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Quaternary stratigraphy and geomorphology of the Lower Thompson Valley , British Columbia Anderton , Lesley Jean 1970

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QUATERNARY STRATIGRAPHY AND GEOMORPHOLOGY OF THE LOWER THOMPSON VALLEY, BRITISH COLUMBIA by LESLEY JEAN ANDERTON B.A., University of Keele s 1965 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ARTS in the Department of Geography We accept t h i s thesis as conforming to the required standard The University of B r i t i s h Columbia August, 1970. 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 deg ree a t t he 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 ee t h a t t he 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 ag ree t h a 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 pu rpo se 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 . I t i s u n d e r s t o o d t h a 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 . Depa r tment The U n i v e r s i t y o f B r i t i s h Co l umb i a Vancouve r 8, Canada Date cW. ^ A /?YO ABSTRACT The Thompson Valley bet-ween Spences Bridge and Lytton, where i t joins the Fraser, merits attention because of the abundance of clear sections of l a t e g l a c i a l and p o s t g l a c i a l deposits i n road and r a i l cuts, and i t s position as an impor-tant t r a n s i t i o n zone between the g l a c i a l lake deposition of the southern I n t e r i o r and the dominantly f l u v i a l aggradation of the Fraser. Most attention was paid to the stratigraphy of road and r a i l cuts, as the only morphological features preserved i n the steep and narrow valley are terraces, fans, landslides and talus slopes. Quaternary deposits y i e l d l i t t l e evidence of the deposit t i o n a l environment prior to the l a s t major advance of i c e , but there i s a good record of conditions during and following de-g l a c i a t i o n . The l a s t ice sheet, which wasted away by down-melting, l e f t a t h i n mantle of t i l l over the uplands and t i l l deposits up to 50 f t . thick i n the v a l l e y . The Lower Thompson Valley, close to the g l a c i e r sources of the Coast Mountains, wa one of the l a s t areas to be free of i c e . Consequently major lakes developed i n the Thompson and Nicola Valleys and were forced to drain into the Okanagan Valley. When the ice plug south of Spences Bridge f a i l e d , some time before 9 S000 B.P., drainage was resumed down the Thompson Valley to the Fraser. During deglaciation, up to 300 f t , of s i l t s and d e l t a i c gravels were deposited i n the l s230 f t . stage of Lake Deadman, which was dammed by ice south of Spences Bridge. Between Skoonka Creek and Seddall, respectively k and 7 miles south of Spences Bridge, are deposits of collapsed s i l t and flow t i l l and i t i s suggested that an ice plug remained here separating lacustrine deposition north of Skoonka Creek s from f l u v i a l aggradation south of Seddall, where the valley was by then largely i c e - f r e e . Aggradation, which was due to the large supply of material from l a t e r a l sources res u l t i n g from recent g l a c i a t i o n and weathering, was extremely rapid; more than 500 f t . of cross-bedded and horizontal gravel and sand were deposited i n probably l i t t l e more than 1,000 years. Aggradation ceased well before 7 S530 B.P. With amelioration of the climate and s t a b i l i z a t i o n of the slopes, the load of the r i v e r was considerably reduced and i t degraded i t s former valley f i l l creating a series of terraces, with a veneer of imbricate cobbles, from 500 f t . down to 30 f t . above present r i v e r l e v e l . Most terraces are n o n ~ c y c l i c s but paired terraces, approximately l^O f t . above the r i v e r , mark a period of r e l a t i v e s t a b i l i t y p rior to 7*530 B.P. The r i v e r was probably within 50 f t . of i t s present l e v e l by 6,600 B.P„ 9 and since then downcutting appears to have proceeded r e l a t i v e l y slowly. At least three phases of a l l u v i a l fan formation occurred during t h i s period of dominant degradation, the terraces acting as temporary base leve l s f o r the mudflows. Mazama ash, deposited on the terraces and fans approximately 6,600 years ago, provides a useful marker horizon. Apart from slow down-cutting by the r i v e r and some a l l u v i a l fan formation, the dominant processes a f f e c t i n g the topography since 6 , 6 0 0 B.P have been la n d s l i d i n g and slumping. The largest s l i d e 3 the Drynoch Earthflow, has been active f o r at least 3*175 years and slumping due to seepage and r i v e r erosion has occurred intermittently along the r i v e r banks. TABLE OF CONTENTS CHAPTER PAGE 1. INTRODUCTION . . . . . . . . . . . . . . . . . . 1 1.1 Geographical Setting . . . . . . . . . . . 1 1.2 Aims and Methods of Present Study . . . . . 2 1.3 Previous Work . . . . . . . . . . . . . . k 1.3.1 Lower Thompson Valley . . . . . . . 1.3.2 Adjacent Areas . . . . . . . . . . . 6 1.3.21 Lower Fraser Valley . . . . 6 1.3.22 The I n t e r i o r 7 2. BEDROCK AND PHYSIOGRAPHY . . . . . . . . . . . . 10 2.1 Geology of the Thompson Basin . 10 2.2 Geology of the Thompson Valley between Spences Bridge and Lytton . . . . . . . 12 2.3 Faulting and Jointing . . . lh 2.'+ Physiography . . . 15 2.H-.1 Outline of Drainage History . . . . 16 2A.2 Physiography of the Thompson Valley between Spences Bridge and Lytton. . 17 2.1+,21 Spences Bridge-Nicoamen . . 18 2A.22 Nicoamen-Lytton 18 3. QUATERNARY STRATIGRAPHY . . . . . . . . . . . . 20 3.1. G l a c i a t i o n of the Lower Thompson Valley . . 20 3.2 Quaternary Deposits. . . . . . . . . . . .22 3.2.1 Material deposited prior to l a s t major ice advance . . . . . . . . . 23 3.2.2 G l a c i a l Deposits . . . . . . . . .25 3.2.21 Upland Regions . . . . . . . 25 3.2.22 Thompson Valley 26 3.2.2 Summary . 3 1 3.2.3 Late G l a c i a l and Early P o s t g l a c i a l DSpOSltS 9 9 * a 9 9 9 9 9 9 9 9 9 • . 3 3.2.31 Lacustrine White S i l t Facies 32 3.2.31.1 Ice contact gravel . . . 32 3.2.31.2 Spences Bridge S i l t . . 3^ . 3.2.31.3 Murray Delta-Fan . . . 37 3.2.31.5 Lake Thompson/Lake Deadman . . . . . . . . 38 3.2.31.5 Late G l a c i a l Lake Stages in Lower Nicola Valley . hi 3.2.31.6 Ice-front complex of Lower Nicola Valley . .. •+ 2 3.2.31.7 Marginal Drainage Channels hh 3.2.31.8 Drainage of Lakes Merritt and Deadman ^5 V CHAPTER PAGE 3.2.31.9 Late G l a c i a l ? Lakes i n Tributary Valleys KQ 3.2.31.10 Other Late G l a c i a l Deposits . . . . . . . . . 52 3.2.31 Summary . . . . . 57 3.2.32 Late G l a c i a l and Early P o s t g l a c i a l Aggradational Deposits . . . « < > . . . . . . 5 8 3.2.32.1 Nicoamen, Shushten and Botanie Slide Deposits . . 6 l 3.2.32.2 Origin of the Aggradation-a l Deposits 63 3.2.32 Summary 69 3.2.'+ Late P o s t g l a c i a l and Recent Deposits. 69 3.2. Ltl A l l u v i a l Fans 70 3.2.^2 Drynoch Slide . . 73 3.2.!+3 Mazama Ash 7^ 3.2.M+ Cul t u r a l Remains Beneath Drynoch Slide . 7^ 3.2.*+5 Progress of Degradation . . . 75 3.2.^ + Summary o . « o . . . > . . . . . . . 7 7 h. GE0M0RPH0L0GY 79 h.l G l a c i a l Landforms . . . . . . 79 *^ • 2 T ©1*1*3 C©S 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 80 k.3 A l l u v i a l Fans 8*+ h.h S l i d e s , Slumps and Talus 85 1+';'+.l Drynoch Earthflow 85 Spences Bridge Slide . . . . . . . 86 k.k.3 Slumps 88 M-.M-.M- Talus . . 88 5. SUMMARY AND CONCLUSION . 89 5.1 Summary of Quaternary History . . . . . . 89 5.2 Conclusion 91 BIBLIOGRAPHY « . . . . . . . . •. . . . . . . . 93 APPENDIX C r i t e r i a by which deposits were distinguished . . . .99 v i LIST OF TABLES TABLE PAGE I Skuhun Creek Fan Exposure 1+3 (facing) v i i LIST OF FIGURES FIGURE PAGE 1. Index Map of Southwestern B r i t i s h Columbia . . 8 (facing) 2. Bedrock Geology - Thompson Basin 11 3.. Bedrock Geology - Lower Thompson Valley. . . . 13 k0 Thompson Canyon . 19 (facing 5. Features Indicating S.S.E. Movement of Ice over I n t e r i o r Plateau . . . . . 20 (facing 6. Location Map - Lower Thompson Valley . . . . . 22 (facing 7. T i l l overlying gravels and sand, Drynoch . . . 23 (facing 8. T i l l overlying cross-bedded gravels at Seddall 23 (facing 9. Striated and faceted stones from v a l l e y t i l l . 27 (facing 10. F l o w t i l l interbedded with gravel . . . . . . . 28 (facing 11. Formation of interbedded deposit of f l o w t i l l and outwash . . . . . . . . . . . . 2 8 (facing 12. T i l l from Skoonka Creek 29 (facing 13. T i l l exposed i n r i v e r bank at toe of Drynoch Earthflow .29 (facing 1*+. ' Exposure above Mud Lake, Spences Bridge . . . 32 (facing 15. Slump f o l d i n ice contact gravel . . . . . . . 32 (facing 16. Axis of f o l d i n r e l a t i o n to ice movement . . .33 17. Folding of s i l t bands i n Spences Bridge s i l t . 36 (facing 18. T i l t e d "varves" near top of white s i l t , Mud 19. The Spences Bridge s i l t s , Murray Delta-Fan and 1905 Slide . . . . . . . . . . . . . . . . 37 (facing 20 a) South Thompson stage of Lake Thompson i n r e l a -t i o n to other G l a c i a l Lakes of the I n t e r i o r . 39 (facing b) Durand stage of G l a c i a l Lake Deadman . . . . k-0 (facing 21. Location Map - Lower Nicola Valley . . . . . hi (facing) v i i i FIGURE PAGE 22. Units h, 5 and 6 of Skuhun Creek Fan Exposure hk (facing! Ice-front complex deposits at Luckachin Creek M+ (facing! 23. 2k. 25. 26. 27. 28. 29. 30. 31. 32. 33. 3^. 35. 36. 37-38. 39. T i l l overlying gravels, 1 mile south of the two road bridges s Nicola Valley . . . . . . S i l t interbedded with t i l l at Nicola twin bridges M+ (facing 0 0 0 0 0 o o a o Ice-marginal Features = Lower Thompson Valley *+5 a) Diagrammatic section of west wall of Murray Creek . . . . . . . . . b) Stages i n the evolution of the above sequence of deposits. . . . . . o o o « o 0 0 0 0 Graded beddings f a u l t i n g and micro-foldings Skoonka sands 0 0 0 o 0 • Graded-bedded sands at Pitquah « o c o 52 52 5k 55 Pitquah sand exposure overlying bedrock bench 55 . 56 Ice-rafted pebbles 3 load casts and graded-bedding i n Pitquah sands Interfingering of sand and gravel at margin of Pitquah sands 0 0 0 0 O • 0 Talus and wedges of c o l l u v i a l material i n t e r -bedded with sand, W. of Pitquah tunnel . . . Tabular cross-bedding i n aggradational gravels near Gladwin . . . . . . . o o o o Trough cross-bedding i n aggradational gravels Lenses of sand interbedded with coarse angular gravels i n aggradational sequence west of Thompson Junction . . . . . o o o Cross-bedded gravels and scoured sand lens at Nicoamen . Horizontally bedded angular gravels near P i t -quah Tunnel . . . . . O 0 0 0 0 O O Sand lens injected into angular granodiorite gravels at Botanie 56 57 59 59 6o 60 61 61 (facing (facing (facing (facing (facing (facing (facing (facing (facing (facing (facing (facing (facing (facing (facing (facing ix FIGURE PAGE kO. Nicoamen Slide taken with telephoto lens . . 62, (facing) *+l. Nicoamen Slide overlying cross-bedded gravels 62 (facing) k2. Imbrication i n r i v e r cobbles opposite Thompson Junction 70 (facing) *+3. Imbricate cobbles overlain by colluvium on Botanie Creek road . . . . . . 70 (facing) kk. Volcanic ash i n aeolian sand overlying f l u v i a l gravels, Drynoch 7k (facing) M-5. Volcanic ash i n a l l u v i a l fan mudflow near M i l d Lei It© 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 o o o 7^" acing) k6. Landforms of the Lower Thompson Valley a) between Spences Bridge and Nicoamen . . . 81 b) between Nicoamen and Lytton 81 k7, Lower Thompson Valley - longitudinal p r o f i l e 82 (facing) k8. Terraces of the Thompson and Fraser near Lytton . . . . . . . 83 (facing) *+9. Generalized section of Quaternary deposits, Spences Bridge 90 (facing) 50. Generalized section of Quaternary deposits, Thompson Canyon . . . . . . . .90 (facing) X ACKNOWLEDGEMENTS The author would l i k e to thank f i r s t Dr., W o H . Mathews fo r his encouragement and advice during the preparation of th i s t h e s i s , and Dr. 0. Slaymaker f o r reviewing the f i n a l draft i n the absence of Dr. J . R. MacKay, who was her depart^ mental adviser. P a r t i c u l a r thanks are due to Dr. June Ryder, who vol u n t a r i l y provided invaluable assistance and discussion both in the f i e l d and during the preparation of thi s t h e s i s . Mrs. Elizabeth Sipes (nee Hamilton) was a most w i l l i n g f i e l d a s s i stant. The author was supported by a Commonwealth Scholarship while doing graduate studies, and f i e l d work was aided by a grant from the University of B r i t i s h Columbia. CHAPTER 1 INTRODUCTION 1.1 Geographical Setting The Thompson River, draining the I n t e r i o r Plateau of B r i t i s h Columbia, i s the p r i n c i p a l t r i b u t a r y of the Fraser. This study i s concerned with the 22 mile stretch of the valley between Spences Bridge, at the confluence of the Nicola and Thompson Rivers, and Lytton, at the confluence of the Thompson and Fraser. The v a l l e y , dissecting the westernmost portion of the I n t e r i o r Plateau, i s a natural focus of routes, and is followed by the Canadian P a c i f i c and Canadian National Railways as well as the Trans-Canada Highway, hence the area i s readily accessible. Furthermore the r i v e r i s bridged at both Spences Bridge and Lytton and can be crossed at intermediate points by means of a cable and an old foot-bridge (see F i g . 6.). As the area l i e s within the rainshadow of the Coast Mountains, the dominant vegetation i s bunch grass, sage brush and scattered Ponderosa pine; with forested areas including spruce and Douglas F i r at higher a l t i t u d e s , and alders, willows, and cottonwoods growing along the water courses. P r e c i p i t a t i o n s t a t i s t i c s f o r Spences Bridge are not available but Lytton, which i s the wettest part of the v a l l e y , has 18 ins. per annum, while Ashcroft to the northeast has 9 i n s . per annum. The climate is c l a s s i f i e d under the Koppen system as Humid Continental with a hot--dry summer (Dsa) (Chapman, 1952). The general a r i d i t y and 2 narrowness of the v a l l e y has tended to discourage farming and settlement. In the past, however, more land was under c u l t i v a -t i o n as i s evidenced by the abandoned i r r i g a t i o n channels and cabins on some of the terraces. Consequently the s u r f i c i a l geology i s obscured neither by dense vegetation growth nor by settlement. Good exposures are abundant too, as the modern Thompson has cut through the Quaternary deposits i n places ri g h t down to the bedrock as at Thompson Canyon, and the numer-ous road and r a i l cuttings have greatly increased the abundance and c l a r i t y of sections. The steep and narrow v a l l e y has not, however, favoured the preservation of g l a c i a l deposits. 1 ,2 Aims and Methods of Present Study The aim of the present study i s to provide as detailed a description and reconstruction of the sequence of Quaternary events as possible, drawing upon f i e l d and laboratory evidence, the results of work i n adjacent areas, and a knowledge of present-day processes. The area merits attention because of:-1 ) An abundance of clear sections of sand and gravel dating from the period of aggradation during and immediately following deglaciation, which makes thi s an i d e a l area f o r the study of t h i s period of a l l u v i a t i o n , which has so f a r received l i t t l e attention. 2) A marked contrast i n width between the northeastern and southwestern sections of the v a l l e y with corresponding differences i n sediment types. 3) Its position at the westernmost edge of the I n t e r i o r Plateau, and hence the p o s s i b i l i t y that i t has been 3 affected by both the C o r d i l l e r a n Ice Sheet and smaller v a l l e y glaciers from the Coast Mountains immediately to the west. k) Its position l i n k i n g the l a t e - g l a c i a l and po s t - g l a c i a l history of the Thompson and Fraser Val l e y s . With t h i s aim i n mind s three months were spent i n the f i e l d i n the summer of 1966, the f i r s t month i n general reconnaisance of t h i s and adjacent areas, and the remaining two concentrating s o l e l y on th i s portion of the Thompson Valley* F i e l d work was consolidated with another month's study i n June 1967. Stereoscopic a e r i a l photographs with a scale of approximately l : l 5 3 0 0 0 were chosen as the basis f o r f i e l d mapping, as they provide more d e t a i l and are more usefu l i n the laboratory than the 1 : 5 0 , 0 0 0 topographic maps ava i l a b l e . The occurrence and extent of exposures, the boundary between s o l i d rock and sur-f i c i a l deposits, and the presence of terraces, a l l u v i a l fans, talus slopes, slumps and landslides were mapped i n the f i e l d . The stratigraphy of each exposure was described, noting where possible the thickness and composition of i n d i v i d u a l units with regard to grain size and shape, rock type and colour according to the G.S.A. colour chart. Such evidence helps to connect separate exposures and provides some clues as to mode of forma-t i o n . Sedimentary structures such as cross-bedding, graded-bedding, imbrication, r i p p l e s , flame structure and load casts were noted, as they provide evidence of conditions at time of deposition and d i r e c t i o n of flow. Where possible, the strata were sampled so that further analysis could be carried out i n the laboratory. The c r i t e r i a by which the various deposits 4 -were distinguished are l i s t e d i n the appendix. The stratigraphy of some of the less accessible exposures was examined with the aid of a t r a n s i t s and thicknesses worked out by t r i a n g u l a t i o n . The terraces were studied i n some d e t a i l by determining the heights of the front edges of some of them with the altimeter i n the f i e l d , and using the f l o a t i n g dot technique i n the laboratory. Clues to chronology were provided by samples of carbonized wood, volcanic ash, and Indian a r t i f a c t s . 1 . 3 Previous work 1 . 3 . 1 The Lower Thompson Valley A.R.C. Selwyn, one-time di r e c t o r of the Geological Survey of Canada, passed through th i s area during his preliminary ex-plorations i n B r i t i s h Columbia i n 1871. He erroneously a t t r i -buted the dryness of the climate to geological f a c t o r s , although noting the occurrence of s i m i l a r conditions east of the Sierra Nevada — "The dryness of the climate i s doubtless due to the absence of f o r e s t , caused probably by the nature of the s o i l not favouring i t s growth; and the s o i l i s dependent on the character of the rocks. Thus geological causes may be regarded as the r e a l o r i g i n of a l l the p e c u l i a r i t i e s observed." Selwyn, 1871, p. 69. Apart from t h i s , he noted the terrace levels and suggested that the material i n which they had been excavated must have been deposited during a lengthened period of depression of the land, which was followed by successive movements of elevation during the terrace epoch. G. M. Dawson, perhaps Canada's most famous geologist, was next on the scene and reported on the area i n more d e t a i l i n 5 1877s 18783 and l89 l +o He paid considerable attention to the s u r f i c i a l deposits 3 commenting upon the general north-south trend of g l a c i a t i o n as suggested by s t r i a e . He distinguished between unmodified boulder clay occurring mainly at heights between 3 S000 f t . and 5S000 f t . 5 and modified deposits, i n -cluding white s i l t below 3 3000 f t . He suggested (1878, p. llf2B) that the white s i l t implied the existence of a t r a n q u i l body of water 3 which must have f i l l e d the entire valley now occupied by Kamloops Lake and the Thompson to a considerable depth. He further attributed the i n t e r c a l a t i o n of gravelly layers to stream deposition at the edge of the lake. Later (l891+, p. 28^-6)3 however, he decided that the lack of moraines or other barriers to account f o r the presence of g l a c i a l lakes, and the general correspondence of the various isolated deposits throughout the I n t e r i o r indicated the s i l t s were marine. He envisaged (1878, p. 122B) the following series of events occurring i n the Thompson Valley and adjacent, areas 1) Glaciation from north to south with deposition of boulder clay. 2) Formation of terraces by lowering of water-surface, accompanied or followed by a warm period. 3) Short advance of glaciers from mountains contemporaneous with formation of the lower terraces. *+) Retreat of glaciers to t h e i r present l i m i t s . C. W. Drysdale i n studying the geology of the Thompson River Valley below Kamloops Lake i n 1912, noted that the topography of the d i s t r i c t had been greatly influenced by topographic forms developed during previous erosion cycles, mainly by those of late T e r t i a r y age, which have been modified by d i f f e r e n t i a l u p l i f t and 6 g l a c i a t i o n . He suggested that the valleys i n the -western section •were youthful and the product of an erosion cycle folio-wing late Pliocene or early Pleistocene u p l i f t and including at least two advances of vall e y i c e . The following series of events i n Quaternary history were envisaged 1) G l a c i a t i o n . 2) Aggradation of f l u v i o - g l a c i a l material. 3) Degradation with cutting of terraces i n f l u v i o - g l a c i a l f i l l s valley-deepening as at Thompson Canyon and l a n d s l i d i n g . He suggested that p o s t - g l a c i a l erosion was invigorated by u p l i f t . 1.3.2. Adjacent Areas 1.3.21. The Lower Fraser Valley •J. E. Armstrong, working i n the Lower Fraser Valley (1955j 1957» I960 a & b) and i n collaboration with geologists i n N. W. Washington (1965)s has proposed s i x geologic climate-units f o r the late Pleistocene sequence of coastal S, W. B r i t i s h Columbia and N. W. Washington:-FRASER GLACIATION This probably represents the same geologic-climate episode as the " c l a s s i c a l " Wisconsin Glaciation of the Mid-western United States and consists of the following subdivisions:-Dates Characteristics Sumas Stade End un-eertain but Indians Rise of land with respect occupied Yale, 9 5 000 + 150 to sea l e v e l 3 withdrawal of yrs. ago. sea from lowland. Advance Began 11,000 yrs. B.P. of v a l l e y g l a c i e r i n E. Fraser Lowland. Everson Interstade Began 13 s 000 yrs. B.P. i n Marine conditions. St. of Georgia. 7 Vashon Stade Ice had reached S. by 15,000 yrs. B.P. Advance of Cor d i l l e r a n Ice into Lowlands. Ice covered the Coast Mountains near Vancouver to altitudes of at least 6,000 f t . Evans Creek Stade Began approximately 2l+,500 yrs. B.P. Advance of Cor d i l l e r a n Ice into Lowlands. Ice covered the Coast Mountains near Vancouver to altitudes of at least 6,000 f t . OLYMPIA INTERGLACIAL Extended from at least 36,000 yrs. B.P. to 2^,500 yrs. B.P. i n the Fraser Lowland. Quadra-Pt. Grey Formation 1.3.22 The I n t e r i o r W. H. Mathews (19M+) has made a study of g l a c i a l lakes and ice retreat i n south-central B r i t i s h Columbia. From a study of old shorelines, lacustrine deposits and cols, he reconstructed the l a t e - g l a c i a l history of the Nicola basin and a p a r t i a l history of the Thompson basin. He suggested that ice remained la t e i n the lower Thompson and lower Nicola Valleys while the surrounding uplands were ice f r e e . Consequently lakes developed i n the i c e - f r e e portions of the v a l l e y s . Lake Merritt, occupying part of the Nicola v a l l e y , was held back by an ice lobe i n the v i c i n i t y of Guichon Creek i n the Lower Nicola Valley (see F i g . 20a). Lake Thompson was held back by ice i n the Thompson Valley which retreated west from Kamloops i n a series of stages. At i t s f i n a l stage, Lake Thompson extended just south of Spences. Bridge and part-way down the lower Nicola Valley (see F i g . 20b). R. J . Fulton mapped the s u r f i c i a l deposits of the Kamloops FIG. I. INDEX MAP OF SOUTHWESTERN BRITISH COLUMBIA Showing study area (cross-hatched) and location of other studies of the Quaternary (outlined) 8 area in 1963 and made a pa r t i c u l a r study of deglaciation« He recognized two t i l l s s separated by an i n t e r = t i l l deposit dated as 32,7^0 yrs. B.P. (G.S.C.=275)s but the lower t i l l i s seen only on Peterson Creek. He suggested that the l a s t ice sheet retreated largely by downwasting, tongues of ice remaining in the main valleys a f t e r the surrounding uplands were bare. He further believed, following Mathews, that the unit of white s i l t occurring here, which he termed the South Thompson S i l t , was deposited i n the ice=dammed g l a c i a l Lake Thompson. The fin e white s i l t i s thought to have been winnowed from t i l l by melt= water, the coarse f r a c t i o n being trapped in the uplands due to the many changes in gradient occasioned by the ir r e g u l a r nature of the drainage at the time. The deposit i s thought to consist of no more than 200 varves and hence occupies a r e l a t i v e l y short span of the history of the area. It i s characterized i n places by disturbances due to grounded ice and slumping. Fulton has also mapped the s u r f i c i a l deposits of the Vernon, Shuswap and Merritt areas, (see F i g . 1), which has enabled him to produce a more detailed history of the g l a c i a l lakes of the Southern I n t e r i o r and to correlate the hi s t o r i e s of the areas discussed by Mathews and Nasmith (Fulton, 1969). H. Nasmith (1962, p. 10) saw the late g l a c i a l record i n the Okanagan Valley as " e s s e n t i a l l y the record of the wasting away of a tongue of ice which was confined to the valley while the adjacent uplands were ice free." He suggested (p. 9) that i n a deep basin=shaped trough l i k e the Okanagan Valley, an ice lobe fed by distant snowfields becomes stagnant when the surface of the ice has no gradient, as would be the case i f climatic change 9 cut off the flow of ice from the source. Consequently stagnation can occur even though the ice i s s t i l l 500 to 1,000 f t , thick. Ice-dammed lakes were a feature of t h i s v a l l e y too. •J. M. Ryder (1970) studied the Quaternary stratigraphy of the Thompson Valley between Ashcroft and Spences Bridge, and the Fraser Valley between P a v i l i o n and Lytton, i n connection with her work on the depositional environment of a l l u v i a l fans. (See F i g . 1 ) . I t i s evident that the Thompson Valley from Spences Bridge to Lytton has not previously been studied i n d e t a i l . G. M. Dawson's and C. W. Drysdale's work i s h e l p f u l i n providing a broad view of the sequence of events and suggesting points to look f o r . W. H. Mathews1' and R. -J. Fulton's work on the late-g l a c i a l lakes i s of considerable value i n interpreting the white s i l t s of the Spences Bridge area. The sequence of events i n the l a t e g l a c i a l history of the Okanagan Valley des-cribed by H. Nasmith i s probably s i m i l a r to that of the Lower Thompson Valley. J . M. Ryder's work i s h e l p f u l i n correlating the l a t e r stages of the recent geological history and i n i n t e r -pretation of the fan deposits of this area. I t i s unfortunate that no detailed study has yet been made of the Thompson Valley from Savona to Ashcroft, of the Lower Nicola Valley, and of the Fraser Valley from Lytton to Hope. Nevertheless i t i s hoped that a broad c o r r e l a t i o n of events w i l l be possible between this area, the Kamloops area, and the Okanagan and possibly the Lower Fraser Valley. 10 CHAPTER 2  BEDROCK AND PHYSIOGRAPHY 2.1 Geology of the Thompson Basin The Thompson drains an area of extremely varied l i t h o l o g y (see F i g . 2). The geology of the lower reaches i s described i n the Ashcroft ( D u f f e l l and McTaggart, 1952), Nicola (Cockfield, 19^8), and Vernon (Jones, 1956) Memoirs, and the upper reaches have been mapped by R. B. Campbell (1961, 196*+, 1968, and Campbell & Tipper, 1966). Mesozoic and Cenozoic igneous rocks predominate i n the south and west with r h y o l i t i c , andesitic and basaltic lavas and t u f f s covering large areas; i n addition there are intrusions of granite and granodiorite of batholithic proportions. Metamorphic rocks of the Shuswap complex pre-dominate i n the north and east with some quartzite, a r g i l l i t e and limestone (Uglow, 1922, Campbell, 1961, 196*+, 1 9 6 8 ) . Permian sedimentary rocks including a r g i l l i t e , quartzite, sheared conglomerate and limestone, with some serpentine, out-crop i n the v i c i n i t y of Kamloops, the Douglas Plateau and Cache Creek, with limestones predominating around P a v i l i o n and the Cornwall H i l l s . Other sedimentary rocks, of Mesozoic and Cenozoic age, include graywacke, shale, conglomerate, sandstone and some coal i n the v i c i n i t y of Hat Creek and Spences Bridge. The Coast and Cascade Mountains to the south and west of the Thompson Valley are composed predominantly of Mesozoic granodiorite, with some serpentinized u l t r a b a s i c rocks, FIG. 2. BEDROCK GEOLOGY - THOMPSON BASIN (modified from B.C. Atlas of Resources 1956) 11 F i g . 2. BEDROCK GEOLOGY - THOMPSON BASIN LEGEND TERTIARY - Lavas and sediments LOWER CRETACEOUS - Shale and sandstones, volcanlcs TRIASSIC-JURASSIC A r g i l l i t e s , lavas, t u f f s , breccia, sandstones, limey s i l t s PERMIAN - A r g i l l i t e s , cherts, lavas, t u f f s , limestones CAMBRIAN-MISSISSIPPIAN Limestone, shales, a r g i l l i t e s , quartzites LOWER CAMBRIAN OR OLDER Quartzites, a r g i l l i t e s , conglomerates, schists and limestone METAMORPHIC - Schists and gneiss etc. INTRUSIVE - Granodiorite, Quartz-diorite c h i e f l y 12 hornblende d i o r i t e i n t r u s i o n s 3 and p h y l l i t e 3 a r g i l l i t e 3 graywacke 3 greenstone and s c h i s t . 2.2 Geology of the Thompson Valley between Spences Bridge  and Lytton Around Spences Bridge and along much of the northern slope of the v a l l e y the Spences Bridge Group (Lower Cretaceous) crops out. (see F i g . 3). This i s mainly an accumulation of andesite 3 d a c i t e 3 r h y o l i t e and basalt lavas and pyroclastic rocks. Most of the lavas are porphyritic with lath-shaped pheno-crysts of f e l d s p a r 3 and may be red, green 9 mauve3 purple, brown3 grey, white or black i n colour. Both vesicular and amygdaloidal types are represented with quartz, c a l c i t e , and less commonly zeolites forming the amygdules. The agglomerates are mainly grey to green and may contain carbonized fragments of stems and twigs. Sandstones, arkoses 3 and t u f f s i n various shades of grey, commonly with a green or yellow tinge, occur. In places the volcanic rocks are much decomposed and commonly show such secondary products as saussurite, carbonate, iron oxides and c h l o r i t e . The Kingsvale Group (Lower Cretaceous) occupies the southern side of the v a l l e y and the Nicoamen Plateau. I t i s somewhat sim i l a r to the Spences Bridge Group, being composed largely of a succession of basaltic and andesitic lavas, agglomerates 3 t u f f s and breccias, but with a zone of sedimentary rocks at the base. The lavas and agglomerates are si m i l a r i n colour and type to those of the above group 3 although some agglomerates may be brown3 red or purple. In many places the FIG. 3 . BEDROCK GEOLOGY - LOWER THOMPSON VALLEY (after Duffell and McTaggart 1952) 13 TERTIARY BEDROCK GEOLOGY = LOWER THOMPSON VALLEY LEGEND Miocene or E a r l i e r (V) |COLDWATER BEDS (?): sandstone, shale and con-CRETACEOUS Lower Cretaceous glomerate, coal KINGSVALE GROUP Basalt and andesite; agglomerate, t u f f and breccia Arkose, conglomerate, shale and greywacke SPENCES BRIDGE GROUP A n d e s i t e , d a c i t e s basalt and r h y o l i t e ; t u f f , \ \ \ breccia and aggomerate; sandstone, greywacke, arkose and conglomerate JACKASS MOUNTAIN GROUP greywacke, a r g l l l i t e , conglomerate, arkose © » o 0 O O TRIASSIC OR EARLIER P h y l l i t e , a r g i l l i t e , conglomerate, greywacke Schist and gneiss f fi < i f PERMIAN AND (?) EARLIER CACHE CREEK GROUP Greenstone; chert, a r g i l l i t e , minor limestone and quartzite; c h l o r i t e and quartz-mica schist INTRUSIVE ROCKS JURASSIC OR CRETACEOUS Lower Cretaceous or E a r l i e r MOUNT LYTTON BATHOLITH: Granodiorite, quartz d i o r i t e and d i o r i t e Fault 14 basalts -weather into a coarse earthy debris containing abundant amygdules, as i n the v i c i n i t y of the Drynoch earthflow. The basal sedimentary beds consist of buff to green arkose and g r i t , soft dark mudstone, grey to greenish grey conglomerate, and hard, dark, thin-bedded a r g i l l i t e with some plant fossils„ The southern slope of the Scarped Mountains i s composed of quartz-mica schists and g r a n i t i c gneiss, thought to be metamorphosed rocks of the Cache Creek Group (Permian), (D u f f e l l & McTaggart, 1952, p. 2k). Adjacent to t h i s is the Mount Lytton Batholith (Lower Cretaceous or e a r l i e r ) , composed of granodiorite, quartz d i o r i t e and d i o r i t e . Two other formations occupy r e l a t i v e l y small portions of the v a l l e y . Just above the confluence of the Nicoamen and Thompson Rivers, on both sides of the Thompson, i s a limited area of hard green andesite, green and red porphyritic flows, and agglomerates cut by granite, assigned to the Nicola Group (T r i a s s i c ) on l i t h o l o g i c a l grounds ( D u f f e l l & McTaggart, 1952, p. 2 9 ) . Near Spences Bridge i s a small and poorly exposed outcrop of Te r t i a r y sediments, te n t a t i v e l y assigned to the Coldwater Beds. Impure sandstones and a r g i l l i t e s containing numerous seed or bud capsules are found i n association with a coal seam. 2.3 F a u l ting and Jointing Faulting i s conspicuous i n the west around Lytton. A series of f a u l t s l i e along the eastern edge of the Coast Mountains and bound the Fraser River Cretaceous Belt. I t has been suggested ( D u f f e l l & McTaggart, 1952, p. 87) that the 15 o v e r a l l nature of the movement along t h i s zone -was to elevate, r e l a t i v e l y , the Coast Mountains to the west -with respect to the I n t e r i o r Plateau to the east. The f a u l t zone that extends from the southern end of Fountain Ridge to Botanie Creek and Thompson River (see F i g . 3) i s perhaps subsidiary to the p r i n c i p a l f a u l t zone. Movement on i t has apparently been such as to elevate the intrusive rocks on the west with respect to the lavas on the east. The crag of granodiorite with a p l i t e veins, immediately east of Botanie Creek, i s thought to owe i t s prominence to recent f a u l t i n g along t h i s zone (Drysdale, 1912, p. 125). Both Botanie Creek and Fraser River appear to follow l i n e s of weakness associated with these f a u l t zones. Apparently j o i n t i n g has influenced the topography too, p a r t i c u l a r l y i n the upland surfaces of the mountains southeast of Lytton. Two directions of j o i n t i n g were noted, one s t r i k i n g N 15-20 E, and the other, less well developed, N 30-70 W. Narrow valleys alternating with abrupt ridges l i e p a r a l l e l to the dominant j o i n t i n g ( D u f f e l l and McTaggart, 1952, p. 8 l ) . 2.h Physiography The Thompson Valley i s deeply incised below the gently undulating surface of the I n t e r i o r Plateau, whose higher eleva-tions range between 6,000 and 6,600 f t . (Mimenuh Mt-. 6030 f t . , Zakwaski Mt. and Lookout Point, 6,600 f t . ) . These heights are reached by gentle slopes from an elevation of 3 3500 - -^,000 f t , , the steepest slopes being those from the valley bottom (500 -700 f t . ) to elevations of 3 3500 - ^,000 f t . Southeast of Lytton are the g r a n i t i c Cascade Mountains (up to 6,500 f t . ) which appear 16 to grade almost imperceptibly into the Nicoamen Plateau to the east. To the -west of the Fraser are the Coast Mountains, -which a t t a i n heights of 8,000 - 9 3600 f t , above sea l e v e l ; Skihist Mt. and a few others exceed 9 S500 f t . Cirques are well developed i n these mountains, 2.J+.1 Outline of Drainage History I t i s believed that the present p r i n c i p a l drainage channels were established i n post-Eocene time, as the Thompson above Ashcroft crosses folds i n Eocene beds. Because of the sharp contrast between the gently r o l l i n g upland surface and the deeply incised valleys, i t i s generally held that the region was eroded to a peneplain and subsequently elevated and deeply dissected by master streams. As thi s peneplain transgresses Mesozoic rocks and folded Eocene beds, and as i t s surface was already well developed before the extrusion of the late Miocene/early Pliocene "plateau basalts", i t i s evidently a post-Eocene/pre-Pliocene erosion surface developed during the period from *+5 to 13 m i l l i o n years ago (Mathews, 196*+, p. M-68). I t i s possible that the Thompson River once flowed more d i r e c t l y south to jo i n the Fraser i n the v i c i n i t y of Anderson Creek instead of swinging sharply west at the mouth of Nicoamen River. There i s a marked col at a height of ^ 250 f t . to the east of Mt. Lytton between the headwaters of t r i b u t a r i e s of Nicoamen River and Mowhokam Creek. This is only speculation, and the c o l could e a s i l y be of g l a c i a l o r i g i n , but the sharp westward swing of the Thompson i s unusual. Former heights of the Thompson Valley f l o o r are well recorded i n the rock benches of the southern slope of the 17 Scarped Mountains, and the l a t e r formed terraces cut i n the f l u v l o - g l a c i a l f i l l . The present Thompson River has cut into i t s former rock f l o o r along the length of i t s course from Nicoamen River to Lytton and is now about 100 f t . below the l e v e l i t had reached prior to the l a s t g l a c i a t i o n (see 3.2.22). Many of the tri b u t a r y streams such as Nicoamen River, Botanie Creek and Murray Creek are characterized, i n the r o l l i n g upland region, by low gradients and gently sloping valley sides, but as they approach the Thompson they assume a more youthful appearance with rapids and waterfalls i n t h e i r lower courses. The causes of t h i s w i l l be discussed i n Section 3.2.31.9. The drainage of the area was greatly disorganized during the period of the Pleistocene g l a c i a t i o n with ponding of some streams and the development of lakes i n the Thompson Valley, the Nicola Valley and Botanie Valley. There are many dry cols i n the Plateau region and i n places i t i s evident that g l a c i a t i o n has resulted i n diversions as f o r instance i n the headwaters of Botanie and Skoonka Creeks. Nevertheless, most of the streams have since managed to re - e s t a b l i s h themselves along or near t h e i r former courses. 2.Lf.2 Physiography of the Thompson Valley between Spences Bridge  and Lytton Following Drysdale (1912, p. 12>+), this part of the valley may be divided into two d i s t i n c t i v e sections, the f i r s t extending from Spences Bridge to Nicoamen River, and the second from Nicoamen River to Lytton. 18 2..M-.21 Spences Bridge to Nicoamen River Here the r i v e r flows v i r t u a l l y from north to south across the Lower Cretaceous Volcanics of the Spences Bridge and Kingsvale Groups. Although c l a s s i f i e d by Drysdale as part of his central section extending from the west end of Kamloops Lake to Thompson siding and "characterized by a great depth of f i l l subsequently terraced by the meandering Thompson River", t h i s area is r e a l l y a t r a n s i t i o n a l zone between the more open central section and the rugged Thompson Canyon (see F i g s . ^6 a & b). Despite the deep i n c i s i o n of the r i v e r , i t has only i n a few places reached bedrock. Its course i s characterized by meandering and point bar deposition, which gradually lessen southwards as the v a l l e y narrows. S i m i l a r l y the aggradational f i l l coarsens south-wards from s i l t s to sands and gravels. Terraces and a l l u v i a l fans are well preserved and r i s e from 30 to 500 f t . above the r i v e r , with t h e i r surfaces sloping gently towards the r i v e r . The lower terraces i n p a r t i c u l a r have a veneer of imbricate cobbles (see F i g s . h2 & h3). Slumping and s l i d i n g have been frequent i n the unconsolidated materials. In addition, the Drynoch earth-flow of weathered bedrock i s s t i l l moving, forcing the r i v e r to cut into bedrock on i t s western bank at Skoonkoon. 2. '+.22 Nicoamen River to Lytton Here the r i v e r swings sharply west to cut through the metamorphic rocks of the Scarped Mountains and the adjacent granodiorite of the Mount Lytton batholith. This i s the Thompson Canyon, where the r i v e r has incised i t s e l f through the aggradational f i l l and well into bedrock. Its course i s > P i g . h. Thompson Canyon. Note rock sheds protecting railway l i n e from scree of Scarped Mountains characterized by rapids. The valley sides are steep and much scree i s s t i l l being produced, p a r t i c u l a r l y on the southern slopes of the Scarped Mountains, (Fig* h) -where rock sheds have had to be erected to protect the C.N.R. l i n e . The aggradational f i l l i s coarse and contains a high proportion of angular l o c a l material. The major t r i b u t a r i e s , Nicoamen River and Botanie Creek, have been l e f t hanging so that the present streams descend to the Thompson River i n a series of f a l l s . Terraces are either absent or very narrow i n t h i s the narrowest section of the whole v a l l e y . However,, as the valley widens from Botanie Creek to the confluence with the Fraser, so the terraces become more pronounced. 20 CHAPTER 3  QUATERNARY STRATIGRAPHY 3.1 G l a c i a t i o n of the Lover Thompson Valley During Pleistocene time, at the height of maximum g l a c i a -t i o n , the whole area was covered by ice to a height of approxi-mately 8,000 f t . a . s . l . (see G l a c i a l Map of Canada, 1958). D u f f e l l and McTaggart (1952, p. 69) note the presence of e r r a t i c s near the head of Texas Creek at 8,300 f t . , and s t r i a -tions south of Skihist Mountain on a ridge r i s i n g above 8,000 f t . The general trend of s t r i a e recorded by e a r l i e r workers i s south-east (see F i g . 5)» Evidence of d i r e c t i o n of ice movement i n the area studied i s sparse, but drumlinoid mounds at the head of Botanie Valley trend S 11° E, and roche moutonnees near Soap Lake on the Nicoamen Plateau are oriented S 26° E. This south-eastward trend would suggest that most of the ice came from the northwest. Ice was probably discharged eastward from the i n t e r i o r of the Coast Mountains, but was forced to the south-east across the I n t e r i o r Plateau ( D u f f e l l & McTaggart, 1952, p. 69 - 70). When the ice surface f e l l i t i s l i k e l y that the trend of the Thompson Valley influenced the d i r e c t i o n of ice movement. On the west side of the Thompson Valley at a height of approximately 2,000 f t . the alignment of drumlinoid ridges bet-ween Cornwall Creek and Oregon Jack Creek, near Ashcroft, i s from north-south p a r a l l e l i n g the trend of the v a l l e y . In the section of the Thompson Valley studied here there i s only evidence of one major glaciation.. But from Peterson Creek i n the Kamloops area there i s evidence of two major gl a c i a t i o n s . R. J . Fulton (Armstrong & Fulton 1965 s p. 99) recognized two t i l l s 3 separated by an i n t e r - t i l l deposit of s t r a t i f i e d sand 3 s i l t , clay and gravel containing oxidized plant fragments and gastropod s h e l l s . P a r t i a l l y oxidized wood gave a radiocarbon date of 32371+0 yrs. B.P. f o r t h i s i n t e r - t i l l deposit. (G.S.C.-275; Dyck, Fy l e s , and Blake s 1966). This date correlates with radio-carbon dates f o r the Olympia I n t e r g l a c i a t i o n recognized i n the Fraser Lowland (see p. 7). Also two t i l l s separated by gravel have been reported from the Bonaparte Delta (Armstrong & F u l t o n 3 1965s p. 101-102). I t i s l i k e l y then that there were at least two major periods of g l a c i a t i o n i n the Lower Thompson Valey, separated by an I n t e r g l a c i a l periods which began more than 3237!+0 years ago. It i s thought that a f t e r the l a s t maximum advance of the glaciers i n south-central B r i t i s h Columbia, there was a rapid and extreme change of climate. As a result the ice-sheet seems to have wasted away by down-melting rather than by a retreat of the ice-terminus along a well-defined front (Nasmith, 1962, p. 10). Consequently the uplands became free of i c e , while ice plugs remained i n the valleys damming late g l a c i a l lakes i n many parts of the I n t e r i o r . F i n a l l y the ice tongues i n the valleys became stagnant and disappeared, and the r i v e r s were able to reassert themselves along t h e i r former courses, at f i r s t deposit-ing large amounts of debris and f i n a l l y downcutting. FIG. 6. LOCATION MAP • - paved highways - LOWER THOMPSON VALLEY — logging roads 22 3.2 QUATERNARY DEPOSITS The unconsolidated sediments s that resulted from the above sequence of events, can be divided into four more or less w e l l -defined units on the basis of age and environment of deposition, the oldest unit representing a non-glacial episode prior to the l a s t g l a c i a l advance. The periods corresponding to the units are as follows: 1 ) Pre-last g l a c i a t i o n - r i v e r and lake deposits. 2) G l a c i a l - represented i n the v a l l e y by scattered out-crops of t i l l underlying most other unconsolidated material; i n addition t i l l mantles much of the plateau region. 3) Late g l a c i a l and early p o s t g l a c i a l - represented by -i ) l a c ustrine s i l t s and sands with d e l t a i c gravels related to the retreating stages of the ice sheet when ice-dammed lakes persisted; i i ) f l u v i o - g l a c i a l and f l u v i a l gravels and sand, t a l u s , landslide debris and a l l u v i a l fan deposits. k) Late p o s t g l a c i a l and recent - marked by terrace forma-t i o n with the deposition of a veneer of terrace gravel on the dissected deposits of the period of aggradation; further a l l u v i a l fan deposits and landslide debris. These deposits are described i n d e t a i l from oldest to youngest. F i g . 8. T i l l overlying cross-bedded gravels at Seddall. Note the preferred orientation of pebbles, rudimentary bedding and the sharp horizontal contact, indicating that t h i s i s f l o w t i l l . 23 3 . 2 , 1 . Material deposited prior to the l a s t major ice advance P l u v i a l gravels and sand are seen underlying t i l l at two places in the Thompson Valley and these are i n close proximity. At Drynoch, on the Canadian P a c i f i c Railway s 100 f t . of well sorted sands and gravels are overlain by approximately 15 f t . of unsorted t i l l , which i s i n turn overlain by f i n e gravel and sand (see F i g . 7). At Seddall, on the Canadian National Railway, t i l l i s seen overlying and washed over similar well sorted sands and gravels. Where the exposure i s clear, i t can be seen that t i l l overlies approximately 60 f t . of rounded gray gravels, interbedded with sand. The stratigraphic .position of these eh' gravels and sands suggests that they predate the l a s t ice advance. However, the t i l l at Seddall has a very sharp horizon-t a l contact with the underlying gravels, and exhibits rudimentary bedding and strong preferred orientation of component pebbles, (see F i g . 8) which suggests that i t i s f l o w t i l l (see 3 . 2 . 2 2 ) from the stagnant ice tongue. If t h i s is the case, these sands and gravels are late g l a c i a l and not p r e g l a c i a l . It i s , however, possible that other f l u v i a l deposits may pre-date rather than post-date the l a s t g l a c i a t i o n , the o r i g i n a l overlying t i l l having been completely eroded away. Just north of Seddall station on the Canadian National tracks i s an ex-posure of t i g h t l y packed, f a i r l y well-rounded gravels, some of which are so badly crushed that they are fractured. The t i g h t -packing and crushing might well be due to the weight of an overlying ice mass. At Nicoamen the lowest sands and gravels appear to have been eroded prior to the deposition of the succeeding gravels leaving a disconformity, but i n the absence of dateable material we cannot determine the significance of the hiatus. Material exposed close to the v a l l e y f l o o r , and hence at or below the general l e v e l of t i l l exposures, could pre-date the l a s t g l a c i a t i o n . Just south of the confluence of the Nicola and Thompson Rivers i s an exposure of 20 f t . of fi n e white horizontally bedded sand i n the east bank of the r i v e r , below the l e v e l of t i l l exposed in the opposite bank. A similar exposure occurs south of the Skoonka tunnels beneath a t r e s t l e bridge over S l e e t s i s Creek, at a height of 700 f t . Both appear to be lacustrine deposits, i n which case Lake Thompson may have been over-deepened very l o c a l l y , or the sands were deposited i n a lake formed prior to the l a s t g l a c i a t i o n . I n t e r - t i l l l acustrine sands have been recognized i n the Bonaparte Delta and Peterson Creek (Fulton, 1965b). On the west bank of the r i v e r , just above the C.N.R. tracks at Spences Bridge i s an exposure of ripple-bedded sand on oxidized gravels, overlain by slumped s i l t , unlike anything exposed elsewhere in the area studied. Possibly the sands and gravels are i n t e r g l a c i a l but t h e i r s t r a t i g r a p h i c a l r e l a t i o n s h i p with known material i s not obvious. The limonization of the deposit i s suggestive of age, and i n t h i s respect the deposit i s si m i l a r to the i n t e r g l a c i a l rusty gravel and sand of Peterson Creek, described by Armstrong and Fulton (1965, p. 99). Summary Very l i t t l e can be said about conditions prior to the l a s t advance of i c e , as much of the evidence has long since been eroded away or buried. Some of the lowest gravels i n the v a l l e y may have been deposited prior to the l a s t ice=advance by an aggrading r i v e r . Possibly the valley was dammed at some time with the development of a lake in the v i c i n i t y of Spences Bridge and S l e e t s i s Creek. 3.2.2 G l a c i a l Deposits 3•2.21 Upland Regions Exposures of t i l l , which mantles much of the plateau region are to be found along the Drynoch, Nicoamen and Botanie logging roads. Usually less than f i f t e e n feet of t i l l i s exposed at any one place. T y p i c a l l y i t i s composed of unsorted angular to rounded fragments of rock, of very variable size and composition in a clayey to sandy matrix. Only i n the more open plateau regions, as f o r instance at the head of Botanie Valley, does the t i l l deposit assume any clear morphological form. The o v e r - a l l composition of the t i l l i n a l l three areas is very s i m i l a r . The unweathered matrix varies in colour from very dark red along the Nicoamen road, dark reddish brown in the Drynoch area, to grey i n Botanie Valley, and appears to r e f l e c t the colour of the underlying bed rock. Caliche i s present i n some places where the matrix has been weathered. Included pebbles are generally less than nine inches i n diameter, but i t should be noted that t h i s represents only the top few feet of the deposit. Pebbles of grey, green and red rhy o l i t e s and andesites, black c r y s t a l l i n e limestone and granite/granodiorite are common i n a l l exposures. Too few exposures were studied and i n i n s u f f i c i e n t d e t a i l to draw many v a l i d conclusions about the source of the component pebbles. The r h y o l i t e s and andesites could have come from the l o c a l Spences Bridge and Kingsvale Groups, and the black limestone from the Cache Creek Group (see F i g . 3 and D u f f e l l & McTaggart p. 21). If this i s the case, a l o c a l and northerly source within twenty miles i s indicated f o r these components. The granodiorite may have come from the Coast Intrusions and would thus be l o c a l too. The degree of rounding of the pebbles i s very variable, the granodiorite generally being well-rounded, whereas the green andesites are angular to sub-angular. The shape of the component pebbles can be ascribed to t h e i r previous environ-ment and degree of resistance, rather than to g l a c i a l action. For instance the rounded granodiorite possibly once formed part of a f l u v i a l gravel deposit, whereas the green andesite may have been t a l u s . According to F l i n t (19573 P° 115) stones i n a gla c i e r can be modified only by crushing or by grinding, and these processes cannot operate on a stone unless i t comes into contact, i n flowing i c e , with other rock fragments or with the bedrock, past which the ice i s moving. Occasionally the more resista n t rocks are highly polished. Very few s t r i a t e d or faceted pebbles were found i n these upland regions, but exposures were few and f a r between. 3.2.22 Thompson Valley The t i l l i s well-exposed at a number of places along the valley sides close to the r i v e r . I t i s distinguishable from other deposits by: 1) absence of sorting (see F i g . 12), 2) presence of s t r i a t e d and faceted pebbles, some of ^ . i . i ^ . i . i . i ^ l . i . i . H N . i . i . i f l . i . i . f f l . i r t T i T l . i . t . f l l . i ' f t ^ . i . i . i J l f t . i . F i g . 9. Striated and faceted stones from valley t i l l . Scale in inches (Photo - J . M. Ryder) c h a r a c t e r i s t i c f l a t i r o n shape (see F i g 9 ) s 3) great v a r i a t i o n i n s i z e , shape and composition of included pebbles. The matrix i s generally s i l t y or clayey, but occasionally may be more sandy. In colour i t i s c h a r a c t e r i s t i c a l l y pale red (10R 6/2) to pale red purple (5RP 6/2) when dry and weathered, although i n places i t i s l i g h t brownish gray (5YR 6/1). The colour v a r i a t i o n may be due to varying degrees of oxidation. In the v i c i n i t y of Spences Bridge, exposures of t i l l are limited to the west bank of the r i v e r . Opposite the confluence of the Thompson and Nicola Rivers, about 15 f t . of t i l l occurs, with angular and rounded fragments up to h f t . i n diameter i n a pale red s i l t y matrix. The t i l l whose base i s concealed, i s overlain i n abrupt succession by sand and gravel, and gravel, possibly outwash deposits. Above Mud Lake the base of the t i l l i s not seen and i t underlies deformed sands and gravels, which are overlain by white s i l t (see F i g . ih) 0 The maximum thickness exposed i s i n the order of 50 f t . Included pebbles vary i n size from a f r a c t i o n of an inch to approximately 15 inches i n d i a -meter. Sub-angular fragments of green, grey and dark red f i n e -grained igneous rock, rounded granodiorite pebbles, and occasional fragments of limestone are t y p i c a l . There are both s t r i a t e d and faceted pebbles, and the s i l t y - c l a y matrix i s t y p i c a l l y pale red. Between Mud Lake and Skoonka Creek, there are few clear exposures of t i l l , but i n the recently slumped material above the C. N. tracks , s t r i a t e d and f l a t i r o n pebbles of varied com-position occur i n combination with a pale red s i l t y clay. At one rock F i g . 11. Formation of interbedded deposit of flow t i l l and outwash. (after J . H. Hartshorn 1958) l o c a l i t y two miles from Spences Bridge, t i l l i s interbedded with horizontally s t r a t i f i e d gravels (see F i g . 10). The i n t e r -bedded gravels are similar to those immediately overlying the t i l l i n other exposures, where they presumably represent outwash. The t i l l i s i d e n t i c a l to that i n nearby exposures and has a f i n e s i l t y matrix. The t i l l bands are up to h f t . thick, whereas the gravel beds average 1 f t . i n thickness. It would appear that the sediments exposed here were deposited by a melting g l a c i e r , with ablation t i l l , derived from the ice i t s e l f , alternating with sediments from supraglacial and englacial streams. Hartshorn (1958) describes a similar phenomenon i n S. E. Massachusetts, where l e n t i c u l a r beds of t i l l overlie or are i n t e r s t r a t i f i e d with stream l a i d sand and gravel, which i s generally undisturbed. He terms such t i l l f l o w - t i l l and notes that i t s essential feature i s i t s o r i g i n i n ablation moraine that moves as a mudflow off gl a c i e r ice onto adjacent lower surfaces, as shown i n F i g . 11. F l o w t i l l is forming today at the margins of stagnant ice; G. S. Boulton (1967) describes an occurrence from the margin of the f r o n t a l stagnant zone of Zorbreen Ny Fri e s l a n d , Vestpitsbergen, and Fahnestock (1963, p. A20-A21) notes a similar occurrence i n the White River area on Mt. Rainier. This exposure of flow t i l l interbedded with gravel indicates the former presence of stagnant ice i n the Thompson Valley. A large quantity of t i l l i s exposed near the mouth of Skoonka Creek. F i f t y feet or more of t i l l with a pale pink silty/sandy matrix i s exposed beneath 100 f t . of f l u v i a l and c o l l u v i a l material. The t i l l appears to be roughly bedded, perhaps F i g . 12. T i l l from Skoonka Creek. Note the va r i a t i o n in size and shape of the component pebbles F i g . 13. T i l l exposed in r i v e r bank at toe of Drynoch Earthflow simply as the res u l t of deposition of ablation t i l l on lodgment t i l l - I t contains boulders up to 3 f t . in diameter, but generally l e s s s varying i n shape from sub-angular to sub-rounded (see F i g . 1 2 ) . As usual, the fine-grained igneous rocks thought to be of l o c a l o r i g i n predominate, but crushed granodiorite and s t r i a t e d black c r y s t a l l i n e limestone are common, so that the valley t i l l i s s i m i l a r i n composition to the upland t i l l (see 3 . 2 . 2 1 ) . Again the base i s not seen. Along the C. P. R. tracks from Spences Bridge to the Drynoch earthflow, t i l l i s f i r s t seen exposed i n a culvert by the track, one mile from Spences Bridge, adjacent to and lower than gravels and white s i l t . There i s about 20 f t . of l i g h t brownish gray t i l l with some rounded granodiorite, but containing mainly f l a t i r o n and sub-angular stones,' some with s t r i a t i o n s . The t i l l i s overlain by r i v e r gravels. Southwards from here, there i s a great deal of recently slumped material with s t r i a t e d and f l a t i r o n pebbles of varied composition i n combination with a pale red s i l t y clay, as on the opposite bank of the r i v e r (see p.27 ). In places, as f o r instance near Wray's F r u i t Stand, one-and-a-half miles from Spences Bridge, the t i l l i s more c l e a r l y exposed. Here f i f t e e n feet of t i l l , apparently interbedded with g l a c i o -f l u v i a l gravels, i s exposed above the CP.R. tracks, and the recently slumped exposure between the railway and the highway below appears to consist e n t i r e l y o f , t i l l (about 50 f t . ) . The t i l l i s overlain by either outwaih or f l u v i a l gravels. T i l l pebbles include limestone, granodiorite, and l o c a l volcanics. T i l l i s again exposed by the r i v e r , beneath the Drynoch earthflow (Fig. 1 3 ) . The t i l l , of which about ^0 f t . is exposed, 30 underlies r i v e r terrace gravels 3 and i t s base is not seen. I t i s s i m i l a r i n colour and composition to the t i l l i n the culvert. The contrast i n colour bet-ween these t-wo l i g h t brownish gray (5 YR 6/1) t i l l exposures and the more common pale red (10R 6/2) t i l l could simply be the result of weathering and oxidation., the l a t t e r t i l l having been exposed f o r a longer period. The other alternative i s that the gray t i l l represents a d i f f e r e n t g l a c i a l episode s but there i s no supporting evidence f o r t h i s . From Skoonka to Seddall, t i l l occurs i n scattered outcrops and is i n a l l respects s i m i l a r to that elsewhere in the v a l l e y . Just south of the Skoonka tunnels i s an exposure of horizontally interbedded sands and gravels and t i l l , very s i m i l a r to that shown in F i g . 10. The t i l l contains s t r i a t e d and f l a t i r o n stones and the i n d i v i d u a l beds vary from 2-k f t . i n thickness. Sandy beds average 6 i n s . and gravel beds 1 f t . i n thickness and are si m i l a r to ice-contact gravels seen elsewhere. I t i s f e l t that these deposits are the res u l t of alternate phases of t i l l and g l a c i a l stream depositions as described on p„28 . The whole outcrop i s about 20 f t . thick. It has already been mentioned.(see 3»2.1) that t i l l , thought to be f l o w t i l l 3 overlies well-rounded gravels and sand at both Seddall and Drynoch. The t i l l i n question has the usual pale red s i l t y matrix with rounded, s t r i a t e d , angular and f l a t -i r o n pebbles. Ah example of a s t r i a t e d f l a t i r o n pebble from the Drynoch t i l l i s shown in F i g . 9. From here to Lytton there appears to be very l i t t l e recog-nizable t i l l . Near Botanie Creek about 10 f t . of t i l l - l i k e material, ( t i l l - l i k e In the sense that the deposit consists of unsorted pebbles i n a f i n e matrix but i t i s poorly exposed) overlies a bedrock bench standing about 100 f t . above the l e v e l of the present r i v e r . This suggests that the Thompsons i n the Botanie section, had excavated a va l l e y to within 100 f t . of i t s present depth before the end of g l a c i a t i o n . 3.2,2 Summary The t i l l exposed i n the va l l e y bottom i s generally less than 50 f t . t hick, and t y p i c a l l y has a pale red s i l t y matrix and contains pebbles of red, green and grey fine-grained igneous rocks, granodiorite and limestone. The upland t i l l has an es s e n t i a l l y s i m i l a r composition. The pebbles are compatible with a r e l a t i v e l y l o c a l source (within twenty miles), granodiorite, limestones, r h y o l i t e s , and andesites being abundant i n the immediate area and farther to the north. For the most part the t i l l i s massive and there i s l i t t l e i ndication of bedding, but in some places t i l l was found horizontally interbedded with sands and gravels, believed to have been deposited by meltwater streams. Such interbedding of t i l l and meltwater deposits i s interpreted as r e s u l t i n g from alternating phases of f l o w - t i l l and stream deposition and indicates the former presence of stagnant i c e . Most of the t i l l occurs in the section from Spences Bridge to Nicoamen, exposures from Nicoamen to Lytton being few and f a r between. As the valley narrows at Nicoamen, i t i s possible that the sc a r c i t y of t i l l exposures i s due to erosion by meltwater and the Thompson i t s e l f , aided by slumping of t i l l down the steep valley sides. The t i l l i s overlain i n places by outwash and i c e -contact sands and gravels, i n which case the contact i s probably Mudflow T i l l White s i l t Slump f o l d (see F i g . 15 F i g . l * f . Exposure above Mud Lake, Spences Bridge Normal f a u l t F i g . 15. Slump f o l d in ice contact gravel Trench t o o l f o r scale 32 conformablej and i n others by f l u v i a l gravels and coILuvium, i n which case some t i l l may have been eroded prior to the deposition of the f l u v i a l and c o l l u v i a l material. 3 « 2 . 3 Late G l a c i a l and Early Post-Glacial Deposits 3 . 2 . 3 1 The Lacustrine White S i l t Facies The most readi l y recognizable fac i e s of l a t e g l a c i a l time i s the lacustrine white s i l t f a c i e s , which i s best developed around Spences Bridge. Where the base of the. white s i l t s i s seen, they rest upon 10-15 f t . of poorly sorted sand and gravel, which i n turn overlies t i l l (see F i g . 1*0. They are overlain i n places by c o l l u v i a l and mudflow material and i n others by a th i n veneer of f l u v i a l gravel. They do not appear to extend any further south than Drynoch, but are well developed northwards towards Ashcroft and extend f o r some distance up the Nicola Valley. In t h i s section of the Thompson Valley they are not found above an a l t i t u d e of 1 ,230 f t . 3 . 2 . 3 1 . 1 Ice-contact gravel The gravel underlying the white s i l t i n the Thompson Valley t y p i c a l l y consists of predominantly sub-angular fragments of red, green, and grey fine-grained igneous rock, and scattered limestone pebbles, generally less than 2 i n s . i n diameter. Beds of s i l t and sand are interbedded with the gravel, i n d i v i d u a l beds averaging 0 . 5 f t . i n thickness or even l e s s . In places the gravel beds are horizontal but near Murray F a l l s they dip at angles up to 2 5 ° - 30° towards the east, and above Mud Lake they are strongly deformed, being folded and faulted (see F i g . 1 5 ) . The deformation could be due either to the action of an over-r i d i n g g l a c i e r or to slumping, perhaps at an ice-margin. The a x i a l l i n e of the main f o l d s t r i k e s southeast and i s therefore at right angles to the trend of the present v a l l e y and to the d i r e c t i o n of ice r e t r e a t , and p a r a l l e l to the main south-easterly d i r e c t i o n of ice advance. Unfortunately the f o l d i s an isolated exposure, hence i t cannot be determined what happens at either side nor at depth, but the beds appear to become more or less horizontal to the northeast. Further, the material above the f o l d i s inaccessible and hence cannot be examined in d e t a i l . As the overlying material i s not c l e a r l y exposed beneath the s i l t , which has washed over the face, i t could be either t i l l or gravel. The material involved in the f o l d i n g resembles that seen d i r e c t l y below the white s i l t in other exposures. On the northern limb of the main f o l d i s a well developed normal f a u l t , and pseudo-rippling i s apparent in the crest of the f o l d (Fig. 15). The evidence suggests slumping at an ice margin. The following factors favour t h i s interpretation: 1) the a x i a l l i n e of the f o l d is at right angles to the d i r e c t i o n of ice r e t r e a t , and p a r a l l e l to the d i r e c t i o n of ice advance. i ce advance ice retreat F i g . 16. Axis of f o l d in r e l a t i o n to ice movement 34 2) normal f a u l t i n g i s compatible with slumping, whereas overthrusts tend to be associated with overriding (see MacClintock & Dreimanis, 1961*, p. 133-l1+2). 3) pseudo-rippling can r e s u l t from compression of the inner layers of a slump f o l d (Kuenen 191+8) h) the s i m i l a r i t y of the material to known ice-contact de-posits, and the lack of d e f i n i t e evidence of overlying t i l l . These sands and gravels with t h e i r abrupt changes in grain size and slumping, immediately overlying t i l l , are t y p i c a l of ice-contact and outwash deposits at the margin of a melting g l a c i e r . The boundary between these ice-contact deposits and the overlying white s i l t , where i t i s not obscured by slumping, i s sharp rather than gradational, and varies from horizontal to highly i r r e g u l a r . R. J , Fulton (1965 5 p. 559=560) found si m i l a r outwash deposits beneath s i l t i n the South Thompson Valley, while -J. M. Ryder (1970, p. 131) interpreted gravels i n a s i m i l a r s t r a t i g r a p h i c position i n the Thompson Valley as ablation moraine dumped d i r e c t l y from the body of stagnating i c e . Presumably the gravels were deposited during a phase of ice-contact deposition with the s i l t l a i d down during a .later phase of lake deposition. 3.2.31.2 The Spences Bridge S i l t The white s i l t s are best exposed at Spences Bridge by Murray F a l l s (Fig. 19) and above Mud Lake (Fig. lM-), hence they w i l l be referred to as the Spences Bridge s i l t . Maximum thick-ness of the s i l t i s i n the order of 200 f t . The s i l t at f i r s t sight appears massive with well developed v e r t i c a l j o inting i n the upper portion. On closer inspection i t i s seen to be ho r i z o n t a l l y layered. The layers are not pronounced here, but i t i s thought that they may represent varves, being very si m i l a r t o 5 although less d i s t i n c t i v e than, the annual rhythmites des-cribed by Fulton (1965» p. 566) i n the South Thompson s i l t . S i l t layers near the base of the deposit may be up to 30 ins. thick but they become thinner upwards. In colour the s i l t i s white (lOYR 8/2) when dry but when wet i t becomes darker (10 YR 7/2). In fact the s i l t i n many places appears to be composed of two d i f -ferent layers, a l i g h t upper layer and a darker lower one. The colour v a r i a t i o n may well be due to moisture content rather than to a fundamental difference i n composition. The s i l t i s so f i n e that compositional analysis using common o p t i c a l procedures is exceedingly d i f f i c u l t . When f i n e sand-sized grains from the s i l t were studied, quartz was seen to be the main constituent, with b i o t i t e and f e l s p a r major constituents, and rock fragments and ferromagnesian minerals minor constituents. S i l t i s of course the dominant grain s i z e . The composition suggests derivation from g r a n i t i c rocks, and i t i s notable that granodiorite above Lytton airport weathers to a material remarkably l i k e the white s i l t . Furthermore the s i m i l a r i t y of the white s i l t and the underlying t i l l matrix was noted in a number of l o c a l i t i e s (3.2.31.6). These observa-tions accord with Fulton's (19659 p. 568-9) suggestion that the f i n e white s i l t was winnowed from t i l l by meltwater. One would expect t i l l derived from ice from the Coast Mountains and the I n t e r i o r Plateau to be r i c h i n g r a n i t i c materials from the F i g . 17. Folding of s i l t bands in Spences Bridge s i l t . Note c h a r a c t e r i s t i c jointing too. F i g . 18. T i l t e d "varves" near top of Spences Bridge s i l t , Mud Lake various batholiths. Rather than being derived d i r e c t l y from winnowing of t i l l , much of the Spences Bridge s i l t could well have been derived from s i l t previously deposited i n the e a r l i e r and higher stages of the g l a c i a l lakes i n the Thompson and Nicola Valleys. This might account f o r the paucity of w e l l -marked varves at Spences Bridge and Tsingkahtle, 8 miles north of Spences Bridge (Ryder, 1970, p. 131+)s as the varves of the South Thompson s i l t were produced by a strong summer i n f l u x of s i l t - l a d e n waters from a large, recently deglaciated watershed. The white s i l t i s f o r the most part of very uniform appearance and grain-size, but i n places i t contains pebbles, and p a r t i c u l a r l y i n the upper sections the bedding i s disturbed. Where there i s a concentration of gravel, t h i s probably indicate the presence of a freshet. More often the pebbles are scattered i n which case i t i s possible that they were i c e - r a f t e d . In some places, as at the apex of the Sportsman Fan i n Spences Bridge, the pebbles scattered throughout the white s i l t s are angular and of immediately l o c a l o r i g i n , i n which case they represent talus f a l l e n from the slopes above into the l a t e - g l a c i a l lake. Where the bedding i s distorted i t may involve s i l t alone, in which case the bands of s i l t may be t i l t e d , as i n F i g . 18, or folded, as In F i g . 17. Fulton (1965 3 p. 56*+) noted similar disturbances i n the South Thompson s i l t and attributed them to the movement of grounded icebergs. In other places the d i s -t o r t i o n involves sand and gravel as w e l l , and what look l i k e balls of sediment occur with concentric rings of s i l t , sand and f i n e gravel, with the grey sand having the appearance of r i p -p l i n g . A number of such balls of sediment were noted above Murray D e l t a - F a n Murray F a l l s Murray F a l l s and a number occur farther north near the top of the white s i l t s exposed i n Tsingkahtle Creek, eight miles north of Spences Bridge (Ryder, p. 1 3 7 ) . The balls of sediment with gravel occur i n close proximity to the Murray Delta, and those in Tsinkahtle Creek must have been close to the shoreline of Lake Thompson. I t i s possible that slumping was responsible f o r the creation of these disturbances. Ph. H. Kuenen (19*+8) attributed the formation of s i m i l a r "slump b a l l s " to the detach-ment of beds of interlaminated s i l t and f i n e sand from the t i p s of overfolds as slumping occurred. The rippled appearance of the sand results from compression of the inner layers of the slump b a l l as the contortion took place; a s i m i l a r phenomenon was noticed i n the slump f o l d beneath the s i l t above Mud Lake ( 3 . 2 . 3 1 . 1 & F i g . 1 5 ) . 3.2.3L3 Murray Pelta-Fan There i s an extensive deposit of gravel, apparently over-l y i n g white s i l t and adjacent to i t , exposed at the top of the Spences Bridge S l i d e (Fig. 1 9 ) . The base of the deposit i s not seen as i t was involved i n the l a n d s l i d i n g ; only 1*+ f t . of gravel i s actually exposed, but the t o t a l thickness of the de-posit i s greater than 50 f t . , as s i m i l a r gravels are exposed 50 f t . below the top of the d e l t a i c feature i n a road cut above Murray Creek. The top ten feet of the gravel i s horizontally bedded, but the lower four feet i s cross-bedded with apparent dips of 2k°to the southeast and southwest. The gravel, which i s composed of l o c a l grey, green and red andesites and r h y o l i t e s , i s poorly bedded and poorly sorted with sub-angular fragments predominating., I t s actual r e l a t i o n s h i p with the white s i l t i s obscure. However, i t can be traced f o r some distance up Murray Creek, where i t overlies t i l l and lacustrine sand (Fig. 27a). The cross-bedding i s sim i l a r to foreset-bedding occurring i n d e l t a i c deposits.. This, coupled with the position of the deposit, i t s a l t i t u d e of 1,280 f t . , and the l o c a l nature of i t s component materials, suggests that the deposit records the former existence of a delta b u i l t into an ice-dammed lake, presumably late g l a c i a l Lake Thompson (3.2.31-h) 3 by a stream draining the uplands of Murray Valley. The upper beds could be the result of fan rather than d e l t a i c deposition. 32.31A Lake Thompson/Lake Deadman The white s i l t s were f i r s t described by G. M. Dawson (l891 3 189^)3 who concluded that they were deposited i n deep tr a n q u i l waters by streams and r i v e r s flowing from melting g l a c i e r s , and that at the time of s i l t deposition ice occupied those parts of the valleys that are free of s i l t today. However, he f e l t that the correspondence of the upper l i m i t s of various s i l t deposits and lack of evidence of dams indicated that the s i l t was deposited i n fjords connected with the P a c i f i c Ocean. Daly (1915) thought i t more probable that the s i l t s were de-posited i n ice-dammed lakes. W. H. Mathews Qykh) from a study of old shorelines and associated deposits^ecognized a series of l a t e - g l a c i a l lakes i n south central B r i t i s h Columbia, one of which was Lake Thompson, He concluded that there were a number of stages of development, as the ice plugging the Thompson Valley to the west slowly melted. Two of the most prominent stages 39 were the 1,800 f t . and 1,600 f t . stages at which time ice occupied the Thompson Valley, south of Ashcroft (Fig. 20a). He envisaged a late stage of Lake Thompson with ice plugging the Thompson Valley just south of Spences Bridge s and an arm of the lake extending down the Nicola Valley approximately as f a r as Kloklowuck Creek (six miles from i t s mouth). A prominent shore-l i n e at the lj'+OO f t . l e v e l and corresponding deposits of gravel and s i l t were noted at and east of Ashcroft. J . M. Ryder (1970) noted that white s i l t occurred i n the section from Ashcroft to Spences Bridge i n connection with a marked bench varying i n height from 1,^00 f t . at Ashcroft to 1,250 f t . near Spences Bridge. A recent paper by Fulton (1969) gives a more detailed account of the various stages of Mathews' Lake Thompson. Fulton (p. 7) refers to the l a t e stages, when'the water level.had f a l l e n low enough to expose the South Thompson s i l t east of Kamloops, as g l a c i a l Lake Deadman, after the Deadman River at the west end of Kamloops Lake. He suggests that t h i s lake had two stages. During the e a r l i e r Tranquille stage, the Thompson Basin outlet was temporarily s t a b i l i z e d at an a l t i t u d e of about l'+OO f t . (at Tranquille waters were at lh05 f t . ) and i t s waters extended west at least to the mouth of the Nicola River and an unknown distance north along the North Thompson River Valley. The l a t e r Durand stage marked the lowest eastward draining lake to occupy the Thompson Basin. Fulton suggests that s t a b i l i z a t i o n of the water plane at an a l t i t u d e of 1,230 f t . occurred when the outlet channel cut i n the South Thompson s i l t became graded to the l e v e l of g l a c i a l Lake Shuswap-Tappen stage at 1,220 f t . elevation. This stage of g l a c i a l Lake Deadman extended north i n the North Thompson Valley and west of the mouth of the Nicola River i n the Thompson Valley (Fig. 20b). As no evidence of i s o s t a t i c t i l t i n g was detected 9 .Fulton -(1969, p* 7) suggested that t h i s stage of g l a c i a l Lake Deadman occurred a f t e r the period of major i s o s t a t i c adjustment. Evidence f o r the height of the lake and the distance i t extended down the Thompson Valley, south of Spences Bridge i s scant. There i s a d e f i n i t e l e v e l at 1230 f t . marking the present top of the white s i l t s by Murray F a l l s and at the con-fluence of the Nicola and Thompson Rivers. From Spences Bridge the white s i l t s can be traced continuously only as f a r as Skoonka Creek, four miles south of Spences Bridge. On the other hand, a few scattered outcrops of white s i l t , very limited i n v e r t i c a l and horizontal extent but s i m i l a r to the Spences Bridge exposure, occur south of Skoonka, as f a r as Seddall. There i s a considerable quantity of t i l l i n Skoonka Creek overlain by gravel (3.2.22), which could have been deposited by a melting ice plug, but the evidence i s by no means conclusive. At Nicoamen the v a l l e y a l t e r s d i r e c t i o n so that the r i v e r i s flow-ing from east to west instead of from north to south, and the v a l l e y narrows considerably. Furthermore, there i s a marked difference i n the character of deposits, large quantities of gravel, often of l o c a l o r i g i n , occurring i n t h i s t r a c t . I t i s possible that the margin of the ice lobe plugging the Thompson Valley once stood f o r a time at Nicoamen. Certainly there i s evidence of the past presence of stagnating ice i n the section from Nicoamen to Lytton (3.2.31.10). The ice would probably FIG. 21. LOCATION MAP - LOWER NICOLA VALLEY 41 be preserved f o r a longer period i n thi s deep narrow canyon, and talus from the steep valley sides would protect the ice from ablation. I t i s possible, too, that the ice was at i t s thickest here, so close to the supply region of the Coast Range Mountains. 3.2.31.5 Late G l a c i a l Lake Stages i n Lower Nicola Valley In Lower Nicola Valley there i s evidence of two phases of white s i l t deposition. The white s i l t s associated with Lake Deadman are most c l e a r l y exposed from the mouth of the r i v e r to the road bridge (121 17' 4-5", 50 2k* 30"), 2.5 miles from the mouth, but can be traced approximately as f a r as Kloklowuck Creek. Their maximum exposed thickness is 150 f t . and they are not found above a height of 1,230 f t . At Spences Bridge the s i l t s form a recognizable u n i t , but going upstream they are found interbedded with t i l l and outwash deposits forming an ice-front complex (3.2.31.6). At Dot there i s another prominent exposure of white s i l t , up to 150 f t . thick and extending up to an a l t i t u d e of 1750 f t . This outcrop, which w i l l be referred to as the Dot s i l t , extends from Shakan Creek to No-name Creek, a distance of 5 miles. The s i l t has well-developed 'varves 1 up to •+ f t . thick and contains pseudo-karst features such as piping. To the northwest i t merges into the ice-front complex, and to the southeast the valley i s constricted and there i s a marked absence of any deposits u n t i l Prospect Creek i s reached. It i s possible that t h i s constricted section of the val l e y was plugged by ice at the time of deposition of the Dot s i l t s . If t h i s was the case, the Dot s i l t s must have been deposited i n a small lake between two ice plugs, as were the South Thompson s i l t s (Fulton, 1965 & F i g . 2 0 a ) s which the Dot s i l t s strongly resemble. A l t e r n a t i v e l y these s i l t s were deposited i n Lake Merritt, though t h i s does not seem very l i k e l y . The former outlet for Lake M e r r i t t , between Stump and Napier Lakes, i s now at a height of 2,550 f t . (Fulton 1969 s p. 5 ) 9 and even allowing f o r the maximum i s o s t a t i c t i l t of 16 feet per mile down to the south (Fulton, 1969s p. 5) the lake l e v e l would have to have been over 1,950 f t . Furthermore, thick deposits of white s i l t are not c h a r a c t e r i s t i c of Lake Me r r i t t , except in l o c a l i z e d areas. The Dot s i l t s could possibly have been deposited i n a l a t e low-level stage of Lake M e r r i t t , which drained towards the Thompson, but t h i s does not explain the gap i n the sedimentary record between Dot and Prospect Creek. Furthermore, there i s a broad col at a height of 2,080 f t . on the south side of the valley from Prospect Creek to Pony Creek which i s well situated to have been an i c e -marginal drainage channel f o r a higher l e v e l of Prospect Creek draining towards Lower Nicola, This would suggest that the main val l e y was blocked by ice at t h i s point, 3 .2.31.6 Ice-front Complex of the Lower Nicola Valley Clues to the drainage history of the lakes i n the Nicola and Thompson valleys are to be found i n the nature of the deposits adjacent to the white s i l t s ; ice-marginal drainage channels; and dating of organic remains i n former lake outlets and along the major r i v e r systems. I t has been mentioned above that there i s a complex unit of interbedded t i l l , outwash and s i l t between the Dot and Spences Bridge s i l t s . This complex i s well exposed at the mouth of Skuhun Creek on the southeastern side of the TABLE I. SKUHTJN CREEK FAN EXPOSURE Unit Thickness Material Colour F t . 10 8« Bedded mudflow 10 YR 6/2 9 10' Rounded to subrounded cobble gravel of variable composition N 6 with much granite/granodiorite 8 10* Sand with some s i l t 5 Y 5/2 7 5' Laminated s i l t 5 Y 8/1 6 l 5 ' Unsorted gravel •= predom. sub-rounded/rounded of varied compo- 10 YR 7/2 s i t i o n , with chunks of s i l t mixed in 5 5' Sand, s i l t and gravel:= .5' f i n e sand with flame structureslO YR 8/3 .5' f i n e gravel (up to .15" diam.) 5 Y k/1 .5" coarse sand w. lumps of s i l t 5 Y 6/1 1.5' f i n e sand with flame structures and lumps of s i l t 10 YR 8/3 2.0' gravel (up to .5" diam. but gen. less) 5 Y k/1 k 50+' Horizontally bedded sand with load structures. Contains some s i l t 10 YR 8/3 bands near top 3 5' S i l t 2 2' Dark o l i v e grey gravel 1 k0* Mixture of s i l t and gravel but not well exposed a l l u v i a l fan. The sequence of beds i s shown i n Table I, and the units may be interpreted as follows. Unit 10 is a t y p i c a l a l l u v i a l fan deposit overlying terrace gravels of the Nicola (Unit 9)s which in turn overlie sand and laminated s i l t (Units 8 & 7)j which are either lacustrine or f l u v i a l deposits from a time of quiescence. Unit 6 could be either a mudflow deposit r e s u l t i n g from slumping of l a t e - g l a c i a l deposits down the valley sides or more probably t i l l . Unit 5 i s similar to the outwash sands and gravels s underlying the white s i l t at Mud Lake (3.2.31.1)s whereas the sands with load structures (Unit k) are t y p i c a l l y l a c u s t r i n e s as are the s i l t s of Unit 3. Units 2 and 1 could represent the e a r l i e s t outwash and t i l l . The next clear exposure downstream from here i s about two miles away. The top unit i s mudflow once again, and t h i s time volcanic ash (3.2.V}) i s present 2 f t . above the base of the 8 f t . mudflow bed. The mudflow rests on 2-8 f t . of s i l t y material with sand and gravel lenses (10YR 7 A ) S which i n turn rests on k f t . of terrace cobbles, probably the equivalent of Unit 9 of the Skuhun exposure. Beneath this i s 10 f t . of cobbles and s i l t , a l l jumbled together, which could well be the equivalent of Unit 6, This i n turn overlies U-0 f t . of pre-dominantly f i n e gravel with some coarser beds dipping upstream, which could be a coarser equivalent of Skuhun Unit h. The upstream dip suggests that water in t h i s section of the Nicola Valley drained southeastwards f o r a time instead of northwest. The downstream coarsening of the sediments, and the orientation of flame structures i n sands and other upstream dips observed elsewhere i n the v a l l e y lend emphasis to t h i s idea. F i g . 23. Units If, 5 & 6 of Skuhun Creek Fan Exposure F i g . 2 3 . Ice-front Complex deposits at Luckachin Creek (Photo J . M. Ryder) F i g . 2h. T i l l overlying gravels 1 mile south of the two road bridges, Nicola Valley (Photo J . M. Ryder) F i g . 25. S i l t interbedded with t i l l at Nicola twin bridges. (Photo J . M. Ryder) Similar deposits were noted at a number of l o c a l i t i e s along the Nicola V a l l e y , as f o r instance at Luckachin Creek s where t i l l - l i k e materials gravel and s i l t were a l l mixed together with s i l t predominating towards the top(Fig. 23), One mile south of the two road bridges over the Nico]a 3 t i l l was observed overlying dark oliv e grey gravels with s i l t lenses (Fig. 2 h ) . At the prominent exposure by the two bridges 3 s i l t was seen to be interbedded with t i l l s with s i l t again pre-dominating towards the top (Fig. 25). The stones i n the t i l l are faceted and s t r i a t e d and of very variable composition. The t i l l matrix and the s i l t are almost i d e n t i c a l i n colour 5 pre-sumably because much of the s i l t represents the f i n e f r a c t i o n of former g l a c i a l sediments. The s i m i l a r i t y of these complex deposits suggests a common o r i g i n . The presence of both t i l l and outwash material indicates deposition very close to the ice margin 5 whereas the s i l t i n -dicates the presence of a lake. I t i s suggested that the deposits mark the slow north-westward recession of an ice-tongue damming the Lower Nicola Valley, so that lacustrine s i l t s at f i r s t alternated with t i l l and outwash from the melting i c e -front and ultimately covered the ice-front deposits, as the lake extended downstream with the recession of the ice-tongue. 3.2.31.7 Marginal Drainage Channels of the Lower Nicola and  Lower Thompson Valleys With the uplands being free of i c e , as i s evidenced from t a l u s , landslide deposits (at Dot) and f l u v i a l gravel incor-porated i n the white s i l t , many tr i b u t a r y streams must have FIG. 26. ICE-MARGINAL FEATURES - LOWER THOMPSON VALLEY < > col (&/22/> approximate extent of lake postulated former course of stream 45 drained down the valley sides to the stagnant ice mass i n the centre of the v a l l e y . Consequently one might expect to f i n d evidence of ice-marginal and even sub-glacial drainage. The evidence i s , however, very sparse as the steep valley sides provided l i t t l e opportunity f o r the development or preservation of ice-contact features. The broad channel at 2,080 f t . above Canford has already been mentioned. A c o l at 1,950' between Twaal Creek and the Sportsman Fan Creek (Fig. 26) near Spences Bridge, i s well situated to have been a l a t e r a l drainage channel. It could, however, simply be the res u l t of stream capture or diversion. There i s a marked col moulded by running water, to the east of Spences Bridge at an a l t i t u d e of approximately 1,250 f t . The c o l is open at both ends and free of deposits, but i t i s very short, about ^ mile, perhaps because the water was flowing largely on or within the valley i c e . Another possible l a t e r a l drainage channel l i e s at a height of 1,650 f t . and p a r a l l e l s the r i v e r f o r one mile near Pitquah (Fig. 26). In a narrow valley such as the Thompson, i t i s d i f f i c u l t to say i f these cols r e l a t e to any of the higher terrace l e v e l s , but i n the wider sections of the Lower Nicola a more detailed study of terrace levels would be worthwhile. The present limited knowledge of marginal drainage i n t h i s area provides l i t t l e help i n working out the d e t a i l s of g l a c i a l lake drainage. 3.2.31.8 Drainage of Lakes Merritt and Deadman Lake Merritt o r i g i n a l l y drained north v i a a c o l , at a present elevation of 2,550 f t . , between Napier and Stump Lakes (Fig. 20a) to the South Thompson Valley at Barnhardt Vale (Fulton 1969s p. 5 ) 3 u n t i l continued ice recession opened the Lower Nicola Valley. Drainage was probably resumed down the Nicola Valley before i t was completely ice f r e e , as i t has already been observed that lacustrine deposits at Dot appear to be too low i n a l t i t u d e to belong to Lake M e r r i t t . Possibly there was a stage when water i n the d i f f e r e n t sections of the Nicola Valley drained i n opposite d i r e c t i o n s 3 as has been suggested f o r the Durand stage of g l a c i a l Lake Deadman (Fulton 1969s P» 8). The northwestward recession of the ice down the Lower Nicola Valley i s c l e a r l y recorded in the ice-contact and lacustrine deposits between Skuhun Creek and Nicola twin bridges (3.2.31.6). Actual reversal of drainage in the Nicola Valley was most l i k e l y aided by d i f f e r e n t i a l i s o s t a t i c t i l t i n g . Fulton (1969s p. 5) states that the Lake Merritt shoreline has been deformed on a N 63° E isobase, with t i l t i n g varying from 3 f t . per mile to 16 f t . per mile down to the south. This i s o s t a t i c t i l t i n g most l i k e l y took place before the development of the Durand stage of Lake Deadman, as Fulton (1969s p. 7) suggests that t h i s stage occurred a f t e r the period of major i s o s t a t i c adjustment. If t h i s i s so 9 modern drainage of the Nicola Basin must have been established prior to t h i s stage of Lake Deadman so that the whole of the Nicola Basin drainage must have emptied into t h i s lake. As the Lower Thompson Valley, south of Spences Bridge 9 was s t i l l blocked by an ice plug, the waters of the Lower Nicola Valley would tend to back up to form an arm of G l a c i a l Lake Deadman u n t i l such time as drainage was resumed southwest down the Lower Thompson to the Fraser. In the Thompson Valley south of Spences Bridge, unlike the Nicola V a l l e y , there is l i t t l e evidence of eastward drainage or of the southwestward recession of an ice tongue. The only i n d i c a t i o n of eastward drainage i s i n the dip of the outwash gravels near Murray F a l l s (3.2.31.1), and of recession i n the slumped exposure above Mud Lake. The lack of evidence i s presumably due to the proximity of the stagnating ice tongue and the steepness of the valley sides. Fulton (1969) suggests that Lake Deadman o r i g i n a l l y had an outlet to the east through a channel cut i n the South Thompson s i l t , from whence i t drained into g l a c i a l Lake Shuswap, which drained into the North Okanagan Basin through the Otter Lake spillway (Fig. 20b). As yet i n s u f f i c i e n t evidence has been found of the actual i n i t i a t i o n of westward drainage. As white s i l t s , as a d i s t i n c -t i v e u n i t , can be p o s i t i v e l y i d e n t i f i e d only as f a r as Skoonka Creek and no possible shorelines have been found south of here, i t seems reasonable to suppose that the ice no longer formed a water-tight barrier once i t retreated south of Skoonka Creek. Collapsed s i l t and t i l l , occasionally interbedded, are found on. the western bank of the Thompson from Skoonka tunnels to Seddall, which suggests that the lake waters extended over the decaying ice-tongue i n t h i s l o c a l i t y . South of Seddall there i s an abrupt change to aggradational gravels, and i t i s thought that an ice-plug remained i n the Seddall area, after the southern section of the v a l l e y was i c e - f r e e (3.2.32.2). Whatever the ultimate mechanism of i n i t i a t i o n of westward drainage, once i t was established headward erosion would capture and drain the lakes i n the Thompson farther east. Fulton (19693 p. 8) suggests that continued headward erosion i n the South Thompson Valley 48 caused, the capture of the Shuswap Basin from the Okanagan-Columbia River drainage. Organic s i l t from the Otter Lake spillway has been dated as 8,900 ± 150 years B.P. (G.S.C.-193, Dyck et a l . , 1965), which suggests that Lake Deadman had been drained by t h i s time and modern drainage established. Furthermore, Borden (196l) has shown that Indians occupied the Fraser Valley at Yale 9,000 ± 150 years ago (S-113, McCallum & Wittenberg, 1962). These two dates suggest that a l l g l a c i a l lakes had been drained and modern drainage established by 9*000 B.P. at the l a t e s t . Radiocarbon dates obtained from bog bottom samples i n the uplands of the I n t e r i o r indicate that deglaciation of the I n t e r i o r Plateau was well advanced by 9,750 B.P. (9750 ± 170 B.P., G.S.C.-526, Lowdon, Fyles and Blake, 1967 and 9,210 ± 150, G.S.C. 5 l l , Lowdon, Wllmeth and Blake, 1969). 3.2.31.9 Late Glacial?.."'. Lakes .in. the -Tributary V^llgys As the ice melted f i r s t from the uplands with an ice plug remaining i n the deep and narrow Thompson Valley, i t i s possible that late g l a c i a l lakes formed i n some of the larger tributary valleys too. This appears to have been the case in Botanie Valley, where there i s both sedimentological and physical evidence of the former existence of a lake. About one-and-a-half miles from the mouth of Botanie Greek, the valley f l o o r widens and the sides slope gently, while the creek i t s e l f i s incised about 100 f t . into lacustrine and other deposits. When the waters were at the 1,800 f t . l e v e l , the lake must have been approximately 3 A mile wide and 2\ miles long (see F i g . 26). Lacustrine deposits at a height of 1,650 f t . are c l e a r l y exposed at a point 2„h miles from the mouth of the valley,, where the creek has incised i t s e l f into the former lake f l o o r . On the western and eastern banks i s the following sequence of deposits: Western bank Eastern bank 3 f t . sand 3 f t . sand 8 f t . sandy/silty 2-3 f t . gravel of l o c a l source 12 f t . sandy/silty 10 f t . sub/ang.-sub/round. gravel of l o c a l source coarsening at base. ho f t . bedded f i n e gravel 25 f t . banded sandy/silty Base not seen Base not seen The sandy/silty material i s composed of bands of green and red s i l t and clay, and rust-khaki coloured sand, generally tr" to 1" thick, which could represent varves. Near the top of t h i s deposit i s a t h i n layer of organic matter, which suggests that the lake was at t h i s time shallow and the climate had ameliorated s u f f i c i e n t l y f o r the growth of vegetation. A l t e r n a t i v e l y t h i s could simply be former s o i l covered by slope wash. A t h i n , somewhat discontinuous band of pale coloured s i l t occurs 8 f t . below the top on the west bank. Unfortunately where i t occurs i n s u f f i c i e n t quantity to be analyzed, i t i s inaccessible. I t may be either volcanic ash or caliche. The interbedded and overlying gravel i s of l o c a l composition, whereas the underlying sand and gravel i s more varied and perhaps represents outwash material. Towards the mouth of the creek, very l i t t l e access-i b l e material i s c l e a r l y exposed i n the steep valley sides, apart from yellow sand and f i n e gravel. Downstream the creek flows over s o l i d rock along the f a u l t l i n e to j o i n the Thompson. I t i s possible that lakes also existed i n the Nicoamen and Murray Valleys, although evidence of t h e i r existence i s less conclusive than i n the case of Botanie Valley. Above Nicoamen F a l l s the v a l l e y widens and a misfi t stream flows across a broad (150 f t . wide) f e r t i l e v a l l e y f l o o r . Cobbles and bedrock outcrop on the northern side of the v a l l e y , but on the south side, sand i s exposed about 50 f t . above the stream. Possible lacustrine deposits are exposed above the road p a r a l l e l i n g Murray Creek, just northwest of Murray F a l l s . Twenty feet of banded s i l t and sand varying i n colour from red, rust and orange to l i g h t buff, similar to the Botanie Lake sand, and ripple-marked at the base, overlies 3 f t . of khaki sand and gravel, which i s probably outwash material. The sand i s overlain by twenty feet of predominantly sub-angular gravel (See F i g . 27). U p h i l l from here and closer to the Thompson i s completely unsorted gravel which contains pebbles of variable composition including granodiorite, and has a clayey matrix so that i t i s most l i k e l y t i l l . This i s overlain by 3-5 f t . of clayey matrix material, with si m i l a r but-.fewer pebbles, i n t e r -bedded with f l u v i a l grey gravel, and capped by subangular grey gravel and sand of the Murray Delta-Fan (3.2.31.3). The l a t t e r sequence may well represent the blocking of Murray Valley by a former ice plug in the Thompson Valley. A l t e r n a t i v e l y the sand i n Murray Valley may simply have been deposited i n an arm of Lake Deadman, but the evidence favours the former interpreta-t i o n . Subsequent deposition i n Murray Valley was no doubt con-tinuous with that i n the Thompson Valley, as i t has already been 51 noted that Murray Creek bu i l t a delta into Lake Deadman. Ponding by ice i n the Thompson Valley has been suggested i n the case of Murray V a l l e y 3 and the same may be true f o r the Nicoamen and Botanie Val l e y s . A l t e r n a t i v e l y the valleys may have been plugged by t i l l and outwash from the melting ice tongue. It i s i n t e r e s t i n g to note that a l l three streams have former channel mouths plugged by gravel, and now descend to the Thompson i n a series of f a l l s cut i n s o l i d rock (Fig. 26). I t would appear that t i l l , white s i l t s and Murray Delta-Fan deposits are blocking the former exit of Murray Creek 3 graded to a Thompson with a s i m i l a r l e v e l to i t s present one 3 or even lower. The present creek has been forced to cut through the rock to the northeast to form Murray F a l l s . At Nicoamen F a l l s the r i v e r bends sharply to cut through the rock to reach the Thompson, i t s old passage, s l i g h t l y north of i t s present position, having been f i l l e d i n by gravel. S i m i l a r l y Botanie Creek i s deeply incised i n a rock channel before reaching the Thompson. In t h i s case there i s a dry v a l l e y 3 containing a large amount of g r a v e l 3 about half a mile to the east, which could well have been the former channel of Botanie Creek. Either the former outlets of these creeks were blocked by ice or outwash so that they were forced to seek a new channel, or a l t e r n a t i v e l y they l a t e r aggraded t h e i r channels to such a height that a very s l i g h t diversion caused them to cut through bedrock at the side of the v a l l e y . If the Murray Creek gravels have been correctly i d e n t i f i e d as l o c a l delta-fan deposits rather than outwash (3.2.31.3), the l a t t e r interpretation i s favoured. The following sequence of events in Murray Creek can be envisaged: g ^Murray Creek N ° ° a •<=> . 0 ° 0 • • ° 0 0 ° . < s 0 o ° 0 • o ' 0 ' o ' 0 ' 0 ' ° ' <» 0 • o ' 0 ' o ' o 0 • 0 o ' ' • 20+- fluvial gravels of local origin Murray • o * • ° • . „ . ° Q . ° . ° 0 . a 0 * e " o . a. „ - Q * Delta - Fan T\^< \ _ _T ~ ~ ~ - ~ - 20 ' lacustrine silt and sand fin ° " 1y± \ \ l\Vvk>;>-:::::":: 0 / a \ \ slumped . — r - • •. • . -. v o outwash gravels FIG. 27a DIAGRAMMATIC SECTION OF WEST WALL OF MURRAY C R E E K Murray Creek Till ICE 1. Ice and till blocking exit of valley c reat ing a lake in which silts and sands were deposited ICE 2. Ice has receded west of Murray Creek allowing it to build a delta into L. Deadman 3. Lake Deadman has drained and Murray Creek is now building 0 fan over the former lake bed 4 Degradation begins with creek located on east side of fan FIG. 27b STAGES IN THE EVOLUTION OF THE ABOVE SEQUENCE OF DEPOSITS damming of the creek by an ice plug and i t s associated deposits, as suggested by the t i l l and lacustrine sands and s i l t s ; followed by retreat of the ice south of the mouth of Murray Creek and the southward extension of G l a c i a l Lake Deadman, into which the Murray Delta-fan was b u i l t . The fan was bu i l t up higher i n the centre of the valley than at the sides so that the period of downcutting may well have begun with Murray Creek located on the northeast side of the fan, immediately over bedrock. With the onset of degradation, the creek continued to i n c i s e i t s e l f into the bedrock to create Murray F a l l s (Fig. 27). Perhaps a si m i l a r sequence of events occurred i n Botanie and Nicoamen Valleys with aggradation occasioning the diversion of the stream channel to the side of the v a l l e y . The present outlet of Botanie Valley follows a f a u l t zone, an obvious l i n e of weakness. Certainly diversion of the stream outlet cannot be taken as evidence of the presence of an ice plug i n the former stream mouth. 3.2.31.10 Other Late G l a c i a l Deposits Apart from the white s i l t f a c i e s , i t i s d i f f i c u l t to i d e n t i f y deposits as l a t e g l a c i a l i n the absence of any surface expression of morphological form. Nasmith (1962) and Fulton (1963) used morphology as an important c r i t e r i o n , i f not the most important one, i n recognizing such deposits. Nevertheless, i t i s possible to recognize pond and lake deposits by sedimen-t o l o g i c a l features, and such c h a r a c t e r i s t i c s as ice- r a f t e d pebbles indicate the presence of i c e . Furthermore, temporary lakes and ponds are t y p i c a l of stagnating ice masses. The deposits described below a l l occur close to the present valley f l o o r and beneath the extensive p o s t g l a c i a l aggradational de-posits, and hence occupy a r e l a t i v e l y early position in the s t r a t i g r a p h i c sequence exposed, which makes i t more probable that they are late g l a c i a l rather than p o s t g l a c i a l . It i s possible that they are even older, but in the absence of over-l y i n g t i l l and dateable material, the deposits w i l l be tenta-t i v e l y assigned to t h i s period. Skoonka Sands At the mouth of Skoonka Creek i s an exposure of 60 f t . of alternating bands of sand and s i l t exhibiting graded bedding, micro-folding and f a u l t i n g , load and flame structures and r i p p l e marks, resting upon apparent t i l l . The bands, which are from 0.25 to 6 inches thick, vary i n colour from very pale brown 10 YR 7A) to l i g h t brownish grey (10 YR 6/2). The v a r i a t i o n in grain size could be due to alternation of d i f f e r e n t condi-tions of deposition i n a lake. The f i n e sand layers may represent more rapid accumulation of lake f l o o r sediment under more active conditions, as when meltwater was entering the lake, and the s i l t s may represent quieter conditions. If the alterna-t i o n i s of seasonal s i g n i f i c a n c e , the s i l t s are representative of the qpiescence of winter, and the deposits could t r u l y be described as varved. However, the graded bedding, load casts and flame structures are t y p i c a l of t u r b i d i t e sequences, often found i n marine deposits. Turbidity currents are not, however, res-t r i c t e d to marine environments, and P. H. Kuenen (195D believes that they are of considerable importance i n g l a c i a l lakes, where » F i g . 28. Graded bedding, f a u l t i n g and micro f o l d i n g , Skoonka Sands 54 the cold sediment-laden meltwater w i l l be s l i g h t l y denser than the lake waters and hence able to flow as a t u r b i d i t y current,, Mathews (1956, p. 550-55D attributes graded laminae of f i n e sand and coarse s i l t i n Garibaldi Lake i n the southern Coast Mountains to the action of t u r b i d i t y currents r e s u l t i n g from subaqueous slumping 3 although he notes that such currents can also be generated by g l a c i a l streams. Stream generated bottom currents might be expected to exhibit a seasonal rhythm and t h i s appears to be true of .the Skoonka sands. The micro f o l d i n g and f a u l t i n g are intimately related (see F i g . 28) and of limited extent. They are probably the res u l t of minor disturbances such as slumping and s e t t l i n g , possibly related to d i f f e r e n t i a l melting of i c e . The deposit i s f o r the most part horizontal but as one goes westward i t has a d e f i n i t e southward t i l t of 10° possibly as a result of d i f f e r e n t i a l s e t t l i n g with i c e melt. The micro s t r u c t u r a l features become more numerous here too. Very occasional pebbles are scattered throughout the deposit. They are of green andesite and could well be locals i n which case they need not be i c e - r a f t e d . This deposit i s s i m i l a r to the lowest laminated beds (Unit 10) of the Tsingkahtle exposure, 8 miles north of Spences Bridge s described by -J. M. Ryder (p. 13D° She suggests that such beds resulted from lacustrine deposition i n close proximity to the i c e , and that the alternating coarse and f i n e sand laminae indicate periodic variations i n the strength of currents carrying material into the lake, which agrees with the observa-tions just made on the Skoonka sands. The Skoonka lacustrine deposits may or may not have any connection with Lake Deadman, F i g . 30. Pitquah sand exposure overlying bedrock bench. 55 but they could well represent the coarse equivalent of the Spences Bridge s i l t s which were deposited at a greater distance from the ice tongue. Some of the coarser material could have been supplied from Skoonka Creek, as well as from the melting ice-tongue. Pltquah Sand •Just west of the tunnel at Pitquah on the C.N.R. l i n e i s a deposit somewhat si m i l a r to the Skoonka sands. Here the material is coarser, with f i n e gravel alternating with sand and s i l t , and the i n d i v i d u a l bands are thicker, being from 3" to 8" thick. Graded bedding, load casts and flame structures are again prominent (Figs. 29 & 3D. In addition to the material being coarser, there are f a r more scattered pebbles than at Skoonka, some being angular and l o c a l i n o r i g i n , whereas others are of disrupted s i l t . None of them appear to have created much disturbance on s e t t l i n g , and could possibly be i c e - r a f t e d . To the west the deposit appears to be resting upon a bedrock terrace, although i t i s not seen i n dir e c t contact because of slumping, but to the east the deposit, at a higher elevation, i s seen to rest on 10-15 f t . of slumped sand overlying 15 f t . or so of subrounded cobble gravel resting upon bedrock (Fig. 30). To the west the graded bedded sand interfingers with s i m i l a r cobble gravel of green, grey and red medium to f i n e grained igneous rock, dark grey limestone and well-rounded granodiorite (Fig. 32). The i n t e r f i n g e r i n g gravel and sand grades westward into a continuous gravel succession. This gravel i s a completely mixed deposit of angular fragments of l o c a l F i g . 31. I c e - r a f t e d p e b b l e s , l o a d c a s t s and graded-bedding i n P i t q u a h Sands. W E F i g . 32. I n t e r f i n g e r i n g of sand and g r a v e l at margin of P i t q u a h sands. Note s l i g h t d i p of g r a v e l s t o east i n d i c a t i n g t h a t they may be d e l t a i c . 56 material and more rounded cobbles of variable composition randomly oriented i n a f i n e matrix. Towards the top of the outcrop i t i s •well-cemented3 probably by p r e c i p i t a t i o n of s a l t s from spring-water. I t appears to represent an admixture of talus and pebble t i l l , and may have resulted from talus f a l l i n g down the bare mountain slopes on to the stagnating ice lobe. In f a c t this talus could well have helped to protect the ice from ablation. The c h a r a c t e r i s t i c s of the Pitquah sands and adjacent gravel deposits indicate deposition i n an ice-marginal or even supraglacial lake or pond, i n very close proximity to the i c e -margin, as evidenced by the coarseness of the deposits and the gradation into apparent t i l l to the west. The Pitquah sands appear to be of only l o c a l s i g n i f i c a n c e , as t h i s i s an isolated deposit, and indicate temporary ponding on or at the margin of a stagnating ice lobe. Pitquah Mudflow To the west of the Pitquah sands and pebble t i l l and at the same a l t i t u d e i s an unusual exposure of laminated sand interbedded with wedges of angular pebbles in a clayey matrix (Fig. 33)« Similar angular pebbles are found i n layers and scattered throughout the overlying gravels, which suggests that they are of l o c a l o r i g i n . The pebbles intercalated with the gravel may be interpreted as talus from the g r a n i t i c gneiss of the Scarped Mountains. Where the pebbles occur within a matrix they are probably a mudflow deposit, largely derived from t a l u s . The talus wedges have caused very l i t t l e disturbance of the interbedded sand, which supports the idea of slow creep. Angular F i g . 33. Talus and wedges of c o l l u v i a l material interbedded with sand, W. of Pitquah Tunnel. 57 blocks of l o c a l rock are i n fact very common i n the lower parts of the aggradational deposits from Sackum to Lytton, and probably represent intense freeze-thaw a c t i v i t y . 3.2.31 Summary Lacustrine s i l t s , apparently derived from g r a n i t e - r i c h t i l l by the winnowing action of meltwater, and associated i c e -contact deposits i n the Thompson and Nicola Valleys indicate the former presence of lakes, dammed by stagnant ice tongues which melted back westwards towards the Fraser. The l a s t and lowest of these lakes was the Durand stage of Lake Deadman, which extended at least as f a r as Skoonka Creek and approximately s i x miles up the Nicola Valley. This lake o r i g i n a l l y drained east via the Otter Lake spillway to the Okanagan-Columbia drainage. Once the ice retreated south of Skoonka Creek i n the Thompson Valley i t appears that i t no longer formed a water-tight barrier and drainage was resumed west to the Fraser. Evidence of the past presence of stagnating ice i n the val-ley south of Skoonka Creek i s seen i n the collapsed s i l t and t i l l at Seddall and i c e -marginal lacustrine sediments at Pitquah, but the evidence i s too scant to d e t a i l the stages of recession and to determine the actual mode of drainage, although i t seems l i k e l y that the lakewaters eventually overtopped the melting ice lobe. Ice-contact and lacustrine deposits between Skuhun Creek and Nicola twin bridges provide a much clearer record of recession of the ice down the Lower Nicola Valley with the lakewaters encroaching upon the i c e . I t i s thought that small temporary lakes were formed i n some of the t r i b u t a r y v a l l e y s , namely Murray, Nicoamen and Botanie Valleys, as the ice tongue remained in the main valley a f t e r the uplands were i c e - f r e e . Lacustrine sediments were found in Murray and Botanie Valleys, and there was evidence of the past presence of an ice plug at the mouth of Murray Valley. There may well be other late- g l a c i a l deposits in t h i s area, but they are d i f f i c u l t to d i s t i n g u i s h i n the absence of any surface expression of morphological form; some of the aggrada-t i o n a l gravels, described i n the following section could be l a t e - g l a c i a l . 3.2.32 Late G l a c i a l and Early P o s t g l a c i a l Aggradational Deposits Aggradational deposits, thought to be largely early post-g l a c i a l , are very prominent i n the section from Skhpowtz to Botanie, and consist of up to 500 f t . of cross-bedded sands and gravels with talus admixed, forming a constructional bench, now considerably dissected, at approximately 1,200 f t . (Fig. Lt7)„ The terms late g l a c i a l and p o s t g l a c i a l have been rather loosely defined, perhaps of necessity, in other reports on the Quaternary of B r i t i s h Columbia. Mathews (1963, p. 11) uses the term post-g l a c i a l " i n the sense that neither ice nor ice-dammed lakes were present at the place or time of deposition", and correspond-ingly terms any ice-dammed lakes late g l a c i a l , as the present author has done. Nasmith (1962, p. 12) correlates such lakes more precisely with the stage of g l a c i a l r e t r e a t , and states that his late g l a c i a l category "contains units formed under climatic and hydrologic conditions d i s t i n c t l y d i f f e r e n t from the present, although deposition did not take place i n contact with g l a c i a l i c e . " He also points out that, because of the F i g . 3*+. T a b u l a r c r o s s -bedding i n a g g r a d a t i o n a l g r a v e l s near Gladwin Note t a l u s f r a g m e n t s . F i g . 35. Trough c r o s s -bedding i n a g g r a d a t i o n a l g r a v e l s . 59 nature of retreat of the ice i n the Okanagan, units of the stage of g l a c i a l occupation at the north end of the valley may be contemporaneous -with g l a c i a l r e t r e a t , late g l a c i a l and even Recent units farther south. The present author prefers to r e s t r i c t the term late g l a c i a l to deposits which owe t h e i r c h a r a c t e r i s t i c s to the presence of stagnant i c e . The term early p o s t g l a c i a l i s used to refer to material deposited immediately after the withdrawal of ice from the area of de-position, and i s used in a s t r i c t l y l o c a l sense. Although i n d i v i d u a l beds within the aggradational deposits cannot be traced f o r any distance, the various outcrops have many ch a r a c t e r i s t i c s i n common and so they w i l l be treated as a single u n i t . As usual, the best exposures are i n the railway and road cuts, and the upper l i m i t s of these deposits are not well exposed. It has already been suggested (3.2.1) that the lowest portions of the gravels may predate the la s t advance of i c e . As t h i s has not been proven and in view of the fact that the lower gravels are very s i m i l a r to the upper ones, they w i l l be included i n the following description. Gravel predominates throughout the exposures and, p a r t i c u l a r l y in the lowest 50 f t . , i t often exhibits apparent tabular cross-bedding (Potter & Pettijohn, 1965* P» $+7) "with a downstream dip of 18° to 23°, which i s i n accord with Potter and Pettijohn's observation (1963, p. 79) that the average i n c l i n a t i o n of cross-beds in undeformed r"ocks i s usually i n the range of 18° to 25°. Individual beds within the cross-bedded units average 0.5 f t . in thickness (Fig. 31*). In both size and appearance these inclined beds resemble the foreset beds of Murray Delta (3.2.31.3)s but they are not well situated to have been the F i g . 36. Lenses of sand i n t e r b e d d e d w i t h c o a r s e a n g u l a r g r a v e l s i n a g g r a d a t i o n a l sequence •west of Thompson j u n c t i o n . F i g . 37. Cross-bedded g r a v e l s and scoured sand l e n s a t Nicoamen. former deltas of tr i b u t a r y creeks, and so i t i s suggested that they may represent very large scale point bar deposits. Where more sand i s present within the section festoon or trough, cross-bedding (Potter & PettiJohn, 1965 9 p. 350) occurs (Fig. 35)° The apparent dip i s again downstream towards Lytton and p a r a l l e l with the present course of the Thompson. This trough-s t r a t i f i c a t i o n could have resulted from dune migration i n the upper part of the lower-flow regime (Harms & Fahnestock 1965)° Horizontal bedding of the gravel i s common too, and occasionally lenses of sand and even s i l t are found interbedded with the gravel, as at Botanie Creek, where lenses of sand, from 1.5 to 2,5 f t . thick and extending f o r at least 20 f t . , occur i n the middle of the Botanie section. Just west of Thompson Junction, 2 f t . thick lenses of sand interbedded with coarse angular gravels are exposed above the C.P.R. tracks (Fig. 36). In some cases, as f o r instance beneath the Nicoamen s l i d e (Fig. 37) 9 i t can be seen that the sand lens has been scoured before the deposition of the overlying gravel. The size of the gravel fragments varies considerably throughout the deposits but remains b a s i c a l l y constant within i n d i v i d u a l beds. Where beds of cobble gravel, with p a r t i c l e s up to 10 i n s . i n diameter, occur they may represent flood deposits. These coarse beds usually have good imbrication with the long axes of the cobbles dipping upstream i n d i c a t i n g deposition by a r i v e r flowing i n es s e n t i a l l y the same d i r e c t i o n as the present Thompson (Fig. 38). There i s considerable v a r i a t i o n i n shape and composition of the gravel p a r t i c l e s . Angular fragments of l o c a l rock are common throughout the deposits. They may be mixed with other V E ing est. F i g . 39. Sand lens injected into angular granodiorite gravels at Botanie. more rounded material, as i n the exposure above the C.P.R. tracks at Gladwin (Fig. 31+)J or occur as d i s t i n c t i v e beds as at Botanie and Pitquah tunnel. At Botanie Creek 5 to 6 f t . of extremely angular fragments of granodiorite, about 2 i n s . i n diameter, o v e r l i e 2 f t . of graded bedded sand, which has been injected into the gravels at one place, probably by loading (Fig. 39). This i s overlain by 8 f t , of angular fragments of granodiorite i n f i n e gravel, which i s succeeded by more sand and gravel. Very badly weathered granodiorite, presumably the source of the angular fragments, i s exposed i n the v i c i n i t y . At the Pitquah tunnel, beds of predominantly coarse angular material alternate with f i n e r beds with only scattered angular fragments (Fig. 38). The angular fragments throughout the deposits are undoubtedly of immediately l o c a l o r i g i n and presumably represent t a l u s . At Gladwin Creek, pink bedrock (10 R 8/2) i s exposed and a great deal of talus derived from i t i s incorporated i n the adjacent aggradational material. Where the gravel i s of more variable composition i t consists of well-rounded to subangular pebbles of granodiorite, red, green and grey f i n e to medium grained igneous rock •— large l y andesite and r h y o l i t e , and minor limestone and quartzite. The pebbles are very similar to those contained i n the t i l l (3.2.22) and granodiorite i s the most rounded except where i t i s of l o c a l o r i g i n . 3.2.32.1 Nicoamen, Shushten and Botanie Slide Deposits Three recognizable landslide deposits are found i n t h i s section of the Thompson Valley, from Nicoamen to Botanie, interbedded with the aggradational gravel and sand. I t seems Nicoamen Slide 25 f t . F i g . kO. Nicoamen Slide taken with t e l e -photo lens. (Photo - J . M. Ryder) F i g . hi. Nicoamen Slide overlying cross-bedded gravels. Note the f l a t top. l i k e l y that they owe t h e i r existence to the period of i n -s t a b i l i t y during and immediately following deglaciation. Nicoamen S l i d e , at the mouth of the Nicoamen River, is the . only one previously described, having been b r i e f l y mentioned by Armstrong i n the INQA guide (p. 103). I t s exposed length paral-l e l to the Thompson i s about 1,230 yards and i t averages 15 to 25 f t . i n thickness, and overlies 25 f t . or more of cross-bedded sands and gravels and i s about 50 f t . above present r i v e r l e v e l . The s l i d e consists of red, green, grey, black and yellow volcanic fragments, s i m i l a r to the bedrock exposed on the west side of the r i v e r and at a higher a l t i t u d e on the east side of the r i v e r . The balance of the evidence i s in favour of an eastern o r i g i n . The rock on the east i s more badly weathered than that on the west, and the s l i d e outcrop r i s e s to the east. The l a t t e r f a c t may not be p a r t i c u l a r l y good evidence, as there was considerable "swash" on the Hope Slide in 1965. The extent of the s l i d e does not match the outcrop on the western bank, and there i s no surface evidence on t h i s side of s l i d i n g . The s l i d e deposit has a l e v e l top suggesting planing off prior to the deposition of the overlying f l u v i a l cobbles. Shushten Slide i s exposed f o r 100 yards on the north bank of the r i v e r just east of Shushten Creek. It has a sloping base so that i t thickens to the west, and averages 25 to 50 f t . i n thickness. I t s base i s approximately 200 f t . above the r i v e r and i t overlies 100 f t . of gravel and sand. It consists of much weathered l i g h t greenish grey rock. According to the geology map (Fig. 3 ) s quartz-mica schists and g r a n i t i c gneiss are found on both sides of the r i v e r here. However, the rock of the 63 south bank weathers pink whereas that of the northern bank i s more usually greenish grey, so i t can be concluded that the s l i d e originated on the side where i t i s found. Botanie Slide consists of a 10 f t . thick bed of red rock 5 R 6 A ) horizontally interbedded with sand and gravel, exposed i n the eastern half of the Botanie Fan section. It i s approxi-mately 60 f t . above the bedrock bench, and 170 f t . above r i v e r l e v e l . The source of the s l i d e material appears to be a very prominent band of red shale, mapped as part of the Kingsvale Group, exposed on the western slope of Botanie Valley, approxi-mately half a mile upstream from i t s confluence with the Thompson. Angular fragments of the same red shale are found i n the under-l y i n g coarse angular to sub-rounded grey gravel. As a l l the slides occur i n the lowest hundred feet of the aggradational deposits, i t seems l i k e l y that they owe t h e i r existence to a common cause. I f the gravels and sands, with which they are interbedded, have been interpreted correctly as early p o s t g l a c i a l deposits, the landslides are most l i k e l y the result of oversteepening and weathering of the valley sides during g l a c i a t i o n and deglaciation. With the disappearance of the i c e , some of the support f o r the weathered materials would be removed, and abundant water would f a c i l i t a t e s l i d i n g . 3.2.32.2 Origin of the Aggradational Deposits Imbrication and the downstream dip of cross-beds indicate that the gravels were deposited by a southwestward flowing r i v e r . As the ice-lobe was melting back southwestwards, the material cannot be considered to be outwash or valley t r a i n from the 64 central ice-lobe. This suggests that the ice had melted back to the Fraser before the gravels were deposited, and t h i s i s further borne out by the fact that there i s no evidence of slumping i n the gravels. Some aggradation undoubtedly did take place during the presence of the ice and the Pitquah Sands and mudflow have already been assigned to the late g l a c i a l period (3.2.31.10), but the bulk of the deposits can be considered early p o s t g l a c i a l . The landslide deposits and talus were a l l supplied from the valley sides, and indicate that the slopes were even more unstable than they are at present. This irs t a b i l i t y can be related to oversteeping during g l a c i a t i o n ; and intense weathering due to an excess of moisture, a large number of freeze-thaw cycles, and the lack of a protective vegetation cover. With the disappearance of the i c e , much of the support f o r the weathered material was removed and consequently i t s l i d down the valley sides. The s i m i l a r i t y of the gravel pebbles to those i n the t i l l suggests that they may have been derived from t i l l or other g l a c i a l deposits. As there i s a marked absence of g l a c i a l d r i f t i n t h i s section, i t i s suggested that g l a c i a l d r i f t slumped down the valley sides and was reworked by the r i v e r . Similar reworked t i l l may have been supplied by the tr i b u t a r y streams. It has been shown that much of the material i n t h i s section of the Thompson was derived from l o c a l l a t e r a l sources. This i s borne out by the fact that the most extensive aggradational deposits are at the creek mouths, so that the r i v e r has been forced over to bedrock opposite each major creek (Fig. k6)o Such aggradation has resulted i n minor diversion of 65 Nicoamen, Botanie and Murray Creeks (3.2.31.9). The source of supply of the materials has been accounted f o r , but the abrupt change from s i l t to arenaceous deposits i n t h i s part of the Thompson needs explaining. Gravels are found overlying or inset i n the white s i l t s at Spences Bridge and north towards Ashcroft (Ryder s 1970s p. 1^2), but they are associated with the entry of tr i b u t a r y creeks and are not continuous throughout the v a l l e y . E i ther the r i v e r did not have the power to transport the gravels f o r any distance or they were deposited e a r l i e r as delta-fans while Lake Deadman was s t i l l i n existence. Undoubtedly there was a greater load available south of Seddall fo r the r i v e r to transport. From Spences Bridge to Ashcroft, the valley sides are less steep and rocky and thus more stable, and, furthermore, the va l l e y sides had been exposed f o r a longer period and many of the t r i b u t a r i e s had already deposited t h e i r reworked g l a c i a l deposits as delta-fans marginal to the ice lobe or as deltas i n Lake Deadman (Ryder, 1970, p. l 6 l ) . If the white s i l t s were deposited before the aggradational gravels, and remnants of both can be found above 1,200 f t . , there must have been very l i t t l e f l u v i a l a c t i v i t y between Ashcroft and Spences Bridge while aggradation of up to 500 f t . of gravel was taking place between Skhpowtz and Lytton. Perhaps t h i s can be partly explained on the basis of the extremely gentle gradient of the Thompson flowing over the white s i l t s . However, the top of the white s i l t s i s above 1,200 f t . while the base of the aggradational deposits i s below 700 f t . , which necessitates a f a l l of approximately 500 f t . at the edge of the white s i l t . Even with extremely rapid melting of the i c e lobe t h i s doesn't seem very reasonable. The lack of any signs of collapse and the d i s t i n c t i v e c r o s s - s t r a t i f i c a t i o n of the aggradational deposits rule against the p o s s i b i l i t y that the gravels were deposited on top of or adjacent to the stagnant ice l o be 3 as drainage was resumed west to the Fraser. It i s possible, however, that some of the white s i l t s were deposited at the same time as the aggradational gravels, as a small ice plug could have remained i n the v a l l e y between Skoonka and Seddall separating lacustrine deposition i n Lake Deadman, north of Skoonka Creek, from f l u v i a l aggradation south of Seddall. Collapsed s i l t and t i l l have already been noted between the mouth of S l e e t s i s Creek and Seddall, and cited as evidence of the encroachment of the lake waters onto the stagnant ice (3.2.31.8). I t i s possible that a mass of dead ice remained i n t h i s area, after most of the ice south of here had melted. This would involve the waters of Lake Deadman draining over the dead ice to the ice-free section of the v a l l e y , where deposition was taking place. Such a mass of dead ice could account f o r the fa c t that considerably more t i l l i s exposed between Mud Lake and Seddall than anywhere else i n the Thompson Valley between Ashcroft and Lytton. It could also explain the presence of f l o w t i l l overlying aggradational sands and gravels at Drynoch and Seddall (3.2.1). Furthermore, Spences Bridge i s the l a s t place i n the Thompson Valley where white s i l t s are found above 1,200 f t . The white s i l t s south of Spences Bridge are overlain by mudflow and f l u v i a l gravels, as i s the case with the white s i l t s below 1,200 f t . between Ashcroft and Spences Bridge 3 which have been terraced by the Thompson. The f l u v i a l gravel overlying -white s i l t between Mud Lake and Seddall could be equivalent to the aggradational gravels, or i t could be simply the terrace veneer l e f t by the Thompson as i t l a t e r cut down through i t s deposits. The l a t e r terrace gravels are t y p i c a l l y cobble gravels with well developed imbricate structure (3.2.*+) and are not cross-bedded. Consequently i t should be r e l a t i v e l y easy to d i s t i n g u i s h between aggradational and terrace gravels. Most of the gravels exposed i n the railway cuts between Seddall and Mud Lake appear to be terrace gravels, i n that they consist of less than 30 f t . of imbricate cobbles. Cobble and pebble gravels are found d i r e c t l y overlying t i l l i n the r i v e r bank at the toe of the Drynoch Slide exposure (Pig. 13) 5 which suggests considerable erosion prior to the deposition of the terrace gravels. Such erosion, combined with slumping, has probably removed much of the evidence connecting the s i l t s at Murray F a l l s with the aggradational gravels at Nicoamen. Finer cross-bedded gravel does, however, overlie the flow t i l l at Drynoch (Fig. 7) and presumably represents continued aggradation in t h i s area a f t e r the deposition of flow t i l l . S imilar aggradational gravels form a 1,000 f t . bench i n the Fraser Valley between Texas Creek and Lytton (Ryder, 1970, p. 105-108). These deposits consist of horizontal and cross-bedded pebble and cobble gravels deposited by the Fraser, and mudflows from l a t e r a l a l l u v i a l fans. Interbedded s i l t s and sands are interpreted as re s u l t i n g from temporary blockage of the main valley by rapid l a t e r a l aggradation. The area south of Lytton has not been examined i n d e t a i l but brief explora-t i o n suggests that the aggradational gravels continue f o r 68 approximately t h i r t y miles down the Fraser. This suggests that the height to which material accumulated in the Thompson Valley was controlled by aggradation i n the Fraser V a l l e y . The cause of aggradation i n both r i v e r s was the excessive load r e s u l t i n g from glaciation'followed by weathering and mass wasting of the valley sides. Variations i n type and size of material were due to l o c a l circumstances, with talus being prevalent i n the Thompson and mudflows i n the Fraser. As yet no dateable material has been found i n the aggrada-t i o n a l gravels, but charcoal i n aeolian sand overlying terrace gravels at the Drynoch Slide s i t e has been dated as 7,530 years B.P. (G.S.C.-530). As considerable downcutting occurred prior to the formation of the terrace, aggradation must have ceased w e l l before 7,530 years B.P. Possibly aggradation had ceased by 9,000 B.P. as aggradation at the Yale s i t e (3.2.31.8) since that date has involved only a l o c a l fan, the Fraser being no longer involved. Accumulation of more than 500 f t . of material could take a considerable length of time, but conditions i n the Fraser and Thompson Valleys were exceptional i n the amount of material a v a i l a b l e . L i t t l e study has been made of rates of aggradation under si m i l a r conditions. Fahnestock (1963) noted an average net increase i n elevation of 1.2 f t . f o r the valley t r a i n of the p r o g l a c i a l White River i n 1958, although such a rate of increase was not maintained over a number of years. G i l b e r t (1917) showed that the Yuba River rose about 0.33 f t . per year i n the period from 1850 - 191^, largely as a result of hydraulic mining i n the Sierra Nevada. If s i m i l a r rates were t y p i c a l of early p o s t g l a c i a l aggradation i n the Thompson and Fraser Valleys, 500 f t . of material could have accumulated i n less than a thousand years. 3.2.32 Summary With the melting of most of the stagnant i c e , the Thompson rapidly aggraded i t s va l l e y from Skhpowtz to Lytton, f i l l i n g i t with more than 500 f t . of gravel to create a constructional bench at about 1,200 f t . above mean sea l e v e l . The source of supply of material was largely l a t e r a l : t a l u s , landslide deposits and reworked g l a c i a l d r i f t from the va l l e y sides and t r i b u t a r i e s . The height to which material accumulated was controlled by s i m i l a r aggradation i n the Fraser. Given the large supply of material r e s u l t i n g from recent g l a c i a t i o n and weathering, i t i s suggested that i t could take less than 1,000 years to accumulate a v a l l e y f i l l 500 f t . thick. I t i s thought that a mass of dead ice remained i n the area between Mud Lake and Seddall after the rest of the v a l l e y was i c e - f r e e , and that th i s separated the white s i l t s from the aggradational gravels. Charcoal dated as 7 S530 B.P. i n sand overlying l a t e r terrace gravels suggests that aggradation had ceased well before that time. 3.2.k Late P o s t g l a c i a l and Recent Deposits Once the slopes were s t a b i l i z e d and the climate had ameliorated s u f f i c i e n t l y to permit the growth of a protective vegetation cover, the load of the r i v e r was considerably reduced, and i t began to degrade i t s aggradational f i l l . The progress of degradation i s marked by a series of terraces at w E F i g . k2. Imbrication in r i v e r cobbles opposite Thompson -Junction. Current from east to west ty F i g . *+3. Imbricate cobbles overlain by coTJuvium on Botanie Creek road. E 70 heights between 500 f t . and 30 f t . above the r i v e r (see F i g . 1+7). Many of the terraces are noncyclic, but i t i s possible to trace a more continuous bench, approximately l*+0 f t . above the r i v e r , between Ashcroft and Lytton, which marks a period of r e l a t i v e s t a b i l i t y during which the v a l l e y f l o o r was widened. These terraces usually have a veneer of imbricate cobbles, varying from 8 f t . to 20 f t . i n thickness, with the cobbles having a modal diameter of 0.5 f t . to 0.75 f t . But cobbles of 1.5 f t . to 2 f t . in diameter are common i n the lower portions of some exposures, and where they occur i n large quantities, as f o r instance at 700 f t . i n Lytton and on the Botanie Road (Fig. *+3) they may indicate a flood deposit. The angle of imbrication i s very variable with a v a r i a t i o n from 11° to 28° i n a single exposure (Fig. *+2). Measurements at a number of l o c a l i t i e s are i n accord with Potter and PettiJohn's (1963s p. 35) observation that imbrication angles generally vary between 10° and 30°. The d i r e c t i o n of dip i s everywhere upstream, as t h i s i s the most stable position f o r densely packed cobbles. The cobble 'exposures are very uniform i n colour, being l i g h t grey (N 6), and they usually contain a high proportion of sub-rounded to rounded granodiorite and grey andesite and r h y o l i t e . 3.2M A l l u v i a l fans Although degradation was the dominant process at this time, several t r i b u t a r i e s b u i l t a l l u v i a l fans on the terraces. The main bench, l'+O f t . above the r i v e r , has the greatest number of fans (Fig. 1+7)s as i t represents a temporarily stable base-level. The fan deposits t y p i c a l l y consist of bedded mudflow of angular fragments of l o c a l rock i n a f i n e matrix (Fig. ^ 5 ) 5 although some of the Nicola fans are composed of s i l t derived from the lacustrine deposits. Fan deposits i n the Thompson Valley are generally t h i n , being from 8 to 15 f t . thick, except around Spences Bridge where the valley i s wider and the fan deposits are up to kO f t . t h i ck. In the less constricted Nicola Valley, fan deposits are generally better developed and are from 20 to >+0 f t . t h i ck. The colour of the fan deposits varies with the parent material, but the colour most frequently encountered i s l i g h t brownish grey (10 YR 6/2). A number of l o c a l i t i e s show evidence of more than one phase of fan formation. Just west of Mud Lake 30 to ho f t . of brown mudflow overlies cobbles of the main bench 1*4-0 f t . above the r i v e r , Mazama ash (3.2.^3) occurs as a narrow band 22 f t . below the surface of t h i s fan (Fig. 1+5)j in d i c a t i n g that the fan was active 6,600 years ago. River cobbles at a height of 160 f t . above the r i v e r are exposed i n a gulley on the east side of t h i s fan, and these are overlain by another ^0 f t . of s i m i l a r mudflow. This indicates that the r i v e r cut down to 160 f t . and while i t was temporarily stable the t r i b u t a r y b u i l t a fan on the f l o o d - p l a i n . Downcutting recommenced u n t i l the r i v e r was approximately l'+O f t . above i t s present l e v e l , and the tr i b u t a r y b u i l t another fan graded to the new l o c a l base-level, which was i n i t s turn subsequently dissected, A s t i l l l a t e r phase of fan development can be seen at the mouth of an unnamed t r i b u t a r y on the west bank of the Nicola, 1.5 miles downstream from Kloklowuck Greek. Here a large fan, with Mazama ash occurring 8 f t . below the surface has been dissected and a 72 small fan bu i l t e n t i r e l y within the confines of the older one. Thus at least three phases of fan formation are indicated f o r the Nicola and Thompson Valleys — one prior to the ash f a l l , one at the time of the ash f a l l , and a l a t e r one. Mazama ash occurs i n many of the Nicola fans and -J. M. Ryder notes i t s occurrence i n some of the Thompson fans between Ashcroft and Spences Bridge. The ash i s a most useful marker horizon and i n t h i s case i t indicates that many fans i n the Thompson and Nicola Valleys were active 6,600 years ago. Hansen (1955) has suggested that the volcanic ash horizon, which he erroneously attributed to the Glacier Peak eruption, marks the time of the thermal maximum i n B r i t i s h Columbia, with the xerothermic i n t e r v a l l a s t i n g from 7 S500 to 3 S500 years ago. It has already been noted that most of the fans are composed of mudflows containing weathered bedrock. B u l l (1968, p. 102) notes that "factors that promote debris flows are abundant water over short periods of time at i r r e g u l a r i n t e r v a l s , steep slopes having i n s u f f i c i e n t vegetative cover to prevent rapid erosion, and a source material that provides a matrix of mud." The f i r s t two conditions could well occur under an arid or even semi-arid climatic regime such as prevailed 6,600 years ago. Certainly much of the fan material i s weathered bedrock and not reworked g l a c i a l debris, as was the case with the e a r l i e r and higher fans, which -J. M. Ryder (1970) terms pa r a - g l a c i a l . Thus climate may have been an important factor i n the development of these l a t e r and lower fans. 3.2.h2 Drynoch Slide Although the slopes -were more stable than they had been e a r l i e r , mass movements were s t i l l taking place and have in fac t continued to the present day. As these have a surface expression they w i l l be considered more f u l l y under the heading of geomorphology. The oldest and most s i g n i f i c a n t of these s l i d e s i s the Drynoch S l i d e , at the mouth of Squianny Creek, 5 miles south of Spences Bridge. It was described by G. M. Dawson (l895 5 p. l&^B) who noted that i t was "slowly subsiding upon the i n c l i n e d surface of hard underlying rocks". The s l i d e i s r e a l l y a slow earthflow and consists of 22,000,000 cubic yards of disrupted material (Brawner, 196*+, p. lM), largely r e s i d u a l s o i l s of volcanic o r i g i n with rock fragments up to 1.5 f t . i n diameter, some of which contain agate amygdules. The s l i d e area extends from an a l t i t u d e of 680 f t . at the Thompson River to an a l t i t u d e of 2,900 f t . , 2.7 miles along Squianny Creek, and i t s width varies from t^OO f t . , where i t i s confined i n Squianny Valley, to 1,200 f t . where i t spreads out at the confluence with the Thompson Valley (Armstrong & Fulton, 1965, p. 103). Due to the i r r e g u l a r i t y of the surface and a r t i f i c i a l interference, the thickness of material varies con-siderably. In the exposure east of the highway bridge over the C.P.R., 20 f t . of yellowish brown (10 YR 5A) earthflow i s ex-posed, but a hummock of si m i l a r material extends at least 60 f t . above the exposed section. The earthflow overlies *+ f t . of mudflow resting on 2 f t . of aeolian sand, containing a layer of ash (Fig. MO. The sand caps f l u v i a l gravels, which are seen to ove r l i e t i l l lower down i n the r i v e r bank (Fig. 13). The mudflow P i g . M+. Volcanic ash i n aeolian sand overlying f l u v i a l gravels. Drynoch Earth-flow overlies sand and the c u l t u r a l deposit i s beneath the ash by the f i g u r e . (Photo - J . M. Ryder) F i g . *+5. Volcanic ash i n a l l u v i a l fan mudflow near Mud Lake. Note angularity of pebbles. 74 i s evidently transgressive and partly contemporaneous -with the sand, as at the eastern end of the exposure, closest to the v a l l e y w a l l , the ash band occurs i n the mudflow which rests d i r e c t l y upon the f l u v i a l gravels. I f the ash beneath the earthflow i s taken to be 6,600 years old (see below) the s l i d e reached the r i v e r less than 6,600 years ago. A piece of wood excavated during recent s t a b i l i z a t i o n f o r the highway was dated as 35175^150 years B.P. (I->+62, Armstrong and Fulton 1965 3 p. 103), which indicates that the earthflow was active at least 3 S175 years ago. 3.2,^3 Mazama Ash A prominent band of white volcanic ash, generally from 1 i n . to 2 i n s . thick was noted within the mudflow deposits of the Nicola and Thompson Basins, at depths from 8 f t . to 22 f t . below the surface. The same ash also occurs i n aeolian sand beneath the Drynoch s l i d e , and i n c o l l u v i a l deposits along the Trans-Canada highway. A sample of ash from the Drynoch s i t e has been examined by R. E. Wilcox of the U.S.G.S., who concluded, with some reservation, that the ash might be correlated with'the Mazama ash f a l l of 6,600 B.P. (personal communication, R. -J. Fulton). The Mazama ash originated from what i s now Crater Lake i n Oregon and the Thompson Valley l i e s within the inferred l i m i t s of i t s d i s t r i b u t i o n (Wilcox, 1965). 3.2.hh C u l t u r a l Remains Beneath Drynoch Slide Charcoal, a microblade and f i s h vertebrae, presumably salmon, have been found beneath the ash layer at the Drynoch Slide s i t e (Fig. Mf) designated E c R i : l (Sanger, 1967). The charcoal has yielded a radio-carbon date of 7 S530 ± 270 B.P. (G.S.C.-530, Lowdon, Wilmeth and Blake, 1969), and thus the vertebrae found here provide the e a r l i e s t s k e l e t a l evidence of salmon f i s h e r i e s i n the Fraser River system (Sanger, 19o73 p. 193). Similar remains are found i n the r i v e r bank, immediately north of the toe of Drynoch S l i d e , where they underlie fan mud-flow. A very t h i n and discontinuous band of ash occurs i n the mudflow but i n i n s u f f i c i e n t quantity to be analyzable. Cul-t u r a l remains, which include charcoal, f i s h vertebrae and ?obsidian cores and chippings are found within sand and mudflow beneath the ash. They appear to be i n a similar stratigraphic position to the main Drynoch Slide s i t e i n that the sand overlies s i m i l a r f l u v i a l gravels, although they are approximately 100 f t . lower. The archaeologists (personal communication C, E. Borden) believe that the c u l t u r a l remains i n the r i v e r bank are part of the same sequence as E c R i : l , although further study i s needed. 3.2A5 Progress of Degradation The most l i k e l y cause of degradation was simply the reduction i n load of the r i v e r r e s u l t i n g from the amelioration of the climate and the fact that most of the g l a c i a l debris had been reworked. W. V. Lewis (191+lf) has shown experimentally that downcutting with terrace formation can result from a diminution i n the supply of material, without any corresponding change of sea l e v e l , t i l t or discharge. Drysdale (1913) suggested that p o s t g l a c i a l erosion was invigorated by u p l i f t , but gave no evidence to substantiate his claim. I s o s t a t i c t i l t could have influenced terrace formation, but Fulton (1969, P« 7) has suggested that the period of major i s o s t a t i c adjustment had ended even before the Durand stage of Lake Deadman. Sea l e v e l changes may have influenced degradation i n that during the period 9 S000 to 6,000 years ago, sea l e v e l stood approximately 35 f t . below the present shore (Mathews, Fyles & Nasmith, 1970), but this amount i s minor when compared with the 500 f t . of downcutting accomplished by the Thompson. It has already been shown that aggradation was extremely rapid, and i t appears that downcutting proceeded quickly too. Apart from the minimum date of the order of 9j000 years B.P. for the establishment of modern drainage, we cannot date the beginning of the period of dominant degradation. The presence of volcanic ash i n the terraces and a l l u v i a l fans indicates how f a r degradation had progressed by 6,600 B.P. The main bench, 1M-0 f t . above the r i v e r had been formed by t h i s time, as ash occurs i n many of i t s fans. It i s less certain whether or not i t had been dissected by thi s time, as fan formation could have continued f o r some time aft e r the main bench had ceased to be the f l o o d p l a i n of the Thompson. The presence of Mazama ash i n aeolian sand at Drynoch approximately 1^0 f t . above the r i v e r suggests that the main bench had already been abandoned as the flood plain of the r i v e r by 6,600 B.P., and possibly as long ago as 7s530 B.P. i n view of the date obtained from charcoal within the sand (3.2.M+). I f the thin discontinuous band of ash near the toe of the Drynoch s l i d e can be i d e n t i f i e d as a primary deposit of Mazama ash, i t can be concluded that the r i v e r was within 50 f t . of i t s present l e v e l by 6,600 B.P. Certainly i t appears that there has been r e l a t i v e l y l i t t l e f l u v i a l a c t i v i t y since 6,600 B.P. when compared with the rapid aggradation and degradation prior to t h i s . Possibly t h i s can i n part be related to the s t a b i l i t y of the shoreline of south-western B r i t i s h Columbia over the l a s t 5,500 years (Mathews, Fyles & Nasmith, 1970), but i t i s more l i k e l y that i t represents s t a b i l i t y of l o c a l slopes. 3.2.h Summary With the amelioration of the climate and completion of reworking of g l a c i a l debris, the load of the r i v e r was con-siderably reduced, and i t began to cut through i t s aggradational f i l l creating a series of terraces, with a veneer of imbricate cobbles, at heights between 500 f t . and 30 f t . above the r i v e r . Although degradation was the dominant process during t h i s period, there were at least three phases of a l l u v i a l fan forma-t i o n , possibly related to the arid climate of the xerothermic i n t e r v a l , and a certain amount of mass movement. The Drynoch earthflow was active at least 3,175 years ago and has been slowly advancing ever since. Volcanic ash from the eruption of Mt. Mazama in Oregon was deposited as a th i n blanket on the terraces and a l l u v i a l fans approximately 6,600 years ago providing a use f u l marker horizon. C u l t u r a l deposits dated 7,530 B.P. were found beneath the ash at the Drynoch Slide s i t e . Degradation appears to have proceeded rapidly at f i r s t , creating a series of non-cyclic terraces at heights between 150 f t . and 500 f t . above the r i v e r . Then there was a period of r e l a t i v e s t a b i l i t y prior to 7,530 B.P. during which the valley f l o o r was -widened rather than deepened, -which l e f t a series of paired terraces at a height of approximately ikO f t . above present r i v e r l e v e l bet-ween Spences Bridge and Lytton. Further down-cutting created non-cyclic terraces below t h i s . The r i v e r was probably within 50 f t . of i t s present l e v e l by 6,600 B.P., and downcutting appears to have proceeded r e l a t i v e l y slowly since then. 79 CHAPTER h  GEOMORPHOLOGY As the history of geomorphic development has la r g e l y been covered in the section on Quaternary stratigraphy and -will be summarized i n the conclusion, t h i s chapter w i l l be primarily concerned with landforms that have a s i g n i f i c a n t topographic expression today. *+.l G l a c i a l Landforms No landforms of g l a c i a l o r i g i n were recognized i n the main v a l l e y , but drumlins and roche moutonnees occur on the plateau. Drumlins composed of t i l l , with many crushed stones, occur at the head of Botanie Valley (see F i g . 5). The drumlins, which average 50 f t . i n height and are up to 200 f t . i n length, are well streamlined and are oriented with the steeper stoss end to the NNW suggesting molding by ice moving i n a SSE d i r e c t i o n . The most c l e a r l y defined roche moutonnee at Soap Lake is 50 f t . high, 300 f t . long and 100 f t . wide, and i t s ice-plucked lee side faces SSE i n accord with the general d i r e c t i o n of ice movement over the I n t e r i o r Plateau. There are a number of cols between drainage divides throughout the area (Ryder, 1970s p. 35-38), of which only one was examined. This c o l l i e s between Pasulko Lakes and the head of Skoonka and Botanie Creeks on the Fraser-Thompson divide, and may have served as a spillway f o r g l a c i a l meltwater 80 draining from the Fraser Valley to the Thompson Valley, via Pasulko Lakes and Skoonka Creek. I t appears to have caused the headwaters of Botanie Creek to be diverted to Skoonka Creek. h.2 Terraces Three broad groups of terraces can be distinguished i n the main vall e y : those formed prior to the development of g l a c i a l Lake Deadman; those associated with Lake Deadman; and those developed during the recent period of degradation. The e a r l i e s t terraces are largely represented by rock benches, which probably indicate former valley f l o o r s , but include probable kame terraces above 1,250 f t . These were not studied in d e t a i l because of i n a c c e s s i b i l i t y and lack of time. Where they have been included on the landform maps (Figs. *+6a & b) they have been l e f t unshaded to d i s t i n g u i s h them from the l a t e r -formed terraces. The most conspicuous rock benches are those of the Scarped Mountains. As well as these high terraces, the rock benches, between 50 and 100 f t . above present r i v e r l e v e l and overlain by l a t e - g l a c i a l and po s t g l a c i a l deposits (Figs, h and 30), belong to t h i s group. This indicates (3.2.22) that the Thompson had excavated a valley to within 100 f t . or less of i t s present depth prior to the end of g l a c i a t i o n . It i s not certain whether the rock benches were created purely by f l u v i a l action or whether some g l a c i a l deepening occurred. The terraces associated with Lake Deadman are confined to the area around Spences Bridge and i t i s inferred that as ice retreated south of Skoonka Creek i t no longer formed a water-ti g h t barrier. A prominent terrace at 1,220 f t . east of Murray F i g s . »f6a) & b). LANDFORMS OF THE THOMPSON VALLEY a) between Spences Bridge & Nicoamen b) between Nicoamen & Lytton LEGEND terraces formed prior to g l a c i a l Lake Deadman white s i l t terrace p o s t g l a c i a l r i v e r terraces landslide scar Hj^X s l i d e or slump a l l u v i a l fan ^ talus cone stream intermittent stream rock face with active weathering FIG. 47 LOWER THOMPSON VALLEY - LONGITUDINAL PROFILE feet a. s.I. 1,500 - i 1,000 5 0 0 -approx. upper limit of aggradation appro* protU e of Thompson white silt o L Y T T O N 10 15 N I C O A M E N 20 m i l e s Compiled from altimeter Survey and air photo interpretation S P E N C E S B R I D G E — bench or terrace Note each symbol indicates the location of a feature but not its extent Creek (Figs. 1 9j *+6a & *+7) marks the top of the white s i l t s . As there i s no gravel veneer on t h i s bench and as i t has an undula-t i n g surface due to compaction of the white s i l t during drying, i t can be assumed to be a remnant of a constructional surface, that was l a t e r dissected by the Thompson. Similar benches of white s i l t at the same al t i t u d e can be recognized at the mouth of the Nicola River, and along the Thompson between Spences Bridge and Ashcroft (Ryder, 1970, p. 14-1). The Murray Delta-Fan bench (Figs. 19 and 4-6a) belongs to the same group and has already been described ( 3.2 . 3 1 . 3 ) . The most,abundant and conspicuous terraces are those formed during the period of degradation following p o s t g l a c i a l aggrada-t i o n . The stratigraphy of these terraces has been described (3.2.4-), but i t should be mentioned that they are usually capped by up to 3 f t . of wind-blown sand. The upper l i m i t of these terraces represents the maximum height of the p o s t g l a c i a l aggradational deposits, but i t i s d i f f i c u l t to determine the maximum height of the f l o o d p l a i n at t h i s time, as so much of the material came from l a t e r a l sources. The best i n d i c a t i o n of the maximum height of the aggradational deposits comes from the 1,24-0 f t . terrace west of Gladwin Creek, where more than 600 f t . of aggradational deposits are c l e a r l y exposed from the rock bench above the r i v e r to the top of the terrace. As the front edge of the terrace i s approximately 350 yards from the r i v e r bank (measured as a horizontal distance) and the upper part of the deposit i s l a r g e l y from a l a t e r a l source, the maximum height of the r i v e r was probably less than 1,24-0 f t . Correlation of the lower terraces i s d i f f i c u l t , as many F i g . M3. Terraces of the Thompson and Fraser near Lytton. Note that the lowest terrace (600 f t . a.s.l.) i n the foreground i s common to both r i v e r s . terraces between Spences Bridge and Seddall have been destroyed by slumping s and i n places the va l l e y i s so narrow that terraces could not be preserved. Erosion and the deposition of c o l l u v i a l material on the surface of some terraces have made i t d i f f i c u l t to determine the o r i g i n a l l e v e l of the terraces. The wider sections of the valley at Spences Bridge and Lytton are the best places to study the terraces. At Lytton, terraces are preserved at varying heights on both sides of the Thompson and can be e a s i l y correlated with those of the Fraser. Lytton i s b u i l t on a terrace, common to both the Fraser and Thompson Rivers at an a l t i t u d e of 600 f t . , which can be correlated with a terrace shared by the Thompson and Fraser on the north bank of the Thompson (see F i g s . Lt-6b, k1? & If 8) and one at the same alti t u d e on the west bank of the Fraser (Ryder, 1970, F i g . 3 . 1 ^ ) . These terraces are approximately l'+O f t . above present r i v e r l e v e l and indicate a period of r e l a t i v e s t a b i l i t y during which the valley f l o o r s of the Thompson and Fraser Rivers were widened. The broad bench with a l l u v i a l fans on the northwest bank of the Thompson at Spences Bridge i s also approximately 1^0 f t . above the r i v e r , and a series of broad terraces at a sim i l a r height above the r i v e r can be traced between Ashcroft and Spences Bridge (Ryder, 1970, p. 1^3)• Terraces of si m i l a r height occur on both sides of the r i v e r between Spences Bridge and Nicoamen River but are not found i n the Thompson Canyon (see F i g . *+7)« It i s reasonable, however, to suppose that the terraces above the Canyon are the same age as those below the Canyon, as they are the same height above present r i v e r l e v e l . This presupposes that the Thompson River has been downcutting at the same rate throughout 84 i t s valley between Spences Bridge and Lytton since the formation of the 14-0 f t . terraces, an assumption which i s not necessarily v a l i d . It i s possible that downcutting has proceeded more rap i d l y southwest of the Nicoamen River, and t h i s could explain why the Thompson i s incised i n bedrock i n t h i s section. If t h i s i s the case, terraces of the same age would be at a greater height above present r i v e r l e v e l at Lytton than at Spences Bridge. There are paired terraces at an a l t i t u d e of 710 f t . , approximately 250 f t . above present r i v e r l e v e l , i n both the Fraser and Thompson Valleys at Lytton, but they are much narrower than those at 600 f t . I f a marker horizon such as Mazama ash could be found i n an equivalent position i n the deposits of the 14-0 f t . terraces at both Spences Bridge and Lytton, correlation would be more precise. As no other terrace levels can be traced fo r any distance along the Thompson, i t can be supposed that the majority of the terraces are non-cyclic. 4-.3 A l l u v i a l Fans The majority of a l l u v i a l fans i d e n t i f i e d i n the Thompson Valley were formed during the period of dominant degradation, although the highest fans such as the one at the mouth of Murray Creek and the 1,24-0 f t . fan, east of Gladwin Creek, were formed during the e a r l i e r period of aggradation when streams were clearing g l a c i a l debris from t h e i r courses. Many of the l a t e r and lower fans are on terraces at the mouths of intermittent streams or dry valleys (Figs. 4-6a & b), as the terraces would act as base levels f o r the debris flows. The morphology of the fans was not studied, but those on the 14-0 f t . terrace at Spences 85 Bridge have average slopes of between 6° and 8°, whereas the fans downstream, where the terraces are narrower, are generally steeper. Most of the fans appear to be ina c t i v e , apart from one beneath a rock basin with recent talus a c t i v i t y near Pitquah (Fig. 1+6b). k.h S l i d e s , Slumps and Talus  Drynoch Earthflow The stratigraphy and dimensions of t h i s feature have already been described, but not i t s o r i g i n . As well as volcanic rocks, the s l i d e material includes fragments of feldspathic sandstone apparently derived from T e r t i a r y coal measures (see F i g . 3 ) s and the clay f r a c t i o n of the s l i d e is largely montmorillonite, which was probably derived from bentonite beds in the coal measures (Armstrong and Fulton, 1965 s p. 103). B. C. Department of Highways engineers suggest that Tertiary coal measures, which occupied the cirque-like scarp area at the head of the s l i d e , s l i d out as a slump or block glide carrying considerable amounts of montmorillonite to the toe of the i n i t i a l s l i d e together with a large quantity of water (Armstrong and Fulton, 1965 s p. 103). The s l i d e then continued as a slow earthflow. Brawner (I960) l i s t e d excess pore pressure and s e r p e n t i n i t i c s o i l s as the major contributory causes of i t s continued movement, and noted that the rate of movement of the s l i d e increased or decreased about one inch per month fo r each one foot r i s e or f a l l i n the water table. Surface drainage, horizontal drains, drainage tunnels and trenches (Brawner, I960) were u t i l i z e d to s t a b i l i z e the s l i d e , which was threatening both 8 6 the Trans-Canada Highway and the Canadian P a c i f i c Railway. These attempts apparently were not completely successful as the railway l i n e had to be relocated i n 1967> major repairs are at present in progress on the highway, and new crevasses have developed on the g l a c i e r - l i k e surface of the s l i d e . k.k.2 Spences Bridge Slide A much more recent s l i d e occurred on August 13th, 1905 near the mouth of Murray Creek, when s i l t and gravel of the Murray Delta-Fan collapsed into the r i v e r damming i t f o r several hours. The horseshoe-shaped scar and debris hummocks are c l e a r l y v i s i b l e today (see F i g s . 19 & ^ a ) . A v i v i d account of the disaster was given i n the Monday ihth 1905 issue of the Vancouver Daily Province: "The side of the mountain slipped into the r i v e r . It came down with a roar that could be heard a mile away. A t i d a l wave of immense proportions was the r e s u l t . This foam-crested wave, which swept up the r i v e r more than two miles, overwhelmed the l i t t l e Indian v i l l a g e . " "In half an hour the swift flowing r i v e r was con-verted into a lake. The s l i d e stretched from bank to bank. Usually the r i v e r i s only a quarter of a mile wide. In half an hour yesterday i t widened to a mile. The water rose between 70 and 80 f t . I t came up almost a foot a minute afte r the f i r s t wave. At one time i t was 5 f t . over the railway tracks at the opposite side of the r i v e r from the point where the s l i d e occurred. Last night at 8 o'clock i t had receded u n t i l i t was :only halfway up the bank. At 3 o'clock t h i s afternoon the swift r i v e r had once more worn i t s way through the wall of gravel-rock." To say that the "side of the mountain slipped into the r i v e r " i s an exaggeration, as the difference i n a l t i t u d e between the r i v e r and the back of the s l i d e i s only 5*+0 f t . The maximum width of the s l i d e i s about 1,^ 50 yards, but i t s maximum length, 87 measured to i t s farthest extent on the east bank, i s only l s100 yards. Flood deposits of cobble gravels and a shallow channel on the low terrace of the north bank of the r i v e r at Spences Bridge may owe t h e i r existence to t h i s s l i d e . The theories suggested to explain the s l i d e a l l involve an excess of water. Armstrong and Fulton (1965 s p. 102) suggest that the s l i d e resulted from the saturation of the s i l t s by water from the valley of Murray Creek during a period of excessive r a i n f a l l . The newspaper reporter blamed i r r i g a t i o n : "One (theory) that i s credited by a great many people i s that the loose sands underlying the high gravel bank had been washed out gradually as a result of i r r i g a t i o n carried out on the top of the bank. The s o i l s are very loose l y i n g and i t i s thought that water from the i r r i g a t i o n ditches gradually worked out the loose lower sands and the heavier top material then s l i d away." (Vancouver Daily Province., Tuesday August l 5 s 1905.) I r r i g a t i o n was cited as the main cause of the Great North Slide of 1881, near Ashcroft, and other s l i d e s i n the v i c i n i t y of the Black Canyon, although the large s l i d e was hastened by the bursting of a reservoir (Stanton 1898). Stanton (1898, p. 16) claimed that no s l i d e s occurred i n the s i l t s along the Thompson River before i r r i g a t i o n was practised. He also experimented with the s i l t , dropping i t into a basin of water so that i t formed "a semi-fluid mass l i k e t h ick pea-soup" and noted that the same mixture could be seen oozing out at many points along the foot of one recent s l i d e (Stanton, 1898, p. 17). Seepage may have been i n part responsible f o r the Spences Bridge S l i d e , as Brawner (1964-, p. 15) points out that noticeable groves of cottonwood appear on the s l i d e i n d i c a t i n g seepage, and the l e v e l of water i n Mud Lake varies noticeably with the l e v e l of the Thompson so that some seepage i s d e f i n i t e l y taking place. Probably the s l i d e resulted from both i r r i g a t i o n and seepage and was hastened by excessive r a i n f a l l . '+.'+.3 Slumps Slumping i s confined to the section of the va l l e y between Spences Bridge and Seddall (see F i g . ^ a ) and coincides with the d i s t r i b u t i o n of s i l t and t i l l . The causes of slumping are basic a l l y the unstable nature of the s i l t s and t i l l when saturated and undercut by the r i v e r . ^ A A . Talus Talus slopes are largely confined to the western slope of the valley between Spences Bridge and Nicoamen and to the northern slope of the va l l e y between Nicoamen and Lytton (Figs. U-6 a and b), la r g e l y because these are the steeper slopes of the valley so that they have many bare outcrops of rock and a sparse vegetation cover. Talus i s s t i l l being produced at the foot of the Scarped Mountains, probably as a result of over-steepening of the slope by the r i v e r undercutting, and in the small rock basins near Pitquah (Fig. l+6b). CHAPTER 5 SUMMARY AND CONCLUSION 5.1 Summary of Quaternary History Evidence of conditions prior to the l a s t major advance of ice i s scant, but i t can be deduced that the Thompson River had by t h i s time carved a valley almost as deep as i t s present one between Spences Bridge and Lytton. During the l a s t g l a c i a t i o n , ice from the Coast Mountains flowed south-south-east over the I n t e r i o r Plateau depositing a thi n mantle of t i l l over the uplands and l o c a l l y moulding i t into drumlinoid forms. This ice sheet wasted away by downmelting so that the uplands were free of ice while ice remained in the valleys damming late-g l a c i a l lakes. The lower Thompson Valley, close to the supply region of the Coast Mountains, was one of the l a s t areas to be free of ice so that major lakes developed i n the Thompson and Nicola Valleys and were forced to drain into the Okanagan Valley. As the ice i n the Thompson melted, the waters of Lake Deadman, the l a s t and lowest of the g l a c i a l lakes i n the Thompson Valley, extended just south of Spences Bridge and about s i x miles up the Nicola Valley depositing s i l t to a height of 1,230 f t . Eventually the waters overtopped the melting ice plug and drainage was resumed down the Thompson Valley to the Fraser. Deglaciation of the I n t e r i o r Plateau was well ad-vanced by 9 3750 B.P. and a l l g l a c i a l lakes had been drained and modern drainage established by 9 5000 B.P. at the l a t e s t . FI6. 49 GENERALIZED SEQUENCE OF QUATERNARY DEPOSITS - SPENCES BRIDGE • 1,250 ft. a.s.l. 0-'5C-V 0 0 0 0 0  0 0 0 0 o o a Q ° •' ° °o °' ° -o- ° • <=>'• 0 ° . ° o •  0 O • ' 'o • o ° ' O , o ° 0 • .° o <= • . © e> o • o 0 • o _ © • o ° 0 ° ° o o ' o o o o 0 . ° ° 0 'o °. o ' O . ° • o " o'. o\ o . o • ° o DELTA IC OR FAN . G R A V E L S i ^  6 (9 slump balls W H I T E S I L T 200+ _ • 1 0 - 3 0 ' • • 7 . W > I C E - C O N T A C T G R A V E L S 50V 850 ft, a.s.l. KA o ' A ^ A ^ A 0 A A T I L L FIG. 50 G E N E R A L I Z E D S E Q U E N C E O F Q U A T E R N A R Y D E P O S I T S - T H O M P S O N C A N Y O N 1,200 ft. as. 500-*-0 - 5 0 5 0 ' 0 - 5 0 0 - 2 0 6 0 0 ft. a s I. 2 - 2 - ^ --2^ -2^ -Z-1 -oa'o- o• o o b o o o ? -o o o o o a 4 / O S 0 , \ \ i - \ AGGRADATIONAL GRAVELS (LANDSLIDE DEPOSITS) CROSS BEDDED SANDS 8 GRAVELS (ICE-CONTACT LACUSTRINE DEPOSITS) (TILL) BEDROCK Apparently a mass of stagnant Ice remained between Skoonka and Seddall, while south of here the Thompson Valley was ice-free and was being rapidly aggraded with reworked g l a c i a l d r i f t , t a l u s , and landslide debris from the valley sides. Probably lacustrine deposition i n Lake Deadman was s t i l l taking place while f l u v i a l aggradation was occurring further south. With the f a i l u r e of the ice plug, normal drainage was resumed i n the Thompson Valley, with the r i v e r p r o f i l e having a very gentle gradient over the former bed of Lake Deadman and steepening southwestwards where aggradation was occurring. Aggradation, which was due to the excessive supply r e s u l t i n g from g l a c i a t i o n and erosion of g l a c i a l debris, was extremely rapid; more than 500 f t . of cross-bedded and horizontal gravel and sand was deposited i n probably l i t t l e more than 1,000 years. Aggradation ceased well before 7 S530 B.P. Once the g l a c i a l debris had been reworked and the climate had ameliorated, the load of the r i v e r was considerably reduced and i t began to degrade i t s former aggradational f i l l . Degradation proceeded rapidly at f i r s t creating a series of non-cyclic terraces between 150 f t . and 500 f t . above present r i v e r l e v e l . This was followed by a period of r e l a t i v e s t a b i l i t y , p rior to 7 S530 B.P., during which the valley f l o o r was widened. Then further downcutting occurred leaving a series of paired terraces at a height of approximately ikO f t . above present r i v e r l e v e l between Spences Bridge and Lytton and non-c y c l i c terraces below t h i s . The r i v e r was probably within 50 f t . of i t s present l e v e l by 6,600 B.P. and since then, down-cutting appears to have proceeded r e l a t i v e l y slowly. At least 91 three phases of a l l u v i a l fan formation occurred during this period of dominant degradation, the terraces acting as temporary base l e v e l s f o r the mudflows. Mazama ash was deposited on the terraces and fans approximately 6,600 years ago providing a u s e f u l marker horizon. Apart from slow downcutting by the r i v e r and some a l l u v i a l fan formation, the dominant processes af f e c t i n g the topography since 6,600 B.P. have been landslides and slumps. The largest s l i d e , the Drynoch Earthflow, has been active f o r at least 391?5 years, and slumping due to seepage and r i v e r erosion has occurred intermittently along the r i v e r banks. 5 .2 Conclusion The history of deglaciation around Spences Bridge and i n the Lower Nicola Valley i s s i m i l a r to that of the Kamloops and Okanagan areas described by Fulton and Nasmith. The evidence agrees with Mathews' and Fulton's observations that the Lower Thompson Valley was one of the l a s t areas to be free of ice and that the lowest and l a s t of the g l a c i a l lakes i n the Thompson Valley extended south of Spences Bridge f o r a few miles only. South of Skoonka, l a t e - g l a c i a l and early p o s t g l a c i a l events were d i f f e r e n t from those i n the more central part of the southern I n t e r i o r Plateau, as melting of the ice plug opened through drainage to the Fraser. The Thompson heavily charged with re-worked g l a c i a l debris and talus from the unstable valley sides rapidly aggraded i t s v a l l e y , depositing more than 500 f t . of sand and gravel, the height to which material accumulated being controlled by s i m i l a r aggradation i n the Fraser Valley, as 92 described by Ryder. •J. M. Ryder i s the only other person who has attempted to determine the progress of degradation i n the I n t e r i o r . Observa-tions i n the Lower Thompson Valley of rapid degradation with the formation of non-cyclic t e r r a c e s 3 followed by a period of r e l a t i v e s t a b i l i t y with valley-widening, and followed by further downcutting creating a series of paired terraces at approximately ikO f t . above the r i v e r are i n basic agreement with Ryder's observations on the Thompson Valley between Ashcroft and Spences Bridge and the Fraser Valley between L i l l o o e t and Lytton. I t can be seen that the Lower Thompson Valley between Spences Bridge and Lytton was an important t r a n s i t i o n zone between the g l a c i a l lake deposition of the more central southern I n t e r i o r and the aggradation of the Fraser. 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K., 1963, "Morphology and hydrology of a g l a c i a l stream", United States Geological Survey Professional Paper i+22-A, 70 pp. F l i n t , R. F., 19575 " G l a c i a l and Pleistocene Geology", John Wiley and Sons, New York, 553 pp. Fulton, R. J . , 1962, " S u r f i c i a l Geology, M e r r i t t , B r i t i s h Columbia", Geological Survey of Canada Map 8=1962, 1:126,720. Fulton, R. J . , 1963, "Deglaciation of the Kamloops Area, B r i t i s h Columbia", Unpublished Ph.D. t h e s i s , Northwestern University, 121 pp. Fulton, R. J . , 1963s " S u r f i c i a l geology of Kamloops Lake, B r i t i s h Columbia", Geological Survey of Canada Map 9=1963, 1:126,720. Fulton, R. -J., 1965s " S i l t deposition i n L a t e - G l a c i a l Lakes of southern B r i t i s h Columbia", American -Journal of Science, v o l . 263s PP- 553-570. Fulton, R. -J., 1967s "Deglaciation studies in Kamloops regions an area of moderate r e l i e f , B r i t i s h Columbia", Geological Survey of Canada B u l l e t i n 15^, 136 pp. Fulton, R. -J., 1969a, " G l a c i a l lake history, Southern I n t e r i o r Plateau, B r i t i s h Columbia", Geological Survey of Canada Paper 69-37, 13 pp. Fulton, R. J . , 1969b, " S u r f i c i a l Geology, Shuswap Lake, west of s i x t h meridian, B r i t i s h Columbia", Geological Survey of Canada Map 12M+A, 1 :126,720. Fulton, R, J . , 1969c, " S u r f i c i a l Geology, Vernon, west of s i x t h meridian, B r i t i s h Columbia", Geological Survey of Canada Map 124- 5A, 1 :126,720. Geological Association of Canada, 1958, " G l a c i a l Map of Canada", 1 : 5 , 0 0 0 , 0 0 0 . G i l b e r t , G. K., 1917, "Hydraulic-mining debris i n the Sierra Nevada", United States Geological Survey Professional Paper 105, 15*+ pp. Hansen, H. P., 1955, " P o s t g l a c i a l forests i n south central and central B r i t i s h Columbia", American -Journal of Science, v o l . 253, pp. 6^0-658. 96 Harms, J . E. and Fahnestock, R. K., 1965? " S t r a t i f i c a t i o n , bed formsj and flow phenomena (with an example from the Rio Grande)", i n "Primary Sedimentary Structures and t h e i r Hydrodynamic Interpretation's edited by G. V. Middleton, Society of Economic Paleontologists and Mineralogists Special Publication No. 12. Hartshorn, J . H., 1958, "Flow t i l l in southeastern Massachusetts", B u l l e t i n of the Geological Society of America, v o l . 69, pp. k77-k82. •Jones, A. G., 1959 9 "Vernon map-area, B r i t i s h Columbia", Geological Survey of Canada Memoir 296, 186 pp. Kuenen, Ph.H., 19*+8, "Slumping i n the Carboniferous rocks of Pembrokeshire", Quarterly Journal of the Geological Society , London, v o l . 10k, pp. 356-369. Kuenen, Ph.H., 195l 3 "Mechanics of varye=formation and the action of t u r b i d i t y currents", Geoiogiska Foreningens Forhandlinger, v o l . 73, pp. 69-8 L. Lahee, F. H., 1961, " F i e l d Geology", McGraw-Hill, New York, 926 pp. Lewis, W. V., 19 ^ 3 "Stream trough experiments and terrace formation", Geological Magazine, v o l . 8 l , pp. 2^1-253. Lowdon, J . A., F y l e s , J . G., and Blake, W. J r . , 1967, "Geological Survey of Canada Radiocarbon Dates VI", Geological Survey of Canada Paper 67-2, Part B, >+2 pp. Lowdon, J . A., Wilmeth, R. and Blake, W. J r . , 1969 s "Geological Survey of Canada Radiocarbon Dates VIII", Geological Survey of Canada Paper 69-2, Part B, h2 pp. Mathews, W. H., 19M+, " G l a c i a l lakes and ice retreat in south-central B r i t i s h Columbia", Transactions of the Royal Society of Canada, 3rd s e r i e s , v o l . 38, section k3 pp. 39-57-Mathews, W. H., 1956, "Physical limnology and sedimentation i n a g l a c i a l lake", B u l l e t i n of the Geological Society of America, v o l . 67, pp. 537-552. Mathews, ¥. H., 1963, "Quaternary stratigraphy and geomorphology of the Fort St. John area, northeastern B r i t i s h Columbia", B. C. Dept. of Mines, 22 pp. Mathews, W. H., 196'+, "Potassium-Argon age determinations of Cenozoic volcanic rocks from B r i t i s h Columbia", B u l l e t i n of the Geological Society of America, v o l . 75 s pp. lf65-1+68. 97 Mathews, W . H., Fyles, J . G.s and Nasmith, H. W . , 1970, " P o s t g l a c i a l c r u s t a l movements i n southwestern B r i t i s h Columbia and adjacent Washington State", Canadian -Journal of Earth Sciences, v o l . 7 s pp. 690=702. MacClintock, P., and Dreimanis, A., 196*+, "Reorientation of t i l l f a b r i c by overriding g l a c i e r i n the St. Lawrence Valley", American Journal of Science, v o l . 262, pp. 133=l l t2. McCallum, K. J . , and Wittenberg, J . , 1962, University of Saskatchewan Radiocarbon Dates I I I , Radiocarbon, v o l , k, Nasmith, H. W., 1962, "Late G l a c i a l History and S u r f i c i a l Deposits of the Okanagan Valley, B r i t i s h Columbia", B r i t i s h Columbia Department of Mines B u l l e t i n No. ^ 6 , ^6 pp. Nasmith, H., Mathews, W. H., and Rouse, G. E., 1967 s "Bridge River ash and some other recent ash beds i n B r i t i s h Columbia", Canadian Journal of Earth Sciences, v o l . k3 pp. 163=170. P e t t i John ? F. J . and Potter, P. E., 196k, "Atlas and Glossary of Primary Sedimentary Structures", Springer-Verlag, New York, 370 pp. Potter, P. E., and Pettijohn, F. J . , 1963, "Paleocurrents and Basin Analysis", Springer-Verlag, New York, 296 pp. Ryder, J . M., 1970, " A l l u v i a l fans of p o s t - g l a c i a l environments within B r i t i s h Columbia", Unpublished Ph.D. t h e s i s . University of B r i t i s h Columbia, ^36 pp. Sanger, D., 1967s "Prehistory of the P a c i f i c Northwest Plateau as seen from the I n t e r i o r of B r i t i s h Columbia", American Antiquity, v o l . 3 2 , pp. 186-197. Selwyn, A.R.C., 1872, "Journal and Report of Preliminary Explorations i n B r i t i s h Columbia", Geological Survey of Canada, Report of Progress 1871=72, pp. 16-72. Stanton, R. B., 1898, "The Great Land Slides on the Canadian P a c i f i c Railway i n B r i t i s h Columbia", Minutes of Proceedings of the I n s t i t u t e of C i v i l Engineering, England, v o l . 132, pt. I I , pp. l-k6. Uglow, W. L., "Geology of the North Thompson Valley", Geological Survey of Canada Summary Report, 1921, pt. A, pp. 72=106. Vancouver Daily Province, August l'+th, 1905. Vancouver Daily Province, August l 5 t h , 1905. Wheeler, J . 0., 196k, "Geology, Big Bend map-area". Geological Survey of Canada Paper 64--329 37 pp. Wilcox, R. E., 1965, "Volcanic-ash chronology" i n "Quaternary of the United States", edited by H. E. Wright J r . , and D. G. Frey, Princeton University Press, Princeton, N. J . s pp. 807-816. APPENDIX C r i t e r i a By Which Deposits Were Distinguished The terms used in t h i s thesis follow conventional geological usage. Such terms as clay, s i l t , sand and gravel simply indicate p a r t i c l e size as outlined in the Wentworth scale (Lahee, 1 9 6 l s pp. 37=38) and have no genetic connotations In order to d i s t i n g u i s h deposits by mode of o r i g i n , terms such as f l u v i a l and lacustrine are used. C r i t e r i a used to c l a s s i f y deposits by o r i g i n are l i s t e d below: T i l l Plow t i l l F l u v l o - g l a c i a l or outwash Ice-contact F l u v i a l deposits are c h a r a c t e r i s t i c a l l y unsorted, and there i s great v a r i a t i o n i n s i z e , shape and composition of included pebbles, some of which may be s t r i a t e d and faceted (see F i g . 9) deposits are distinguished from t i l l deposits i n that they exhibit rudimentary bedding and preferred orientation of component pebbles (see F i g . 8 ) . I d e n t i f i c a t i o n i s sim p l i f i e d i f they are interbedded with water-laid gravels, p a r t i c u l a r l y i f the interbedded gravels are undisturbed, deposits generally have a l l the ch a r a c t e r i s t i c of f l u v i a l deposits so that i t i s not always possible to d i s t i n g u i s h them i n the absence of supplementary evidence such as proximity of known g l a c i a l deposits and morphology. f l u v i o - g l a c i a l and lacustrine deposits exhibit signs of collapse and/or slumping. In order to ascertain whether they are genuinely i c e -contact deposits, other evidence such as proximity of known g l a c i a l deposits and t h e i r r e l a t i o n s h i p to the d i r e c t i o n of ice retreat i s required. deposits are bedded, moderately to well sorted and may exhibit evidence of current d i r e c t i o n such as cross-bedding and imbrication. P a r t i c l e s are sub-angular to well rounded and vary from s i l t to cobble gravel, with gravels predominating. 100 Lacustrine D e l t a i c Mudflow deposits are well bedded and well sorted and may be laminated or graded bedded. Bottom deposits consist of c l a y s s i l t and sand. deposits have a l l the c h a r a c t e r i s t i c s of f l u v i a l deposits and i n addition have recog-nizable foreset beds. They are easiest to d i s t i n g u i s h when they are i n close proximity to lacustrine bottom deposits with which they may i n t e r f i n g e r . deposits consist of angular to sub-angular pebbles 9 generally of l o c a l material in a f i n e matriXj and are noticeably bedded. They may be confused with flow t i l l i f they are derived from s u p e r f i c i a l deposits rather than bedrock. Fabric analysis may help in distinguishing mudflow from flow t i l l , but t h i s can be done more quickly by examining the r e l a t i o n s h i p of the deposit concerned with the topography and other deposits. For instance, the majority of mudflow deposits in the Thompson Valley are found overlying p o s t g l a c i a l deposits at the mouths of t r i b u -tary v a l l e y s . 

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