i c l e s and bubbles. Probably the l o c a l accumulations of f i n e peat fragments at the i n t e r f a c e caused v a r i a t i o n s i n ra t e s of i c e growth and bubble n u c l e a t i o n and' growth. Elsewhere there occur elongate bubbles orthogonal tb the l a y e r s , and a l s o Figure 67. Block slump on coast exposing i c e i n the a c t i v e layer adjacent to wedges. '.JkT. i o m m i i Figure 63. Peat and bubble pattern, vertical section par a l l e l to w Figure 69 . P e t r o f a b r i c s of v e r t i c a l s e c t i o n , p a r a l l e l to i c e wedge, a c t i v e l a y e r i c e . 34 c r y s t a l s . c = compositional l a y e r i n g 195 l a r g e r bubbles.at the lower p a r t of the body, adjacent to the organic matter. The above p a t t e r n i s d i s r u p t e d as shown i n F i g . 63. A s e p a r a t i o n of 15 mm occurs, i n the upper l a y e r s which are upturned l o c a l l y . The : v e r t i c a l . 3 0 mm long d i s r u p t i o n zone comprises c l e a r i c e surrounding a c e n t r a l core of elongate,' and s p h e r i c a l bubbles, the zone does not pene- t r a t e to .the base of the i c e but terminates c e n t r a l l y . T h i s p a t t e r n suggests f r a c t u r e due to pressure associated, w i t h m u l t i - d i r e c t i o n a l f r e e z i n g i n the a c t i v e l a y e r . C r y s t a l C h a r a c t e r i s t i c s . - . C r y s t a l s i z e and shape, and dimensional and l a t t i c e o r i e n t a t i o n s . v a r y throughout the body. At. the upper and lower boundaries of the i c e occur zones of s mall c r y s t a l s . , which widen away from those boundaries and give r i s e to v e r t i c a l l y elongate', d e n d r i t i c c r y s t a l s >50 mm long x 5 mm wide. These c r y s t a l s d i s p l a y h o r i z o n t a l o f f s e t s at bubble and peat l a y e r s , but,no. t e r m i n a t i o n occurs. No pronounced sub- s t r u c t u r e was observed. . These c r y s t a l c h a r a c t e r i s t i c s are d i s t u r b e d at the d i s r u p t i o n zone, c r y s t a l s are s h o r t e r and wider, but m a i n t a i n a den- d r i t i c shape.. L a t t i c e o r i e n t a t i o n s f o r 34 c r y s t a l s are shown i n F i g . 69 f o r c r y s - t a l s i n the main.mass, and d i s r u p t i o n zone. C-axes tend to be contained i n a broad h o r i z o n t a l g i r d l e which suggests e x t e n s i o n i n the b a s a l plane. A second v e r t i c a l s e c t i o n p a r a l l e l to the wedge, but adjacent to, the s o i l , was analysed. those C r y s t a l C h a r a c t e r i s t i c s . - C r y s t a l s d i f f e r i n s i z e i n the previous s e c t i o n . • Although most have v e r t i c a l and shape from dimensional - 196 orie n t a t i o n s . , h o r i z o n t a l boundaries are frequent ( F i g . 70) and o f t e n c o i n - " ci d e w i t h bubble l a y e r s . Sub-boundaries, are w e l l developed, and e x e m p l i f i e d i n the l a r g e upper c r y s t a l . Here the sub-boundaries are a s s o c i a t e d w i t h bubble bands. Above sub-boundaries, bubbles are elongate up to 6 mm, whi l e i n the sub-boundaries, bubbles are smaller and s u b - s p h e r i c a l or s l i g h t l y elongate i n the sub-boundary. Many elongate bubbles have s l i g h t l y bulbous and f l a t ends. The f l a t end i s o f t e n o b l i q u e to the bubble, a x i s , but p a r a l l e l to the b a s a l plane i n a given c r y s t a l . . The presence of the c r y s t a l s w i t h h o r i z o n t a l dimensional o r i e n t a t i o n s remains to be explained. The l a t t e r s e c t i o n was adjacent to the s o i l whereas the previous s e c t i o n was not; thus the i n f l u e n c e of c r y s t a l growth at the s o i l i c e i n t e r f a c e was i n v e s t i g a t e d i n a s e c t i o n orthogonal to th a t i n t e r f a c e . Sample Adjacent to S o i l , Orthogonal to S o i l - I c e I n t e r f a c e • Bubble C h a r a c t e r i s t i c s : In t h i s , sample the outer contact of. i c e w i t h organic matter i s i n c l u d e d , and the a s s o c i a t e d - b u b b l e p a t t e r n ( F i g . 71a) d i f f e r s from that i n the previous samples. Bubble t r a i n s orthogonal to the s o i l curve upwards Into h o r i z o n t a l i t y . Most bubbles are elongate p a r a l l e l to the t r a i n s , and range downwards In s i z e from 4 mm x 0.5 mm adjacent to the s o i l . C r y s t a l C h a r a c t e r i s t i c s : Crystal.shape i s r e l a t e d to bubble trends i n that dimensional o r i e n t a t i o n i s p a r a l l e l to. the t r a i n s ( F i g . 71b). C r y s t a l s i z e i s v a r i a b l e - s m a l l c r y s t a l s o c c u r . a djacent t o the s o i l and s i z e increases ax^ay from the s o i l . There. Is no tendency f o r bubbles t o . 197 F i g u r e 70. V e r t i c a l s e c t i o n , , normal and adja- cent to s o i l , a c t i v e l a y e r i c e . Note h o r i z o n t a l l y elongated c r y s t a l s . , compare /yf i'y. F i g . 71b f o r i n f l u e n c e of pfa growth normal to s o i l , , s ubgrain boundaries F i g u r e 71. V e r t i c a l s e c t i o n orthogonal to s o i l , a c t i v e l a y e r i c e . (a) Bubble p a t t e r n , (b) C r y s t a l p a t t e r n . I: '/•"•••v:-: s o i l --'j ti ..s t Figure 72. • "-'\\ P e t r o f a b r i c s , c r y s t a l s i n F i g . 71b. . x c r y s t a l s adjacent \t to s o i l . s o i l V r 198 occur p r e f e r e n t i a l l y on c r y s t a l boundaries; some h o r i z o n t a l o f f s e t s of upward growing c r y s t a l s occur at.bubble l a y e r s i n the lower i c e . C-axis o r i e n t a t i o n s f o r 63 c r y s t a l s are shown i n F i g u r e 72. The sample was l i m i t e d by the small s i z e . o f the specimen; the p a t t e r n i s a broad concentra- t i o n , w i t h c r y s t a l s adjacent to peat being c e n t r a l l y s i t u a t e d . Thus the c r y s t a l s w i t h h o r i z o n t a l dimensional o r i e n t a t i o n i n Figure 70 are exten- , sions of c r y s t a l s which grew orthogonal to tha s o i l . Samples w i t h F r a c t u r e s I n t r o d u c t i o n : In a d d i t i o n to the previous samples there are some which c o n t a i n f r a c t u r e s . . Here wa i n v e s t i g a t e the i n f l u e n c e of bubble and c r y s t a l c h a r a c t e r i s t i c s on . f r a c t u r i n g . The f r a c t u r e s were observed In the f i e l d , p r i o r to sampling, and thus are not due to sampling, or thermal shock d u r i n g handling. The f r a c - tures were open, .and thus, occurred under "dry" c o n d i t i o n s . Bubble C h a r a c t e r i s t i c s : In common w i t h other a c t i v e l a y e r b o d i e s , there are a l t e r n a t i n g l a y e r s of high and low bubble contents, which are h o r i z o n t a l and p a r a l l e l away from the i n f l u e n c e of the s o i l . The contained bubbles are s p h e r i c a l , or elongated normal to the l a y e r s . C r y s t a l C h a r a c t e r i s t i c s : . A range of c r y s t a l shapes occurs, and the i n f l u e n c e of bubble bands on shape i s found- as before;, many c r y s t a l s terminate a b r u p t l y at h o r i z o n t a l bubble bands. F i g u r e 73 demonstrates the i n f l u e n c e of s e v e r a l bands. C-axis o r i e n t a t i o n s are given i n Figure 74(a), (b ) ; the o v e r a l l p a t t e r n i s an incomplete h o r i z o n t a l g i r d l e and a minor v e r t i c a l p o i n t c o n c e n t r a t i o n . A f u r t h e r s e c t i o n ( F i g . 74(c)) shows' a broad C F i g u r e 74. P e t r o f a b r i c s of a c t i v e l a y e r i c e . (a),(b) v e r t i c a l s e c t i o n , 95 c r y s t a l s ; (c) v e r t i c a l s e c t i o n , 40 c r y s t a l s . contour i n t e r v a l 2, 4, 6, 8, 10 cr F i g u r e 75. V e r t i c a l s e c t i o n showing i n f l u e n c e of bubble layers, on f r a c t u r e propagation. a o <y o o o O O O o o o o o o o O OO * a . . i II ' . !J o o o ^ 3 o a o o a 0 o o o o 3 0 „ „ = <> :> . 0 0 • 1 ,.,,,1 j JJ 0 rj:) gag nn/inn 0 0 5 30*0 0 oV) 0 fl *)') oV08" 5 A 0 010 0t 0 0 0 1F0 200 h o r i z o n t a l g i r d l e a l s o , t h i s suggests basal plane growth i n a p l e n t i f u l water supply. Fractures: The above pattern of c r y s t a l growth i s disturbed by fractures, of which the surfaces of separation, are p a r a l l e l to bubble layers , but stepped l o c a l l y (Fig.. 75). Due to the close a s s o c i a t i o n of bubble bands and c r y s t a l boundaries i t i s l i k e l y that the fractures have propagated along the weak zones. In t e r p r e t a t i o n ,. The ice bodies grew i n the previous season's active l a y e r , and there- fore represent one winter's growth; any p o s t - s o l i d i f i c a t i o n m o d i f i c a t i o n has had l i m i t e d time f o r development.. The o v e r a l l i n c l u s i o n pattern i s ho r i z o n t a l bubble bands and v e r t i c a l l y elongate bubbles, r e s p e c t i v e l y p a r a l l e l and orthogonal to the freez i n g front, but also l o c a l l y curved t r a i n s orthogonal to s o i l . C r y s t a l dimensional o r i e n t a t i o n i s e s s e n t i a l l y v e r t i c a l , although c o n t r o l l e d l o c a l l y by h o r i z o n t a l bubble bands. L a t t i c e o r i e n t a t i o n s are such that basal planes are v e r t i c a l , p a r a l l e l to the growth d i r e c t i o n , although some are h o r i z o n t a l , i n c r y s t a l s which have hor i z o n t a l dimensional orientations and occur at bubble bands, i n d i c a t i n g l a t e r a l growth from the s o i l . ' P o s t - s o l i d i f i c a t i o n features are the f r a c - tures which are concentrated on bubble bands. The stress system responsible for the fractures i s not c l e a r , the fractures are h o r i z o n t a l , i n contrast to v e r t i c a l thermal contraction cracks. The c o a s t a l block slumping occurred i n early June 1973, thus fracture may have been due to sudden exposure to warm a i r temperatures, but t h i s i s speculative. There may a l s o have been an influence of the c o l l a p s i n g blocks. 201 (b) Tuktoyaktuk S i t e F i e l d C h a r a c t e r i s t i c s An area of high centred polygons l i e s above f l a t s surrounding a creek south of Tuktoyaktuk. During e a r l y June 1974 s m a l l excavations were made i n the polygon area during a study of wedges and•polygons. Small i c e bodies were found at. a depth of. 0.3 m,. and r e p o r t e d to the author. There was an abrupt contact between the i c e and the organic s o i l above and below, which was v i r t u a l l y i c e - f r e e . Ice C h a r a c t e r i s t i c s The i c e bodies g e n e r a l l y extended l a t e r a l l y up to 120 mm and v e r - t i c a l l y f o r 80 mm. No s t r u c t u r e s , e.g. f r a c t u r e s , were apparent; few organic m a t e r i a l i n c l u s i o n s occurred, but bubbles were abundant. The i n c l u d e d s o i l was l a r g e l y c l o s e to the i c e - s o i l c o n t a c t , w h i l e bubbles formed curved, converging t r a i n s . Bubble C h a r a c t e r i s t i c s The curved bubble t r a i n s begin adjacent and orthogonal to the i c e - s o i l c o n t a c t s , which i n d i c a t e s m u l t i p l e f r e e z i n g d i r e c t i o n s . I n a d d i t i o n , i n some cases' a zone of small s p h e r i c a l bubbles l i e s p a r a l l e l to the s o i l , from which the. t r a i n s o r i g i n a t e ( F i g . 76(a)). A l s o a few s p h e r i c a l bubbles are incorporated i n t o the zone of bubble t r a i n s . The s p h e r i c a l bubbles are ^ 2 mm i n diameter, and elongate bubbles are ^ 3' mm long and 1 mm i n diameter. 202 s o i l ; s o i l . i _ 1 • Figure 76. A c t i v e layer i c e , Tuktoyaktuk, (a) bubble pattern, (b) -crystal pattern. v e r t i c a l s e c t i o n scale 10 mm 203 C r y s t a l C h a r a c t e r i s t i c s • C r y s t a l s i z e v a r i e s , ranging from 2 mm x 3 mm i n i r r e g u l a r c r y s t a l s adjacent to the peat, to 10 mm x 5 mm i n elongate c r y s t a l s . The l a t t e r ' c r y s t a l s have simply shaped compromise boundaries, w i t h o c c a s i o n a l s e r r a - t i o n s . The p r e f e r r e d dimensional o r i e n t a t i o n of the elongated c r y s t a l s i s orthogonal to the s o i l i The r e l a t i o n s h i p of bubbles to t e x t u r e i s such, that bubble t r a i n s and c r y s t a l dimensional o r i e n t a t i o n s are p a r a l l e l and l a y e r s of s p h e r i c a l bubbles are confined to grain, boundaries.. I n t e r p r e t a t i o n The a c t i v e l a y e r p o s i t i o n and l a c k of i c e i n the surrounding s o i l suggest that water was confined during, downward f r e e z i n g from the ground surface and upward f r e e z i n g from the top of permafrost. The.two f r e e z i n g ' f r o n t s met and confined-the water body which f r o z e . o m n i d i r e c t i o n a l l y . The bubble t r a i n s and dimensional o r i e n t a t i o n of elongate c r y s t a l s i n d i c a t e the change.in f r e e z i n g d i r e c t i o n during p r o g r e s s i v e s o l i d i f i c a t i o n i n t o .enclosed water. I t i s l i k e l y that at the time of s o l i d i f i c a t i o n a steeper temperature gradient e x i s t e d i n the upper part of the a c t i v e l a y e r than below, as a i r temperatures were lower than those i n the s o i l below. E v i - dence f o r t h i s i s that downward growing c r y s t a l s have cut o f f the growth of other c r y s t a l s , as has been found i n metal c a s t i n g s where d i f f e r e n t temperature gradients have been maintained on d i f f e r e n t faces of a s o l i d i - f y i n g body. A d d i t i o n a l l y bubble p a t t e r n s [ d i f f e r s l i g h t l y i n the two zones. 204 Ice Bodies w i t h M u l t i p l e Freezing H i s t o r i e s I n t r o d u c t i o n In the i c e bodies discussed so f a r i t i s evident that.one major growth p e r i o d has been r e s p o n s i b l e f o r . t h e features observed ( e x c e p t . i n wedge and t e n s i o n crack ice)„ However, i t i s known that the c r e s t s of pingos may rupture and expose the i c e core, to meltdown. I c e wedge i c e i n troughs may be subject to melti n g and thermokarst development; a l s o a t h i c k e n i n g of the a c t i v e l a y e r , by n a t u r a l ..or a r t i f i c i a l means, may l e a d to thaw of i c e bodies. I f , at a l a t e r date, lower mean annual temperatures p r e v a i l i t i s to be expected that r e f r e e z i n g may occur, w i t h a s s o c i a t e d growth of i c e bodies w i t h d i f f e r e n t features from the pr e v i o u s . Sedimenta- t i o n , s o i l creep or peat growth may produce s i m i l a r r e s u l t s . . Thaw uncon- f o r m i t i e s and subsequent r e f r e e z i n g have been recognized at s e v e r a l s i t e s ' i n the f i e l d area. Such c o n d i t i o n s are discussed for.two s i t e s : (a) Tuktoyaktuk Coast, (b) P e l l y I s l a n d . (a) Tuktoyaktuk Coast . I n t r o d u c t i o n This i s a g e n e r a l l y f l a t - l y i n g area about 2 m above present sea l e v e l w i t h a complex p a t t e r n of high-centred ice-wedge polygons. I t i s not known whether the wedges are • p r e s e n t l y a c t i v e . In a d d i t i o n to wedges there i s abundant i c e i n the form of h o r i z o n t a l and d i p p i n g l a y e r s outcropping i n c o a s t a l exposures, causing r a p i d c o a s t a l r e t r e a t . I t i s t h i s r e t r e a t which led to lake drainage and the growth of pingos shown i n Mackay (1973a, Fig.15) 205 The polygon patterns but not the l a y e r s of i c e are apparent from such a e r i a l photographs. These l a y e r s are of v a r y i n g s i z e , shape and o r i e n t a t i o n , ranging from t h i n seams to 1 m t h i c k tabular blocks 3 m i n ex t e n t . The o r i g i n of these bodies- i s not immediately d i s c e r n i b l e on the b a s i s of previous d i s c u s s i o n s . F i e l d C h a r a c t e r i s t i c s I n June.1973 a storm caused r a p i d c o a s t a l r e t r e a t and exposure o f many ice,bodies by c o l l a p s e of l a r g e blocks of organic s o i l and i c e wedges ( F i g . 67). . . . Most i c e bodies were at l e a s t 0.3 m below the present a c t i v e l a y e r , the l o c a l s o i l having high organic and i c e contents i n c l u d i n g a g g r a d a t i o n a l ice.. Ice body s i z e and shape v a r i e d from s m a l l lenses through bodies 0.5 m by 0.2 m, to l a t e r a l l y extensive sheets over 3 m long. I n c l u s i o n s , were mainly bubbles and s o i l fragments, both i n h o r i z o n t a l l a y e r s and v e r t i c a l to c u r v i n g t r a i n s . Some bodies appeared to have f r o z e n o m n i d i r e c t i o n a l l y , some had truncated bubble bands which suggested l a t e r m e l t i n g and subsequent r e f r e e z i n g . I n a d d i t i o n some near surface bodies occurred a t the base of •the previous season's a c t i v e l a y e r . These three major types of body ware sampled f o r t h i n s e c t i o n a n a l y s i s . ( i ) Ice Bodies with O m n i d i r e c t i o n a l Bubble T r a i n s I n t r o d u c t i o n • • Such features suggest that a po o l of water froze inwardly from a l l d i r e c t i o n s . Thus a s e r i e s of samples was taken to. include i c e - s o i l con- t a c t s and bubble t r a i n s from a l l p a r t s of tha body. . . . . 206 Body No.. 1 Bubble C h a r a c t e r i s t i c s . •• T h i s i c e t y p i c a l l y has a high content of bubbles i n l a y e r s and groups, . 1 mm diameter bubbles occupying up to 40% by volume. Fra c t u r e s d i s t u r b t h i s general p a t t e r n . For example a sample . : c o n t a i n i n g a l t e r n a t i n g c l e a r and bubbly l a y e r s of s p h e r i c a l 1 mm and e l o n - gate 2.5 mm bubbles contained a f r a c t u r e s u r f a c e w i t h voids or gas i n c l u - s i o n s . Although the bubble l a y e r s may dip at up to 40° , bubbles are elongate^, v e r t i c a l l y , suggesting m o d i f i c a t i o n of bubble o r i e n t a t i o n by thermomigration i n a v e r t i c a l temperature.gradient. I n comparison, bubbles i n t r a i n s i n a c t i v e l a y e r i c e are p a r a l l e l to- the t r a i n s ( F i g . 71(a)). C r y s t a l C h a r a c t e r i s t i c s . - I n a h o r i z o n t a l s e c t i o n ( F i g . 77) at the top of the body, large c r y s t a l s (long axes ^50 mm) occur throughout the s e c t i o n , but i n the f r a c t u r e zone, c r y s t a l s are s m a l l e r (< 2 mm) . The large c r y s t a l s are anhedral, and s t r o n g l y s e r r a t e d (3-4 mm amplitude) but not deeply intergrown, whereas c r y s t a l s i n the f r a c t u r e are a nhedral, approximately e q u i g r a n u l a r , w i t h s i n g l y curved or s t r a i g h t boundaries and no s e r r a t i o n s . Lineage substructure occurs i n the l a r g e r "grains, but not w i t h i n f r a c t u r e c r y s t a l s . No pronounced dimensional o r i e n t a t i o n occurs i n t h i s plane. L a t t i c e o r i e n t a t i o n s are shown i n Fig."78 f o r c r y s t a l s , i n the l a y e r e d i c e and f r a c t u r e . C-axes i n the l a y e r e d i c e are hear the plane of the bubble l a y e r s , and i n a point c o n c e n t r a t i o n . C r y s t a l s i n the f r a c - t u r e have grown w i t h a l e s s p r e f e r r e d o r i e n t a t i o n , at 20 to 65° to the f r a c t u r e plane. • A s e r i e s of s e c t i o n s was prepared from a v e r t i c a l , face d i s p l a y i n g bubble t r a i n s converging toward the centre of the body, i n d i c a t i n g f r e e z i n g from the peat on a l l s i d e s . Again bubbles are v e r t i c a l . i n t r a i n s of a l l 207 Figure 78. P e t r o f a b r i c s of i c e Figure 80. P e t r o f a b r i c s of i c e i n i n F i g . 77. x = f r a c t u r e c r y s t a l . F i g . 79. "208 o r i e n t a t i o n s , clue to l a t e r temperature gradient e f f e c t s . Peat forms another i n c l u s i o n type, trending inwards from the surrounding' peat mass, and i n separate, i n c l u s i o n s . . C r y s t a l C h a r a c t e r i s t i c s . C r y s t a l s are large and elongate', par- a l l e l to the bubble t r a i n s , thus dimensional o r i e n t a t i o n v a r i e s s y s t e m a t i c - a l l y around the i c e body, being everywhere orthogonal to the ice-peat contact and c u r v i n g to the centre of the body ( F i g . 79). G r a i n boundary s e r r a t i o n s are orthogonal to long axes, i n d i c a t i n g d e n d r i t i c growth, and c r y s t a l s became narrower as they converged i n the growth d i r e c t i o n . Sub- s t r u c t u r e occurs i n bands p a r a l l e l to the e l o n g a t i o n ; these o f t e n trend from peat i n c l u s i o n s , i n the growth d i r e c t i o n , suggesting a s l i g h t l a t t i c e o f f s e t where the c r y s t a l has grown round the peat. Where m u l t i p l e bands occur r a d i a l l y from i n c l u s i o n s , a.form of p o l y g o n i z a t i o n (Knight 1962b) has • occurred due t o growth.stresses. Bubbles occur i n groups near c r y s t a l boundaries, and as s i n g l e bubbles on boundaries. No major changes occur .: i n boundaries at the bubbles, d e s p i t e other evidence of thermomigration. In the c e n t r a l zone, c r y s t a l s become more equiaxed, and smaller.. Fewer peat i n c l u s i o n s occur and bubbles are c l o s e to g r a i n boundary i r r e g u l a r i - . t i e s . L a t t i c e o r i e n t a t i o n s f o r 37 c r y s t a l s are shown i n Fig.. 80. I n the outer zone, c-axes are p a r a l l e l to the e l o n g a t i o n d i r e c t i o n . I n t e r p r e t a t i o n ' These i c e bodies occur, i n a . l o w - l y i n g area of large tundra polygons and abundant organic matter. P r e s e n t l y some thermokarst a c t i v i t y i s o c c u r r i n g adjacent to the l a r g e r wedges. 209 From the o r i e n t a t i o n of bubble t r a i n s and the dimensional o r i e n t a t i o n of elongate c r y s t a l s i t i s evident that the ice body grew o m n i d i r e c t i o n a l l y . Thus the surrounding material was i n a frozen s t a t e ; i t i s thus argued that the body i s a frozen melt pond. The c r y s t a l s i n the h o r i z o n t a l s e c t i o n from the top of the body have horizontal'c-axes which, frequently occurs i n the freezing of bulk water whereas at the curved margin.of the body c r y s - t a l s have c-axes orthogonal to the boundary which in d i c a t e s growth normal to. the basal plane, which i s less frequently observed, although reported., by Michel and Ramseier (1971) i n lake i c e . There i s no c h i l l zone of competitive growth evident i n the v e r t i c a l s ections, so the c r y s t a l s grew i n l a t t i c e continuity with c r y s t a l s i n the peat. Toward the centre of the body l a t t i c e o r i e n t a t i o n tends toward that c h a r a c t e r i s t i c of basal plane growth. Some p o s t - s o l i d i f i c a t i o n modification has occurred i n that bubbles are not elongate, p a r a l l e l to bubble t r a i n s , but i n a v e r t i c a l d i r e c t i o n , i n d i c a t i n g thermomigration i n a v e r t i c a l temperature gradient. The lineage- substructure i s present only i n pre-fracture grains and i s thus due to freezing conditions, probably the incorporation of i n c l u s i o n s , but i t may have been exaggerated by stresses produced by f r e e z i n g of confined water. The c r y s t a l c h a r a c t e r i s t i c s of t h i s i c e body are quite d i s t i n c t from those of lens i c e where growth i s u n i d i r e c t i o n a l . In lenses bubble and c r y s t a l dimensional o r i e n t a t i o n are not m u l t i - d i r e c t i o n a l . From the a i r or ground surface the area appears t y p i c a l of ice-wedge polygon f l a t s . There i s no surface expression of the -thermokarst-type- i c e . 210 Body No. 2 f • • • A second i c e body has s i m i l a r gross c h a r a c t e r i s t i c s , i n d i c a t i v e of f r e e z i n g i n a c a v i t y w i t h i n f r o z e n peaty m a t e r i a l . However, the i n c l u s i o n content d i f f e r s from the previous case and i n f l u e n c e s c r y s t a l c h a r a c t e r i s - t i c s . I n c l u s i o n C h a r a c t e r i s t i c s . - There occurs a r e d u c t i o n i n peat con- c e n t r a t i o n from dense i n the top l e f t hand corner through a zone of d i s - persed p a r t i c l e s and o c c a s i o n a l s t r e a k s , to c l e a r at the base; t h i s corresponds to the sequence: - elongated and s p h e r i c a l bubbles, s p h e r i c a l bubbles, bubble-free. C r y s t a l C h a r a c t e r i s t i c s . - Texture i s a g a i n r e l a t e d t o i n c l u s i o n 2 content. G r a i n s i z e In the.peaty zona averages 12 mm , compared w i t h 2 544 mm i n the p e a t-free zona ( F i g . 81). The former grains a r e a n h e d r a l , approximately equigranular and l a c k s e r r a t i o n s and s u b s t r u c t u r e ; t h e . l a t t e r grains are anhedral. w i t h m u l t i p l e curved or s e r r a t e d boundaries, and are elongate w i t h w e l l developed, s u b s t r u c t u r e . V a r i a t i o n of dimensional o r i e n - t a t i o n i n d i c a t e s p r o g r e s s i o n of the f r e e z i n g interface;. In the upper i n c l u s i o n zone, peat i s g e n e r a l l y confxned to g r a i n boundaries or d i s p e r s e d pockets, i n c r y s t a l s , whereas i n the lower peaty zone the peat i n c l u s i o n s are concentrated on p a r a l l e l l i n e s i n i n d i v i d u a l c r y s t a l s , a pparently the b a s a l planes. The f r e e z i n g i n t e r f a c e advanced downwards from the top and i m p u r i t i e s were r e j e c t e d except at g r a i n boundaries, whareas i n growth, from the base, c r y s t a l s extended i n , the b a s a l p l a n a , and i n c l u s i o n s ware trapped. .In the absence of d e t a i l e d thermal data i t i s d i f f i c u l t to compare downward and upward f r e e z i n g . However, i t . i s to be expected t h a t a steeper 211 F i g u r e 83. Schematic diagram of m u l t i p l e growth periods. (a) Bubble p a t t e r n . Note v e r t i c a l bubbles i n lower, c u r v i n g t r a i n s , and h o r i z o n t a l t r u n c a t i o n , , (b) C r y s t a l p a t t e r n . Note competitive growth zone at i c e - s o i l contact and above t r u n c a t i o n zona. Note, a l s o curved, elongate c r y s t a l s , p a r a l l e l to lower bubble t r a i n s . 212 thermal gradient would, e x i s t i n . t h e o v e r l y i n g m a t e r i a l , and thus i n f l u e n c e f r e e z i n g r a t e and the i n c o r p o r a t i o n of in c l u s i o n s . . This i s t r u e e s p e c i a l l y i n the case of buoyant i n c l u s i o n s which would be trapped at the top of the c a v i t y . ; /..''.' L a t t i c e o r i e n t a t i o n s f o r tha. c r y s t a l s i n F i g u r e 81 are shown i n F i g u r e 82. A change In l a t t i c e o r i e n t a t i o n w i t h depth i s recognized. A broad h o r i z o n t a l maximum (a) becomes more di s p e r s e d w i t h depth .(b),'(c).-. A second sample shows a change i n l a t t i c e o r i e n t a t i o n from a broad h o r i z o n t a l g i r d l e (d) to a 45° g i r d l e (e) to a p a r t i a l v e r t i c a l ' g i r d l e ( f ) . ( i i ) Truncated Bubble Pat t e r n s I n t r o d u c t i o n The above d i s c u s s i o n described bodies which had not been subject, to great p o s t - s o l i d i f i c a t i o n changes. Nearby occurs an i c e body w i t h d i f f e r e n t mesoscopic f e a t u r e s . The curved, r a d i a t i n g bubble trains- occur at the lower edges of the body, but i n the top centre the p a t t e r n i s d i s t u r b e d . I n c l u s i o n C h a r a c t e r i s t i c s . At the base of the i c e , bubble t r a i n s are normal to the contact w i t h the peat and includ e some peat fragments, then curve upwards at 45° to the h o r i z o n t a l ( F i g . 83(a)). Bubbles: are approximately s p h a r i c a l or e l l i p s o i d a l , e l o n g a t e . v e r t i c a l l y w i t h i n the t r a i n s which cross i n d i v i d u a l c r y s t a l s , w h i l e dispersed bubbles are contained mainly w i t h i n c r y s t a l s . Above, t h i s p a t t e r n includes groups 10/100 m m - of bubbles 1-2 mm i n diameter. A surface of t r u n c a t i o n can be traced l a t e r a l l y and i s seen to cut across o r i g i n a l bubble t r a i n s of s e v e r a l o r i e n t a t i o n s . The surface contains peat  214 fragments, above which i s a l a t e r a l l y extensive l a y e r of s m a l l s p h e r i c a l bubbles, i n marked co n t r a s t to the u n d e r l y i n g i c e . Above, groups and contained elongated bubbles a r e . o r i e n t e d d i f f e r e n t l y from those i n the i c e beneath the t r u n c a t i o n zone, but orthogonal to t h a t zone, i n d i c a t i n g upward i c e growth. • C r y s t a l C h a r a c t e r i s t i c s We consider f i r s t l y , c r y s t a l s below the apparent " t r u n c a t i o n zone", then the e f f e c t of t h a t zone, f o l l o w e d by c r y s t a l s above t h a t zone. I n the lower i c e , c r y s t a l s increase i n s i z e upwards, reaching > 600 mm" a t the c u t - o f f . C r y s t a l shape i s anhedral w i t h s t r o n g l y s e r r a t e d boundaries. Dimensional o r i e n t a t i o n i s p a r a l l e l to the bubble t r a i n s , and s e r r a t i o n s are orthogonal to c r y s t a l long axes, as in. the p r e v i o u s l y d i s c u s s e d i c e bodies ( a ) . C r y s t a l c h a r a c t e r i s t i c s change at the t r u n c a t i o n zona ( F i g . 8 3 ( b ) ) . 2 2 C r y s t a l s i z e i s 6 mm , i n c r e a s i n g upwards to 45 mm . C r y s t a l shape.in t h i s c o m p etitive growth zone i s l e s s complex than i n tha u n d e r l y i n g i c e , many s t r a i g h t compromise boundaries occur, and curved boundaries have s i n g l e c u r v a t u r e , which i s t y p i c a l of competitive c r y s t a l growth. Upwards a dimensional o r i e n t a t i o n develops p a r a l l e l to bubble e l o n g a t i o n . Tha r e l a t i o n s h i p of bubbles to c r y s t a l c h a r a c t e r i s t i c s v a r i e s w i t h p o s i t i o n . In the lower i c e , bubbles occur i n groups w i t h i n c r y s t a l s r a t h e r than on boundaries, as was. found i n lake i c e by Swinzow (1966). A s m a l l amount of i c e growth occurred above the t r u n c a t i o n zone before'bubble . n u c l e a t i o n ; bubbles were at f i r s t e s s e n t i a l l y randomly d i s t r i b u t e d , as found i n c o m petitive growth zones elsewhare ( i c i n g mound i c e , t e n s i o n crack ic e ) then became p r e f e r r e d l y s i t e d i n l a y e r s . ' .' '. . • . . • ' L a t t i c e , o r i e n t a t i o n s are shown f o r the lower i c e and upper i c e i n Figure 84(a)-(d). F i g u r e 84(a) includes c r y s t a l s from the base of the lower i c e up to the t r u n c a t i o n zone; an upward increase i n c - a x i s p r e f e r r e d o r i e n t a t i o n occurs i n a s s o c i a t i o n w i t h an increase i n c r y s t a l s i z e . F i g ure 84(b)-(d) s i m i l a r l y shows the change i n c - a x i s d i s t r i b u t i o n upwards from the t r u n c a t i o n zone. I n t e r p r e t a t i o n From the bubble patterns,, c r y s t a l s i z e , shape, dimensional and l a t t i c e o r i e n t a t i o n s , i t i s evident that i n i t i a l l y growth of i c e occurred i n a hollow w i t h i n frozen.peat, as i n the p r e v i o u s l y discussed bodies. L a t e r some melt-down occurred, as i s seen from the t r u n c a t i o n of bubble t r a i n s and the h o r i z o n t a l l a y e r of organic matter and gas i n c l u s i o n s . A l s o a major change i n c r y s t a l c h a r a c t e r i s t i c s occurs where new upward c r y s t a l ' growth took place. The t r u n c a t i o n zone i s not a temporary s t a n d s t i l l i n growth of the body, as i t truncates bubble t r a i n s of s e v e r a l o r i e n t a t i o n s and may be traced l a t e r a l l y i n t o the adjacent organic matter. From f i e l d r e l a t i o n s h i p s i t appears that s e v e r a l melt-down and regrowth events occurred i n the area. A second such body occurs nearby, d i s p l a y i n g s i m i l a r features.. Tha ; e a r l y growth has been subject to melt-down; large c r y s t a l s terminate a b r u p t l y upwards at a l a t e r a l l y e x tensive bubble l a y e r c o n t a i n i n g vegeta- . t i o n a l d e b r i s . Copious c r y s t a l n u c l e a t i o n occurred at t h i s l a y e r , f o l l o w e d by upward growth. Thus the previous more d e t a i l e d d e s c r i p t i o n i s , n o t of a rare occurrence, f i e l d c h a r a c t e r i s t i c s suggest the growth h i s t o r y a p p l i e s to many bodies along the coast. However the presence of such i c e can not be r e a d i l y i n f e r r e d from surface expression. igure 84. P e t r o f a b r i c s , c r y s t a l s below and above t r u n c a t i o n zone. (a) c r y s t a l s below t r u n c a t i o n zone; symbols i n sequence i n d i c a t e . • distance from s o i l - i c e c ontact; (b) ,(c),(d) c r y s t a l zones p r o g r e s s i v e l y upward from t r u n c a t i o n zone. Diagrams p a r a l l e l to sections-, t = t r u n c a t i o n zone 217 ( H I ) Ice at tha Base of the Active Layer - Introduction . . The ice bodies discussed i n sections (a) and (b) were..overlain by several.metres of organic s o i l , but there also occur ice. layers immediately at the base of tha a c t i v e layer. These are of p a r t i c u l a r i n t e r e s t as s l i g h t v a r i a t i o n s i n a c t i v e layer thickness occur from year to year and thus the upper part of such i c e would be expected to r e f l e c t these v a r i a - t i o n s . F i e l d C h a r a c t e r i s t i c s The i c e body to be discussed l i e s near those i n (a) and (b), above. I t was exposed i n June 1973 and sampled before a c t i v e layer thaw reached the top of the i c e . Measurements i n August 1973 showed that l o c a l a c t i v e layer depth was greater than overburden thickness. The ice was lensoid i n shape, 0 to 0 . 5 m i n thickness, with a f l a t lower surface and convex upper surface, o v e r l a i n by 0 . 3 m of organic s o i l ( F i g . 8 5 ) . Similar material underlay the. i c e , but with a higher i c e con- tent than the overlying s o i l . Ice C h a r a c t e r i s t i c s The upper contact was less abrupt than the lower, and vege t a t i o n a l fragments, including roots,, were observed near the top, as w e l l as c y l i n - d r i c a l bubbles ( F i g . 8 6 ) . Bubbles at tha top of the body are c y l i n d r i c a l and' trend normal to the upper surface. These extend 8 mm i n t o the sample, below which is a 5 mm thick bubble-free band, abova a surface containing peat and roots. These roots l i e on the surface, b,ut below are'orthogonal 218 F i g u r e 8 5 . L e n s o i d i c e body e x p o s e d by b l o c k c o l l a p s e . F i g u r e 8 5 . S c h e m a t i c d i a g r a m o f O v e r b u r d e n t h i c k n e s s 0 . 3 m. t op o f F i g . 8 5 . R o o t s h a p e s , i n f i l l e d b u b b l e s and b u b b l e s . F i g u r e 8 7 . D e t a i l o f b u b b l e s h a p e s i n i c e o f F i g . 8 5 . L o n g s i d e i s 8 mm. F- igure 8 8 . B u b b l e p a t t e r n t h r o u g h o u t l e n s o i d body (Fig. 8 5 ) . No te zones o f e l o n g a t e and s p h e r i c a l b u b b l e s a t b a s e , above p e a t . 219 to tha s u r f a c e . These features are i n t e r p r e t e d as i n d i c a t i n g melt-down of the i c e body to the peaty s u r f a c e , f o l l o w e d by upward r e f r e e z i n g . The roots were o r i g i n a l l y orthogonal to the ground s u r f a c e , and p a r a l l e l t o the f r e e z i n g d i r e c t i o n ; thus o r i e n t a t i o n i s maintained below the melt-down - surface. Bubble C h a r a c t e r i s t i c s Below the melt surface the only i n c l u s i o n s are bubbles, i n groups c o n t a i n i n g 2-3 mm long i n d i v i d u a l s , 1 mm i n diameter, and some d i s p e r s e d s p h e r i c a l bubbles, ^ 1 mm diameter. Elongate bubble o r i e n t a t i o n v a r i e s from orthogonal to the ground surface at the top, towards v e r t i c a l at 100 mm depth. ,The d e t a i l e d shapes of bubbles are complex, elongate bubbles have bulbous ends and l o c a l promontories ( F i g . 87). Narrow or wide p o i n t s on i n d i v i d u a l bubbles do n o t . c o r r e l a t e . w i t h one another i n d i c a t i n g v a r y i n g f r e e z i n g c o n d i t i o n s or p o s t - s o l i d i f i c a t i o n changes. Tha former presence of bubbles- at the peaty surface i s seen by i n f i l l e d c y l i n d r i c a l pockets of peaty m a t e r i a l ( F i g . 86). F u r t h e r down.the i c e body, bubbles are confined.to a c u r v i n g zona ( F i g . 88). A l s o at the contact of the i c e and u n d e r l y i n g o r g a n i c - r i c h s o i l i s a 10-20 mm h o r i z o n t a l band of v e r t i c a l l y o r i e n t e d ( 4 5 mm) bubbles, w i t h bulbous ends above which i s a zone of s p h e r i c a l bubbles ( 4 1 . 5 en),- . ( F i g . 88). C r y s t a l C h a r a c t e r i s t i c s C r y s t a l s d i s p l a y a range of s i z e s , shapes and dimensional o r i e n t a t i o n s throughout the body. In the t h i n , s e c t i o n p a r a l l e l to the upper s u r f a c e , c r y s t a l s i z e i s l o c a l l y d i f f i c u l t to estimate due to the h i g h l y developed 220 sub-boundaries. The o r i g i n a l growth boundaries are taken to be s e r r a t e d compared w i t h sub-boundaries; a l s o the l a t t e r are the s i t e s of embayment at t h e i r contacts w i t h boundaries. On t h i s b a s i s , g r a i n s i z e i s 20 to 30 mm long by 5 to 10 mm wide. ' W i t h i n these are elongate subgrains, u s u a l l y p a r a l l e l to the c r y s t a l long axes, but l o c a l l y sub-boundaries converge. C r y s t a l shapes are h i g h l y s e r r a t e d , w i t h s e r r a t i o n amplitudes of 1-2 mm; some are superimposed on s t r a i g h t s i d e s . • The bubbles show no p r e f e r r e d p o s i t i o n s on boundaries, nor are sub-boundaries g e n e r a l l y r e l a t e d to bubbles. Where bubbles are i n boundaries there are d i s t i n c t changes of • curvature of boundaries and f l a t t e n i n g of bubbles; away from such boundaries bubbles are approximately s p h e r i c a l . Bubble s i z e ranges from 0.1 mm to 0.5 mm. The sub-boundaries are w e l l developed; not a l l a r e • s t r a i g h t , but. curve to maintain approximately 120° i n t e r s e c t i o n s w i t h boundaries. C r y s - t a l c h a r a c t e r i s t i c s i n the v e r t i c a l s e c t i o n s vary w i t h depth. I n the c e n t r a l i c e , c r y s t a l s i z e i s very l a r g e , ^.90 mm x 4^30 mm, elongate v e r t i c a l l y , w i t h s m a l l c r y s t a l s -\5 mm across at g r a i n boundaries of l a r g e c r y s t a l s , thus i n v e r t i c a l t r a i n s ; a. f u r t h e r t r a i n crosses from the l e f t hand s i d e of F i g u r e 89a, dipping at 40°.. Lower t h i s p a t t e r n changes, l a r g e (80 mm) c r y s t a l s have long axes at 45° to those above, again w i t h s m a l l ( <.5 mm) c r y s t a l s i n t h e i r boundaries ( F i g . 8 9 ( b ) ) . These boundaries are h i g h l y indented, and i n t e r n a l s t r a i n bands i n t e r s e c t the boundaries a t i n d e n t a t i o n s . These bands are 2-3 mm wide and .'mostly continuous across the c r y s t a l . In a h o r i z o n t a l s e c t i o n the i n f l u e n c e of these, bands i s e v i d e n t ; g r a i n boundaries p a r a l l e l to the bands are approximately s t r a i g h t w h i l e boundaries trending normal are h i g h l y indented. The small c r y s t a l s tend to be equidimensional w i t h s t r a i g h t to g e n t l y curved boundaries and no su b s t r u c t u r e . Bubbles are not present i n the s m a l l c r y s t a l zone, elsewhere p o s i t i o n s are apparently random r e l a t i v e to g r a i n boundaries. F i g u r e 89. V e r t i c a l s e c t i o n s , l e n s o i d body ( F i g . 85). (a) Large., v e r t i c a l grains .with sub-boundaries, crossed by and separated by zones of s m all c r y s t a l s . . 10. mm g r i d . (b) Below F i g . 89(a). Note h o r i z o n t a l l y elongated c r y s t a l s i n b a s a l peaty zone. 10 mm g r i d . 1 — 1 Crossed p o l a r i z e r s : ..'•'.•' Fig u r e 90., P e t r o f a b r i c s of i c e i n F i g . 85. (a) H o r i z o n t a l s e c t i o n , top of sample, 60 c r y s t a l s . . (b) V e r t i c a l s e c t i o n s , 120 c r y s t a l s , i n c l u d i n g bands of s m all c r y s t a l s . . . . 1 . x 18 h o r i z o n t a l l y elongated c r y s t a l s at base. Diagrams p a r a l l e l to sections' 222 .At the base of the i c e body l i e s a zone of markedly d i f f e r e n t texture ( F i g . 89(b)). The c r y s t a l s are very elongated h o r i z o n t a l l y up to 80 mm, and boundaries are d i f f i c u l t to define due to peat content. . P e t r o f a b r i c diagrams are given i n F i g u r e 9 0 ( a ) , ( b ) . F i g u r e 90(a) represents the c r y s t a l s i n a . s e c t i o n p a r a l l e l to the upper s u r f a c e , which, give a p o i n t c o n c e n t r a t i o n . Figure 90(b) represents v e r t i c a l , s e c t i o n s i n which a l l c r y s t a l s are contained i n a point c o n c e n t r a t i o n , except f o r the lower zone of h o r i z o n t a l l y elongated c r y s t a l s which are contained In a minor h o r i z o n t a l g i r d l e . I n t e r p r e t a t i o n The f i e l d , bubble and c r y s t a l c h a r a c t e r i s t i c s of the body i n d i c a t e a. f a i r l y complex h i s t o r y . U n f o r t u n a t e l y the l a t e r a l f e a t u r e s of the. body are not known, thus the o r i g i n of the lower zone of h o r i z o n t a l l y elongate c r y s t a l s i s not c l e a r , although an abrupt change from the o v e r l y i n g i c e i s e vident, i n terms of shape and l a t t i c e o r i e n t a t i o n . The geomorphic p o s i t i o n of the body, namely i n an area of i c e wedge polygons sub j e c t to ~ thermokarst a c t i v i t y (see (a) and ( b ) , above) and c o a s t a l r e c e s s i o n means that thermokarst and thermal e r o s i o n (seaward water flow through wedge, troughs) processes have, operated. A l s o Mackay (1972d) has shown that Ice lensing . occurs i n ridges adjacent to wadges. Thus a complex thaw and freeze h i s t o r y may have taken p l a c e . The upper r o o t p a t t e r n i s evidence of. a recent malt-down and r e f r e e z i n g c y c l e , but t h i s does not e x p l a i n the major part of the body. There i s no s m a l l c r y s t a l zone t y p i c a l of c h i l l type growth, as discussed i n the cases of t e n s i o n crack, i c i n g mound and thermokarst depression i n f i l i c e s . Had upward growth occurred from above the zone of h o r i z o n t a l l y elongated c r y s t a l s , , a zone of competitive growth. 223 or growth, i n l a t t i c e c o n t i n u i t y would be expected, but n e i t h e r are found. Nor i s there an upper c h i l l zone. I t appears t h a t such a c h i l l zone occurred at the top and was removed by downmelting to the' i n c l u s i o n zone, then upward f r e e z i n g i n l a t t i c e c o n t i n u i t y occurred. However, i f the body, grew e s s e n t i a l l y by downward f r e e z i n g , f o r example water being drawn up p r o g r e s s i v e l y i n t o the polygon rim frota the adjacent deep wedge trough, the contact w i t h the lower zone of h o r i z o n t a l l y elongated c r y s t a l s must be explained. In the absence of knowledge of the l a t e r a l extent of the body, but knowing the p a t t e r n of wedges i t i s suggested' that the body tapered o f f l a t e r a l l y , and the growth d i r e c t i o n was o f f s e t from the v e r t i c a l at depth as i n d i c a t e d by the bubble o r i e n t a t i o n ( F i g . .88). T h i s c o n c l u s i o n must be considered s p e c u l a t i v e , but the complexity of the h i s t o r y i s recog- n i z e d . (b) P e l l y I s l a n d . " . - F i e l d C h a r a c t e r i s t i c s On the northwest coast of .Pelly I s l a n d is a low l y i n g a r e a of polygon f l a t s comprising l a c u s t r i n e . c l a y s w i t h a w e l l developed ice-wedge system. Many wedges are over 2 m across and some greater than 3 m. Polygons have diameters of. up to 10.m w i t h rims reaching 1 m h i g h and deep troughs. • These troughs have been subject to thermal e r o s i o n . C o a s t a l exposures i n d i c a t e that s e v e r a l periods of melt-down and freeze-back have occurred, and s e v e r a l wedges have i r r e g u l a r upper s u r f a c e s . Thaw zones may be t r a c e d i n t o the adjacent sediments, i n d i c a t e d by surfaces of i r o n s t a i n i n g , and . d i f f e r i n g lens s t r u c t u r e s i n the c l a y s . W i t h i n.the r e f r o z e n peat and c l a y s over the wedge-margins are "pond i c e " bodies ( F i g . 91) c h a r a c t e r i z e d on 224 Figure 91. F i e l d p o s i t i o n of "pond" i c e over wedges, P e l l y I s l a n d . Figure 92. "Pond" i c e body. Wedge i c a below. Figure 93. I n c l u s i o n p a t t a r n , "pond" i c e , v e r t i c a l s e c t i o n normal to s i d a . F i g ure 94. C r y s t a l p a t t e r n v e r t i c a l s e c t i o n s normal to s i d e . 10 im g r i d . Crossed p o l a r i z e r s 225 m e l t i n g surfaces by etched out c r y s t a l boundaries and bubble t r a i n s . These patterns i n d i c a t e m u l t i p l e f r e e z i n g d i r e c t i o n s ; the i c e type i s thus r e a d i l y d i s t i n g u i s h a b l e i n the f i e l d from lens i c e or wedge i c e . I n some cases an i n d i v i d u a l "pond i c e " body has been subject apparently to l a t e r m e l t i n g and upward refreezing.. Both peat.and clay, appear as i n c l u s i o n s w i t h i n , the i c e . A sample of t h i s i c e was taken from the area shown i n Fi g u r e 92 and hand specimen c h a r a c t e r i s t i c s are given i n terms of sediment and bubble content. Ice C h a r a c t e r i s t i c s F i r s t l y the features normal to a si d e of the pond.are g i v e n , then those i n a v e r t i c a l plane p a r a l l e l to the side.. ( i ) Features Normal to the Side \ . . The nature of the f r e e z i n g process i s best understood from study of a v e r t i c a l sample orthogonal to the s i d e of,the body. No f r a c t u r e s were observed i n hand-specimen; i n c l u s i o n s are discussed i n terms of peat, sediment, and bubbles. Peat and sediment content i s confined to a s m a l l d u s t i n g of p a r t i c l e s i n the top 20 to 30 mm, the zone of s p h e r i c a l bubbles. Bubble C h a r a c t e r i s t i c s . The bubble pattern, comprises seven d i s t i n c t zones: (a) at the top of the sample i s a i0-20 mm deep zone of s m a l l •(''••4 1 mm diameter) s p h e r i c a l to e l l i p s o i d a l bubbles i n a g e n e r a l l y random p a t t e r n w i t h some l o c a l l y higher c o n c e n t r a t i o n s ; (b) a narrow e s s e n t i a l l y bubble-free zona; (c) a 40 ram deep band crosses' the sample s u b - h o r i z o n t a l l y , c o n t a i n i n g v e r t i c a l l y elongated bubbles 10 mm long and ^-1 mm i n diameter. Some have v a r i a b l e t h i c k n e s s , i n c l u d i n g bulbous ends. These elongate, bubbles may . . occur i n l o c a l groups or i n t e r s p e r s e d w i t h s m a l l ' 1 mm) s p h e r i c a l bubbles; (d) the next lower band comprises t r a i n s of elongate and s p h e r i c a l bubbles c u r v i n g down from band (c) and away from the.side o f the pond, and i n t o a lower narrow band of small s p h e r i c a l bubbles ( F i g . 93);. (e) below occurs a s e r i e s of t r a i n s t r e n d i n g upwards, i n the m i r r o r . image of.band ( d ) . The t r a i n s are curved but contained,elongate bubbles. (5 mm) are more ne a r l y v e r t i c a l l y oriented,'and i n t e r s p e r s e d w i t h s m a l l (1 mm) s p h e r i c a l and e l l i p s o i d a l bubbles. T r a i n s are 30-50 mm long, separated by 20-50 mm of c l e a r i c e , and become narrower upwards; ( f ) beneath the t r a i n s i s a t h i n band of bubble-poor i c e , then a h o r i z o n t a l ; discontinuous band of s l i g h t l y elongated (3-5 mm) v e r t i c a l • bubbles, i n t e r s p e r s e d w i t h some s p h e r i c a l and i r r e g u l a r l y shaped -bubbles, i n d i c a t i v e of melt; (g) below i s a zone of low bubble content, c o n t a i n i n g patches of s p h e r i c a l and i r r e g u l a r bubbles i n a tren d s i m i l a r to th a t . i n . z o n e (d) but formed during a separate f r e e z i n g p e r i o d . Sediment occurs as pods and s t r e a k s above zone ( f ) , and p a r a l l e l to t r a i n s i n zone (e) and become . narrower upwards. ' C r y s t a l C h a r a c t e r i s t i c s C r y s t a l s i z e i s discussed w i t h reference to bubble zones ( F i g . 94). 227 ( a ) , (b) and (c) contain c r y s t a l s averaging 3 mm x 2 mm, and ranging up to•12 mm x 10 mm; , . (d) at. the base of the zone of elongated bubbles begins a zone of narrow elongated c r y s t a l s > 30 mm x 6 mm, which curve round i n t o h o r i z o n - t a l i t y i n zone ( e ) ; (e) c r y s t a l s are s l i g h t l y l a r g e r than i n zone (d) and tha dimensional o r i e n t a t i o n i s a m i r r o r image to that of zone (d) thus corresponding t o the bubble p a t t e r n . ( f ) , (g) and (h) c o n t a i n l a r g e r c r y s t a l s , up to 50 mm l o n g , < 10 mm wide i n a p a t t e r n shown i n Figure 94 . C r y s t a l shape v a r i a t i o n s c o r r e l a t e w i t h bubble zones. Zones ( a ) , (b) and (c) c o n t a i n anhedral e q u i g r a n u l a r c r y s t a l s w i t h no s t r o n g i n t e r - growths or s e r r a t i o n s . In zone ( d ) , shape changes to anhedral, s a r r a t e d , elongated c r y s t a l s w i t h a curved dimensional o r i e n t a t i o n . Wedging out of c r y s t a l s has occurred during downward growth. Zones (e) , ( f ) and (g) c o n t a i n anhedral, s e r r a t e d elongate c r y s t a l s w i t h a r a d i a l v a r i a t i o n i n dimensional o r i e n t a t i o n , shown i n F i g u r e 9 4 ( b ) , ( c ) . Substructure i s confined to l a r g e r c r y s t a l s . In zones ( a ) , (b) and (c) some la r g e c r y s t a l s are embayed by s m a l l e r c r y s t a l s , the boundary segments at embayments. are s t r a i g h t . In the lower zones elongate c r y s t a l s c o n t a i n complex sub-boundaries, i n patterns p a r a l l e l and normal to long axes. The broad r e l a t i o n s h i p between bubble p a t t e r n and t e x t u r e i s evident from the above d i s c u s s i o n and Figures 93, 94. 223 P e t r o f a b r i c diagrams for t h i s s e r i e s of samples are shown i n F i g u r e 9 5 . C r y s t a l s i n the upper zones ( F i g . 9 b ( a ) ) have c-axes i n a zone p a r a l - l e l to the top of the body An i n t e r e s t i n g d i s t r i b u t i o n of c-axes e x i s t s i n the lower zones, shown n Figure 9 5 ( b ) . The i n i t i a l f r e e z i n g p a t t e r n was from a l l sides towards a c e n t r a l p o i n t , thus as the f r e e z i n g i n t e r f a c e changed, so the c r y s t a l dimensional o r i e n t a t i o n s , changed. But the c r y s t a l s were large and growth continued i n l a t t i c e c o n t i n u i t y r a t h e r than r e q u i r i n g f u r t h e r n u c l e a t i o n . A f t e r a period of melt-down, r e f r e e z i n g occurred, again from a l l s i d e s . Upward growth occurred on the a l r e a d y e x i s t i n g l a t t i c e s i t e s , thus the l a t t i c e o r i e n t a t i o n s are maintained. Downward growth occur- red as an extension of the c h i l l zone, i n the form of a columnar zona. Where the two zones approached, dimensional o r i e n t a t i o n changed to remain . orthogonal to the f r e e z i n g i n t e r f a c e , but l a t t i c e c o n t i n u i t y was maintained, even i n h o r i z o n t a l c r y s t a l s . . Thus the g i r d l e of c-axes i s e x p l a i n e d . Sub- s t r u c t u r e s observed i n the l a r g e r c r y s t a l s are due to i n t e r n a l s t r a i n . The enclosed f r e e z i n g discussed above,, and l a t e r temperature f l u c t u a t i o n s which would lead to expansion and c o n t r a c t i o n , gave r i s e to the small-angle bound-, a r i e s . ( i i ) Sections P a r a l l e l to Side of Ice Body Sediment bands are not continuous throughout the body, but. taper i n - wards from the contact w i t h the surrounding c l a y , of which thay are composed. S l i g h t curvatures of the bands r e s u l t from v a r i a t i o n s i n the shape of tha f r e e z i n g i n t e r f a c e . W i t h i n the bands, sediment occurs as s m a l l ( < 5 mm) "pods" and as p a r a l l e l s t r e a k s , probably on c r y s t a l basal planes. Charac- t e r i s t i c a l l y a band has ona d i f f u s a and one abrupt boundary, tha former being the f i r s t to f r e e z e , i n d i c a t i n g l a t e r a l l y uniform f r e e z i n g c o n d i t i o n s . 229 Figure 95. P e t r o f a b r i c s of "pond" i c e . (a) upper zones, v e r t i c a l s e c t i o n normal to side; (b) lower zones, v e r t i c a l s e c t i o n normal to. side; (c) upper zones, v e r t i c a l s e c t i o n p a r a l l e l to side; (d) ,(e) lower zones, v e r t i c a l s e c t i o n p a r a l l e l to side (converging c r y s t a l s ) . Diagrams p a r a l l e l to sections c. = compositional l a y e r i n g 230 Bubble C h a r a c t e r i s t i c s Bubble content i s v a r i a b l e . ( F i g . 96): (a) A high c o n c e n t r a t i o n of bubbles occurs i n the i c e above the top sediment band, comprising two.types: (1) s p h e r i c a l , ^ 1 mm diameter, randomly p o s i t i o n e d w i t h respect to elongate bubbles and sediment, (2) elongate < 1 mm diameter, up to 13 mm long, w i t h bulbous ends, o r i e n t e d orthogonal to the sediment banding, and arranged i n groups; (b) i n the top sediment band i s a much lower bubble c o n c e n t r a t i o n than i n zone ( a ) , s p h e r i c a l and elongate bubbles occur where the sediment content i s lower; • (c) below the sediment band i s a bubble-poor zone grading i n t o more bubbly i c e comprising (1) elongate bubbles which occur s i n g l y or w i t h i n patches of s p h e r i c a l and mainly c l o s e to the sediment band, (2) s p h e r i c a l bubbles < 1 mm diameter i n a 20-30 mm t h i c k zone immediately above the sediment, (3) curved t r a i n s , 70 mm long and 10 mm i n diameter which c o n t a i n s p h e r i c a l and e l l i p s o i d a l bubbles, trend upward from the second sediment band, (4) bubble groups occur midway between sediment bands and not asso- c i a t e d w i t h a t r a i n ; (d) . i n the upper p a r t of second sediment band occur both s p h e r i c a l and elongate bubbles, but none were observed i n the. lower p a r t ; . (e) between the second and t h i r d sediment bands are few.bubbles, there being o c c a s i o n a l s p h e r i c a l and elongate bubbles above the sediment; ( f ) the lower sediment band i s bubble-free. 23L F i g u r e 96. Bubble and c l a y i n c l u s i o n zones, v e r t i c a l face p a r a l l e l to side of "pond:' i c e . Figure 97. V e r t i c a l s e c t i o n , top of F i g . 96. T e x t u r a l change at c l a y i n c l u s i o n zone. 10 mm g r i d 1 1 Crossed p o l a r i z e r s 2 3 2 C r y s t a l Characc.erisic.es Textures are discussed i n r e l a t i o n to sediment and bubble bands. W i t h i n the upper bubble and sediment band are c r y s t a l s averaging 5 mm x 4 mm, and ranging from 2 mm x 1 mm to 15 mm x 8 mm. . Below t h i s sediment band are l a r g e r v e r t i c a l l y elongate c r y s t a l s averaging > 3 0 mm x 1 0 mm, and ranging from 6 mm x 4 mm to > 4 0 mm x 2 0 mm ( F i g . 9 7 ) . Lower there occur much l a r g e r c r y s t a l s , up to 48.mm x 3 0 mm but o c c a s i o n a l l y around 7 mm x 2 mm. There i s a f u r t h e r change i n g r a i n s i z e to more eq u i g r a n u l a r below the lower sediment band. G r a i n shape v a r i e s w i t h . g r a i n s i z e . I n the small c r y s t a l zone c r y s t a l s are anhedral, boundaries are s t r a i g h t or have simple curvature. There i s a s l i g h t dimensional p r e f e r r e d o r i e n t a t i o n p a r a l l e l to bubble e l o n g a t i o n . The elongate c r y s t a l s are anhedral w i t h w e l l developed s e r r a t i o n s and embayments; again the dimensional o r i e n t a t i o n i s orthogonal to the sediment band. I n the lower part of the sample elonga- t i o n i s more complex. Substructure i s only p o o r l y developed i n the upper s m a l l c r y s t a l zone but i s strong i n s e v e r a l of the elongate c r y s t a l s , f r e q u e n t l y p a r a l l e l to the dimensional o r i e n t a t i o n ; a second r e c t a n g u l a r s u b s t r u c t u r e occurs lower down. There i s no strong r e l a t i o n s h i p between bubbles and t e x t u r e , other than p a r a l l e l , dimensional o r i e n t a t i o n . In the case of elongate bubbles, some pass, across' c r y s t a l boundaries, others terminate at boundaries, w h i l e s p h e r i c a l bubbles are s c a t t e r e d randomly w i t h respect to boundaries. An exception to the above p a t t e r n occurs where a bubble t r a i n w i t h i n otherwise c l e a r i c e i s contained w i t h i n one l a r g e elongate c r y s t a l . 'The r e l a t i o n - ship between sediment and texture i s two-fold: ,. 233 (a) "pods" of sediment are not contained w i t h i n c r y s t a l s ; . (b) " s t r e a k s " of sediment tend to be v e r t i c a l or l o c a l l y orthogonal to the trend of the band, but not r e l a t e d to c r y s t a l s t r u c t u r e or g r a i n boundaries, as i n d i v i d u a l l i n e s cross more than one c r y s t a l . . . I t i s not c l e a r whether the sediment p a t t e r n i s a f u n c t i o n of a complex i n t e r f a c e shape during f r e e z i n g , or l a t e r thermomigration. C-axis o r i e n t a t i o n s f o r the sample are given i n F i g u r e 95. Small c r y s t a l s i n the zones ( a ) , (b) and (c) give an approximately v e r t i c a l maximum which may occur during I n i t i a l downward f r e e z i n g ( M i c h e l and Ramseier 1971) and a minor v e r t i c a l g i r d l e ( F i g . 9 5 ( c ) ) . The c r y s t a l s i n the g i r d l e do not d i f f e r i n other c h a r a c t e r i s t i c s from the surrounding c r y s t a l s , although some deviate from the v e r t i c a l dimensional o r i e n t a t i o n . C r y s t a l s i n the zone of elongate c r y s t a l s are shown i n Fi g u r e 97 and t h e i r c-axes p l o t t e d i n F i g u r e 95(d). The maximum deviates from t h a t i n F i g u r e 9 5 ( c ) . Below, the p a t t e r n i s more complex ( F i g . 95(e)) due t o the change i n f r e e z i n g d i r e c t i o n and dimensional o r i e n t a t i o n , but from comparison w i t h diagrams f o r the orthogonal s e r i e s of s e c t i o n s i t i s seen t h a t c-axes are orthogonal to the l o c a l f r e e z i n g d i r e c t i o n , and thus the p a t t e r n i s e f f e c - t i v e l y an extension of the c-axis h o r i z o n t a l p a t t e r n r o t a t e d f o r the l o c a l f r e e z i n g i n t e r f a c e . I n t e r p r e t a t i o n '. . From the f i e l d r e l a t i o n s and i n c l u s i o n , p e t r o f a b r i c and c r y s t a l c h a r a c t e r i s t i c s the h i s t o r y of the. i c e body can be described. A f t e r , c o n s i d e r a b l e development of the wedge system a greater depth of thaw, i n d i c a t e d by the surface of i r o n s t a i n i n g t r a c e a b l e l a t e r a l l y . f r o m wedges 234 into the adjacent sediment, occurred and d i f f e r i n g lens structure i n the c l a y s . A d d i t i o n a l l y thermokarst and thermal erosion processes have opera- ted over the wedges. Refreezing occurred from above and below, and above the wedge pools of water froze to give the pattern shown i n zones (g)-(h) ( F i g . 96). A subsequent melt-down removed the upper part of t h i s i c e and caused changes i n bubble shape and the i n t r o d u c t i o n of sediment (above zone ( f ) ) . Refreezing again occurred simultaneously from, above and below g i v i n g r i s e to the equigranular c h i l l zone c r y s t a l s i n zone (a) to (c) and the curving, converging bubble t r a i n s and curved, elongate c r y s t a l s of zones (d) and (e), those i n (e) being extensions of c r y s t a l s i n zone(f). The zone of competitive growth above zone (f) from the previous f r e e z i n g was removed by melt-down. Bubbles wi t h i n the t r a i n s i n zones (d) and (e) are now elongate v e r t i c a l l y having been subject to thermomigration i n a v e r t i c a l temperature gradient. C r y s t a l c-axis orientations change from v e r t i c a l at the top to more nearly h o r i z o n t a l then change pattern due to the freezing d i r e c t i o n change. Relationship of Ice Type to Surface Form A e r i a l photographs of the area show the well-developed pattern of i c e wedge polygons with associated ridges. F i e l d examination of the area di s c l o s e d the presence of very deep troughs over some wedges which obviously did not freeze each winter. Also i t i s evident that some thermal erosion i s occurring l a t e r a l l y at the top of some troughs, and considerable over- hangs of organic s o i l are developing . ( F i g . 91). However, these l a t t e r features are not always obvious from a i r photographs. Nor i s the presence of the ice bodies discussed above immediately evident from a i r photographs 235 or surface f i e l d examination; th e i r c h a r a c t e r i s t i c s and d i s t r i b u t i o n were well displayed only on coastal sections. Thus, i n summary, the i c e type had no s p e c i f i c surface expression, but i s an e x c e l l e n t i n d i c a t o r of the complex thermal h i s t o r y of the area. Similar processes of melt-down adjacent to wedges are now occurring nearby. A f u r t h e r , more minor, point to be extracted from the discussion i s tha importance of preparing t h i n sections of several o r i e n t a t i o n s . Aggradational Ice Introduction Aggradational i c e i s ice which grew at the base of an a c t i v e l a y e r and became incorporatad into permafrost as the permafrost t a b l e rose. This r i s e i n tha permafrost table may ba dua to thinning of tha a c t i v e layer i n a c l i m a t i c change or sedimentation on the ground surface. A subsequent change i n surface conditions could destroy such i c e . As surface conditions vary considerably i n space and time i t i s to be a n t i c i p a t e d that aggrada- t i o n a l i c e c h a r a c t e r i s t i c s w i l l be v a r i a b l e l a t e r a l l y . We discuss aggradational ice from two s i t e s : (1) an involuted h i l l near Tuktoyaktuk, (2) a construction s i t e at Tuktoyaktuk. (a) Involuted H i l l S i t e Introduction In a previous s e c t i o n (Involuted h i l l i c a - f o l d e d i c e penetrated by wedga) wa di s c u s s e d a s i t e where paat had accumulated and an i c a wedga had 236 grown i n a m u l t i p l e f a s h i o n (Mackay 19 74a, p. 1379, F i g . .18). This i s evidence of a r i s e i n the permafrost t a b l e , and we now discuss i c e i n the adjacent s o i l . F i e l d C h a r a c t e r i s t i c s i The i n v o l u t e d h i l l on the coast near Tuktoyaktuk i s t y p i c a l of a l l such h i l l s i n the area, i n terms o f • s u r f i c i a l form, and c o a s t a l r e c e s s i o n has. exposed some of i t s i n t e r n a l f e a t u r e s . I t i s evident t h a t s e v e r a l metres of peat has accumulated i n i n t e r - r i d g e depressions ( F i g . 93), and that some minor thawing a c t i v i t y has occurred adjacent to some of the l a r g e r i c e wedges. Here we discuss i c e which i s , by d e f i n i t i o n , aggrada- t i o n a l , and which has not bean di s t u r b e d by l a r g e s c a l e tharmokarst a c t i v - i t y . Most of the i c e i s i n a dis p e r s e d , p a r t i c u l a t e form and unsuited f o r t h i n s e c t i o n p r e p a r a t i o n , but at what was probably a depression i n a one- time permafrost t a b l e , there occurs a l e n s o i d i c e body which i s s u i t a b l e f o r a n a l y s i s . . Ice C h a r a c t e r i s t i c s The lower surface of the body i s un d u l a t i n g and thare i s no abrupt : boundary w i t h the s o i l , r a t h e r there i s a g r a d a t i o n from peat through i c y peat to^ peaty i c e to i c e . W i t h i n the i c e body proper a r e b u b b l e • t r a i n s of v a r y i n g o r i e n t a t i o n ; these are truncated and-naw t r a i n s occur above ( F i g . 9 9 ( a ) ) . The upper i c e - s o i l contact i s again not abrupt.. Bubble C h a r a c t e r i s t i c s The bubble p a t t e r n comprises tha f o l l o w i n g zones: 237 AI M. 1. / M I Figure 93. Schematic diagram, peat accumulation and wedge ice growth, involuted h i l l . A.L. - a c t i v e layer. 1,2,3 indic a t e old ac t i v e layers. P = peat, M..I. = massive i c e . Figure 99a. Schematic diagram, peat and bubble pattern, aggradational i c e . ''OX aHa. o ••/»«• .. . ^̂ -/j A- <>«••• 238 ( i ) Lower zone: This zone comprises v e r t i c a l l y elongate and some s p h e r i c a l bubbles which are <1 mm diameter, and lengths are </ 4 mm. There i s no o v e r a l l r e g u l a r p a t t e r n i n the form of l a y e r s of a given bubble s i z e or shape, r a t h e r there are groups of l o c a l l y higher concen- t r a t i o n , surrounded by s p h e r i c a l bubbles. This zone has a h i g h peat c o n c e n t r a t i o n i n p a r t i c u l a t e form which may have c o n t r o l l e d bubble growth,, but i f any p a t t e r n of bubbles e x i s t e d immediately a f t e r s o l i d i f i c a t i o n , I t has s i n c e been modified. ( i i ) C e n t r a l zone: The p a t t e r n i n zone ( i ) i s a b r u p t l y t r u n c a t e d i n a curved s u r f a c e , which i s not a growth f e a t u r e . W i t h i n zone ( i i ) i s a s e r i e s of discontinuous 1-2 mm l a y e r s c o n t a i n i n g bubbles t r e n d i n g away from the contact between zones ( i ) and ( i i ) . Above, the i c e becomes c l e a r e r and i s truncated approximately h o r i z o n t a l l y . ( i i i ) Upper zone: Above the t r u n c a t i o n of zone ( i i ) i s a p a t t e r n of elongate bubbles t r e n d i n g orthogonal to the t r u n c a t i o n zone and upper s u r f a c e , which i s t y p i c a l of o m n i d i r e c t i o n a l f r e e z i n g as discussed e l s e - where. The contacts are abrupt, and bubble shape i s t h i n , <C 1 mm diameter and elongate, 3 mm. C r y s t a l C h a r a c t e r i s t i c s The c r y s t a l s i z e and shape c h a r a c t e r i s t i c s a r e c l o s e l y a s s o c i a t e d w i t h bubble zone f e a t u r e s , and are discussed i n terms of those zones ( F i g . 99(b)). ( i ) Lo-wer zone: This zone has a high i n c l u s i o n , content i n the form of . . peat and bubbles, and c r y s t a l s i z e i s g e n e r a l l y 1-2 mm f o r equant c r y s t a l s , w i t h some elongated c r y s t a l s up to 5 mm. G r a i n boundaries are. u s u a l l y 239 s t r a i g h t from one i n c l u s i o n t o : t h e next, but some l a r g e r c r y s t a l s are very i r r e g u l a r . ( i i ) C e n t r a l zone. At the lower edge a mass of small c r y s t a l s grows up from the t r u n c a t i o n of zone ( i ) . The discontinuous bubble l a y e r s have a marked e f f e c t on c r y s t a l form - c r y s t a l s . t e r m i n a t e at these zones and new- growth occurs on the other s i d e . Where no such bubble l a y e r s occur elonga- ted c r y s t a l s reach 30 mm long and 3 mm wide. . ( i i i ) Upper zone. C r y s t a l shape corresponds to the bubble p a t t e r n , c r y s t a l s have grown orthogonal to both the upper surface of zone, ( i i ) and the top of the i c e body. In some cases the lower c r y s t a l s are i n l a t t i c e c o n t i n u i t y w i t h those i n zone ( i i ) but i n general new growth has occurred. Competitive growth zones on a l l s i d es of the body give r i s e to elongate c r y s t a l s up to 20 mm long which trend toward the centre of the body. C r y s t a l s i n the upper p a r t are longer. - L a t t i c e o r i e n t a t i o n s are shown i n Figure 100; s l i g h t l y d i f f e r i n g c - a x i s patterns correspond to the s e v e r a l bubble and c r y s t a l zones. I n the lower zone the p a t t e r n lacks p r e f e r r e d o r i e n t a t i o n ( F i g . 100(a)) from which develops a s l i g h t c o n c e n t r a t i o n i n zone ( i i ) ( F i g . 100(b)). A t r a n s i t i o n occurs i n the upper zone, where a random.. o r i e n t a t i o n f o r c r y s t a l s around the edges . ( F i g . 100(c)) becomes a weak g i r d l e orthogonal to the dimensional o r i e n t a t i o n ( F i g . 100(d)) i n the elongate c r y s t a l s . The samples are too small f o r r i g - orous d i s c u s s i o n , but F i g . 100(d) .corresponds to elongate c r y s t a l s . I n t e r p r e t a t i o n The growth sequence of l a y e r s i s zone ( i ) , zone ( i i ) , zone ( i i i ) . Zone ( i ) represents a l o c a l l y higher water content i n the peat which.gave r i s e to the t r a n s i t i o n from peat w i t h a high i n t e r s t i t i a l i c e content to 240 Figure 100. Petrofabrics for aggradational i c e . (a) lower zone (b) second zone (c) edge c r y s t a l s , upper zone (d) elongate c r y s t a l s , upper zona. 241 peaty i c e . The peat and bubbles (which, i n a d d i t i o n to d i s s o l v e d a i r , may i n c o r p o r a t e gases from r e a c t i o n s i n the peat) may have encouraged copious n u c l e a t i o n , and have a strong c o n t r o l on c r y s t a l s i z e and shape. T h i s i c e body was l a t e r subject to me It-down from above as i s evident from the i r r e g u l a r upper surface of zone ( i ) and the abrupt t r u n c a t i o n of c r y s t a l f e a t u r e s . Zone ( i i ) represents a new growth p e r i o d as seen by the t r a n s i - t i o n i n c r y s t a l and bubble dimensional o r i e n t a t i o n - growth i s everywhere orthogonal to the i r r e g u l a r . u p p e r surface of zone ( i ) . This zone i s i n t u r n truncated but the lower part of the zone became incorp o r a t e d i n t o permafrost. A f u r t h e r type of growth occurred i n zone ( i i i ) . Here.the bubble p a t t e r n and c r y s t a l dimensional and l a t t i c e o r i e n t a t i o n s i n d i c a t e simultaneous growth v e r t i c a l l y from above and below, and l a t e r a l l y . T h i s l a t t e r zone i s at the base of the present a c t i v e l a y e r and may be q u i t e recent. I t i s thus i n a p o s i t i o n where disturbance i s l i k e l y w i t h a s l i g h t change i n surface c o n d i t i o n s . In the case of such an event zone ( i i i ) might be melted down and d i s p l a y the features t y p i c a l of zones ( i ) and ( i i ) . (b) Tuktoyaktuk S i t e I n t r o d u c t i o n The P o l a r C o n t i n e n t a l S h e l f P r o j e c t has a base i n Tuktoyaktuk, which was b u i l t , i n 1956. This e n t a i l e d the l a y i n g of a g r a v e l pad on the e x i s t i n g a c t i v e l a y e r , on which b u i l d i n g s ware s i t e d . G r a d u a l l y permafrost aggraded through the o l d a c t i v e l a y e r u n t i l e q u i l i b r i u m was r e - e s t a b l i s h e d . In June 1973, the M e t a l l u r g y D i v i s i o n of the Department of Energy, Mines and Resources dug small e x c a v a t i o n pits, i n t o the g r a v e l pad, f o r the purpose of e r e c t i n g a t e s t compound.. During these excavations i c e was 242 encountered i n s e v e r a l forms:- p a r t i c u l a t e , and i n s m a l l bodies; samples of the l a t t e r i c e type- were s u p p l i e d to the author. F i e l d C h a r a c t e r i s t i c s The m a t e r i a l of the-"gravel pad" comprised a mixture of g r a v e l , sand . and organic matter (bulldozed from the.nearby.tundra). Ice samples were removed from beneath 0.9 m of overburden, at which depth the i c e formed discontinuous l a y e r s , *C 75 mm t h i c k and <3Q0 mm diameter. Ice C h a r a c t e r i s t i c s No s t r u c t u r e s were evident i n the i c e (some small f r a c t u r e s were probably a r e s u l t of the d i g g i n g ) . I n c l u s i o n s occurred i n s o l i d and gaseous form; bubbles d i s p l a y e d a s l i g h t tendency t o banding l o c a l l y , p a r a l l e l to the ground surface and orthogonal to bubble dimensional o r i e n t a t i o n , w h i l e sediment appeared to be randomly p o s i t i o n e d . . Bubbles were g e n e r a l l y c y l i n - d r i c a l i n shape, ^ 1 mm i n diameter, <cl5 mm long and v e r t i c a l l y o r i e n t e d . Sand g r a i n s and fragments of organic m a t e r i a l were <0.1 mm. C r y s t a l C h a r a c t e r i s t i c s C r y s t a l s i z e and shape vary w i t h i n c l u s i o n content. Small c r y s t a l s , 2-3 mm i n diameter, were found near the top of the specimen, where a higher sediment content occurred, and s i z e i n creased to 5-25 mm below. Most c r y s t a l boundaries ware s t r a i g h t o r only s l i g h t l y curved, an i n d i v i d u a l c r y s t a l g e n e r a l l y had 6 s i d a s , both convex and concave boundaries; these c h a r a c t e r i s t i c s are complicated by the presence of s o l i d and gaseous i n c l u - sions. G r a i n shape becomes more complex i n zones of high i n c l u s i o n content, w i t h i r r e g u l a r boundaries, a l s o sub-boundary development i s c l o s e l y a s s o c i a t e d w i t h i n c l u s i o n s . 243 L a t t i c e o r i e n t a t i o n s are given i n Figure 101, which shows a hig h degree of p r e f e r r e d o r i e n t a t i o n , w i t h a s i n g l e maximum p a r a l l e l to the d i r e c t i o n of bubble e l o n g a t i o n . I n t e r p r e t a t i o n The equidimensional c r y s t a l s are clo s e to an e q u i l i b r i u m c o n f i g u r a - t i o n . G r a i n boundaries are mutually a d j u s t e d , probably sub-boundaries have become boundaries, except where i n t e r f e r e d w i t h by i n c l u s i o n s owing to the hig h content of v e r t i c a l l y elongated bubbles. C-axis o r i e n t a t i o n s are p a r a l l e l to i n f e r r e d heat flow d i r e c t i o n s , thus c r y s t a l s extended p a r a l l e l to the c-ax i s as found i n the experimental work of Kapl a r (per- sonal communication, 1974). The i c e i s a g g r a d a t i o n a l , i n the sense t h a t the permafrost t a b l e has r i s e n due to a r t i f i c i a l sedimentation. Figure 101. Petrofabrics, aggradational ice, Tuktoyaktuk. Plane of diagram horizontal,parallel to section': r 245 Chapter 5 SUMMARY AND CONCLUSIONS The widespread d i s t r i b u t i o n of ground i c e along the A r c t i c Coast and Mackenzie Va l l e y had been recognized previously, and a c l a s s i f i c a t i o n , of i c e types had been established i n terms of water supply, t r a n s f e r mechanisms and associated ground ice forms. This was based on long-term observations and precise surveys of a range of growing i c e s , but l i t t l e p e t r o f a b r i c study. In the present study a f i e l d area was chosen near Tuktoyaktuk i n order to undertake both d e t a i l e d f i e l d study of i c e i n good exposures and i n bodies of known h i s t o r y , and p e t r o f a b r i c a n a l y s i s of those i c e s . Thus i t was possible to compare ice of known age ( <C 1 year) with older bodies and thus to elucidate both growth features and p o s t - s o l i d i f i - c a t i o n changes. C r y s t a l and i n c l u s i o n c h a r a c t e r i s t i c s t y p i c a l of growth of i c e i n bulk water, as reported i n lake i c e and laboratory studies, were found to apply to some i c e bodies. Although previous f i e l d and laboratory work on pet r o l o g i c aspects of segregated i c e and pore i c e growth has been l i m i t e d , the r e s u l t s here are i n agreement with e a r l i e r r e p o r t s . M o d i f i c a t i o n of growth f a b r i c s was recognized i n seve r a l i c e types, for example i n ice wadges where progressive changes i n c r y s t a l features occurred from the centre (recent growth) to the wedge margin; also the f a b r i c of segregated ice adjacent to a wedge changed due to the growth of that wedge. :• o 246. . Thus each i c e type has a p a r t i c u l a r texture r e l a t e d to i t s h i s t o r y , and petrologic c r i t e r i a are added to a modified ve r s i o n of Mackay's (1972b) i c e c l a s s i f i c a t i o n given i n Appendix 1. A b r i e f summary of each.ice type is. given below: 1. Lake i c e comprised an upper zone of small c r y s t a l s from which grew v e r t i c a l l y elongate c r y s t a l s , p a r a l l e l to the heat flow d i r e c t i o n , with optic axes i n a ho r i z o n t a l g i r d l e . 2. Icing mound ice comprised an upper c h i l l zone of small c r y s t a l s , adjacent to the s o i l , from which grew elongate c r y s t a l s normal to the upper surface, with optic axes normal to the long axes. Bubble bands were p a r a l l e l to the upper i c e surface and contained elongate bubbles normal to the bands. A l a t e r fracture i n f i l comprised i c e with d i f f e r - ent c r y s t a l and i n c l u s i o n c h a r a c t e r i s t i c s r e l a t e d to the l o c a l thermal gradient. 3. Ice from three pingos was studied and found t o d i f f e r i n a l l three. A small pingo of known age displayed c r y s t a l features t y p i c a l - o f i c e growth i n bulk water and i s interpreted as fr e e z i n g of a r e s i d u a l pond, as freezing was u n i d i r e c t i o n a l , rather than i n j e c t i o n i c e which would give evidence of m u l t i d i r e c t i o n a l freezing. A second pingo exhibited.a c l e a r i c e and bubbly ice core overlying pore i c e . An increase i n freez i n g rate during growth was indicated by a change i n i n c l u s i o n content and c r y s t a l l a t t i c e o r i e n t a t i o n ( o p t i c axes normal to freezing d i r e c t i o n ) . This i s at t r i b u t e d to u p l i f t of the lake bottom and exposure to cold a i r temperatures. In tha case of Tuktoyaktuk pingo there, i s a pattern of a l t e r n a t i n g layers of segregated ice and pore i c e , i n marked contrast to the previous cores. A general tendency e x i s t s f o r o p t i c axes to be normal to the compositional layering, but markedly d i f f e r e n t patterns e x i s t . Some flow has occurred since cessation of growth. Involuted h i l l i c e represents an extension of segregated i c e growth, with considerable upward movement of the i c e . An increase i n c-axis concentration orthogonal to the compositional layering and i n dimensional o r i e n t a t i o n p a r a l l e l to the layering occurred i n a n t i c l i n e s i n the i c e . Where an ic e wedge penetrated such a f o l d the p e t r o f a b r i c pattern changed adjacent to the wedge, and c r y s t a l s i z e decreased toward the wedge, due to the stress system associated with wedge growth. Tension crack i c e from two s i t e s was studied. Crack i c e which grew i n the 1973-74 season had fresh growth features markedly d i f f e r e n t from one season's wedge i c e growth. Older tension crack i c e , which had been subject to coastal recession, and thus unknown stress and temperature gradients, displayed features due to p o s t - s o l i d i f i c a t i o n m o d i f i c a t i o n . " . . . - ' Thermal contraction crack and wedge ice from several s i t e s was studied. Individual thermally-induced cracks i n segregated (involuted h i l l ) i c e were investigated; l i t t l e r e l a t i o n s h i p to the f a b r i c of the massive ice was found, nor were subsequent fractures influenced g r e a t l y by e a r l i e r ones. Large wedges on P a l l y Island showed the prograde f a b r i c of growing wedges. Progressive increases i n c r y s t a l size,, degree of preferred dimensional o r i e n t a t i o n ( p a r a l l e l to compositional layering) with distance from the wadge centre were recognized. The influence of 248 a growing wedge on surrounding i n v o l u t e d h i l l i c e was po i n t e d out above ( c o n c l u s i o n No. 5). R e t i c u l a t e v e i n i c e was examined a t one s i t e o n l y , where v e r t i c a l veins dominated. I n c l u s i o n and c r y s t a l c h a r a c t e r i s t i c s v a r i e d i n each sample. Narrow veins contained medium s i z e d c r y s t a l s w i t h c-axes normal to the v e i n . In large v e i n s , c e n t r a l c r y s t a l s were l a r g e r , without a strong p r e f e r r e d c-axis o r i e n t a t i o n , and surrounded by elongate c r y s t a l s , normal to the v e i n . 8. A c t i v e l a y e r i c e i n d i c a t e d m u l t i p l e f r e e z i n g d i r e c t i o n s . 9. Ice w i t h m u l t i p l e f r e e z i n g h i s t o r i e s was discussed from two s i t e s . O m n i d i r e c t i o n a l growth w i t h i n f r o z e n s o i l was observed i n c o a s t a l areas of ice-wedge polygons. C r y s t a l s and bubble t r a i n s t r e n d orthog- o n a l l y from surrounding s o i l , w h i l e contained bubbles are v e r t i c a l i n response to a l a t e r v e r t i c a l temperature g r a d i e n t . In some cases l a t e r m e l t i n g and r e f r e e z i n g was evident. The p o s i t i o n of such bodies sug- gests that they o r i g i n a t e from o m n i d i r e c t i o n a l f r e e z i n g of pools or channels produced by coastward flow of water. A g g r a d a t i o n a l i c e grew between two ri d g e s of an i n v o l u t e d h i l l , and i n a s s o c i a t i o n w i t h the upward growth of permafrost under a gravel pad. I n the former case upward growth f o l l o w e d by melt-down and sub- sequent upward growth was recognized. ".' I t i s emphasized that any one c r i t e r i o n i s i n s u f f i c i e n t f o r i n t e r - p r e t a t i o n of the h i s t o r y of a given i c e body. For example the o r i e n t a t i o n of bubble long axes i n the i c i n g mound i c e ( F i g . 5) was p a r a l l e l to the 249 g r o w t h d i r e c t i o n as i n d i c a t e d by c r y s t a l ' z o n a t i o n , c r y s t a l d i m e n s i o n a l o r i e n t a t i o n , c r y s t a l c - a x i s o r i e n t a t i o n and t h e f o r m o f t he mound. I n c o n t r a s t , i n o l d e r , n e a r - s u r f a c e i c e s w h i c h h a v e been s u b j e c t t o m e l t - d o w n a n d s t r o n g t e m p e r a t u r e g r a d i e n t s (no t measu red ) t he b u b b l e s have d e v e l o p e d v e r t i c a l d i m e n s i o n a l o r i e n t a t i o n s ( F i g . 8 3 ( a ) ) d e s p i t e t h e d i p o f t he b u b b l e t r a i n . Howeve r , t he b u b b l e t r a i n s , c r y s t a l z o n a t i o n and d i m e n - s i o n a l o r i e n t a t i o n s and shape o f t he i c e - s o i l i n t e r f a c e i n d i c a t e t h e f r e e z i n g d i r e c t i o n , and t h e b u b b l e o r i e n t a t i o n i s i n t e r p r e t e d a s a r e s u l t o f t h e r m o m i g r a t i o n i n a v e r t i c a l t e m p e r a t u r e g r a d i e n t . A l s o , i n t h e c a s e o f f o l d e d i n v o l u t e d h i l l i c e , b u b b l e s we re m o d i f i e d f r o m t h e i r o r i g i n a l f o r m , b u t some u n d e r s t a n d i n g o f t h e h i s t o r y o f a n i n d i v i d u a l body may be g a i n e d f r o m c o n s i d e r a t i o n o f a l l p e t r o f a b r i c p r o p e r t i e s . T h i s i s t r u e e s p e c i a l l y where o n l y l i m i t e d s a m p l e s a r e a v a i l a b l e , f r o m a c o r e , f o r e x a m p l e . I n t h e p r e s e n t s t u d y s a m p l e s were o b t a i n e d f r o m w e l l e x p o s e d i c e b o d i e s , o r f r o m a r e a s o f w h i c h t h e g e o m o r p h i c h i s t o r y was w e l l known. E l s e w h e r e , e x p o s u r e may be l a c k i n g o r t o p o g r a p h i c e x p r e s s i o n may n o t r e f l e c t s u b s u r f a c e i c e f o r m . I n s u c h i n s t a n c e s t h e p e t r o f a b r i c f e a t u r e s summar i zed h e r e may be u s e d as a i d s i n c l a s s i f y i n g a n i c e s a m p l e , b u t a l l a v a i l a b l e s e d i m e n t a r y and o t h e r d a t a s h o u l d be e m p l o y e d . Know ledge o f c r y s t a l c - a x i s o r i e n t a t i o n s i s i m p o r t a n t i n t h e i n t e r - p r e t a t i o n o f r e s u l t s o f g e o p h y s i c a l i n v e s t i g a t i o n s on g r o u n d ' i c e . F o r e x a m p l e , s e i s m i c wave v e l o c i t y p a r a l l e l t o t h e c - a x i s i s 200 m.s ^ f a s t e r t h a n e l s e w h e r e , t hus p r e d i c t i o n o f f a b r i c s w o u l d be h e l p f u l . 250 Suggestions for further work The aim of the present study has been an understanding of p e t r o l o g i c a l aspects of the growth and deformation-of i c e bodies i n permafrost. The r e s u l t s summarized above indicate that some progress has been made i n that a foundation has been established on which to base further f i e l d and lab- oratory study. In p a r t i c u l a r , ' p e t r o l o g i c a n a l y s i s of i c e i n lenses grown under known conditions of heat-flow and water supply should be attempted. Those pingos and other i c e bodies of which the recent growth h i s t o r i e s are well.known would provide excellent f i e l d comparisons. In terms of deforma- t i o n , much could be gained from a knowledge of st r e s s d i s t r i b u t i o n s around wedges, and creep rates of the larger Ice masses. Detailed thermal data for. the upper layers of permafrost are a v a i l a b l e and i t i s apparent that . contained i c e bodies have complex, c y c l i c , thermal s t r a i n h i s t o r i e s which merit further study. Our understanding of the influence of i n c l u s i o n s on the flow of i c e i s l i m i t e d and more experimental work i s necessary to a i d i n f i e l d studies. 251 LITERATURE CITED AD HOC STUDY GROUP ON PERMAFROST 1974. P r i o r i t i e s f o r b a s i c r e s e a r c h on p e r m a f r o s t . Committee on P o l a r R e s e a r c h , N a t i o n a l R e s e a r c h C o u n c i l . Nat. Acad. S c i . , Washington, D.C, 54 p. ANDERSON, D.L. , and WEEKS,W.F. 1958. A t h e o r e t i c a l a n a l y s i s o f s e a i c e .. ' s t r e n g t h . T r a n s . A.G.U., v . 39, no. 4, pp. &32-&40. 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The development of blowholes i n the ice/water/C02 system.. J . C r y s t a l Growth, v. 28, no. 1, pp. 85-92. VTIURINA, E.A., and. VTIURIN, B.I. 1970. Ice formation i n rocks ( i n . Russian) I z d a t e l s t r o "Nauka", Moscow, 280 p. WEEKS, W.F., and ASSUR, A. 1964. Growth, s t r u c t u r e and s t r e n g t h of sea i c e . U.S. Army CRREL, Research Report 135, 19 p. WILLIAMS, P.J. 1967. The nature of f r e e z i n g s o i l and i t s f i e l d behaviour. Norwag. Geotech. I n s t . P u b l . no. 72, pp. 91-119..  Appendix 2 260 GLOSSARY Anhedral - Descriptive of c r y s t a l s on which c r y s t a l faces are absent. Annealing - Recovery and r e c r y s t a l l i z a t i o n due to heating of deformed material'. Basal plane - (0001) plane, normal to c-axis or o p t i c axis, i n Ice c r y s t a l . C h i l l zone - A boundary layer, adjacent to a co o l i n g surface, of small, equigranular c r y s t a l s with random o r i e n t a t i o n s . Columnar c r y s t a l s - Elongate c r y s t a l s aligned p a r a l l e l to the d i r e c t i o n of heat flow. Defect - Imperfection i n cr y s t a l ' l a t t i c e , e.g. missing or e x t r a atoms, impurities, d i s l o c a t i o n s . • . _ • Deformation band - Deformation-induced c r y s t a l l a y e r d i f f e r i n g i n o r i e n t a t i o n from remainder. • . Dendrites - L a t t i c e c o n t r o l l e d projections of c r y s t a l s at advancing i n t e r f a c e . D i s l o c a t i o n - Linear l a t t i c e defect, produced during growth or under subse- quent s t r e s s ; important i n deformation (see s l i p ) . D i s l o c a t i o n climb - Movement of d i s l o c a t i o n out of i t s s l i p plane, e.g. , over an obstacle. Grain boundary migration - Displacement of a boundary perpendicular to i t s . tangent plane. Grain growth - Increase i n grain s i z e of p o l y c r y s t a l l i n e aggregate at elevated temperature under a d r i v i n g force of reduction i n surface energy. Lineage structure - D i s l o c a t i o n arrays p a r a l l e l to growth d i r e c t i o n , formed during growth. Polygonization - Formation of low angle boundaries separating unstrained l a t t i c e segments during recovery of strained l a t t i c e . Recovery -Removal of re s i d u a l stresses of deformation. R e c r y s t a l l i z a t i o n - Nucleation of new, s t r a i n - f r e e grains i n a deformed . matrix. S l i p - S l i d i n g displacement of one part of a c r y s t a l r e l a t i v e to another by d i s l o c a t i o n motion p a r a l l e l . t o planes of high atomic density. Subgrains - L a t t i c e segments of a grain, separated by low angle boundaries.

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