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UBC Theses and Dissertations

Quaternary stratigraphy and geomorphology of lower Bridge River valley, British Columbia Howes, Don Edwin 1975

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Q U A T E R N A R Y STRATIGRAPHY A N D G E O M O R P H O L O G Y O F LOWER BRIDGE RIVER V A L L E Y , BRITISH C O L U M B I A by D O N EDWIN HOWES B . A . , University of Western Onta r io , 1971 A THESIS SUBMITTED IN PARTIAL FULFILMENT O F THE REQUIREMENTS FOR THE DEGREE O F MASTER O F SCIENCE in the Department of Geography We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH C O L U M B I A April, 1975 In presenting th is thes is in par t ia l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree that the L ibrary sha l l make it f ree ly ava i lab le for reference and study. I fur ther agree that permission for extensive copying of th is thesis for scho la r ly purposes may be granted by the Head of my Department or by his representa t ives . It is understood that copying or pub l i ca t ion of th is thes is fo r f inanc ia l gain shal l not be allowed without my wri t ten permission. Department of The Univers i ty of B r i t i s h Columbia Vancouver 8, Canada i i ABSTRACT Bridge River is an east-f lowing tributary of Fraser River originating in the Coast Mountains of British Columbia . Its confluence with the Fraser River is near the v i l lage of L i l l ooe t . This study encompasses the lower 15 miles (24 km) of Bridge River V a l l e y . The purpose of the study is: (1) to describe and reconstruct the local Quaternary events and relate them to the regional chronology, (2) to produce a surf icial geology map, at a scale of 1:50,000. Lacustr ine, ice contact f l uv i o -g l ac ia l , f l uv io -g lac ia l ( lacking ice contact features), g l a c i a l , a l luv ia l fan and recent deposits occur here. From their characteristics and stratigraphic relationships, two alternative Quaternary chronologies can be reconstructed: A : Silts and clays deposited in a proglacial lake were subsequently overridden and partly deformed by ice of the last g lac ia t ion . La te-g lac ia l aggradation of as much as 400 feet (120 m) of outwash material was succeeded by the f luvia l degradation and terracing which continues at present. B: The outwash material represents two phases of proglacia l deposition that were preceded by a g lac ia l in terval , and separated by the g lac ia l lake phase and the ensuing g lac ia t i on . The latter, Chronology B, is favoured. This has been tentat ively correlated with the Pleistocene chronology in the adjacent part of Fraser River V a l l e y . i i i TABLE OF CONTENTS INTRODUCTION 0.1 LOCATION 1 0.2 OBJECTIVES - 1 ' 3 0.3 GENERAL PROCEDURES 3 , 5 0.3.1 Mapping Procedures 3 0.3.2 Stratigraphic and Geomorphic Procedures 5 0.3.2.1 Surveying 5 0.3.2.2 Stratigraphic Studies 5 CHAPTER ONE 1.1 INTRODUCTION 6 1.2 G E O L O G Y OF STUDY AREA 6-8 1.3 DESCRIPTION OF THE STUDY AREA 9-15 1.3.1 Interior Plateau 9,11 1.3.2 Coast Mountains 11 1.3.3 Central Valley Region 14 CHAPTER TWO 2.1 LATE-PLEISTOCENE C H R O N O L O G Y OF FRASER LOWLAND 16,17 2.2 GENERAL PLEISTOCENE C H R O N O L O G Y FOR INTERIOR BRITISH COLUMBIA 2.2.1 Evidence of Multiple G laciations 18 2.2.2 Olympia Interglacial 18,19 2.2.3 Fraser G laciation Advance 19,21 2.2.4 Fraser Glaciation Retreat 21 I V Page 2.2.5 Fraser Glaciation During Sumas Stade 22 2.2.6 Post-Glacial Climare and Ashfalls 22,23 2.3 RELATED PLEISTOCENE STUDIES 23-25 2.4 BRIDGE RIVER VALLEY DURING FRASER GLACIATION 25-27 CHAPTER THREE 3.1 INTRODUCTION 28 3.2 LACUSTRINE DEPOSITS 23-32 3.3 GLACIAL DEPOSITS: TILL 33 3.3.1 Description of Til l 33-42 3.4 ICE CONTACT FLUVIO-GLACIAL DEPOSITS: KAME TERRACES 42-45 3.5 FLUVIO-GLACIAL DEPOSITS: AGGRADATION GRAVELS OF VALLEY TRAIN 3.5.1 Introduction _ 45*46 3.5.2 A Typical Section - The "Horseshoe Section" 46-52 3 .5 .3 Downstream Sections of Aggradation Gravels 52-56 3.5.4 Discussion 56-61 3.6 ALLUVIAL FAN DEPOSITS 62-63 3.7 RECENT DEPOSITS 64 CHAPTER FOUR 4.1 BRIDGE RIVER C H R O N O L O G Y 65-71 4.2 CORRELATION WITH FRASER RIVER VALLEY PLEISTOCENE C H R O N O L O G Y 72-76 V Page SUMMARY STATEMENT 77 BIBLIOGRAPHY 78-81 APPENDIX A Classification Used for Surficial Geology Map 82-83 APPENDIX B Procedures Conducted at Each Stratigraphic Section 84-86 v i LIST OF TABLES Page TABLE I SUB-UNITS OF FRASER GLACIATION 17 TABLE II POST-GLACIAL ASHFALLS -i 23 TABLE III TENTATIVE C H R O N O L O G Y FOR FRASER VALLEY BETWEEN LYTTON A N D LILLOOET 25 TABLE IV DESCRIPTION OF TILL SITES 35 TABLE V DESCRIPTION OF HORSESHOE SECTION 47,48 V I I FIGURE 0.1 FIGURE 0.2 FIGURE 0.3 LIST O F FIGURES L O C A T I O N O F STUDY AREA Page .. 2 INDEX M A P O F BRITISH C O L U M B I A S H O W I N G facing 3 AREAS O F OTHER Q U A T E R N A R Y STUDIES FLOW D I A G R A M O F PROCEDURES 4 CHAPTER O N E FIGURE 1.1 REFERENCE M A P O F BRIDGE RIVER BASIN facing 6 FIGURE 1 .2 G E O L O G Y O F STUDY AREA V FIGURE 1 .3 REFERENCE M A P O F STUDY AREA 10 FIGURE 1.4 SURFICIAL G E O L O G Y O F STUDY AREA 12 CHAPTER TWO FIGURE 2.1 FIGURE 2 .2 FIGURE 2 .3 ICE M O V E M E N T S D U R I N G FRASER G L A C I A T I O N 20 TENTATIVE C H R O N O L O G Y O F FRASER RIVER 24 V A L L E Y BETWEEN L Y T T O N A N D LILLOOET G L A C I A L STRIAE I N D I C A T I N G DIRECTION O F 26 ICE M O V E M E N T CHAPTER THREE FIGURE 3.1 FIGURE 3.2 FIGURE 3.3 FIGURE 3 .4 FIGURE 3 .5 FIGURE 3.6 C L A Y PIT SECTION facing 28 STEP-LIKE PATTERN O F SILT BED OBSERVED O N EASTERN F L A N K O F C L A Y PIT OVERRIDDEN LACUSTRINE SEDIMENT TILL SITES A N D U N D E R L Y I N G BEDROCK D E F O R M A T I O N TILL A N EXAMPLE O F H O W DEBRIS C A N BE INCORPORATED I N T O A N ICE SHEET facing 29 31 34 39 41, ' v i i i FIGURE 3 .7 FIGURE 3.8 FIGURE 3 .9 FIGURE 3 .10 C O R R E L A T I O N O F KAME DEPOSITS C R O S S - S E C T I O N S H O W I N G STRATIGRAPHIC RELATIONSHIP O F LACUSTRINE, G L A C I A L , F L U V I O - G L A C I A L , A N D I C E - C O N T A C T • F L U V I O - G L A C I A L DEPOSITS ILLUSTRATION O F "HORSESHOE SECTION'- ' Page facing 44 facing 45 facing 47 C O R R E L A T I O N O F OUTCROPS O F V A L L E Y TRAIN GRAVELS 53 CHAPTER FOUR FIGURE 4.1 FIGURE 4 .2 FIGURE 4 . 3 POSSIBLE C H R O N O L O G I E S O F BRIDGE RIVER V A L L E Y A POSSIBLE M E C H A N I S M A C C O U N T I N G FOR S L U M P I N G IN LACUSTRINE SEDIMENTS RELATIONSHIP O F D E G R A D A T I O N TERRACES TO V A L L E Y TRAIN SURFACE A N D PRESENT PROFILE O F BRIDGE RIVER 66 68 70 FIGURE 4 .4 TENTATIVE C O R R E L A T I O N O F BRIDGE A N D FRASER RIVER V A L L E Y S 73 ix LIST O F PLATES CHAPTER O N E PLATE 1.1 PLATE 1.2 PLATE 1.3 PLATE 1.4 PLATE 1.5 BEDROCK RIVER SCARPS O F LILLOOET G R O U P M O R P H O L O G Y O F UPPER PART O F BRIDGE RIVER V A L L E Y (in study area) C O L L U V I A L F A N DRAPED OVER CENTRAL V A L L E Y FILL FLUVIAL TERRACES (foreground) A N D STEP-LIKE M O R P H O L O G Y O F M I S S I O N RIDGE (background) DUE TO FAULT ING P O T - H O L E IN C H A N N E L BED NEAR THE HORSESHOE EXPOSURE Page facing 9 facing 9 13 13 15 CHAPTER THREE PLATE 3.1 PLATE 3 .2 PLATE 3 .3 PLATE 3 .4 PLATE 3 .5 PLATE 3 .6 PLATE 3.7 PLATE 3 .8 PLATE 3 .9 OVERALL VIEW O F C L A Y PIT SECTION 2 9 FRACTURED A N D FAULTED C L A Y F R O M 29 C L A Y PIT S E C T I O N L O D G M E N T TILL (MEMBER A AT HORSESHOE 37 SECTION) STRIATED CLAST F R O M U P L A N D - Z O N E A : TILL 37 D E F O R M A T I O N TILL: A N EXAMPLE (NOTICE 38 FRACTURES A N D C O N T O R T E D SILT BANDS) LENSES O F S M A L L , A N G U L A R CLASTS IN 38 L O D G M E N T TILL (MEMBER A AT HORSESHOE SECTION) KAME TERRACE A L O N G SOUTH SIDE O F BRIDGE 43 RIVER V A L L E Y NEAR M O U T H O F BRIDGE RIVER MEMBERS N , M , L, K, J , AT HORSESHOE SECTION 49 (140 F E E T / 42 METRES) UPPER PART O F HORSESHOE SECTION 49 (NOTICE C O N T I N U I T Y O F MEMBERS) X PLATE 3.10 PLATE 3.11 PLATE 3.12 PLATE 3.13 PLATE 3.14 PLATE 3.15 PLATE 3.16 PLATE 3.17 PLATE 3.18 MEMBER H A T HO R S E S H O E S E C T I O N ( N O T E : S E E P A G E STAIN) I N D U R A T E D MEMBERS A T O E A T HO R S E S H O E S E C T I O N SITE O N E O F D O W N S T R E A M S E C T I O N S A S Y M M E T R I C A L RIPPLES I N MEMBER H A T SITE FOUR O F D O W N S T R E A M S E C T I O N S S C O U R A N D FILL STRUCTURES A T SITE FIVE O F D O W N S T R E A M S E C T I O N S C O L L U V I A L MATERIAL I N C O R P O R A T E D IN A G G R A D A T I O N G R A V E L S (SITE FIVE) Page 50 .50 57 57 58 58 M U D F L O W B A N D I N A L L U V I A L F A N DEPOSIT facing 62 C L O S E - U P O F M U D F L O W B A N D I L L U S T R A T I N G facing 62 TEXTURE V O L C A N I C A S H S U R R O U N D E D BY C O L L U V I A L MATERIAL 63 CHAPTER FOUR PLATE 4.1 K A M E TERRACES IN BRIDGE RIVER ( F O R E G R O U N D ) 75 A N D FRASER RIVER V A L L E Y ( B A C K G R O U N D ) x i ACKNOWLEDGEMENTS The author would like to thank first Dr. June Ryder for her encouragement and advice during the preparation of this thesis, and Dr. W.H . Mathews for reviewing the final draft. I am also grateful to Mr. P . N . Sprout, Chief of the Soils Division, B . C . Department of Agriculture, for the "loan" of Mr. Andrew Holmes. Mr. Holmes was a most willing field assistant. Special thanks are due to my wife, Peggy Howes, for her assistance and patience in the preparation of this thesis. Field work was financially supported by NRC Grant #A8171 to Dr. June Ryder. 1 INTRODUCTION 0.1 LOCATION Bridge River is an east flowing, glacier-fed tributary of Fraser River originating in the Coast Mountains of British Columbia. Its confluence with Fraser River is approximately 5.2 miles (8.3 km) north of the village of Lillooet. This study encompasses the lower 15 miles (24 km) of the Bridge River Valley, between Fraser and Yalakom Rivers (FIGURE 0.1). This segment of the river is the boundary between the Interior Plateau to the east and the Coast Mountains to the west (Holland, 1964). Most of the land in the study region is in the Bridge River Indian Reserve. The dominant occupation of the inhabitants is part-time ranching. 0.2 OBJECTIVES The study has two objectives. The first is to provide a detailed description and reconstruction of the local Quaternary events and relate them to the regional chronology. The second is to produce a surficial geology map, at a scale of 1:50,000. The area merits attention for several reasons. These are: (1) There are a number of thick, continuous exposures of fluvio-glacial gravels within the region that have received no previous attention. These present an ideal situation for reviewing an aggradational sequence following a glacial episode. (2) In recent years, there has been an increased interest in Pleistocene research of interior British Columbia (FIGURE 0.2). Bridge River Valley can be related to these studies and the locally derived chronology can be correlated in a regional context. 50 N 49 N 50 N - 49 N 124 W 123 W 122 "W 121"W 120"VV FIGURE 0.2 : INDEX MAP OF SOUTHWESTERN BRITISH COLUMBIA - SHOWING LOCATION OF STUDY AREA AND OTHER QUATARARY STUDIES CONDUCTEDIN INTERIOR 119 W 3.b (3) No terrain analysis or surficial geology mapping has been carried out in the area. The data obtained, thus, will become background information for related studies, such as soil research, and planning. 0.3 GENERAL PROCEDURES With these objectives in mind, field research was conducted from May through to August, 1973. Research advanced through a series of steps as outlined below (FIGURE 0.3). 0.3.1" MAPPING PROCEDURES : Production of the terrain and surficial geology map involved four major steps. These were: (1) initial air photo interpretation of landforms and surficial deposits, • (2) field mapping using the air photo base, (3) re-interpretation of units and boundaries using field data, : (4) transformation of data onto the final map. Terrain and surficial geological units were classified according to a system developed by R .J . Fulton of the Geological Survey of Canada. i Each unit is defined by four major components: composition, morphology, texture, and erosional modification (refer to Appendix A for details). Field verification proved to be important for determination of the textural component and exact location of boundaries. Air photos at a scale of two inches to the mile (40 chain) were used, as this provided sufficient information for the final map scale (refer to Bibliography for flight lines and photo numbers). 4 FIGURE 0.3 : FLOW DIAGRAM OF PROCEDURES PRELIMINARY PROCEDURES ,1. LITERATURE REVIEW 2. FAMILIARIZATION OF STUDY AREA MAPPING PROCEDURES INITIAL AIR PHOTO INTERPRETION FIELD VERIFICATION RE-INTERPRETION OF BOUNDARIES AND UNITS STRATIGRAPHIC AND GEOMORPHIC PROCEDURES SURVEYING OF TERRACE HEIGHTS DETAILED ANALYSIS OF INDIVIDUAL SECTIONS ' CORRELATION OF SECTIONS RESULTS .1 .CHRONOLOGY OF STUDY AREA 2. SURFICIAL GEOLOGICAL MAP OF STUDY AREA 5 0.3.2 STRATIGRAPHIC AND GEOMORPHIC PROCEDURES 0.3.2.1 SURVEYING Fluvial and kame terrace levels were obtained with a Paulin Altimeter. The Single Altimeter Method (Hodgson, N . D . , p. 25) with corrections for temperature and barometric pressure, provided a reasonable estimate of the terrace heights, ±10 feet (^ 3 m). The rate of change of barometric pressure was assumed to be constant and each run from one of the established bench-marks was limited to two hours. The heights of the inaccessible terraces were obtained using a clinometer and the altimeter. The clinometer was levelled with the outer lip of the terrace from the opposite side of the valley. When a zero reading was obtained, the altimeter was read. This technique, though not as accurate as the former, provided a sufficient satisfactory result, +20 feet (+6 m). 0.3.2.2 STRATIGRAPHIC STUDIES Observations at single exposures included texture and colour of sediment types, lithology and shape of clasts, and descriptions of primary sedimentary structures (refer to Appendix B for details). This information was usually supplemented by field sketches and photographs. The critical variables for correlation of single sections proved to be elevation, colour, texture, and relative location with respect to the main valley floor. This eventually led to the formulation of a local chronology, that was, in turn, related to the regional sequence of events (Chapter 4). • STUDY AREA FIGURE 1.1 : REFERANCE MAP OF BRIDGE RIVER BASIN BOUNDARY BETWEEN PHYSIOGRAPHIC REGIONS 6b CHAPTER ONE  THE SETTING 1.1 INTRODUCTION Most of Bridge River drainage basin lies within the Coast Mountains, except for the extreme eastern portion which is part of Interior Plateau (Camelsfoot Range) (FIGURE 1 .1). There are five major tributaries of Bridge River. Four of these, Gun Creek, Caldwallader Creek, Tyaughton Creek and Hurley River drain the Coast Mountains. Yalakom River occurs along the boundary between the two physiographic regions and originates in the Camelsfoot Range. Bridge River Glacier and several small cirque glaciers occur in the western-half of Coast Mountains. The terminus of Bridge River Glacier is approximately 4700 feet (1410 m) a .s . l . (Pemberton Map Sheet, 1:250,000, 1958). The existence of these present-day glaciers probably reflects the fact that this area was an ice accumulation zone during Pleistocene time. The relative relief of the basin is 9600 feet (2880 m). Broad, U-shaped valleys surrounded by high, local ranges (FIGURE 1.1) characterize the region. 1.2 G E O L O G Y OF STUDY AREA The geology of the study area has been described by Duffel I and McTaggart (1952) and Roddick and Hutchison (1973). The lower 5 miles (8 km) of Bridge River valley has been mapped by the former of the Ashcroft Map Sheet. The recently completed geological map of the Pemberton (East-Half) Map-area, by the latter, includes the rest of the study area. A brief description of the local geological units has been paraphrased, below, from these reports (FIGURE 1.2). GEOLOGY OF STUDY AREA MIDDLE TRIASSIC : BRIDGE RIVER GROUP. CHERT, ARGILLiTE, PYLLITE, GREENSTONE, LIMESTONE. AS ABOVE, METAMORPHIC ROCK OF BRIDGE RIVER GROUP. BIOTITE SCHISTS. UPPER TRIASSIC: HURLEY FORMATION. THIN-BEDDED ARGILLITE, PHYLLITE, LIMESTONE, TUFF, CONGLOMERATE. UPPER TRIASSIC: ULTRA BASIC. UPPER JURASSIC/LOWER CRETACEOUS : RELAY MOUNTAIN GROUP. ARGILLITE, GREYWACKE, CONGLOMERATE. LOWER CRETACEOUS : LILLOOET GROUP. ARGILLITE, VOLCANIC CONGLOMERATE, TUFFACEOUS SANDSTONE. LOWER CRETACEOUS: JACKASS MOUNTAIN GROUP. SUBUNITS A. INTERBEDDED ARGILLITE AND GREYWACKE. B. GREYWACKE, PEBBLE CONGLOMERATE,ARGILLITE,SANDSTONE. C. ARGILLITE, CONGLOMERATE, GREYWACKE. D. GREENISH GREYWACKE, ARGILLITE, SANDSTONE, PEBBLE CONGLOMERATE. UPPER CRETACEOUS : KINGSVALE GROUP. ARKOSE, GREYWACKE , SHALE, MINOR CONGLOMERATE. LOWER TERTIARY. SHALE, SILTSTONE, SANDSTONE, SANDSTONE, ARKOSE, CONGLOMERATE. TERTIARY RHYOLITE, DAC1TE BRECCIA. TERTIARY REXMOUNT PORPHYRY. PLUTONIC : GRANODIORITE. BOUNDARY OF STUDY FAULTS 8 The Bridge River Group, Middle Triassic Age, are the oldest rocks in the area. Dark to grey weathering chert and dark cherty argil lite are the most common. The chert commonly forms lensoid and nodular layers up to 3 inches (7.6 cm) thick that are separated by thin films of dark argil lite. Pods of light grey to buff grey limestone are scattered throughout the group. The group exhibits a pumpellyite-prehnite metamorphic grade. In the Shulaps Range (Mission Ridge), however, the group is represented by a higher degree of metamorphism. These include phyllites, biotite-quartz schists, chlorite and graphitic schists. Upper Jurassic - Lower Cretaceous rocks of the Relay Mountain Group outcrop to the north of the study region in Yalakom River Valley. The unit is not well-exposed in the Bridge River Basin, but in Taseko Lakes Map-area to the north, it consists of a thick sequence of argillaceous seiments intercalated with greywacke, pebble conglomerate and volcanic rock (Jeletzky and Tipper, 1968). The Lillooet Group (Lower Cretaceous) is exposed near the mouth of Bridge River in river cut scarps (PLATE 1.1). The group is made up of argiMite, greywacke, volcanic conglomerate and tuffaceous sandstone. The Jackass Mountain Group, also of Lower Cretaceous age, is restricted to the Camelsfoot Range. It forms an open, northwest-trending syncline plunging to the northwest. The group has been subdivided into four units (FIGURE 1.2). It consists of more than 14,000 feet (4200 m) of greywacke, argi I lite and conglomerate. There are some rare, thin coal beds. 9a PLATE 1.2 Morphology of Upper Part of Bridge River Valley (in study area) 9b 1.3 DESCRIPTION OF THE STUDY AREA Bridge River flows through the study area in a south southeasterly direction to the Fraser River. It is bounded to the east by Camelsfoot Range and to the west by Mission Ridge (part of Shulaps Range) (FIGURE 1.3). The highlands have a smooth, rounded form and range between 6000 - 8000 feet (1800 - 2400 m) a . s . l . Applespring, Moon, Antoine and Camoo Creek are the major tributaries of Bridge River originating in these local ranges. Upstream, above a canyon zone in the study area (FIGURE 1.2 above Applespring Creek), the valley displays a distinct U-shaped morphology (PLATE 1 .2). The lower segment, however) below this canyon narrows and has a V-shaped form. This downstream change in morphology can be observed in other east-flowing tributary valleys of Fraser River drainage basin in the Coast Range (for example, Stein, Kwoiek, Nahatlach River Valleys - Lytton Map Sheet, 1:125,000). This basin form could possibly have resulted from an extended occupation of ice in the upper portions of these valleys; for example, during a relatively long "Intense Alpine Stage" in the development or deterioration of a Continental Ice Sheet (Chapter 3.2). During this stage the valley glaciers would be actively eroding at their base, even though the margins were probably semi-stationary. The study area can be divided into three units for the purpose of description: Interior Plateau, Coast Range and Central Valley Region. 1.3.1 INTERIOR PLATEAU The Interior Plateau encompasses the Camelsfoot Range and the down-stream portion of Mission Ridge. Lower and Upper Cretaceous classic sedimentary rocks (predominantly Jackass Mountain Group) dominate the geology of the area (FIGURE 1.2). Camelsfoot Range possesses the steepest slopes, COAST \ MOUNTAINS V FIGURE 1.3 : REFERANCE MAP OF STUDY AREA. \ i INTERIOR PLATEAU •<p. HORSESHOE EXPOSURE %\ </ u\./ "S. * \ V & A 6852 I 5 \ ~ \ . Q C L A Y \ *• PIT ' V \ \ \ B O U N D A R Y O F S T U D Y 7650 , SPOT ELEVATION U CANYON PHYSIOGRAPHIC BOUNDARY i \ \ ^ • I'LILLOOET S £ T C W Z./4 A T * 111) - " ' ^ ^ V \ 11 approximately 30 - 35 degrees, in the study area. The upper portion of the range, above 3000 feet (900 m) a . s . l . , is a semi-continuous outcropping of Jackass Mountain Group. Associated with these exposures, are active talus deposits that tend to concentrate along the slopes of the intermittent streams (FIGURE 1.4). The lower slopes are covered by colluvial deposits, including large fan deposits that drape over the central valley fill (PLATE 1 .3). In the vicinity of a clay pit (FIGURE 1.3), however, a small pocket of drift was observed (FIGURE 1.1). Where the slopes are moderately steep, open and park-like stands of good timber, ponderosa pine, provide the major vegetation cover. 1.3.2 COAST MOUNTAINS The western portion of the study area (Mission Ridge) is part of the Coast Mountains (FIGURE 1.3). The region is dominated by Middle Triassic sediments of the Bridge River Group (FIGURE 1.2). The valley side, upstream from the canyon area, displays a step-like form (PLATE 1.4). This is a result of faulting. The Yalakom Fault Zone, in this section of the valley, cuts across the boundary of Coast Mountains and Interior Plateau (FIGURE 1.2). This zone is thought to be the northwestern extension of the western part of the Fraser Valley fault system (Duffell and McTaggart, 1952). It has been suggested, by these authors, that the overall nature of the movement along this zone was to elevate the Coast Mountains with respect to the Interior Plateau. Several outcrops of serpentine, associated with the faults in the Bridge River Valley, were observed. A large portion of the upland has a till/drift blanket. On some of the higher and steeper slopes, colluvial slope deposits and bedrock make up the surficial geology (FIGURE 1 .4). The drift blanket has a heavy timber cover, dominated by spruce and Douglas fir (PLATE 1 .4). 12 Fig . 1.4 i S U R F I C I A L G E O L O G Y O F S T U D Y A R E A interpretation of letter code, Appendix A ] Cb/Db 60% Cb 40% Db Steep Bedrock Colluvial Blanket Talus Slope Fluvial Terraces, Steep Drift Slopes Kame Terraces, Steep Drift Slopes — Gullied Alluvial, Colluvial Fans Steep Drift Slopes Drift Blanket Till Till Blanket, Till Hummocks, Ridges Bog, Fine Lacustrine A:x=Ds g A G t - V dAf,gAf Ds Db Mb.Mrh PLATE 1.4 Fl uvlal Terraces (foreground) and Step-1 ike Morphology of Mission Ridge (background) Due to Faulting • 14 1.3.3 CENTRAL VALLEY REGION The average gradient of the river, in this segment of the val ley, is approximately 52 feet/mile (9.7 m/km). The river has incised the thick, val ley-f i l l sediment, resulting in the formation of numerous fluvial terraces. These are usually well-preserved and range from 20 feet (6 m) to 400 feet (120 m) above the present channel (FIGURE 1.4 and PLATE 1.4). Several of the larger terraces serve an agricultural function. As a result of fluvial degradation, the previously mentioned river cut scarps, up to 100 feet (30 m) in height, have been developed in Lillooet Group sedimentary rocks (PLATE 1.1). These scarps can be traced approximately 5 miles (8 km) upstream from the mouth of the river. Numerous pot-holes, observed during period of low discharge, in the channel bedrock suggest that downcutting has been recently active (PLATE 1 .5). The major tributaries, Applespring and Antoine Creeks, have also been actively lowering their beds. This is especially evident at their confluence with Bridge River, where former alluvial fans have undergone dissection. Recently, the available discharge of the river, in the study area, has been decreased with the completion of the Terzaghi Dam at Carpenter Lake (FIGURE 1.1). The diversion of water (for power purposes) via tunnels to Shalath, and retention of water during flood peaks, appears to have reduced the rate of downcutting. 15 PLATE 1.5 Pot-hole in Channel Bed Near the Horseshoe Exposure CHAPTER TWO LITERATURE REVIEW This review is concerned with outlining the Pleistocene chronology of various regions in British Columbia for eventual comparison to the Bridge River area. 2.1 LATE-PLEISTOCENE CHRONOLO G Y OF FRASER LOWLAND Armstrong et a I.(1954', 1957, 1960a and b, 1965) have proposed six geologic-climate units for the late Pleistocene record in the lower Fraser Valley of British Columbia. These are briefly described below. Olympia Interglacial^ The Olympia Interglacial was originally defined as the "climatic episode immediately preceding the last major glaciation" (Armstrong et a l . , 1965, p. 324). Initially, finite radiocarbon dates indicated that the non-glacial period lasted from before 36,200 until 19,150 years ago (Armstrong e t a l . , 1965). Fulton (1971), however, feels that the Olympia Interglacial time can be extended to at least 51,000 years ago by including the basal 'Quadra Sediments' unit of Fyles (1963) in the interglaciation. Little informa-tion is available concerning the climate of this period. According to Fyles (1963), the climate was probably cool and temperate. Fraser Glaciation The Fraser Glaciation has been defined as "the last major glaciation of British Columbia" (Armstrong, et a l . , 1965) and is probably equivalent to Late Wisconsin Glaciation of mid-western United States (Flint, 1971, p. 560), Fraser Glaciation consists of the following subdivisions: Evans Creek Stade, 1 All references to years in B.P. 17 Vqshon Stade, Everson Interstade and Sumas Stade. The major characteristics of these events are outlined in the following table. TABLE I SUB-UNITS OF FRASER GLACIATION Unit Date C M y r s . , B . P . Characteristic /Comment References Evans Creek Stade Vashon Stade Everson Interstade Sumas Stade Maximum Age 19,150 +250 Overrode Area after 20,000 Ended 11,500 +200 Began soon after 11,400. Ended 9,000. Alpine Glaciation Stage. Little known about extent or significance. Cowichan River Valley on Vancouver Island - only site where distinction made between Alpine Advance and later Vashon Stade ice sheet in British Columbia. Last major climatic episode during which drift was deposited by ice originating in mountains of British Columbia and occupying lowlands of southwestern British Columbia. Episode of Vashon ice retreat during which glacio-marine, marine and related deposits accumulated. Cordilleran ice re-advance into eastern Fraser Low-land. Armstrong et al . Halstead, 1968 Fulton, 1971 1965 Armstrong et al, Fulton, 1971 1965 Armstrong et a I, Mathews at a I. i Fulton, 1971 Armstrong et a I, Mathews at a I. , Fulton, 1971 , 1965 1970 , 1965 1970 2 The Sumas Stade dates represent approximate maximum and minimum. Recently, there has been some question as to the accuracy of these date (c.a. 1950's), thus the Sumas Stade is currently undergoing reinterpretation. R. Mathewes et a l . (1972) reports dates from Yale, British Columbia, of 11,430 ±150 and 11,140 ±260 marking the beginning of post-glacial lacustrine sequences. 18 GENERAL PLEISTOCENE CHRONOLOGY FOR INTERIOR BRITISH-COLUMBIA 2.1 EVIDENCE OF MULTIPLE GLACIATIONS Stratigraphic evidence of multiple glaciations in the Interior is rather scant. This is a consequence of three factors: burial of older sediment by thick silt blankets from post-glacial lakes, erosion of sediment by Fraser Glaciation ice and lack of field analysis in many areas. Tipper (1971) has suggested that an ice dome developed in Central British Columbia prior to Fraser Glaciation. The ice flowed radially from the dome, over and through the Coast and Rocky Mountains. The pattern of erratic distribution in the Redstone area (Tipper, 1971, p. 84), coupled with his observations of glacial grooves at 8,800 feet (1740 m) a .s . l . in the Rocky Mountains (McLeod Lake Map Sheet) led him to this conclusion. This is supported by Mathews (1946, 1963) who reported ice was active over Rocky Mountains, south of Peace River at 6000 feet (1800 m) a . s . l . and crossed Rocky Mountains, nearly reaching Fort St. John. It is not known to which episode the above event should be ascribed. Easterbrook tentatively correlated it to the Salmon Springs Glaciation in Puget Lowland (Easterbrook, 1967). However, a recent date from Peace River area suggests that the ice stayed at or beyond the eastern front of Rockies until about 11,600 B.P. (Mathews, pers. comm.). This date falls within Fraser Glaciation. 2.2 OLYMPIA INTERGLACIAL Finite radiocarbon dates for the Olympia Interglacial for the Interior range from 43,800 1800 to 19,100 +240 years B.P. (Fulton, 1971). Little is known about the climate during this interval, however, it was warm and humid enough to maintain forest and large vertebrates such as Bison sp. and Equus cf. conversidens, and Mammuthus cf. M. columbi (Harington et a l . , 1974). With one exception, dates from exposures of Olympia Interglacial sediment are either infinite, or fall in the 19,000 to 23,000 years B.P. interval (Fulton, 1971). The exception is in the Purcell Trench where a nonglacial radiocarbon-date sequence extends from 25,840 1320 to 43,800 1800 years B.P. (Fulton, 1971). This distribution of dates suggests that the Olympia Interglacial contained three phases. These are as follows: Date Comment 37,200 Base level higher or as high as present 37,200 - 22,900 t Dissection of earlier deposits 22,900 l l 5 0 - Fraser Base level higher or as high as present G laciation (Fulton, 1971) . 2 . 3 FRASER GLACIATION ADVANCE Traditionally (cf. Kerr, 1934, and Davis and Mathews, 1944), it has been considered that the Fraser Glaciation and other earlier glaciations proceeded through four stages to achieve their maxima . These are Alpine, Intense Alpine, Mountain Ice-Sheet and Continental Ice-Sheet Stages. A . Alpine Stage Ice built-up initially in the areas of heavy precipitation, the Coast and Columbia Mountains. It is not known whether these alpine glaciers developed simultaneously. B. Intense Alpine Stage v A mass of coalescent valley glaciers formed in the accumulation zones. The valley walls exerted a guiding influence on the flow direction of the ice. APPROXIMATE CONTACT', BETWEEN COAST AND; CARIBOO ICE ! 21 C. Mountain Ice-Sheet Stage Coast and Columbia Mountains became capped by ice sheets that flowed outwards from the axis of the ranges. Only in the outer zone did topography influence flow patterns, as the ice advanced from the mountains to form irregular piedmont glaciers. D. Continental Ice-Sheet Stage The piedmont glaciers expanded and formed a dome mass over the Interior. This dome stood higher than the rimming mountains. Movement of ice was uncontrolled by topography and flowed in a radial direction from the centre of the dome. Subsequent evidence, made available by Tipper (1971), suggests that during Fraser Glaciation there was only a great expansion of the piedmont glaciers into one coalescent mass. The ice flowed in a northwesterly and southeasterly direction over the Interior Plateau (FIGURE 2.1). This represents the Fraser Maximum but should only be considered as an initial Continental Ice-Sheet Stage. 2.2.4 FRASER GLACIATION RETREAT Insufficient research has been conducted for reconstruction of a composite deglaciation pattern for the Interior. Two hypotheses, however, have evolved regarding this problem. They are: A . Ice recession was accomplished by downwasting with a frontal retreat beginning in the southern Columbia Mountains and proceeding north-west (Fulton, 1971). B. Several broad ice masses became successively stranded as the ice front retreated in central Interior. Along the mountain rims, the ice retreated in a normal manner (Tippper, 1971). 22 Obviously, the complex topography of Interior British Columbia, resulted in a complex deglaciation pattern. Perhaps, both alternatives are acceptable, each being applicable to different regions. On a regional scale, deglaciation in Kamloops-Okanagan area appears to have been accomplished by downwasting, resulting in ice occupying the major valleys. A consequence of this process is the complicated history of late-glacial lakes reported in these valleys (Flint, 1935; Mathews, 1944; Nasmith, 1962; Fulton, 1965, 1969a; Anderton, 1970). 2.2.5 FRASER GLACIATION DURING SUMAS STADE There is no evidence, according to Fulton, 1971, of a Sumas Ice advance in Interior. The oldest post-glacial date, 11,000 -180 years B.P. falls well within the Sumas Stade defined in Fraser Lowland (Chapter 2.1). Tipper (1971), however, suggests that there was a late readvance of Coast and Cariboo Ice into Interior. In Taseko Lakes Map-area, immediately north of Bridge River Valley, the ice is presumed to have readvanced from Coast Mountains to Chilcotin River (Tipper, 1971, Figure 34). Al l the valley glaciers are considered to be separate entities. Dates from bog sediments outside the terminus of present-day Tiedemann Glacier indicate that the Coast Mountains were as ice-free as they are at present by 9,510 1160 years B.P. (Fulton, 1971). Most low-level cirques have not been occupied by local bodies of ice since the disappearance of Cordilleran Ice Sheet (Mathews, 1951). 2.2.6 POST-GLACIAL CLIMATE AND ASHFALLS It is believed that the Interior underwent a rapid change from glacial to non-glacial climate without a tundra vegetation stage. This is based on the arboreal pollen record from the lowest organic sediments (Hansen, 1955) and the 23 inclusion of a Bison cf. occidentalis skull in glacial lake material near Vernon (Fulton, 1971). A thermal maximum occurred about 6000 years ago. The effect of climatic deterioration (about 3000 years ago) has been observed at the Tiedemann Glacier (Fulton, 1971). The coldest period or greatest post-Wisconsin glacial advance, however, did not occur until the last few hundred years (Mathews, 1951). There were three major post-glacial ashfalls in British Columbia. These are summarized in TABLE II. TABLE II POST-GLACIAL ASHFALLS Ashfall Date, B.P. Characteristic References Mazama 6,640+250 Extends from Victoria to Saskatchewan. Rubin and Alexander, 1960 Westgate et a l , 1970 St. Helens Y 3200 Narrow, northeast-trending plume across southern British Columbia Westgate et a l , 1970 Fulton, 1971 Bridge River 2,290+130 -2,400 ±140 Extends east from head-waters of Bridge River Nasmith et a l , 1967 2.3 RELATED PLEISTOCENE STUDIES J . Ryder (pers. comm.) has detailed the only major Pleistocene chronology directly related to Bridge River region. The following chronology for Fraser Valley between the villages of Lytton and Lillooet has been tentatively outlined in TABLE III and FIGURE 2.2. F I G U R E 2.2: T E N T A T I V E C H R O N O L O G Y O F F R A S E R V A L L E Y B E T W E E N L Y T T O N AND L I L L O O E T L Y T T O N L I L L O O E T CORDILLERAN ICE-SHEET GLACIATION z LU u o H INKOIKO PRO-GLACIAL PHASE . FLUVTAC •bEGRADATION SPINTLUM GLACIAL PHASE GLACIAL LAKE LILLOOET FLUVIAL DEGRADATION ROT AT I ON A L SLU M PING 6 F SI LT tSJ OUTWASH AGGRADATION OF ALLUVIAL FANS AND OUTWASH SETON GLACIAL READVANCE DEPOSITION OF KAMES H Z LU u U l FLUVIAL DISSECTION AND TERRACING OF VALLEY FILL, AGGRADATION AND DISS E CTI0N OF A LL U V|"AL~F^NS*TA"LUS SI OPES D E V A L O P M E N T , A E O L I A N ACTIVITY |AFTER RYDER ( pcrs. comm:)| 25 TABLE III TENTATIVE CHRONOLOGY FOR FRASER VALLEY BETWEEN LYTTON AND LILLOOET (J. Ryder) Phase Characteristic Cordilleran Ice-Sheet Glaciation Inkoiko Pro-glacial Phase Fluvial Degradational Phase Spintlum Glacial Phase Glacial Lake Lillooet Major glaciation of region resulting in erosion of any pre-existing sediments. Aggradation of outwash gravels in lower part of study area. Near Lillooet, ice persisted from 'Cordilleran Ice-Sheet'. Erosion of Inkoiko outwash gravels. Extent unknown, probably occupied all of study area. Initial deposition in presence of Spintlum ice. Minimum lake level of 1240 feet (372 m) a . s . l . Drainage via Thompson Valley to Okanagan System. Fluvial Degradational Degradation and rotational slumping of Lillooet silt. Phase Seton Glacial Readvance Readvance of ice from Seton Lake Basin (FIGURE 2.3). Partial erosion of Lillooet silt, 1000 feet (300 m)a.s.l . Deglaciation accompanied by formation of kame terraces, north of Lillooet, between 1200 - 1400 feet (360 - 420 m) a . s . l . Followed by aggradational phase. Cessation of aggradation considered end of Pleistocene era. 2.4 BRIDGE RIVER VALLEY DURING FRASER GLACIATION With the onset of Fraser Glaciation, the alpine regions located at the headwaters of Bridge and Yalakom Rivers are presumed to be the principal sites of ice accumulation. This includes the higher peaks of Coast Mountains, Dickson and Leckie Range, and Camelsfoot Range (FIGURE 1.1). These local, isolated 2.6, 27 glaciers commenced to expand into the tributary, and eventually into the main valleys of Bridge and Yalakom Rivers (FIGURE 1.1), until the region was a mass of coalescent valley glaciers. The ice is believed to have advanced out of Bridge River Valley and was diverted southwards by Clear Range down the Fraser Valley (FIGURE 2.3). Eventually, the accumulation zones became capped by a mountain ice sheet. The ice was probably sufficiently thick to flow in an eastward direction over Clear Range (FIGURE 2.3). This east-flowing ice coalesced with south-west-flowing Cariboo ice, and moved in a southeasterly direction across the Thompson Plateau (FIGURE 2.1). At the Fraser Maximum, the ice surface in Bridge River area had probably built-up to a mean height of well over 8000 feet (2400 m) a .s . l . (Glacial Map of Canada, 1958). With the onset of deglaciation, the higher peaks of Clear Range probably became ice-free. This is thought to have resulted in the separation of east-flowing Bridge River and southeasterly-flowing Thompson Plateau ice. It is more than likely that Thompson Plateau ice downwasted, while Bridge River ice retreated in a 'normal' fashion. The late glacial readvances noted by Tipper (1971) and Ryder (pers. comm.) in the vicinity of the study area (Chapter 2.2.5 and 2.3), strongly suggest that this event, also, occurred in Bridge River Valley. 518 509 501 498.5 F I G U R E 3.1 : C L A Y PIT S E C T I O N METRES FEET 526.2 1 754 522.5 1 730 A 513.5 17154 1700 504.5 1685 -1670 1 663! 496.5 1655 -^  ! III ill! II | || I I  Ij !|H tillMli 111 IH || i I  I Ii I ll MlHlllllllllllllllhlll lllllil mi III I'i CLAY SILT } • ' [ SAND | "* | TRACER BED CENTRAL PART OF VALLEY NORTH VALLEY WALL 2 8 ^ CHAPTER THREE THE QUATERNARY DEPOSITS 3.1 INTRODUCTION This Chapter provides a detailed description of the various Quaternary deposits of Bridge River Valley. To facilitate the description, the sediments are subdivided as follows: lacustrine deposits, ice contact fluvio-glacial deposits, fluvio-glacial deposits lacking ice-contact features, glacial deposits, alluvial fan deposits and recent deposits. Although the discussion follows a chronological order of deposition of the sediments, the descriptive aspect will be stressed. A more complete outline of Quaternary events is given in the following chapter (Chapter 4.1). 3.2 LACUSTRINE DEPOSITS Interbedded clay and silt, with well bedded, medium textured sand was observed at three exposures, all located within 4 miles (6.4 km) of Bridge-Fraser River confluence. The thickest exposure is at a clay pit (FIGURE 1,3 and PLATE 3.1) that was excavated during the construction of Terzaghi Dam. The exposure is approximately 100 feet (30 m) thick (FIGURE 3.1). Elevation at the top of the section is 1754 feet (526 m) a . s . l . The lower 85 feet (25.5 m) consists of bedded clay with thin contorted beds of silt that are 12 to 36 inches (25 - 90 cm) thick. Resting above these, is 14 feet (4.2 m) of wavy, laminated, fine sand. The clay displays a variety of colours ranging from dark brown (7.5R 4/4)^ and greyish brown (2.5y 5/2), to dark grey (5y 4/1). The silt has a distinctive light olive grey colouring (5y 5/2). The sediment is fractured and faulted (PLATE 3.2) It is compact and exhibits an interesting structure (FIGURE 3.1): the basal beds 1 Al l references to colour according to Munsell Colour Chart. FIGURE 3.2 : STEP-LIKE PATTERN OF SILT BED OBSERVED ON EASTERN FLANK OF THE'CLAY PIT' 29b 3Qf. dip northwards towards the valley wall at 9 -10° , while the upper beds dip outwards at an angle of 5 ° . This orientation of beds, coupled with a step-like pattern of the 1663 foot (499 m) silt bed displayed on the eastern flank of the clay pit (FIGURE 3.2), suggests that the sediment settled and underwent rotational slumping after termination of lacustrine deposition. The structures observed in the exposure indicate that slumping occurred prior to drying of the sediment. The compaction, faulting and fracturing, and the presence of over-lying till (Chapter 3.3 Uplands - Zone B) has been interpreted as evidence of subsequent glacial overriding. These clays and silts can be correlated with similar downstream exposures by their colour, texture and elevation. A north-facing gully exposure, 1.6 miles (2.5 km) upstream from the mouth of Bridge River, reveals the strati-graphic relationship of the sediment with respect to the local t i l l . The exposure is 25 feet (7.5 m) thick. The lower portion of the section (base elevation is 1760 feet (528 m) a.s. I.) is composed of alternating beds of medium and coarse sand. The beds vary from 12 to 36 inches (30-90 cm) in thickness and display normal faulting and fracturing. At 1770 feet (531 m) a . s . l . , this unit is over-lain by 15 feet (4.5 m) of deformation till (Chapter 3.3 for definition). This is made up of contorted beds of silt, clay and fine sand that has a similar texture and colour to the sediment observed at the "Clay Pit". The contact between the two units is undulating with blocky clay (deformation till) overlying wavy, platy clay (lacustrine sediment). The till is very compact and some of the fine sands possess their original bedding. The till is overlain by slopewash deposits of variable thickness. Half a mile (.8 km), downstream, from the above site is another gully exposure. The lacustrine sediments, here, have been overridden but not displaced 31 FIGURE 3.3: OVERRIDEN LACUSTRINE SEDIMENTS , 1725 H 517.0. 1720-1 L513.5 CLAY AND SILT BEDDED SAND i VEGETATION by glacial ice (FIGURE 3.3). The lower unit, at 1730 feet (519 m) a.s. I., is correlated with the medium and coarse sand unti detailed in the previous paragraph. An undulating contact separates this unit and the overlying relatively horizontal, thinly bedded clay. The clay is very compact and contains a wavy lense of thinly bedded, fine and medium sand. The internal structure of the sediment, coupled with the character of the unit to unit boundary, suggests that there has been no actual displacement by ice. The precise extent of the lake in which these sediments were laid down cannot be delimited because of lack of shore features and too few exposures. At the northwest end of the study area, there is a large "horseshoe" exposure (FIGURE 1.3) of aggradation gravels. Here there are two sequences of aggradation gravels separated by 60 feet (18 m) of interbedded sand and clay (FIGURE 3.9, Member H). This member is between 1620 and 1680 feet (480-504 m) a . s . l . The upper sequence of gravels rests unconformily above these fine sediments. This interbedded sand and clay member may be tentativel correlated with the silt and clay of the "Clay Pit". This suggests that the lake occupied the central portion of Bridge River Valley in the study area. (The character and possible interpretations concerning the origin of Member H are discussed, more fully, in Chapter 3.4). The contact between the bedded sand unit and deformation ti l l , described previously in this section, indicates the minimum level of the lake is about 1770 feet (531 m) a . s . l . After deposition, the lacustrine sediment was modified by rotational slumping (at some localities) and deformed and/or eroded by glacial ice. 33, 3.3 GLACIAL DEPOSITS: TILL Two types of t i l l , lodgment and deformation were recognized in the study area. Lodgment till (defined by Flint, 1971, p. 171) is a compact sediment characterized by the presence of faceted clasts, some with striations (PLATES 3.3 and 3.4), lack of size sorting of clasts and variable clast lithology. Deformation till (A. Dremanis, pers. comm.) may be defined as a non-glacial sediment ( i .e. , fluvial, lacustrine) that has been overridden, deformed and displaced by glacial ice. The sediment, however, has not been sufficiently reworked to destroy all sedimentary structures inherited from its original mode of deposition ( i .e. , thin bedding in sands) (PLATE 3.5). The term "overridden sediment", as opposed to this, implies deformation in situ (Chapter 3.2). Most of the till recorded is thought to be the result of the last major glaciation. This conclusion is reflected by the stratigraphic relationship demonstrated at several sections (TABLE IV). Till located at higher elevations may, possibly, be an exception: if the last ice occupying the valley was a minor readvance, these till deposits may well then be the result of earlier glacial episodes. 3.3.1 DESCRIPTION OF TILL (TABLE IV and FIGURE 3.4) A . Main Valley Floor Exposures Only two till exposures were recorded in this part of the valley. In both instances, lodgment till displays similar characteristics. The texture of both is a silty matrix with subangular clasts that grade upwards from large pebbles and cobble(at the base) to medium pebble size (at the top). The tills are very compact with clasts consisting predominantly of serpentine. At each section, the till is resting on serpentine bedrock and is overlain by aggradation gravels. TABLE. IV DESCRIPTION OF TILL SITES 3 5 Texture Location From „Eigure 4.3.1 Matrix Clast Size Thickness/Base Height Colour (Munsell Colour Chart) Stratigraphic Position Percentage Lithology C lasts Shape of C lasts Compaction ::.oJ .'.gi.3— Other ! ; Main Valley Floor Sites ! - ! 1 r 1 • Dl D. -Silt Large pebble/cob-ble grading upwards to pebble size 6-30' (variable) / l . 8 - 9 m 1410' / 423 m Ol iveGrey 5Y 5/2 Rests on serpentine bedrock;glacial out-wash gravels above Serpe ntine schists 25-30 Subangular; flat soles Very r Thin bands of serpentine > clasts (angular.shape) 1 ••..'l, . • •. •- i * A 2 . no - ' . no •' Silt As Site 1 45'/13 .5 m 1030 ' /309 m As Site 1 As Site 1 Serpentine argi Mites 15-20 As Site 1 As Site 1 As Site 1; Plus striations j on some clasts,caliche j on face Uplands: Zone A - Uplands ; ~ A 3 . Silty Cobble/pebble Clay grades upwards to „, pebble size 70-80'/21-24m 1700'/510 m As Site 1 Rests on serpentine bedrock, surface expression Serpentine 30-Base 15-Top As Site 1 As Site 1 p — — • • ( - ' • . . Actively slumping 1 • • ; • one. Silty Clay Pebble, odd cobble 6-8'/ l .8-2.4m 3400'/1020 m LightGrey 5Y 7/2 Overlain by col lu-vium; base not seen Shale,sandstone conglomerate, cherts 20 As Site 1 As Site 1 « Utriations on some shales !• V'V':,, -Silt Pebble, some cobbles 6-8'/l .8-2.4m 1600'/480 m LightOlive Grey 5Y 6/2 Surface expression base not seen Sandstone,shale, conglomafe,chert, 15 serpentine, odd granitic As Site 1, but granitic tend to be more rounded As Site 1 ?~ "XTot ser bles are al l Dentine 6 : Silt Pebbles,scattered cobbles 1574.5 m 1670'/501 m Ol ive Grey 5Y 5/2 Rests on serpentine bedrock; surface expression Serpentine 10 Subangular As Site 1 ; v 1 7 . Silt Pebbles/cobbles odd boulder 4 0 ' / 12 m 1500'/450 m Ol iveGrey 5Y 5/2 As Site 6 As Site 6 35-Top 15-Base Subangular Flat soles As Site 1 r _ • so ' l ; : 8 . Zone B - Uplands. C lay , Silt Bands 3 0 ' / 9 m 1539 7462 m Ol iveGrey Surface expression 5Y,5/2(silt) base not seen Greyish Brown 2.5Y,5/2(clay) As Site 1 .8 Beds dip 20 downstream silt beds contorted; clay frajctured, faulted c : •' 9 . - ! He : . Sand,Lenses of Clay - 25' /7.5 m 1530'/459.3m — As Site 8 As Site 1 0 Sand bedded, fractured faulted,dip 25° down-stream, clay dikes contorted ? J TO J Sil t , Lenses Contorted C1 Pebbles ay 1 5 ' / 4 . 5 m 1515 /454.8 m Pale Yellow 5Y 7/4 As Site 8 Schists,ser-pentine argi Mite 2 Subangular Some rounded As Site 1 QF Lenses of contorted clay I 3 8 i i i ^ :rlt Sil t , Sand, Clay Pebble in clay beds 15.1 / 4 . 5 m As Site 10 As Site 8 Serpentines, Schists 1 As Site 10 As Site 1 r r Bea*s dip 30° upwards fractured; sand, clean, thinnly bedded pockets of(clay in sand 3 - 12. Clay Silt Pebble 1875.4 m 1763'/528.9 m Sil t : Ol ive Grey ;5Y5 /2 As Site 8 As Site 11 Less than 1 As Site 10 As Site T r Contorted bands of medium sand, fractured, faulted *G • 13i ' Si It, C lay , Some Sand Pebble 18' / 5 . 4 m 17707531 m Silt: Ol ive Grey ;5Y5 /2 Rests on bedded, faulted sand;over-lain by colluvium As Site 11 Less than 1 As Site 10 As Site Ifr Described in Chapter 3.2 14. Overridden Lacustrine Sediment. Detailed in Chapter 3 .2 - - 1 | 36 The tills include also unusual lenses of small, angular clasts of serpentine. These lenses are concentrated in the lower portions of the exposure and range from 2 to 5 inches (5-12 cm) in thickness (PLATE 3.6). These features may be the product of rock fragments incorporated into the sole of the glacier by a regelation process induced by a bedrock obstacle, and ' eventually deposited by a melt-out process (Boulton, 1970a and b). B. Uplands-Zone A Lodgment t i l l , in this zone, appears to occur only on the gentler north-facing slopes (FIGURE 1 .4). Slope erosion probably has removed most of the till from the steeper, south-facing slopes. The thickness of the till, generally decreases upslope. At higher elevations, 3500 to 6000 feet (1050-1800 m) a.s. I., it is overlain by colluvium. It is suggested that the till is thicker on the flat steps of Yalakom Fault Zone (Chapter 1.3). These areas are characterized by poor drainage and numerous small ponds. The texture of the till is a silt or silty clay matrix with predominantly pebble size clasts. Those sections resting on bedrock (TABLE IV, Sites 3 and 7) display the same gradation of clast size as discussed under "Main Valley Floor-, Exposures". Colour and lithology vary according to location. There is a strong correspondence between clast lithology and local bedrock (FIGURE 3.4 and" TABLE IV). The remnants of a hummocky moraine can be observed near Camoo . Creek (FIGURE 1 .4). The deposition of this moraine resulted in the ponding of Camoo Creek. Several small sections of recent clay and organic material were observed near the present shrunken pond. The moraine was probably formed during the retreat of the last ice occupying the valley. 37 PLATE 3.4 Striated Clast from Upland-Zone A : T i l l PLATE 3.5 Deformation TiII: An example (Notice Fractures and Contorted Silt Bands) PLATE 3.6 Lenses of Small, Angular Clasts in Lodgment Till (Member A at Horseshoe Section) FIGURE 3.5 : DEFORMATION TILL ( SITE 11, TABLE 3.1 ) METRES FEET 439.5 1465 435 1450 BASE NOT SEEN 10.5 METRES 35 FEET NOTE : (1) DIP 30° (2) SAND FRACTURED AND FAULTED (3) SAND POCKETS IN SILT fcfi;:^ MASSIVE FINE/MEDIUM SAND f~-~j SILT H CLAST LENSE 40 C. Uplands-Zone B The deformation till occurs in this area. It is characterized by fine texture (silt and clay with some bedded sand), low clast percentage, high compaction, the presence of fracturesand faults, dipping or contorted beds of alternating sediment (sand and silt) types (FIGURE 3.5) and thinly bedded sand in contorted lenses. The character of the ti l l , as previously mentioned, suggests it originated from the lacustrine sediment detailed in Chapter 3.2. Two mechanisms could account for the formation of this deformation till. (1) Weertman (1961) described a situation in which basal sediment may be incorporated into ice by a basal freezing mechanism. According to him, the frontal portion of the marginal ice may be divided into three zones. Near the terminus, the ice is frozen to its bed, while at some point inward, the base of the ice is at pressure melting point. An intermediate zone, between these, exists where melfwater forced outwards by the pressure gradient freezes to the base. Hence, the 0°C isotherm is at some depth in the underlying sediment near the outer edge of ice, and rests along the base of the ice inwards from the intermediate zone. FIGURE 3.6 illustrates how fluctuations in the 0°C isotherm may result in the incorporation of slabs of basal debris. These blocks would eventually be released, resulting in a till possessing the previously outlined characteristics. If this situation existed in Bridge River Valley, it would probably occur seasonally during the latter stages of deglaciation. It is at this time that the temperate ice would be sufficiently thin to allow penetration of a winter cold wave into the underlying sediment. 41 FIGURE 3.6 : AN EXAMPLE OF HOW DEBRIS CAN BE INCORPORATED INTO THE ICE SHEET (a) FROZEN TO BED WATER FREEZING TO BOTTOM ICE 0° ISOTHERM (b) FROZEN TO BED ICE WATER FREEZINF TO BOTTOM 0 ISOTHERM (c) FROZEN TO BED WATER FREEZING TO BOTTOM ICE ^ " ^ C r ^ ^ v h i ^ DEBRIS LAYER (a) TO LEFT X 2 , 0 ° C ISOTHERM DESCENDS INTO BED.TO RIGHT X 2 ,BOTTOM AT MELTING POINT, WATER BEING FROZEN TO BASE. (b) LESS WATER FLOWING FROM RIGHT AND POINT OF DESCENT OF 0° ISO-THERM SHIFTED TO RIGHT. (c) GREATER WATER SUPPLY FROM RIGHT. WATER FREEZES TO 0 ° ISOTHERM POSITION IN (b) AND PUSHES UP INTO ICE DEBRIS FROZEN ON BOTTOM SHOWN IN (b) POINT X 2 MARKS THE LIMIT OF FLOW OF WATER HAS MOVED TO LEFT. [ AFTER WEERTMAN, 1961. p. 972 ] (2) Deformation and displacement may, also, have resulted from ice thrusting at the margin of the ice. This has been observed by several authors (i . e . , Fuller, 1914; Hopkins, 1923). Thrusting may have been induced by the presence of high pore pressures and consequent loss of shear strength in the underlying sediment (Mackay and Mathews, 1960). This implies that the basal material was saturated and overlying an impervious zone. Thus, as the ice advanced the pore pressure increased, reducing the critical shear stress of the clay and silt; resulting in thrusting of the sediment. Assessing the relative roles of these mechanisms is rather difficult. Former ice and sediment conditions cannot be fully reconstructed. These mechanisms, however, operating either individually or together, provide a reasonable explanation for the formation of the deformation ti l l . In summary, two important points concerning the till deposits merit restating. These are: (1) the clast lithology and texture (deformation till) reflect a local origin of the ti l l , (2) the till is resting on lacustrine silt and clay or bedrock and overlain by aggradation gravels or colluvium. ICE CONTACT FLUVIQ-GLACIAL DEPOSITS: KAME TERRACES A Kame terrace extends parallel to Bridge River along the south valley wall from the mouth of the river to a small Indian farm, approximately 3.3 miles (5.2 km) upstream. The surface of the bench is between 1400 and 1500 feet (420-450 m) a . s . l . (PLATE 3.7) . The downstream portion of the 4 3 Kame Terrace PLATE 3.7 Kame Terrace Along South Side of Bridge River Valley Near Mouth of Bridge River 44a FIGURE 3.7 : CORRELATION OF KAME DEPOSITS FEET 4 4b kame terrace is a continuation of the kame terraces, north of Lillooet in Fraser River Valley (Chapter 2.3). The surface morphology of the larger remnants of the terrace has been destroyed by gulling, former Indian occupation (several Indian pit houses were observed), and agricultural use. No clear exposures of the kame gravels were observed. The upper surface is generally blanketed by an aeolian sand cover of variable thickness. On the north slopes, in approximately the same part of the val ley, three road cuts reveal 'typical' kame gravels. These are characterized by coarse texture, poor sorting and stratification, and subrounded to rounded clasts. Erosional features and recent colluvium, however, have masked the original surface expression of the bench. Two units were recognized in these kame deposits and have been correlated in various outcrops by their texture, stage of weathering of clasts and elevation (FIGURE 3.7). Unit A is composed of cobble and pebble gravels with scattered boulders . These are poorly sorted and exhibit poor stratification. The clasts tend to be predominantly sub-rounded, through some are rounded and subangular. Their lithology is mainly serpentine, argil l i te, chert and granitic rocks. Many of the clasts are moderately weathered (Stage 2 on a arbitrarily defined scale of 1 (least weathered) to 3). Resting above these gravels, at two localities, are interbedded sand and clay (Unit B). The sand is thinly bedded with a medium to coarse texture. This unit is best displayed at Site Three (FIGURE 3.7). The sand beds here vary from 2 to 3 feet (.6 - .9 m) in thickness and tend to predominate towards the base of the unit. Thickness of the clay beds ranges from 6 to 18 inches (15-45 cm). Correlation of the units along a 3-mile stretch of the valley allows an estimate to be made of the longitudinal slope of the kame terrace. This is 54 feet/mile (10.1 m/km) which is very similar to the present river gradient of 45a F I G U R E 3.8 : CRO S S - S E C T I O N SHOWING S T R A T I G R A P H I C R E L A T I O N S H I P O F L A C U S T R I N E , G L A C I A L , F L U V I O - G L A C I A L , A N D ICE C O N T A C T F L U V I A L - G L A C I A L DEPOSITS M FT 522 1740 i - ' w r . - sJSs . n V ^ v ^ D E F O R M A T I O N TILL 348 1160 V A L L E Y ! TRAIN- . A B 45b 50 feet/mile (9.4 m/km). Unit A was probably deposited when available discharge and sediment load were high. It is thought that Unit B represents ponding and backwash sediment deposited as a consequence of a waning ice sheet (resulting in a decreasing discharge) and channel migration away from the valley wall towards the thinning ice occupying the central valley. Alternatively, the kame terrace gradient suggests that the ice edge was relatively flat, possibly indicating a stagnating ice margin. Hence, Unit B could possibly have been deposited in ponds along the edge of the ice. The clarity and number of exposures viewed, however, makes these suggestions tentative. FIGURE 3.8 illustrates the stratigraphic relationship between kame terrace, deformation till and overridden lacustrine sediment. A series of small road cuts and gully sections provided most of the detail required for construction of the figure. The kame terrace is inset in front of the till and lacustrine clay and silt, and is considerably higher than the aggradation gravels of the valley train. 3;5 FLUVIO-GLACIAL DEPOSITS: AGGRADATION GRAVELS OF  VALLEY TRAIN 3.5.1 INTRODUCTION There are a number of excellent exposures of fluvio-glacial gravels revealed in the present-day river banks. These gravels are characterized by: (a) coarse texture; pebble to boulder size, with beds of finer sediment, (b) geographical location in the central portion of the valley, (c) abrupt textural changes, (d) presence of primary structures such as ripple marks, cross-bedding, and pinchouts, 46 (e) variable bedding and stratification, (f) stratigraphic position; resting on basal till (TABLE IV). 3.5.2 A TYPICAL SECTION - THE "HORSESHOE SECTION" At the northwest end of the study area is an extensive exposure of planar bedded aggradation gravels, approximately 400 feet (120 m) thick (for location refer to FIGURE 1.3). The "Horseshoe Section" is a large, semi-circular shaped river bank formed by fluvial erosion. Fourteen members can be identified in the exposure by their texture, colour and internal characteristics ( i .e . , stratification, bedding). These are illustrated and described in FIGURE 3.9 and TABLE V . Several general observations may be made concerning the nature of these gravels: (1) There is a distinct cyclic pattern of textural variation on three scales. Considering the overall section, three phases of deposition of alternately coarse and fine material are represented by the three formations (FIGURE 3.9). Formations One and Three consist of coarse gravels, while Formation Two is made up of fine sediments (silty sand and clay). Within each of these formations, the members display a second cyclic variation of texture. Formation One (represented by Members B to G) and Formation Three (represented by Members I to L) consist of alternating members of coarse gravel and fine gravel and sand. At the most detailed level of observation, within each member, a third cyclic variation of textures is again noted. In the finer textured members ( i .e . , C , E, G , J , and L), there are generally alternating beds of sand and coarse cobble and 47a METRES 559.5 550.5 540.6 529.5 525 517.5 504 486 483 463.5 459.9 450 447 441 438 430.5 420 F E E T 1865 1835 1802 1765 1750 1725 168a 1620 1610 1545 1533 1500 1490 1470 1460 1435 1400 ILLUSTRATION k< OF FORMATION ' HORSESHOE SECTION' ~T~ I i n r i - * 3fi1 - -Joi £ 3; A FIGURE 3.9 : v: SAND & gri CLAY PEBBLE GRAVEL ' ••' l c COBBLE GRAVEL BOULDER GRAVEL CROSS^BEDDING :1 ic TILL OBSCURE UNCONFORMITY BEDDING i = ni I ! A TILL TABLE V DESCRIPTION OF HORSESHOE SECTION Member from., Figute 3.9 Texture Thickness Stratifi-Bedding cation Sorting Colour of Shape Member* of Clast :r.-0 4 7 b Lithology er N ! fine sand variable T-30' (30 cm - 9m) massive, windblown sand, topographic expression: sand dunes at some localities of Horseshow Section downward texture change through: a) pebble gravel b) cobble gravel c) boulder/cobble gravel  32' (9.6 m) clearly moderate rounded, some sub-rounded schists,serpentine, argillite, chert, some granitic loose, well wasr pd (clean) , rests uncomformly above MEMBER L,terrace capping deposit (Chapter 3.8) H: beds of: L3: coarse sand 8' (2.4 m) L2: pebble gravel and cobble gravel 5' (1.5 m) laminated clearly L]: alternating beds of coarse sand and pebble gravel with occasional cobbles 24' (7.2 m) moderate-ly thick 2-4'(.6-1.2 m) well light •- medium moderate grey N.6 moderate predominantly argillite, chart, subrounded, serpentine, slate, some rounded granitic and off volcanic great lateral variation in texture downstream beds are pinched out (PLATE 3.8) continuous throughout Horseshoe Section cross bedding in L3 -sse-ic alternating strata of pebble gravel, odd 15' (4.5 m) boulder and cobble gravel with pebbles moderate moderate pale yellowish brown 10YR 6/2 mostly sub-angular, some subrounded/ rounded mostly serpentine, slates, some argil-lite, cherts and scattered granitic finer textured strata are thinner, that are pinched out by coarseristrata becomes coarser towards the base continuous along face beds of: ''J'2: medium sand i t ID: J ] : pebble gravel, scattered cobbles 14' (5.2 m) IT (3.3 m) thin wel moderate (in J] only) moderate light medium grey N .6 predominantly subangular, some sub-rounded/ rounded as Member D only two sub-members at this site at other localities, pinchouts of different textural units similar to MEMBER L continuous along face (PLATE cross bedding in J] 3.9) 1 1 N I ' •'dM upper portions: alternating strata of cobble gravel, occasional boulder and "pebble gravel lower portion: same texture as above but more boulders present 45' (12.5 m) moderate to poor moderate to poor poor to non-existent pale yellow brown 10YR 6/2 poor subangular with some subrounded predominantly subangular mostly serpentine schists, some chert, argillite, sand-stone, odd granite continuous in the exposure similar to MEMBER K except coarser the strata exhibits pinchouts throughout the whole member, pinch-outs of fine sand beds 4-6' (.9-1.8 m) thick fine tex-ture pinchouts N.6 light medium grey H 3 6* : 12 ti, sand and clay (interbedded) sand-fine texture, dirty — silt present 60' (18 m) decreases downstream (wedged-shape) clay beds: .9-3 m sand: .5-2' (\l-.6m) beds become progressively thinner higher in the member 1 • not continuousj in the Horseshoe Section • it is pinched out end replaced by MEMBER H (see 3.5.3 - SlJTE TWO) • can be identified by the vegetation growth at its top and seepage stains on face during wet periods (PLATE 3.9-} 3.10)  G alternating beds of: a) scattered cobbles in bedded granules, and b) pebble gravels with odd cobble 10' (2.7 m) bed thickness moderate -well light medium grey N .6 subangular/ subrounded mostly serpentine schists, some argillite, chert - MEMBERS Hj — O obscured in most parts of the Horseshoe by scree - no pinchouts (evident at the single site TABLE V (cont'd) Member from Figure 39 Texture Thickness Bedding Stratifi-cation Sorting Colour of Member* Shape of Clast < Lithology . 48 Other- iv ; -F alternating strata of a) pebble gravel with scattered cobble,: and b) cobble and pebble gravel 65' (19.5 m) - moderate moderate pale yellow brown 10YR -6/2 predominantly subangular, odd sub-rounded mostly argillite, chert, sandstone, some serpentine - strata exhibit pinchoiFfs - pinchouts of bedded granules, grey colour(N .6) about 32' (9.5 m)frornJop of member E2: alternating beds of pebble gravels and granules with scattered pebbles 8' (2.4 m) bed thickness 1-2' (.3-.6 m) moderate-well light medium grey N.6 mostly sub-angular, some sub-rounded mostly schists, serpentines, some chert, - beds have appearance of book pages, thinner beds hollowed out accentuating thicker beds - moderately indurated, cemented by calcite Ey. bedded granules with scattered pebbles 4' (1.2 m)' bed thickness 1-3' (.3-.9m) - moderate • r ' L>t IT.C-b:V-. 9310 ;• '• r alternating strata of: .a) pebble gravel and scattered cobbles b) cobble gravel scattered boulders 43' (12.9 m) (boundary , approximately) poor-moderate poor-moderate pale yellow brown 10YR6/2 predominantly subangular, some angular mostly cherts, some serpentine - about 10' (3 m),from tfcp pinchout of alternating beds of granules and pebbles in granules that have grey colour N .6 - base obscured by scree , - pinchouts of strata s b p q ' C3: beds of coarse sand with granules, bands of pebble gravels and medium sands, thinly bedded 20' (6 m) bed thickness 4-7' (1.2-2.1 m) moderate-well moderate yellow brown 10YR5/4 predominantly subangular, some sub-rounded mostly cherts, serpentine, schists, some igneous granitic and vol-canic - cross bedding :evidentDn sub-member C3 - indurated, book page appearance, bonded by calcite (PLATE 3.11) ~ C2' coarse sand 25' (10.5 m) thin C ] : alternating beds of pebble gravels and coarse sand bed thickness 1-3' (.3-.9m) alternating strata of: a) cobble gravel, scattered boulder :b) pebble gravels with cobbles 35' (10.5 m) poor poor pale yellow brown 10YR6/2 mostly sub-angular and angular mostly serpentine, slate, some chert - similar to MEMBER ©but coarser, poorer sorting and stratification - pinchouts of strata - boulder gravel near base ( PLATE 3.11) A Till - Described in TABLE IV - SITE 1 ( PLATE 3.10) A * Munsell Colour Chart 49 PLATE 3.8 Members N , M, L, K. J . at Horseshoe Section (140 Feet/42 Metres) PLATE 3.9 Upper Part of Horseshoe Section (Notice Continuity of Members) 50 PLATE 3.11 Indurated Members B to E at Horseshoe Section 51 pebble gravels. The sand is laminated of fine or coarse texture, and commonly displays cross-bedding and pinchouts; the coarser beds are massive or moderately stratified. On the other hand, coarser members ( i .e . , B, D, F, I, and K) consist of cobble, pebble and boulder gravels, poorly to moderately stratified, alternating with pinchouts of fine sand and granule beds. Member H of Formation Two consists of alternating beds of sand and clay. Two unconformities were noted. One occurs between Members L and M and marks the upper limit of the aggradation gravels. Member M is a terrace capping (Chapter 3 .7) formed during the degradational phase that followed aggradation. The lower unconformity, between Members H and I, signifies the start of Phase Three in the sedimentary history of the gravels. Member H becomes progressively thinner (wedged shaped), downstream, along "Horseshoe Section". At the extreme downstream end of the site, this member is absent and is replaced by coarse, poorly sorted gravels (Chapter 3 . 5 . 3 - Site Two). Progressing upwards from the basal members of each of Formations One and Three to their upper limits, several trends are visible in the character and appearance of the gravels. These are: (a) overall texture becomes relatively finer, (b) clast shape tends to become rounder, (c) sorting and internal stratification become more pronounced. The gravels are resting on basal till (Chapter 3 . 3 , TABLE IV), Site One) and overlain by terrace cappings that usually have a surficial veneer of aeolian sand. 52 (5) No conclusion can be reached regarding the provenance of the aggradation gravels. There is no definite indication of either Upper Bridge or Yalakom River origin when the lithology of the gravel (TABLE V) is compared to the local geology (FIGURE 1.2). Although the high proportion of serpentine clasts, in numerous members, suggests a Yalakom provenance, this observation requires substantiating. Analysis of clast lithology in exposures upstream in Yalakom River would probably clarify the issue. 3.5.3 DOWNSTREAM SECTIONS OF AGGRADATION GRAVELS There are several other sections in the same fluvio-glacial gravels downstream from "Horseshoe Section". Their member to member relationship is diagrammatically presented in FIGURE 3.10. The key variables for correlation of the members proved to be colour, texture, elevation and internal characteristics such as stratification. The upper surface of the valley train gravels on FIGURE 3.10 is considered an approximation. Its recon-struction was based upon the correlation of Member L, the upper constructional terrace elvations (FIGURE 1.4) and a small road-cut of gravels near Fraser River confluence (believed to be near the upper limit of aggradation) (FIGURE 3.10, Site Eleven). The following is a capsule description of these downstream sections. Members that have been correlated with "Horseshoe Section" members are not discussed within the text but described in TABLE V . Site One Three members, here, correlate with "Horseshoe Section". Member C retains its 'book-pages' appearance (PLATE 3.12) and exhibits excellent 53 54 examples of cut and fill structures that illustrate the rapid textural changes that occur within members. Site Two At this locality, Member H is replaced by the previously mentioned coarse gravels (Chapter 3.5.2). This member, Member H', consists of approximately 60 feet (18 m) of coarse gravels. The lower portion is made up of non-sorted, poorly stratified, cobble and boulder gravels. At the base of this member is an extremely coarse boulder gravel, approximately 10 feet (3 m) thick. Texture becomes finer, upwards, grading into a pebble and cobble gravel that displays poor to moderate sorting and stratification. The member contains several light, medium grey (N.6) lenses of bedded granules and coarse sand. These lenses have an erosional contact with the overlying coarser gravels. The clasts are predominantly serpentine and cherts, with some granitics. The majority are subangular; though there are more angular clasts towards the base. The member is a pale yellowish brown 10YR 6/2) colour. A gradational boundary occurs between Member H 1 and I. The two differ with respect to texture, clast shape, sorting and stratification. Site Three Al l members observed are similar to "Horseshoe Section" description. Most of the upper part of the section was obscured by scree, accounting for the tentative correlation. Site Four Members F through Lwere distinguished at this site. Three differences from the upstream sites were observed: 55 (a) Member H consists of laminated beds of alternating coarse sand and granules for the lower 8 feet (2.4 m). These are overlain by 12 feet (3.6 m) of alternating beds of medium and fine sand. This member displays asymmetrical ripples orientated S10°W (PLATE 3.13), and is pinched out downstream. (b) Clast shape in most members tended to be subrounded to subangular. (c) Colluvial fan sediments overlie Member L. Site Five The top part of the section consists of about 30 feet (9 m) of thinly, bedded sand. This is considered to be part of Member C because of its relative elevation and similar appearance to Member C^ . The lower two outcrops have not been placed within any chronological member. The upper one displays some excellent scour and fill structures of pebble gravels and sand and granules (PLATE 3.14). These features are probably the result of a laterally migrating stream channel. Several thin bands of angular pebbles occur within these sediments. The lower outcrop is composed of angular pebbles and cobbles, approximately 15 feet (4.5 m) thick. They are poorly sorted and stratified (PLATE 3.15). Site Six About 50 feet (15 m) of moderately, well stratified cobble and pebble gravels are displayed in this section. Numerous, light, medium grey (N.6) lenses of coarse sand and granules occur within these gravels. These lenses vary from 4 to 10 feet ( 1 . 2 - 3 m) in thickness. The overall appearance is similar to Member C , although there is no sign of induration. It has been tentatively correlated with Member C . Site Seven This small road cut merits comment because the outwash gravels are interfingered with alluvial fan material deposited from Applespring Creek (FIGURE 1.3). The fan sediment is detailed in Chapter 3.6. Sites Eight and Nine Member C ? , in both instances, is identical to Member C of Site Six. The material above and below Member C ? consists of poorly sorted, alternating strata of pebble and cobble gravels; hence, the tentative correlation with Members B and D, respectively. The clast shape in all members is predominantly round to subrounded. Sites Ten and Eleven Site Ten is a second locality where basal till occurs underneath the outwash gravels. These gravels are obscured by slope debris. The till has been described, previously, in Chapter 3.3. The final section, Site Eleven, comprises 5 feet (1.5 m) of thinly bedded , medium sand surrounded by granules with scattered pebbles. It is considered to be part of Member Land its significance has been previously mentioned. .5.4 DISCUSSION Combining the information from all the sections leads to some valuable insights into the nature and depositional history of the outwash gravels. The slope of the pre-aggradation river channel was probably similar to the present-day river gradient. The slope for the segment above the canyon region can be reconstructed at 64 feet/mile (12 m/km) from the slope of the basal contacts of Members H and L, respectively, between "Horseshoe Section" and Site Four (FIGURE 3.10). Between these localities, the present river 57 PLATE 3.13 Asymmetrical Ripples in Member H at Site Four of Downstream Sections. 58 PLATE 3.15 Coll uvial Material Incorporated in Aggradation Gravels (Site Five) 59 gradient is 55 feet/mile (10.3 m/km). Using the two till contacts (Horseshoe Section and Site Ten) as an estimate, the former river gradient as measured now is 46 feet/mile (8.6 m/km) compared to 50.6 feet/mile (9.5 m/km) for the present river gradient between these sections. This suggests that there has been no tectonic movement in the study area in Holocene time. The aggradation gravels become thinner downstream. Their maximum thickness, observed at "Horseshoe Section" is about 400 feet (120 m), while near the Bridge-Fraser River confluence 350 feet (105 m) of gravel was measured (FIGURE 3.10). This decreasing thickness, downstream, is probably the result of the increasing distance from the source of the sediment and water (ice terminus) to the zone of deposition. Correlation of individual members is easily carried out from "Horseshoe Section" to the canyon zone, downstream. Beyond this, correla-tions become tenuous. This is probably the result of the instability of the canyon walls during deglaciation (as suggested by the angular colluvial bands and outcrops noted at Site Five), the introduction of sediment from Applespring Creek (Site Seven) and the narrowing width of the valley (Chapter 1.3). The introduction of large amounts of material into the fluvial system and a narrowing channel, downstream, probably affected the character of the deposits in the lower part of the study area, making upstream correlation of members difficult. Clast shapes tended to become rounder, in most correlated members, downstream. This is probably a function of distance travelled, allowing for greater attrition of clasts. 6 0 Formations One and Three are characterized by tabular set stratification, rapid textural changes and cut and scour structures. Thus, these formations are considered to be the result of deposition during upper-flow regime by a braided, high gradient stream (Harmes and Fahnestock, 1965). The member to member variation in texture of these formations is believed to be the result of seasonal fluctuations in discharge and sediment load. The internal variation of texture within each member is thought to be the result of shifting channel bed. The trends concerning clast shape and stratification within Formations One and Three observed at "Horseshoe Section" are believed to have been controlled by a retreating ice margin in the upper portion of Bridge and Yalakom River Valleys ( i .e . , the ice margin was progressively retreating to greater distances from "Horseshoe Section"). Two possible suggestions for the deposition of Formation Two are: (1) Formation Two Is the result of a period of lower-flow regime (Harmes and Fahnestock, 1965) that is characterized by fine sand and clay at "Horseshoe Section" and fine textured sand with ripple marks, noted at Site Four. This quiet phase of deposition is believed to be the consequence of a fluctuating ice margin. Formation One was deposited during initial melting and retreat of the ice, upstream, in Bridge and Yalakom River Valleys. With increasingly great distance the ice front, Formation Two, may have resulted from: (a) a sediment load consisting of only fine textured material in the meltwater stream (due to the increased distance between the ice margin and "Horseshoe Section") in the study area, (b) or, the rapid retreat may have been accompanied with the development of a basin, upstream (at the ice front), thus trapping most of the coarse sediment. 61 A readvance of the ice, to the west of the study area, followed this quiet phase. This was accompanied by the fluvial erosion of the upper levels of Formation Two and subsequent aggradation of Formation Three (including Member H'). This implies that Formations One to Three are deposits of one deglaciation phase in which the ice margin fluctuated in the upper parts of Bridge and Yalakom Basins. [2) Formation Two may be tentatively correlated with the clay and silt at "Clay Pit" (Chapter 3.2). This suggests that Formations One and Three represent individual phases of aggradation separated by a glacial lake phase (Formation Two). The unconformity between Member H and I is interpreted as a glacial erosion surface. Under this hypothesis, Member H 1 (or the lower portion of this member) (Chapter 3 .5 .3. , Site Two) is interprted as a former till that has been washed by subsequent fluvial action. This process could account for the lack of lodgment till in this area. The minor difference in texture between "Clay Pit" sediments and Member H may be attributed to the upstream location in the lake where Member H was deposited. Or, the interbedded sand and clay (Member H) may be part of an earlier sequence of this lacustrine phase. The interpretation of Formation Two is significant in determining the chronology of the study area (Chapter 4.1). It appears that alluvial fan deposition commenced prior to the termination of the last aggradational sequence and continued after cessation of aggradation. Colluvial fan sedimentation began after the termination of aggradation (Site Four). PLATE 3.17 Close-up of Mudflow Band Illustrating Texture ALLUVIAL FAN DEPOSITS The alluvial fan deposits occur at the outlets of large tributaries, such as Applespring and Antoine Creek (FIGURE 1.4). The larger fans are located on the steeper, south-facing slopes and form the present land surface. Only two, small road-cut exposures, however, were accessible for observation; the previously mentioned Applespring exposure (Chapter 3.5.3 - Site Seven) and at the next creek downstream from Applespring Creek (about 1.3 miles - 2 km). The Applespring fan outcrop contains 15 feet (4.5 m) of moderately stratified, fluvial, pebble gravels with scattered cobbles. These rest unconformably below outwash gravels. The clast shape is predominantly sub-angular, with lithologies ranging from argillites, sandstone, and arkose to conglomerate. The fan has a surface gradient of approximately 3 degrees. The second fan has a surface slope of 4 degrees and overlies outwash gravels. The overall thickness of material in the exposure is 17 feet (5.1 m) with an upper elevation of approximately 1300 feet (390 m) a.s. I. (PLATES 3.1 and 3.17). The fan sediment consists of alternating strata of granules with scattered pebbles and pebble gravel. The lithology of the subangular clasts is predominantly sandstone, conglomerate and shales. In the middle part of the exposure is a mudflow band. It is about 6 feet (18 m) thick and consists of coarse pebble, cobble and boulder, argillite clasts. The local origin (reflected by the lithology of the clasts) and shape of the clasts indicate the fluvial and mudflow sediments travelled only a short distance prior to deposition. More important, the surficial relationship of the fans and stratigraphic position displayed by these exposures, indicates deposition commenced and continued after the last Bridge River aggradation sequence. 63 PLATE 3.18 Volcanic Ash Surrounded by Colluvial Material 64 3.7 RECENT DEPOSITS Recent sediments observed in Bridge River Valley include fluvial gravels, volcanic ash, aeolian deposits, alluvial fans (Chapter 3.6), colluvial slope deposits (FIGURE 1.4). A terrace capping of clean, cobble and boulder gravel occurs on the fluvial terraces formed during the recent phase of degradation. These deposits rest unconformily over the outwash gravels detailed in Chapter 3.5. They range between 20 and 30 feet (6-9 m) in overall thickness. The gravels are moderately to well stratified. Clasts are predominantly rounded and subrounded and have a similar lithology to the clasts in the valley train. Usually, a fine, wind-blown sand covers these gravels, forming the surface sediment of the terraces and parent material of the soil. These sands are characterized by their fine, uniform texture, lack of internal structure, variable thickness (1 to 3 feet - 3-9 m) and isolated, surface expression (dunes). An undulating ash band, between 1 and 3.5 inches (2.54 - 8.9 cm) thick, outcrops in a south-facing road cut, approximately 10 miles (15 km), upstream, from Fraser-Bridge River confluence (PLATE 3.18). The ash has a coarse, gritty texture; consisting of glass shards and coarse sand sized fragments pf pumice and dark, olive green phenocrysts of hornblende. It is surrounded by angular slope sediment and rests between 6 to 8 feet ( 1 . 8 - 2 . 4 m) below the surface (surface elevation is 1525 feet (457 m) a .s . l . ) . Although the site location falls within the inferred limits of Bridge River Ashfall (FIGURE 1, Nasmith et a l . , 1967), the texture suggests it is Mazama Ash (W. • Mathews, pers. comm.). Only a single exposure was observed in the study area, thus, one can only conclude that since 6,640 years B.P. (Chapter 2.2) the valley sides, at this side, have been relatively active. 65 CHAPTER FOUR PLEISTOCENE CHRONOLOGY 4.1 BRIDGE RIVER CHRONOLOGY Two alternate chronologies of Bridge River Valley can be reconstructed from the characteristics of the various deposits that were described in Chapter 3 (FIGURE 4.1). These are as follows: Chronology A The lacustrine silt and clay of "Clay Pit" are the oldest sedimentary unit in the sequence if the sediments at "Horseshoe Section" represent a more or less continuous phase of fluvial aggradation (Chapter 3.5.4). These lacustrine sediments are representative of a glacial lake phase that occurred prior to the last glacial episode. This is suggested by their stratigraphic position beneath the deformation till (Chapter 3.3) and internal structure resulting from glacial overriding (Chapter 3.2). The duration of this "Glacial Lake Phase" is unknown. There is no indication if it is part of an Interglacial or Interstadial interval. A minimum estimate of lake level is provided by the upper glaciated surface at 1770 feet (531 m) a . s . l . (Chapter 3.2). It is tentatively suggested that a fluvial degradation phase followed the "Glacial Lake Phase". The internal structure of the sediment at "Clay Pit" suggests that the silt and clay underwent rotational slumping after termination of the lacustrine environment. Slumping could have occurred as consequence of fluvial erosion of the silt and clay occupying the main valley floor. Subsequent undercutting by the river of the sediment on the valley walls would have resulted in rotational slumping. Alternatively, there is the RGURE 4.f: POSSIBLE CHRONOLOGIES OF BRIDGE RIVER VALLEY CHRONOLOGY A*' COMMENT CHRONOLOGYB GLACIAL LAKE PHASE; FLUVIAL DEGRADATION! PHASE ? 'GLACIAL PHASE! 'FLUVIAL'AGGRADATIONr-' PHASE : INCLUDES ' CLAY PIT'. :CLAYS AND SILTS INCLUDES TILL OVERLAIN BY j OUTWASH GRAVELS (TABLE 3.1 ITNCLIJDESTORMATfoN 6NTAT '"HORSE siToESECTION7 INCLUDES MEMBER H AT 'HORSESHOE SECTION' AND_ 'CLAY PIT' CLAYS'AND SILTS ^SLUMPING OF LACUTRINE SEDIMENT ?-L_ , INCLUDES DEFORMATION;.TILL__. ;AND TILL OVERLAIN BY OUTWASH: 'GRAVELS (TABLE 3.1 )' INCLUDES DEFORMATION TILL. UNCONFORMITY "BE-TWEEN FORMATION, ... TWO AND THREEE-GLACIAL IEROSION SURFACE U—--—• KAME TERRACE DEPOSITS • — INCLUDES FORMATION THREE ATl'HORSESHOE SECTION"; .'INCLUDES FORMATION ONE T O THREE AT 'HORSESHOE - S E C T I O N ' L ^ ... _. ;UNCONFORMITY BETWEEN FORMATION. _ UTWO AND THREE-FLUVIAL EROSION SURFACE * ^ ^ _ _ _ FAN AGGRADATION GLACIAL PHASE: 1FLUVIAL AGGRADATION ' PHASE IGLACIAL LAKE PHASE. jFLUVIAL DEGRADATION PHASE .? . GLACIAL PHASE: .FLUVIAL.AGGRADATION PHASE^ FLUVIAL..DEGRADATION PHASE'' FLUVIAL DEGRADATION PHASE iDISSriOTONC rMOZ AM A' ASH- D EPOS IT comINfuED'CO'LLUvIAL SLOPE DEPOSITION possibility that the lacustrine sediment slumped during the last deglaciation (FIGURE 4.2). There is no further evidence to lend support to either of these observations. This "Fluvial Degradation Phase" was followed by a glacial phase (for pattern of ice development refer to earlier description of Fraser Glaciation, Chapter 2.2 and 2.4). As the valley glaciers of this "Glacial Phase" progressed, downstream, through the study area, the pre-existing valley sediments were eroded by the ice. This is suggested by the stratigraphic position of many of the till exposures (TABLE IV) and the previously mentioned upper glaciated surface of the lacustrine sediment. The ice occupied all of Bridge River basin during the maximum of "Glacial Phase". The deformation till and basal till observed beneath the aggradation gravels are representative of this phase. The local lithology of clasts in the till and texture of the deformation till suggests the sediment was transported a short distance in the ice prior to deposition. Initial waning of the ice was accompanied with the deposition of the kame terraces in the downstream part of the study area (Chapter 3.4). The surface of this bench is between 1400 and 1500 feet (420-450 m) a . s . l . The kame gravels are inset in front of the deformation till and the overridden lacustrine sediment (FIGURE 3.8). The retreating ice front appears to have stood for a short period of time near Camoo Creek (Chapter 3.2). As a result, the previously mentioned moraine was deposited. As the valley glaciers of the "Glacial Phase" continued their retreat, upstream, towards Coast Mountains, a phase of fluvial aggradation commenced. Initial retreat of the ice was accompanied with the deposition of Formation One (Chapter 3.5). This was followed by a retreat of the ice to the distant, upper parts of Bridge and Yalakom River Valleys. Interbedded sand and clay 68 F IGURE 4.2 :l A POSSIBLE MECHANISM ACCOU NT IN G FOR SLUM PING IN LACUSTRINE SEDIMENTS j ICE O V E RIDES A N D : E R O D E S L A K E ] SEDIMENT] " " . ' . " " • "' EROSION IS G R E A T E R I N - C E N T R E ' OF V A L L E Y ; ICE COMMENCES TO WANE I CONTINUED WANING OF ICE. L A K E SEDIMENT A L O N G V A L L E Y : WALLS SLUMPS AS SUPPORTING ICE! MELTS ; (Member H - Chapter 3.5.2), separating Formations One and Three at "Horseshoe Section" are thought to have been deposited during this phase. A readvance of these valley glaciers followed this rapid retreat. The limits of this readvance are unknown. This final resurgence of the ice was accompanied by the deposition of Formation Three , the last aggradation sequence. Deposition of these gravels was accomplished by an overloaded, braided outwash stream. The slope of this river was similar to the present-day gradient. Similar outwash deposits can be traced, upstream, in Yalakom and Bridge River Valleys and downstream, in Fraser River Valley (Ryder, pers. comm.). Alluvial fans were being built up at the mouths of the large tributaries during this "Fluvial Aggradation Phase". Stratigraphic contacts, between outwash gravels and fan sediment, however, indicate fan aggradation was not a dominant process until the latter stages of valley train accumulation (Chapter 3.6). The complete retreat of the ice and cessation of aggradation of the valley train is believed to mark the end of Pleistocene time. Fan aggradation, however, continued after this time. The sequence is terminated by the presently active phase of fluvial degradation. This "Fluvial Degradation Phase" has resulted in dissection of the valley train, alluvial fans and incision of the river into the bedrock. Consequently there are several well preserved, non-paired degradation terraces cut into the outwash gravels (FIGURE 4.3). Eight terrace levels were observed. The regular height interval between levels suggests downcutting was relatively con-tinuous. Fluvial erosion of the bedrock, near the mouth of Bridge River, has resulted in steep, bedrock river scarps. Colluvial fan sedimentation commenced during this phase. These have accumulated over the aggradation gravels, particularly, on the north side of the valley (Chapter 3.5.3 - Site Four and FIGURE 1.4). Vegetation appears to have slowed recent fan development to F I G U R E 4.3 : R E L A T I O N S H I P O F D E G R A D A T I O N T E R R A C E S T O V A L L E Y T R A I N S U R F A C E A N D P R E S E N T - D A Y P R O F I L E O F B R I D G E R I V E R 15 MILES LENGTH OF CROSS SECTION 24 KILOMETRES 71 some extenr. The active talus slopes along Camelsfoot Range provide the source material for these fans. Approximately 6,640 years B.P. , Mazama ash was deposited (Chapter 3.7). In recent years, the discharge of Bridge River has been controlled by the activity of man at Terzaghi Dam. Chronology B Member H, at "Horseshoe Section", may be correlated with the lacustrine silt and clay at the "Clay Pit" (Chapter 3.5.4). Under this hypothesis the "Glacial Lake Phase" was preceded by glacial and fluvial aggradation phases, and the unconformity between Formation Two and Three is possibly a glacial erosion surface. The earlier "Glacial Phase" is represented by the two till exposures overlain by outwash gravels (TABLE IV). The geographical location of these sites (FIGURE 3.4) indicates the ice of this phase occupied the complete Bridge River basin. The retreat of this ice was accompanied by fluvial aggradation of Formation One at "Horseshoe Section". This "Fluvial Aggradation Phase" was followed by "Glacial Lake Phase" and subsequent phases outlined under Chronology A . The unconformity between Formations Two and Three is representative of the last "Glacial Phase". Both chronologies are acceptable when the recorded description of the various sediments is considered. The latter, however, is favoured. In viewing the three formations at "Horseshoe Section", there is an observable difference in colour between Formations One and Three . Formation One has a tan or yellow-brown appearance while Formation Three has a silver-grey colour. This may indicate that Formation One is an older and more weathered gravel than Formation Three. There is no further evidence to warrant this tentative conclusion. 72 CORRELATION WITH FRASER RIVER VALLEY PLEISTOCENE CHRONOLOGY A renrarive chronology between the Pleistocene stratigraphy of Bridge and Fraser River Valleys (Chapter 2.3) is suggested in FIGURE 4.4. Bridge River Chronology B has been used for the correlation. The similarity in the sequence of phases, stratigraphic relationships of sediments, events within phases and continuation of sedimentary units up Bridge River from Fraser River Valley is the basis for correlation. The earlier "Glacial Phase" in Bridge River basin has been correlated with "Spintlum Glacial Phase" in Fraser River Valley. The position of these phases within their respective chronologies accounts for the correlation. Although evidence of this glacial episode is poor in each valley, it is suggested that both areas were completely glaciated at this time (Ryder, pers. comm. and Chapter 4.1 - Chronology B). Initial retreat of the ice of this glacial phase was accompanied by the development of Glacial Lake Lillooet in Fraser River Valley and the deposition of fluvial gravels ("Fluvial Aggradation Phase") in Bridge River Valley. (This phase is represented by Formation One, Chapter 3.5.2). Inflow from Bridge River at the confluence of the two rivers, at this time, probably resulted in the construction of a delta. The lack of stratigraphic evidence supporting this conclusion is more than likely the result of subsequent fluvial and glacial erosion or burial by colluvial slope sediment of the delta deposit. (The valley walls along Fraser River Valley at the confluence with Bridge River are relatively steep and mass wasting is presently active.) With the complete retreat of the ice out of the area and cessation of aggradation, a glacial lake formed in Bridge River Valley. This "Glacial Lake Phase" has, also, been correlated with "Glacial Lake Lillooet". The most obvious flaw with the 73 F I G U R E 4.4 : T E N T A T I V E C O R R E L A T I O N O F B R I D G E A N D F R A S E R R I V E R V A L L E Y BRIDGE RIVER CHRONOLOGY B FRASER RIVER CHRONOLOGY BETWEEN LYTTON AND LILLOOET GLACIAL PHASE FLUVIAL AGGRADATION PHASE GLACIAL L A K E PHASE ; 'CORDILLERAN ICE-SHEET PHASE': MNKOIKO PRO-GLACIAL PHASE' ' FLUVIAL DEGRADATION PHASE' ' SPINTLUM GLACIAL PHASE'": 'GLACIAL L A K E L ILLOOET' i FLUVIAL ! DEGRADATION PHASE ? — =— 'FLUVIAL DEGRADATION PHASE' GLACIAL PHASE : FLUVIAL AGGRADATION PHASE 'SETONi GLACIAL R E A D V A N C E ' FLUVIAL AGGRADATION PHASE' FLUVIAL DISSECTION, TERRACING, AGGRADATION, AND DISSECTION OF FANS, TALUS SLOPES, S AEOLIAN ACTIVITY, MAZAMA ASHFALL, 74 correlation of these lake phases is the discrepancy between the level of "Glacial Lake Lillooet" and the glacial lake in Bridge River Valley. The latter has a minimum of 1770 feet (531 m) as compared to the 1240 foot (372 m) level of the former. A possible explanation is Bridge River glacial lake represents an earlier, higher stage of "Glacial Lake Lillooet" ( i .e . , same sequence of events but slightly different timing). On the other hand, glacial erosion of lacustrine sediments, in Fraser River Valley, could account for the lack of stratigraphic evidence of this higher stage of the lake. The presence of a 1000 foot (300 m) a . s . l . glacial erosion surface of "Glacial Lake Lillooet" silt (Chapter 2.3). lends some support to this conclusion. It is suggested that fluvial erosion and removal of the above glacial lake deposits either preceded or occurred simultaneously with the subsequent glacial phase. The tentative "Fluvial Degradation Phase" in Bridge River basin has been correlated with "Fluvial Degradation Phase" in Fraser River Valley. The rotational slumping observed in the lacustrine deposits of each area and their position within their respective chronologies is the basis of this correlation. This is a tentative correlation, however, because of the poor stratigraphic evidence in Bridge River Valley (Chapter 4.1) supporting this phase and the 'condition' of the sediments during slumping. The lack of secondary structures in slumped Glacial Lake Lillooet silts, as compared to "Clay Pit" exposure, suggests drying occurred prior to movement (Ryder, pers. comm.). The contorted silt bands at "Clay Pit" suggests movement occurred prior to drying. The most recent "Glacial Phase" in Bridge River Valley has been correlated with Seton Glacial Readvance in Fraser River Valley. The similarity of stratigraphic relationship and continuation of sedimentary units were the key factors in correlation. The ice proceeded out of Bridge River Valley and PLATE 4.1 Kame Terraces in Bridge River (foreground) and Fraser River Valley (background) 76 probably coalesced with readvancing Seton ice. The ti l l , deposited by this ice, in both valleys, overlies glacial eroded lacustrine sediment (Chapter 2.3 and 3.3). A kame terrace complex that can be traced from Fraser River Valley, north of Lillooet, up the Bridge River Valley was deposited during the early stages of deglaciation of this ice (PLATE 4.1). The kame terrace, north of Lillooet, between 1200-1400 feet (350-420 m) continues up Bridge River basin between 1400-1550 feet (420-450 m) a . s . l . As deglaciation continued, Bridge and Yalakom Rivers functioned as a channel for east-flowing meltwater from Coast Mountains (FIGURE 1.1). This water flowed into Fraser River and drained through the Fraser Canyon to the coast (Ryder, pers. comm.). The continuation of the gravels, deposited during this "Fluvial Aggradation Phase", above and below the study region, supports this observation. The cessation of aggradation in Bridge and Fraser Valleys, plus the retreat of the ice to near present-day limits, is considered the termination of Pleistocene time. SUMMARY STATEMENT The last deglaciation sequence in Bridge River Valley lends support to the conclusion of Tipper (1971) (Chapter 2.3) that the Fraser Glaciation ice retreated in a normal manner along the mountain rims of Interior. The ice tongue retreat with its associated fluvial aggradation gravels in Bridge River Valley sharply contrasts the mode of deglaciation in the Thompson River basin (Chapter 2.3). In this area, deglaciation was accomplished by downwasting, and resulted in a complicated late-glacial lake history. In order to gain further insight into the Pleistocene history of Bridge River Valley, it is felt that future research should proceed in several directions. These are: 1. A detailed, quantitative analysis of the gravels at "Horseshoe Section". This would aid in the interpretation and verification of the qualitative conclusions deduced from this aggradation sequence. 2. Inspection of upstream sections in Yalakom and Bridge River Valleys for possible correlation with the results of this study. 3. Comparison of lithologies of the aggradation gravels in Yalakom River to those at "Horseshoe Section" to gain insight concerning the provenance of the aggradation gravels. 78 BIBLIOGRAPHY Anderton, L . , 1970, "Stratigraphy and Geomorphology of Lower Thompson Valley, British Columbia," Unpublished M.A . Thesis, University of British Columbia. Armstrong, J . , 1954, "Late Wisconsin marine drift and associated sediments of lower Fraser Valley, British Columbia, Canada," Geological Society of  American Bulletin, vol. 78, pp 13-20. , 1957, "Surficial Geology of New Westminster Map-Area, British Columbia," Geological Survey of Canada, Paper 57-5, 25 pp. , 1960a, "Surficial Geology of the Sumas Map-Area, British Columbia," Geological Survey of Canada, Paper 59-9, 27 pp. , 1960b, "Surficial Geology, Chilliwack (West Half), British Columbia," Geological Survey of Canada, Map 53-1959, 1:63, 360. Armstrong, J . , Crandell, D . , Easterbrook, D . , & Noble, J . , 1965, "Late Pleistocene stratigraphy and chronology in southwestern British Columbia and northwestern Washington," Geological Society of American Bulletin, vol . 76, pp 321-330. Boulton, G . , 1970a, "On the deposition of subglacial and melt-out tills at the margins of certain Svalbard Glaciers," Journal of G laciology, vol . 9, no. 56, pp 231-245 . , 1970b, "On the origin and transport of englacial debris in Svalbard Glaciers," Journal of Glaciology, vol. 9, no. 56, pp 213-229. Davis, N . , & Mathews, W . , 1944, "Four phases of glaciation with illustrations from southwestern British Columbia," Journal of Geology, vol . 52, pp 403-413. Duffel, S . , & McTaggart, K . C . , 1952, "Ashcroft Map-Area, British Columbia," Geological Survey of Canada, Memoir 262, 122 pp. Easterbrook, D . , 1967, "Pre-Olympia Pleistocene Stratigraphy and Chronology in the central Puget Lowland, Washington," Geological Society of America Bulletin, v.. 78, pp 13-20. : Flint, R., 1935, "White silt deposition in Okanagan Valley, British Columbia," British Columbia Royal Society of Canada Transactions, Section 4, pp 107-114. , 1971, Glacial and Quaternary Geology, New York, John Wiley& Sons. Fuller, M . , 1914, "The geology of Long Island, New York," United States Geology  Survey Professional Paper 82, 231 pp. Fulton, R., 1962, "Surficial Geology, Merritt, British Columbia," Geological  Survey of Canada, Map 8-1962, 1:126, 720. , 1963, "Surficial Geology of Kamloops Lake, British Columbia," Geological Survey of Canada, Map 9-1963, 1:126, 720. 79 , 1965, "Silt Deposition in Late-Glacial Lakes in southern British Columbia," American Journal of Science, vol. 263, pp 553-570. , 1969a, "Glacial lake history, southern Interior, British Columbia," Geological Survey of Canada, Paper 69-37, 14 pp. , 1969b, "Surficial Geology, Shuswap Lake, west of sixth meridian, British Columbia," Geological Survey of Canada, Map 1244A, 1:126, 720. , 1971, "Radiocarbon Geochronology of Southern British Columbia," Geological Survey of Canada, Paper 71-37, 28 pp. Fyles, J . , 1963, "Surficial Geology of Home Lake and Parksville Map-Area, Vancouver Island, British Columbia," Geological Survey of Canada, Memoir 318, 142 pp. Halstead, E . C . , 1968, "The Cowichan ice tongue, Vancouver Island," Canadian  Journal of Earth Science, vol. 5, pp 1409-15. Hansen, H . , 1955, "Postglacial forests in south-central and central British Columbia," American Journal of Science, vol. 253, pp 640-658. Harington, C . , Tipper, W . , & Mott, R., 1974, "Mammoth from Babine Lake, British Columbia," Canadian Journal of Earth Science, vol . 11, no. 2, pp. 285-303. Harmes, H . , & Fahnestock, R., 1965, "Stratification, bed forms and flow phenomena (with an example from Rio Grande)," in Primary Sedimentary Structures and  their Hydrodynamic Interpretation, edited b y G . V . Middleton, Society of Economic Paleontologiots and Minerologists Special Publication, no. 12. Heginbottom, J . , 1972, "Surficial Geology of Taseko Lakes Map-Area, British Columbia," Geological Survey of Canada, Paper 72-14, pp 9. Hodgson, R., N . D . , Precision Altimeter Survey Procedures, Los Angeles, American Paulin System (Educational Division). Holland, S . , 1964, "Landforms of British Columbia, a physiographic outline," British Columbia Dept. of Mines and Petroleum Resource, Bulletin 48, 138 pp. Hopkins, O . , 1923, "Some Structural Features of the plains of Alberta caused by Pleistocene glaciation," Geological Society of America Bulletin, vol . 34, pp 419-30. Jeletzky, J . , & Tipper, W . , 1968, "Upper Jurassic and Cretaceous rocks Taseko Lakes Map-Area and their bearing on the geological history of southwestern British Columbia, Geological Survey of Canada, Paper 67-54, pp 218. Kerr, F . , 1934, "Glaciation in northern British Columbia," Royal Society of Canada  Transactions, vol . 28, section IV, pp 17-32. 80 Mackay, J . R . , & Mathews, W . , 1960, "Deformation of soils by glacier ice and the influence of pore pressure and permafrost," Royal Society of Canada Trans- actions, Ser. 3, v . 54, pp 27-36. Mathewes, R., Borden, C , & Rouse, G . , 1972, "New radiocarbon dates from the Yale area of the lower Fraser River canyon, British Columbia," Canadian  Journal of Earth Science, vol. 9, number 8, pp 1055-57. Mathews, W . H . , 1944, "Glacial lakes and ice retreat in south-central British Columbia," Royal Society of Canada Transactions, vol . 38, section IV, pp 39-57. , 1946, "Geology and coal resources of the Carbon Creek-Mount Bickford Map-Area," British Columbia Dept. of Mines, Bulletin 24. , 1951, "Historic and prehistoric fluctuations of alpine glaciers in the Mount Garibaldi Map-Area, southwestern British Columbia," Journal of  Geo logy, vol . 39, pp 357-380. , 1963, "Quaternary stratigraphy and geomorphology of the Fort St. John Map-Area, Northwestern British Columbia, British Columbia Dept. Mines  and Petroleum Resources, pp 1-22. Mathews, W . H . , Fyles, J . , & Nasmith, H . , 1970, "Postglacial crustal movements in southwestern British Columbia and adjacent Washington State," Canadian  Journal of Earth Science, vol . 7, pp 609-702. Nasmith, H . , 1962, "Late Glacial History and Surficial Deposits of the Okanagan Valley, British Columbia," British Columbia Dept. of Mines Bulletin, no. 46, pp 163-170. Roddick.:, J . , & Hutchison, W . , 1973, "Pemberton (East Half) Map-Area, British Columbia," Geological Survey of Canada, Paper 73-17, 21 pp. Rubin, M . , & Alexander, C . , 1960, "U .S . Geological Survey radiocarbon dates," Radiocarbon, vol. 2, pp 129-185. Tipper, W . H . , 1971, "Glacial Geomorphology and Pleistocene history of Central British Columbia," Geological Survey of Canada, Bulletin 196 , 88 pp. Weertman, J . , 1961, "Mechanism for the formation of inner moraines found near the edge of cold ice caps and ice sheets," Journal of G laciology, vol. 3, no. 30, pp 965-78. Westgate, J . , Smith, D . , & Tomlinson, M . , 1970, "Late Quaternary tephra layers in southwestern Canada," in Early Man and environments in northwest North  America, University of Calgary Archaeology Assoc . , The Students' Press, Calgary, pp 13-34. Nasmith, H. f Mathews, W.H., and Rouse, G,, 1967, "Bridge River ash and some recent ash beds in British Columbia," Canadian Journal of Earth  Science, vol. 4, pp. 163-70. """"""" 81 Secondary Sources Air Photographs Flight Line BC 5145 Photo Numbers: 004 - 011 Flight Line BC 5144 Photo Numbers: 237 - 242 Maps Geological Association of Canada, 1958, "G lacial Map of Canada, 1:5,000,000. National Topographic System, "Lytton Map," 1:125,000. National Topographic System, 1958, "Pemberton Map," 1:250,000. 8 2 APPENDIX A Classification Used for Surficial Geology Map (after R. Fulton, Geological Survey of Canada) 1 . Compositional Categories morainal (till) M alluvial (fluvial: sand, gravel, silt) A lacustrine (clay, silt, sand, gravel) L colluvial (mass-wasting processes) C aeolian (wind: silt, sand) E organic (peat) O undistinguished Quaternary Deposit D 2. Morphologic Expression plain (relatively flat) p rolling plain (undulating) m hummocky (small but steep-side hillocks) h ridged (small but steep-side linear hills) r terraced (flat, bounded by scarps) t fan (cone segment <. 15° slope) f cone (cone segment ^ 15° slope) c veneer (thin (< 6 ft.; 2 m.) cover of material deriving topographic expression from underlying unit) v blanket (as veneer except > 6 ft.; 2 m.) b complex (a mixture of several of above) x 83 3 . Unconsolidated Component Structure bouldery (abundance of boulders) b gravel ly (gravel and coarse sand) g sandy (granule and sand) s si l ty (fine sand silt) ) > f clayey (fine silt and clay) ) rubble (rock) r 4 . Erosional Modi f ica t ion glaciated G washed (affected by body of standing water) W f luvial-erosion (modified by through flowing stream) R gul l ied (modified by local channel cutting) V flowage (modified by slow flowage of water, i . e . , solif luction) F mass-wastage (modified by downslope movement of loose material) M frost-heaved H avalanche modification A piping modification P aeol ian modification (deflation) E Each Uni t is coded: 312-4 (3 . r • For example: ta lus slope =/ l . C = rCa deposits 2 . a A. -84 APPENDIX B  Procedures Conducted at Each Stratigraphic Section For Each Section 1 . Assign ID number. 2 . Locate on air photo and mark with ID number. 3. Absolute elevation top or base. 4. Orientation. 5. Associated landforms. 6. Sub-divide into units (see below). 7. If not overlying bedrock, write "base not seen'. For Each Unit 1 . Thickness, continuity, variation in thickness. 2 . Relationship to adjacent units, nature of contacts . 3. Bedding and Stratification: bedded = bedding planes present "well bedded" = bed immediately apparent, clearly defined "moderately - well bedded" = . . . intermediate "poorly bedded" = beds are only discerned after careful scrutiny stratified = alignment within unit due to variations of sorting or texture, no defined bedding planes use "well stratified" . . . to . . . "poorly stratified" 4. Thickness of beds: laminated (less than 1 cm. or 1/2 inch) thinnly bedded . . . massive (measure thickness) 85 5. Texture: For sorted sediments: boulder g rave l , cobble g rave l , pebbled g rave l , (or combinations of these), very coarse sand, coarse sand, medium sand, fine sand, silt and c l a y . C lean = wel l washed = l i tt le or no silt If coarse material subsidiary to fine ( i . e . : clasts do not touch), i .e . , c lay with boulders = boulders common throughout c lay silt with scattered boulders = several boulders visible at a g l ance . Diamict ion = a heterogeneous mixture of sizes . 6. Shape of Clasts: / O O O weM rounded rounded subs-rounded su^ang^j lar aj!gT\ar 7. Sorting: wel l sorted moderate sorting poorly sorted 8. Identify any vert ical or lateral gradational changes in any above characteristics 9 . Compaction: loose = crumbles upon contact with hammer very compact - does not crumble upon contact (i .e . " thud" from hammer blow) 10. Induration: wel l indurated, indurated in places . . . note nature of cementing agent. 11. Colour: according to MunseII Scheme . 12. Weathering of clasts: Stage One : breaks under hammer noticeably more easi ly than if unweathered . 8 6 12. Stage Two: between one and three . Stage Three: granular disintergration (granitic rocks) original rock unrecognizable (volcanic rocks) 13. Continuity of beds, lenses, pinchouts. 14. Sedimentary structures present: i .e . , ripple marks, cross-bedding. 15. Identify lithology of clasts. For Til I 1 . Matrix texture, clast texture. 2 . Colour by Munsell Scheme . 3. Percentage coarse particles (visual estimation chart). 4. Lithogies. 5 . Check for striations . 6. Describe compaction. 7. Is site suitable for fabric measurement? For Volcanic Ash 1 . Sample (35 mm film can). 2. Section description: depth ash below surface, any variation in depth, thickness of ash (maximum, minimum mean), any evidence of rework enclosing material, associated landform. 

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