Quaternary Stratigraphy and Glacial History of the Peace River Valley, Northeast British Columbia1

549 Quaternary stratigraphy and glacial history of the Peace River valley, northeast British Columbia1

Gregory M.D. Hartman and John J. Clague

Abstract: Two Cordilleran and three Laurentide glacial advances are recorded in Quaternary sediments and landforms in the Peace River valley, northeast British Columbia. The advances are inferred from fluvial gravels, glaciolacustrine sediments, and tills within nested paleovalleys excavated during three interglaciations and from the distribution of granitoid clasts derived from the Canadian Shield. Till of the last (Late Wisconsinan) Laurentide glaciation occurs at the surface, except where it is overlain by postglacial sediments. The advance that deposited this till was the most extensive in the study area, and the only advance definitively recognized in western Alberta south of the study area. Late Wisconsinan Cordil- leran till has not been found in the study area, but Cordilleran and Laurentide ice may have coalesced briefly during the last glaciation. Support for this supposition is provided by the inferred deflection of Laurentide flutings to the southeast by Cordilleran ice. The earliest Laurentide advance may have been the least extensive of the three Laurentide events recognized in the study area. Erratics attributed to this advance occur only east of the Halfway River – Beatton River drainage divide. Re´sume´ : Deux avanceés glaciaires de la Cordille`re et trois de l’Inlandsis laurentidien sont e´videntes dans les se´diments et la topographie du Quaternaire de la valleé de la rivie`re de la Paix, dans le nord-est de la Colombie-Britannique. Les avanceés sont de´duites a` partir des graviers fluviatiles, des se´diments glacio-lacustres et des tills dans des paleóvalleés emboıˆteés excaveés durant trois pe´riodes interglaciaires et a` partir de la distribution des clastes granitoı¨des provenant du Bouclier canadien. Un till de la dernie`re glaciation laurentidienne (Wisconsinien tardif) se retrouve a` la surface, sauf la` ou` il est recouvert de se´diments post-glaciaires. L’avanceé qui a de´pose´ ce till a e´te´ la plus extensive dans le secteur a` l’e´tude et c’est la seule avanceéde´finitivement reconnue dans l’ouest de l’Alberta, au sud du secteur a` l’e´tude. Aucun till de la Cordille`re datant du Wisconsinien n’a e´te´ trouve´ dans le secteur a` l’e´tude. Cependant, la glace de la Cordille`re et celle de l’Inlandsis laurentidien pourraient s’eˆtre unis durant une bre`ve pe´riode au cours de la dernie`re glaciation. Cette hypothe`se est soutenue par la de´flexion infe´reé de rainures glaciaires laurentidiennes vers le sud-est par la glace de la Cordille`re. L’avanceé glaciaire laurentidienne la plus prećoce pourrait eˆtre la moins e´tendue des trois e´veńements laurenti- diens reconnus dans le secteur a` l’e´tude. Les blocs erratiques attribue´sa` cette avanceé ne se retrouvent qu’a` l’est de la ligne de partage des eaux entre les rivie`res Halfway et Beatton. [Traduit par la Re´daction]

Introduction and Laurentide glaciations of the area. Moreover, the Quater- nary history along the entire Canadian Rocky Mountain The Quaternary stratigraphy of Peace Valley was docu- front has, in recent years, been revised. Early researchers mented 30 years ago by Mathews (1978) but, since then, inferred up to four Laurentide glaciations of the Alberta some researchers working in northeast British Columbia and Plateau, but recent work has reduced this number, and northwest Alberta have come to conclusions incompatible some workers have suggested that only one advance with Mathews’ interpretation of the history of Cordilleran reached Edmonton, Simonette, and Watino (Fig. 1). The number of Cordilleran advances has also been reduced Received 30 March 2007. Accepted 30 October 2007. Published from that proposed by early workers. on the NRC Research Press Web site at cjes.nrc.ca on 25 June These revisions have been driven, in part, by improve- 2008. ments in describing and interpreting sediments, especially Paper handled by Associate Editor R. Gilbert. diamictons (Eyles et al. 1983). Equally important, Mac- Donald et al. (1987) demonstrated that radiocarbon ages on G.M. Hartman.2 Department of Earth Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, gyttja and some freshwater aquatic plants suffer an ‘‘old Canada; Present address: AMEC Earth and Environmental, 5681 carbon effect.’’ These and other developments led to the re- 70th Street, Edmonton, AB T6B 3P6, Canada. evaluation of interpretations based on incorrectly described J.J. Clague. Department of Earth Sciences, Simon Fraser deposits or spurious radiocarbon ages. The work of early re- University, 8888 University Drive, Burnaby, BC V5A 1S6, searchers however, cannot be completely discounted. Canada. Henderson (1959), Tokarsky (1967), St-Onge (1972), and 1This article is one of a selection of papers published in this Mathews (1978) present evidence for multiple glaciations Special Issue on the theme Geology of northeastern British that is not dependent on lithofacies interpretations and radio- Columbia and northwestern Alberta: diamonds, shallow gas, carbon ages. gravel, and glaciers. Even recent researchers, however, do not agree on the ex- 2Corresponding author (e-mail: [email protected]). tent and times of Cordilleran advances and whether or not

Can. J. Earth Sci. 45: 549–564 (2008) doi:10.1139/E07-069 # 2008 NRC Canada 550 Can. J. Earth Sci. Vol. 45, 2008

Fig. 1. Study area. The map at the upper left shows adjacent areas where Quaternary research has been completed.

Cordilleran glaciers ever coalesced with the Laurentide ice Laurentide glaciers. It was affected by both the Cordilleran sheet. Jackson et al. (1997), Bednarski (1999, 2001), Dyke and Laurentide ice sheets, and Quaternary sediments are et al. (2002), and Bednarski and Smith (2007) have argued well exposed along Peace River and its tributaries. In addi- for extensive Late Wisconsinan Cordilleran glaciation, in- tion, interstadial and interglacial paleovalleys have been par- volving coalescence of Cordilleran and Laurentide ice over tially preserved, allowing an assessment of the stratigraphic a considerable distance of the Rocky Mountain front. relations between valley-fill sediments of different age. Bobrowsky (1989), Bobrowsky et al. (1991, 1992), Bobrow- sky and Rutter (1992), and Catto et al. (1996), on the other hand, concluded that the Late Wisconsinan Cordilleran ad- Study area vance was restricted, and they attributed the most extensive This study was conducted in the Charlie Lake map area Cordilleran advance to an earlier glaciation. (National Topographic System (NTS) 94A), which is located The Peace River valley in northeast British Columbia is within the Alberta Plateau subprovince of the northern ideally located for a study of interactions of Cordilleran and Interior Plains (Fig. 1). The area is a flat to gently rolling

# 2008 NRC Canada Hartman and Clague 551 plateau deeply incised by the valleys of the Peace River and Fig. 2. Topographic provinces of the study area (modified from its tributaries. Mathews 1978; reproduced and adapted with the permission of the Mathews (1978) identified three distinct topographic Minister of Public Works and Government Services Canada, 2007 provinces in the study area (Fig. 2): (1) uplands with steep and courtesy of Natural Resources Canada, Geological Survey of to gently rolling ridges and cuestas; (2) river valleys deeply Canada). incised into the uplands; and (3) benches with little relief adjacent to incised valleys. Uplands are areas of low to moderate relief where near-surface bedrock controls the topography. River valleys are incised into the upland topographic province. They range from broad, flat-floored features to V-shaped and canyon-like forms. Benches adjacent to incised river valleys are nearly flat and are thus morpho- logically distinct from the uplands. Depths to bedrock beneath the benches range from about 15 to 180 m. Gently northeast-dipping Cretaceous sedimentary rocks underlie the study area (Stott 1982). The rocks include marine shale and siltstone of the Smokey Group, sandstone and conglomerate of the Dunvegan Formation, and marine shale and minor sandstone of the Fort St. John Group. The Clear Hills and Chinchaga Hills are gravel-capped surfaces that lie 300–600 m above the level of the surrounding Interior Plain and dip gently to the northeast (Fig. 1). Math- ews (1978) inferred that the hills are erosional remnants of a peneplain that once extended across the study area.

Methods Exposures of Quaternary sediments were visited and described during the summers of 2002 and 2003 (Fig. 3). This task entailed locating intact exposures, defining and describ- face was interpolated over known and estimated bedrock ing sedimentary units, measuring elevations of unit contacts, surface points using inverse distance weighting. and collecting samples. Sediments were described and grouped using the litho- Paleovalleys, planation surfaces, and deep facies analysis method (Eyles et al. 1983). Characteristics bedrock basins and features that were noted include grain size, sorting, sedimentary structures, colour, clast lithology, compaction, Three relict fluvial drainage systems are graded to differ- jointing, weathering, sliding planes, rupture surfaces, and ent base levels, providing a framework for understanding the the angle of the exposure face. Basal and upper contacts of Quaternary stratigraphy and history of the study area. Post- units were also described, and fossils, erratics, striated sur- glacial Peace River has incised almost 50 m deeper into faces, fabric, and deformation features, if present, were bedrock than any of its precursors; the river today flows noted. over bedrock through most of the study area. Two incised The colours of moist sediment samples were determined bedrock straths at different elevations, and thus different using the Geological Society of America Rock Color Chart ages, are exposed in the valley walls of Peace River and its (Goddard et al. 1963). Pebble fabrics were determined by tributaries (Fig. 4). The straths and overlying fluvial sedi- measuring the trends and plunges of 50 (Woodcock 1977) ments have been identified in water well drill logs away rod-shaped clasts (a:b ‡ 2) from a small area of exposure. from valley wall exposures. A broad gravel-covered fluvial Elevations of unit contacts were determined using a baro- planation surface, which is not incised, occurs locally on metric altimeter and a laser range finder (Hartman 2005). the uplands at a higher elevation than the higher of the two Pink orthoclase-rich granite and granitoid gneiss pebbles, incised straths. which are exotic lithologies in western Canada, were col- The two incised straths are the floors of paleovalleys that lected for examination in the laboratory. are buried beneath up to 180 m of Pleistocene sediments. As Additional stratigraphic information was acquired from Mathews (1978) suggested, the paleovalleys define drainage water well drill logs, and geotechnical drill logs. The drill systems that pre-date the glaciations responsible for their logs were obtained from a database maintained by the BC burial. The straths are best delineated where they are near Ministry of Sustainable Resource Management. Geotech- and intersect the modern Peace River. Limited exposure along nical drilling and water well logs also provided data on the tributaries of Peace River precludes detailed assessment of elevation of the bedrock surface in drift-covered areas. the tributary paleovalleys beyond their lower reaches. Stratigraphic data were entered in a geographic informa- The lower incised strath is locally well exposed about tion system (GIS) to examine lithostratigraphic units in three 50 m above Peace River and has been encountered in water dimensions. The GIS was also used to interpolate a bedrock wells elsewhere. It slopes 0.0006 down to the east, a gra- surface and discern the form of buried paleovalleys. The sur- dient similar to that of modern Peace River. The higher in-

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Fig. 3. Locations of measured sections.

cised strath has the same gradient, is about 225 m above tion of the upper paleovalley is preserved, what remains indi- Peace River, and is exposed on the north side of the valley cates that it was almost four times as wide as the modern between the confluence of Peace and Moberly rivers and the Peace River valley and wider than the lower paleovalley. The British Columbia – Alberta boundary. It has also been en- lower paleovalleys of Peace and Beatton rivers are incised countered in water wells. The high broad gravel-covered into the higher paleovalley, as are the modern valleys of planation surface is 330 m above Peace River. Its extent is Peace, Beatton, and Alces rivers and many smaller tributaries. poorly known because it is exposed at only one location Deep bedrock basins occur below the Portage Mountain and is intercepted by only a few water wells, but it appears end moraine and near Lynx Creek (Fig. 5; Rutter 1977; Math- to slope 0.0005 to the east. ews 1978; Hartman 2005). The basins are close to the Rocky The lower paleovalley system is dominated by a large val- Mountain front, where a former lobe of the Cordilleran ice ley that trends eastward across the southern part of the study sheet flowed onto the Alberta Plateau. The location and depth area near the modern Peace River valley. Its tributaries are of the basins suggest that they were carved by a Cordilleran close to present-day Beatton, Pine, Moberly, Halfway, and outlet glacier. This interpretation is supported by the presence Doig rivers. The lower paleovalley system diverges from the of diamicton near the base of a drill hole through the Portage modern drainage along the canyons of Kiskatinaw, Beatton, Mountain end moraine (Rutter 1977). The basins are local- Doig, and Halfway rivers and along two reaches of Peace ized and do not appear to be structurally controlled. River—one near its confluence with Moberly River, and the other at the east edge of the study area. The lower paleoval- Quaternary stratigraphy leys are two to four times wider than their modern counter- parts and have flat floors and fairly steep walls. Fluvial deposits of the high planation surface The higher paleovalley system is more difficult to delineate Fluvial gravel overlies a bedrock strath that stands slightly than the lower one. It was only definitively identified on the higher than the adjacent plateau (Fig. 6; Table 1). Erosion north side of Peace River between Moberly River and the has removed most of this unit and lowered the surface of British Columbia – Alberta boundary. Although only a por- the uplands around its remnants.

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Fig. 4. Limits of buried paleovalleys. a.s.l., above sea level.

Fig. 5. Interpreted geology between drill-log M-1 near the Portage Mountain end moraine and drill-log M-2 at Lynx Creek. The bedrock surface in this area is below the level of past fluvial erosion. a.s.l., above sea level.

The unit is, on average, 6 m thick; its upper surface is Fluvial deposits of the upper paleovalley about 330 m above present-day Peace River. It is composed A younger fluvial gravel unit overlies the higher of two in- mainly of well-sorted, cross-bedded, pebble–cobble gravel cised bedrock straths, within the upper paleovalley of Peace with minor lenses of silt and sand. Cross-beds at section M- River (Fig. 6; Table 1). The strath is located on the north side 3 (Imperial gravel pit) record northward flow. All clasts are of Peace River valley between the confluence of Peace and of Cordilleran or local provenance. Moberly rivers and the British Columbia – Alberta boundary.

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Fig. 6. Representative stratigraphic sections from the four paleo-drainage systems. a.s.l., above sea level.

The unit has an average thickness of about 8 m, and its Table 1). They occur up to 613 m above sea level (a.s.l.) top is about 225 m above present-day Peace River. The within the upper paleovalley. dominance of lithofacies interpreted to be channel deposits This lithostratigraphic unit is dominated by lithofacies suggests that the sediments were deposited by a braided indicative of deposition from suspension and underflows in river (Miall 1977). The gravel consists mainly of Cordilleran a lake, including laminated mud and sand, massive sand, lithologies, notably quartzite, chert, sandstone, granitic and rippled sand, and trough cross-bedded sand. Dropstones and volcanic rocks, and carbonates. Eastern exposures near diamicton pods, some containing Canadian Shield clasts, Alces River also contain rare granite gneiss clasts derived increase in abundance toward the top of the unit. The tran- from the Canadian Shield. No gneiss clasts were found in sition upwards from ice-distal to ice-proximal lithofacies western exposures of the unit near Old Fort. indicates the approach of Laurentide ice.

Advance-phase glaciolacustrine deposits of the upper Pre-Late Wisconsinan Laurentide till? paleovalley Mathews (1978) hypothesized that tills of two Laurentide Up to 66 m of glaciolacustrine silt and sand conformably glaciations are present in exposures on the north side of overlie fluvial gravel of the upper paleovalley (Fig. 6; Peace River valley near Old Fort (Fig. 6). He assigned the

# 2008 NRC Canada Hartman and Clague 555 lower of the two tills to a pre-Late Wisconsinan glaciation. 8; Table 1). They record damming of the paleo-Peace valley An attempt was made to test this hypothesis by measuring and its tributaries early during the last glaciation. The unit the clast fabric of the lower till. A clast fabric from the is dominated by laminated mud and horizontal and drape- base of section SS-26 has a polymodal to girdle distribution laminated sands deposited from suspension and underflows with a dominant mode oriented 1208, perpendicular to the in ice-distal, deep-water environments (Gustavson et al. regional ice flow direction during the last glaciation 1975; May 1977; Ashley 1988; Wolfe and Teller 1995). (Mathews 1978). Striations on a boulder pavement at the Coarser sediments are present in the glaciolacustrine unit base of this unit trend 1128, similar to the direction of the near paleovalley walls. They include clast-supported mass- dominant fabric mode. However, at section SS-24, a clast ive diamicton deposited by subaqueous debris flows (May fabric from the base of a similar diamicton has a spread un- 1977) and cross-bedded, horizontally bedded, and rippled imodal distribution with a dominant mode of 2188, parallel sands deposited by energetic density underflows (Wolfe and to surface ice-flow indicators. Teller 1995). Similar sediments have been interpreted to be Two Laurentide tills directly overlie one another at sec- basin-marginal glaciolacustrine deposits (Ashley 1988; tion SS-50 near Old Fort. They can be distinguished by their Wolfe and Teller 1995). clast content, colour, and the presence of a recessive band at The dominant, laminated mud facies generally contains no the sharp undulating contact between the two units. The dropstones. However, at sections where the unit has not lower till has a strong bimodal fabric with clusters oriented been extensively eroded by Laurentide ice, dropstones and 2488 and 688. A large anvil-shaped boulder at the base of diamictic pods increase in abundance upward through the the lower till appears to have been lodged and has a flat unit. upper surface with two sets of striations, an older set oriented 2408 and a younger set oriented 2208. The fabric and Advance-phase glaciolacustrine sediments may extend striations are roughly parallel to the surface ice-flow indica- into the Rocky Mountain Foothills between Portage and tors. The upper till corresponds stratigraphically with the Bullhead mountains. The log of the drill hole through the Late Wisconsinan Laurentide till. Portage Mountain end moraine records silt, silty sand, and clayey sand overlying interbedded gravel, sand, and silt Western-provenance till (Campbell 1959). Based on its thickness and elevation Up to 60 m of till of western provenance is exposed in range, the gravel is assigned to fluvial deposits of the lower lower Halfway River valley (Fig. 6; Table 1). The high clast paleovalley. The overlying finer sediments likely record content (about 30%), silty sand matrix, and greyish-orange deposition in a lake dammed by advancing Laurentide ice. (10YR 7/4) colour distinguish the unit from Laurentide till The highest known elevation of the lower paleovalley (clast content typically 10%, clay-rich matrix, and olive glaciolacustrine unit is 632 m a.s.l. The lake thus extended grey (5Y 3/2) colour). Clasts are almost entirely of Cor- to at least this elevation at its maximum stage. dilleran or local provenance; only two possible Laurentide pebbles were found at section SS-75. A clast fabric taken Till of the last glaciation near the middle of the unit at section SS-75 has a strong un- imodal distribution with a primary eigenvector of 2218, par- Laurentide till is a widespread surface or near-surface unit allel to the orientation of a nearby gap in the mountains in the study area. It is a massive matrix-supported diamicton through which Peace River once flowed (Beach and Spivak with <15% clasts and a silt and clay matrix. Many of the 1943). The western-provenance till is overlain across an clasts are granite and granitoid gneiss derived from the unconformable contact by Late Wisconsinan outwash, glacio- Canadian Shield. lacustrine sediments, and till, and thus is older than the last Drumlins and flutings mapped by Mathews (1978) suggest glaciation. that the Laurentide ice sheet flowed to the south-southwest during the Late Wisconsinan. The flow indicators shift to Fluvial deposits of the lower paleovalleys the south-southeast in the southeast part of the study area, Fluvial gravel lies on the floors of the lower paleovalleys indicating an interaction between Cordilleran and Lauren- (Fig. 6; Table 1). It is well exposed in the walls of the mod- tide ice. Previously mentioned pebble fabric orientations ern Peace River valley and its larger tributaries (Fig. 7). and striated pavements measured at the base of the till at The unit is 13 m thick on average; its top is about 50 m two sections (SS-24 and SS-50) match the direction of above modern river level. It is interpreted to be channel regional ice flow. However, at sections SS-26 and SS-91, deposits of a braided river (Miall 1977). Gravel transported the fabrics are parallel to the local paleovalley axis and by the paleo-river is about the same size as that carried by transverse to the regional flow direction. These data may the modern river. Eastward flow is indicated by paleoflow be interpreted in two ways—either till from two separate measurements. The gravel is dominated by Cordilleran and glaciations are superposed at sections SS-26 and SS-91 local lithologies, but it contains rare clasts derived from the (Mathews 1978), or initial Laurentide ice flow into these Canadian Shield in exposures between Lynx Creek and the areas was topographically controlled (Catto et al. 1996). eastern boundary of the study area. The ice sheet that deposited the till extended west beyond the study area into the Rocky Mountain Foothills, but no Advance-phase glaciolacustrine deposits of the lower terminal moraine related to this advance has been identified. paleovalleys Canadian Shield erratics occur above the maximum limit of Up to 100 m of glaciolacustrine sediments conformably Glacial Lake Peace in east-draining valleys of the foothills overlie fluvial gravel of the lower paleovalleys (Figs. 6, 7, (Bobrowsky et al. 1991; Catto et al. 1996).

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Table 1. Characteristics of lithostratigraphic units recognized in the study area.

Avg. elev. above Thickness Lithostratigraphic unit Peace River (m) (m) Areal extent Dominant lithofacies Postglacial deposits Variable 17 (mean) valley slopes and gravel, matrix-supported floors diamicton, massive silt Late-glacial glaciolacustrine Variable 10 (mean) various stages of laminated mud deposits Glacial Lake Peace Till of the last glaciation Variable 12 (mean) Regional Matrix-supported diamicton

Advance-phase glaciolacustrine Variable 18 (mean) Lower paleovalley Laminated mud, massive, deposits of the lower paleovalley 100 (max) rippled, and trough cross- bedded silt and sand Fluvial deposits of the lower 50 13 (mean), Lower paleovalley Horizontally bedded gravel paleovalley 67 (max) Western-provenance till Variable 40 (mean), Lower Halfway Matrix-supported diamicton 60 (max) valley

Pre-middle Wisconsinan eastern- Variable 32 m (one Local Matrix-supported diamicton provenance till? locality only) Advance-phase glaciolacustrine Variable 33 (mean), Upper paleoalley Laminated mud, massive, deposits of the upper paleovalley 66 (max) rippled, and trough cross- bedded silt and sand Fluvial deposits of the upper 225 8 (mean) Upper paleovalley Horizontally bedded gravel paleovalley Fluvial deposits of the high plana- 330 6 (mean) Regional Horizontally bedded gravel tion surface

Late-glacial glaciolacustrine deposits in the study area established themselves at successively Glacial Lake Peace was impounded behind retreating lower elevations during middle and late Cenozoic time fol- Laurentide ice at the end of the last glaciation (Fig. 6; Math- lowing the Laramide Orogeny. Mathews (1989) noted local ews 1978, 1980). Sediments deposited in this lake consist dissected gravel-capped peneplains of early Oligocene to mainly of laminated mud with <5% dropstones. Several late Tertiary age in the Rocky Mountain Foothills and west- stages of Glacial Lake Peace have been identified from re- ern Interior Plains from Montana north to the Richardson lict shorelines, deltas, and spillways (Rutter 1977, Mathews Mountains in Yukon Territory. Remnants of lower drainage 1980, Woolf 1993). systems are probably Pleistocene in age. Successive drainage systems cross-cut and nest within one another, produc- Postglacial deposits ing a palimpsest of valley in-fills that provide a framework Fluvial gravel underlies terraces cut during postglacial on which a relative chronology of geologic events can be incision of the modern Peace River valley (Fig. 6). The placed. most prominent terrace is 45 m above present-day Peace Incision of the drainage systems is probably due to a River; its capping gravel is about 17 m thick. Postglacial combination of epeirogenic uplift (Pazzaglia et al. 2001; gravel is similar to older gravel units, except for its much McMillan and Heller 2002), increased erosion due to cli- greater proportion of eastern-provenance clasts. matic forcing in the late Cenozoic (Dethier 2001), and Landslide deposits are ubiquitous in Peace River valley base-level lowering resulting from drainage integration and and its major tributaries. Most of the deposits are massive glaciation (Pederson and Pazzaglia 2002). matrix-supported diamicton that is less compact than typical Laurentide till and generally has a lower clast content. Pre-Laurentide river system Recognition of postglacial landslide deposits, however, is Fluvial gravel of the high planation surface is the oldest based less on lithofacies than on geomorphology. Landslide post-Cretaceous lithostratigraphic unit in the study area. It deposits typically have hummocky surfaces with gentle to lacks eastern-provenance clasts and thus was deposited prior moderately inclined slopes. The surfaces of active landslides to the first Laurentide glaciation of the study area. The are marked by tension cracks, tilted trees, and sag ponds, gravel records a period of deposition following planation of and are located down-slope from scalloped head scarps. bedrock surfaces by an east- to north-flowing river system. Its wide distribution on low-relief upland bedrock surfaces Geologic history suggests deposition on a broad peneplain. Four drainage systems of different age, including the The gravel is younger than higher gravels of middle to modern drainage system, are recognized in the study area late Tertiary that cap peneplains at the west edge of the (Fig. 9). A fifth, older system is recorded by fluvial gravel Interior Plains, including the Clear Hills, Saddle Hills, and overlying an eastward-sloping peneplain east of the study Chinchaga Hills (Mathews 1978; Liverman 1989; Stott and area (Mathews 1978; Liverman 1989). East-draining rivers Aitken 1993). It is tentatively assigned a late Tertiary or

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Lithofacies Lower association contact Bounding units (above / below) Shield clasts Event Fluvial, colluvial Erosional None above / all units Yes, west to Rocky Holocene fluvial and Mountain front colluvial deposition Glaciolacustrine Conformable None above / till of the last glaciation Yes, west to Rocky Late Wisconsinan Lauren- over till Mountain front tide glacial retreat Glacial Erosional Late-glacial glaciolacustrine deposits or Yes, west to Rocky Late Wisconsinan none above / many different units below Mountain front Laurentide glaciation Glaciolacustrine Conformable Till of the last glaciation / fluvial deposits Yes, west to Portage Late Wisconsinan Lauren- of the lower paleovalley Mountain end tide glacial advance moraine Fluvial Erosional Advance-phase glaciolacustrine deposits of Yes, west to Lynx Middle Wisconsinan river the lower paleovalley / bedrock Creek system Glacial Erosional Outwash and advance-phase glaciolacus- No Pre-Middle Wisconsinan trine deposits of the lower paleovalley / Cordilleran glaciation bedrock Glacial Erosional Till of the last glaciation / advance-phase Yes, west to Old Pre-Middle Wisconsinan glaciolacustrine deposits of the upper Fort? Laurentide glaciation paleovalley Glaciolacustrine Conformable Till of the last glaciation / fluvial deposits Yes, west to Old Pre-Middle Wisconsinan of the upper paleovalley Fort Laurentide glacial advance Fluvial Erosional Advance-phase glaciolacustrine deposits of Yes, west to Golata Early or middle the upper paleovalley / bedrock Creek Pleistocene Fluvial Erosional Till of the last glaciation / bedrock No Pliocene or early Pleistocene river system early Pleistocene age because it pre-dates the oldest Lauren- integration of drainage were important causes of base-level tide glaciation of the study area and because it lies on highly lowering. dissected, fluvially eroded bedrock. Drainage subsequently became incised into this broad planation surface. Penultimate glaciation

Early nonglacial river system Laurentide advance The next younger deposit in the study area is fluvial Advance-phase glaciolacustrine sediments of the upper gravel of the upper paleovalley. It contains the same litholo- paleovalley conformably overlie nonglacial fluvial gravel. gies as the older, pre-glacial gravel unit, with one important They were deposited when Laurentide ice advanced up the exception. Rare clasts of granite and granitoid gneiss de- regional slope of the western Interior Plains and blocked rived from the Canadian Shield occur in exposures near east-flowing paleo-Peace River. The glaciolacustrine sedi- Alces River and Golata Creek and at section SS-109, but ments occur in outcrop west to Old Fort, but the lake not farther west. The Canadian Shield clasts must have reached to at least 613 m a.s.l. and thus extended farther been transported westward up the regional slope of the west. Interior Plains during an early Laurentide glaciation. Math- Advance-phase glaciolacustrine sediments have not been ews (1978) concluded that the clasts were transported as far dated, but they are older than the lower paleovalley system west as Golata Creek by Laurentide ice. Alternatively, they into which they are cut. The lower paleovalley is no younger could have been rafted on icebergs in a lake dammed by the than Middle Wisconsinan (see later in the text), con- Laurentide ice sheet somewhere to the east, or they could sequently the glaciolacustrine sediments filling the upper have been transported into paleo-Peace River valley by a paleovalleys, as well as the Laurentide ice sheet that im- northern tributary or by meltwater diverted along the pounded the lake in which these sediments accumulated, are Laurentide ice margin. At the very least, the presence of Early Wisconsinan or older. these clasts indicates that an early Laurentide advance Older advance-phase glaciolacustrine sediments and approached the study area. Canadian Shield clasts within the lower paleovalley fluvial This early glaciation may be younger than the Matuyama gravel provide definitive evidence for the penultimate Reversed Polarity Chron (0.78 Ma). Barendregt and Irving Laurentide advance. Although uncommon, shield clasts (1998) found that all glacial sediments on the central Interi- were recovered from lower paleovalley gravel as far west or Plains are normally magnetized and concluded that the as Lynx Creek, corroborating Mathew’s assertion that the Laurentide ice sheet first reached the western Interior Plains penultimate Laurentide glaciation transported eastern- after 0.78 Ma ago. provenance clasts farther west than an earlier Laurentide Glaciation may have been partly responsible for entrench- advance. ment of the high bedrock planation surface. Pederson and The western limit of the penultimate Laurentide advance Pazzaglia (2002) concluded that glacial repositioning and is unknown. No definitive landforms or till of the pen-

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Fig. 7. Contact between fluvial deposits of the lower incised paleovalley and Glacial Lake Mathews sediments in an exposure along Peace River between Halfway and Moberly rivers.

Fig. 8. Fluvial deposits of the lower paleovalley conformably over- the clasts were rafted on icebergs or carried by streams to lain by advance-phase glaciolacustrine sediment, section SS-13. the positions they are found today. The penultimate and last glaciations were separated by an interglaciation during which the lower paleovalley system developed. Based on this evidence, the penultimate event is Early Wisconsinan or older.

Cordilleran advance A till in lower Halfway River valley is the only western- provenance glacial unit found in the study area. It may have been deposited by the glacier that eroded the bedrock basins beneath the Portage Mountain end moraine and Lynx Creek. The basins contain thick glaciolacustrine sediments that are truncated by fluvial deposits of the lower paleovalley. These stratigraphic relations suggest that the early Cordilleran advance is older than the lower paleovalley. Furthermore, Cordilleran till is not present in any of the exposures between the Rocky Mountain front and Halfway River, making it unlikely that the Cordilleran till at Halfway River is Late Wisconsinan in age. This inference is in agreement with Bobrowsky (1989), Bobrowsky and Rutter (1992), and Catto et al. (1996), who assigned the early Cordilleran till to an Early Wisconsinan or older glaciation. ultimate glaciation have been found, thus the possibility that Canadian Shield clasts were transported into the study area The last interstade by an agent other than a glacier must be considered. Perhaps The lower paleovalley fluvial deposits record an east-

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Fig. 9. Cross-section A–A’ through Beatton and Peace River valleys showing the fluvial deposits of three relict drainage systems and overlying valley fills.

flowing river system. They are the oldest sediments in the than 26 450 ± 310 14C years BP, which is the age of a bison study area that have been securely dated. Mathews (1978) vertebra recovered from underlying fluvial gravel. recovered a mammoth tooth that yielded a radiocarbon age The approach of the Laurentide ice sheet is indicated by of 27 400 ± 580 14C years BP (GSC-2034) from paleovalley an upward transition from ice-distal to ice-proximal glacio- gravel beneath postglacial terrace gravel at Taylor. A bison lacustrine sediments in sections in the western part of the vertebra from the same unit at section SS-29 gave an age of study area. Diamictic pods containing Canadian Shield clasts 26 450 ± 310 14C years BP (Beta-188305). The two ages in- are restricted to the upper part of the glaciolacustrine se- dicate that Peace River was aggrading its channel within the quence. lower paleovalley late during the Middle Wisconsinan. The waters of Glacial Lake Mathews appear to have risen Ancestral Peace River flowed north of Portage Mountain quickly, inundating paleo-Peace valley without significant before the last glaciation. Fluvial gravel encountered between interruption. The transition from fluvial channel sedimenta- 527 and 547 m a.s.l. during drilling through the Portage tion to lacustrine sedimentation spans <1 m in most sections Mountain end moraine (Campbell 1959) is the same height (Fig. 8). above modern Peace River as gravel of the last interstade else- Fluvial gravel at some sections (SS-18, SS-19, SS-44, and where in the study area and probably was deposited by ances- possibly SS-47) records a minor lake regression followed by tral Peace River during the Middle Wisconsinan. Drainage renewed glaciolacustrine sedimentation shortly after Glacial north of Portage Mountain was blocked when Cordilleran ice Lake Mathews inundated the sites. The gravel occurs about built the end moraine there. During deglaciation, Peace River 3–4 m above the base of the glaciolacustrine sequence at was forced through a saddle south of Portage Mountain, where these sites, and it sharply and unconformably overlies it excavated Peace River Canyon (Beach and Spivak 1943). bedded and laminated mud. Possible explanations for a lake regression include (1) minor retreat of Laurentide ice, ex- Late Wisconsinan glaciation posing lower lake outlets that previously had been blocked by ice, (2) erosion of the lake outlet or development of new Glacial Lake Mathews and lower spillways when the lake overtopped drainage Advance-phase glaciolacustrine sediments record dam- divides across which channels were subsequently eroded by ming of east-flowing rivers by advancing Laurentide ice. out-flow, and (3) draining of the lake during an outburst The lake in which the sediments accumulated is here termed flood (jo¨kulhlaup). No other lake-lowering event is recog- ‘‘Glacial Lake Mathews’’ in recognition of the many contri- nized in Glacial Lake Mathews deposits. butions of the late Dr. W.H. Mathews to our understanding To our surprise, we found no exposures of deltaic sedi- of the Quaternary geology of western Canada. The sediments deposited by paleo-Peace River in Glacial Lake Math- ments have not been directly dated, but they are younger ews. However, a buried delta may be present beneath the

# 2008 NRC Canada 560 Can. J. Earth Sci. Vol. 45, 2008 end moraine in ancestral Peace River valley north of Portage time after 13 970 ± 170 14C years BP (TO-2742) (Catto et al. Mountain (Beach and Spivak 1943). Well sorted gravel 1996) and was well advanced or complete before 10 500 14C occurs from 481 to 527 m a.s.l. in the borehole through the years BP (Geertsema and Jull 2002). The rock-walled, in- end moraine (Campbell 1959). We correlate this unit with cised canyons of Beatton, Halfway, and Kiskatinaw rivers the lower paleovalley gravel. It is overlain by 66 m of inter- have a meandering planform. Meandering rivers typically stratified silt, sand, and gravel, which are likely Glacial are alluvial; lateral channel migration produces the meanders Lake Mathews deposits. (Miall 1992). The walls of the Beatton, Halfway, and Kiska- tinaw canyons are bedrock, and the meandering planform Late Wisconsinan Laurentide glaciation could not have developed after the rivers incised to their Bobrowsky et al. (1992) and Catto et al. (1996) found present levels. Rather, it developed when the rivers flowed Canadian Shield clasts up to about 950 m a.s.l. in the Rocky over the floor of Glacial Lake Peace soon after the lake Mountain Foothills west of the study area and argued that drained. Incision was so rapid that the rivers did not migrate the Laurentide ice sheet could not have ascended valleys in laterally, preventing the canyons from being widened and the foothills if it had been opposed by east-flowing Cordil- preserving the meandering form. leran ice. This observation is consistent with the conclusions of Bobrowsky (1989) and Bobrowsky and Rutter (1992) that Comparison with previous work the Cordilleran advance was restricted and that its limit in Peace River valley was at Portage Mountain. Radiocarbon Two models have been developed to explain the Quater- ages from Finlay River valley, a major tributary of Peace nary stratigraphy and landforms of the westernmost Alberta valley, bracket the time of Late Wisconsinan Cordilleran Plateau (Bobrowsky et al. 1991). One model is based on the glaciation there to between 15 180 ± 100 14C years BP (TO- work of Rutter (1976, 1977) and Mathews (1978, 1980). It 708) and 10 100 ± 90 14C years BP (GSC-2036) (Bobrowsky features two or three Laurentide advances, three or four Cor- 1989). A series of Middle Wisconsinan radiocarbon ages on dilleran advances, and coalescence of the Cordilleran and sediments underlying till indicate that only one advance Laurentide ice sheets during one glaciation. The second occurred in Finlay River valley during the Late Wiscon- model is based on Bobrowsky (1989) and includes one sinan. Laurentide advance and two Cordilleran advances without These data are inconsistent, however, with the interpreta- coalescence of the Laurentide and Cordilleran ice sheets. tion that flutings east of Fort St. John and adjacent to the The stratigraphy presented in this study (Fig. 10) supports, mountain front formed beneath coalescent Laurentide and with some modification, the interpretation of Mathews Cordilleran ice during the last glaciation (Mathews 1978). If (1978, 1980), specifically that there were three Laurentide the flutings are indeed Late Wisconsinan, Cordilleran ice and two Cordilleran advances in the region. This stratigra- probably reached east of Portage Mountain, albeit perhaps phy is discussed in the following two sections in the context only for a short time. Restricted Cordilleran glaciation in of previous stratigraphic work. Peace valley also appears to be inconsistent with the evidence presented by Bednarski (1999, 2001) and Bednarski Laurentide stratigraphy and Smith (2007) for overtopping of high ridges in the Several researchers have argued that the Laurentide ice Rocky Mountains northwest of the study area by the Cordil- sheet reached Edmonton, Watino, and areas to the south leran ice sheet (see below). and west only once in the Pleistocene, during the Late Wis- Glacial Lake Peace formed behind the retreating Lauren- consinan (Westgate et al. 1971, 1972; Catto 1984; Liverman tide ice sheet. Wood recovered from the contact between 1989; Liverman et al. 1989; Young et al. 1994). The Late Wisconsinan Laurentide till and overlying Lake Peace stratigraphy reported in this study requires three Laurentide sediments near Fort St. John yielded a radiocarbon age of glacial events separated by interglaciations or long inter- 13 970 ± 170 14C years BP (TO-2742) (Catto et al. 1996), stades. Only the last of the three glacial events is Late Wis- which is a minimum for local Laurentide deglaciation. consinan. Glacial Lake Peace drained in stages, perhaps with rapid Laurentide glaciations were centred in northern Quebec lowering of the lake between stages. The lake was east of and Labrador, Keewatin, and the Foxe Basin (Dyke et al. Fort St. John before 10 770 ± 120 14C years BP (SFU-454), 2002; Kleman et al. 2002; Dyke 2004). To reach the study which is the age of the lowest cultural layer at the Charlie area, ice had to flow southwest to south-southwest from the Lake cave archaeological site (Fladmark et al. 1988). Geert- Keewatin ice dome. Flutings in the study area that match sema and Jull (2002) and Jull and Geertsema (2006) report a this flow direction were noted by Mathews (1978, 1980). series of radiocarbon ages on charcoal layers in a debris- Ice flowing into the study area from the northeast or north- flow fan on a prominent river terrace at Bear Flat. The char- northeast during pre-Late Wisconsinan glaciations may have coal layers increase in age consistently with depth to about terminated short of Edmonton, 500 km to the southeast, and 10 500 14C years BP, indicating that Glacial Lake Peace had Watino, 160 km to the east-southeast. drained and Peace River had incised its modern valley to the Two, pre-Late Wisconsinan Laurentide advances are in- level of the terrace before 10 500 14C years BP. ferred in the study area from the presence of Canadian Shield erratics in gravels of the upper and lower paleovalley Postglacial Peace River valley systems and from glaciolacustrine sediments. The penulti- Peace River may have excavated its modern valley rapid- mate Laurentide advance is interpreted to have been the ly, soon after deglaciation. Entrenchment did not commence more extensive of the two based on the distribution of shield until Glacial Lake Peace drained from the study area, some- erratics in the paleovalley gravels.

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Fig. 10. Stratigraphy of the study area (modified from Mathews 1978; reproduced and adapted with the permission of the Minister of Public Works and Government Services Canada, 2007 and courtesy of Natural Resources Canada, Geological Survey of Canada).

The limits of these pre-Late Wisconsinan advances must sheds would not have introduced shield clasts into the be north and east of Edmonton and Watino, yet close sediments on which the stratigraphies are based. enough to the study area to introduce shield clasts into the The distribution of the Canadian Shield clasts on the paleo-Peace River drainage and to impound lakes in its val- westernmost Interior Plains and the paleogeography of the leys. No Laurentide glacier extended beyond the most distal watersheds in which they occur constrain pre-Late Wiscon- shield clasts that it transported, but the locations of the sinan Laurentide ice limits. An early Laurentide glacial clasts do not necessarily delineate the ice margin. The pres- advance was extensive enough to deposit shield clasts in the ence of eastern-provenance clasts in paleovalley fluvial watersheds of northern tributaries of ancestral Peace River, gravels merely indicates that Laurentide ice entered the including ancestral Beatton River. This early Laurentide watershed of the paleo-drainage system. advance did not cross the drainage divide that separates an- The interpretation presented in this study can be recon- cestral Beatton and Halfway rivers, nor did it extend south ciled with those of Catto (1984), Liverman (1989), Liverman and west of Watino. The penultimate Laurentide glacial et al. (1989), and Young et al. (1994) by considering the pa- advance extended west at least as far as the British Colum- leogeography of the rivers that deposited the sediments on bia – Alberta boundary because ancestral Peace River in the which these researchers have based their stratigraphies and study area was dammed by this advance. In addition, Cana- within which Canadian Shield clasts are absent. All stratigra- dian Shield clasts are present in the lower paleovalley gravel phies from which only a single Laurentide advance has been as far west as Lynx Creek. postulated come from north- and east-draining watersheds The pre-Late Wisconsinan, Laurentide glacial history pro- south and west of Watino and Edmonton. Pre-Late Wiscon- posed here is similar to that of Mathews (1978). It is also sinan advances that terminated north and east of these water- consistent with the finding of Liverman et al. (1989) that

# 2008 NRC Canada 562 Can. J. Earth Sci. Vol. 45, 2008 the early Laurentide advances recognized by Mathews valley northwest of the study area. The upper part of Finlay (1978) did not reach Edmonton and Watino. valley remained ice-free until at least 15 180 ± 100 14C years BP (TO-708). Cordilleran ice thus advanced to its Cordilleran stratigraphy eastern limit and retreated significantly in <5000 years. The stratigraphy presented in this paper argues for two Bobrowsky (1989) also argued that the western- Cordilleran glacial advances—an earlier advance sometime provenance clasts noted by Mathews (1978) probably were prior to the Middle Wisconsinan, and a Late Wisconsinan transported and deposited by a pre-Late Wisconsinan Cordil- advance that reached at least as far east as Portage Moun- leran glacier and were subsequently entrained by Late tain. Cordilleran till has not been found east of Portage Wisconsinan Laurentide ice and deposited as part of its till. Mountain, but Mathews (1978) argued that the Cordilleran Catto et al. (1996) suggested that the marked decrease in glacier in Peace valley during the Late Wisconsinan was eastern-provenance lithologies in Mathews’ mixing zone is extensive and that it merged with the Laurentide ice sheet, due more to entrainment of a greater amount of surficial depositing a single till consisting of mixed eastern and west- sediment by the Laurentide ice sheet in this area than to co- ern lithologies. He was unable to distinguish Cordilleran and alescence of Laurentide and Cordilleran ice. Laurentide tills, which he attributed to the absence of a dis- Canadian Shield erratics occur in east-draining valleys in tinct boundary between the two till sheets and to a shifting the Rocky Mountain Foothills to 950 m a.s.l. (Bobrowsky et zone of interaction between the two glaciers. He noted that al. 1991, 1992; Catto 1991; Catto et al. 1996). This eleva- the eastern-provenance clasts decrease significantly in a tion is higher than the upper limit of Glacial Lake Peace, narrow zone in the western third of the study area, therefore the erratics must have been deposited by the Lau- coincident with an increase in the abundance of western- rentide ice sheet. In addition, Laurentide ice could not have provenance clasts. This area coincides with the zone of co- ascended the valleys in contact with Cordilleran ice. alescence of Cordilleran and Laurentide ice delineated by If Bobrowsky (1989), Bobrowsky and Rutter (1992), and flutings (Mathews 1978). The flutings suggest that Cordil- Catto et al. (1996) are correct, the flutings noted by Math- leran ice emanating from ancestral Peace valley was de- ews (1978) can only be a product of coalescence of the Lau- flected to the southwest by Laurentide ice. rentide and Cordilleran ice sheets prior to the Late Several researchers have presented evidence for an exten- Wisconsinan. This scenario, however, requires that the flut- sive Cordilleran advance onto the plains and foothills adja- ings survive overriding by the Laurentide ice sheet during cent to the Rocky Mountains during the Late Wisconsinan the Late Wisconsinan. An argument to this effect was made (Jackson et al. 1997; Bednarski 1999, 2001; Dyke et al. by Catto et al. (1996) to explain the Redwillow Fluting fan 2002; Dyke 2004; Bednarski and Smith 2007). Jackson et south of the study area. They argued that the Redwillow al. (1997) showed that the Foothills Erratics Train was Fluting fan formed at the base of a tongue of pre-Late deposited during the Late Wisconsinan by coalescence of Wisconsinan Cordilleran ice that extended from the Rocky Laurentide and Cordilleran ice on the westernmost Interior Mountains through Redwillow valley and onto the Alberta Plains. The erratics extend more than 1000 km from Atha- Plateau. The flutings are overlain locally by Late Wiscon- basca valley in the north to Lethbridge in the south. Dyke sinan Laurentide till. et al. (2002) suggested that coalescence of the ice sheets Any coalescence of the Cordilleran and Laurentide ice in over such a distance would not be possible without the ice the study area during the Late Wisconsinan must have been sheets also coalescing in the vicinity of Peace River. This brief and would have ended with rapid retreat of Cordilleran assertion is, however, debatable. Significant coalescence of ice to Portage Mountain. At the same time, the Laurentide the Cordilleran and Laurentide ice sheets south of Athabasca ice sheet may have spread westward into the Rocky Moun- valley does not require coalescence more than 400 km north tain Foothills (Bednarski and Smith 2007). of the head of the Foothills Erratics Train. In summary, the pattern of restricted Late Wisconsinan Bednarski (1999, 2001) and Bednarski and Smith (2007) Cordilleran glaciation envisioned by Bobrowsky (1989), have argued for an extensive Late Wisconsinan Cordilleran Bobrowsky and Rutter (1992), and Catto et al. (1996) is in- ice sheet in the Rocky Mountains northwest of the study compatible with the more extensive glacial model of area. They found striae on mountain ridges that trend per- Bednarski (1999, 2001) and Bednarski and Smith (2007). pendicular to the ridge crests. The implication is that the The area where Bednarski and Smith worked and the Rocky striae were produced by ice flowing unhindered by topogra- Mountain Trench, studied by Bobrowsky and Rutter, are 36 phy. Cl exposure dating of the striated surfaces suggests separated by the Rocky Mountains, but such a fundamental that they were deglaciated between 14 020 ± 760 and difference in extent of Late Wisconsinan Cordilleran ice 13 100 ± 1560 cal years ago. The presence of limestone and cannot be explained by the different locations of the two schist erratics of western provenance on the plains east of study areas. Our study fails to resolve this issue. Late Wis- the Rocky Mountains supports extensive Late Wisconsinan consinan Cordilleran ice may have reached only as far east Cordilleran glaciation. Bednarski (1999, 2001) and Bednar- as Portage Mountain, or it may have advanced about 50 km ski and Smith (2007) also noted eastern-provenance erratics farther east, where it coalesced briefly with Laurentide ice. and Laurentide till in east-draining valleys of the Rocky Mountain Foothills, and suggested that Laurentide ice advanced into these valleys shortly after Cordilleran ice re- Conclusion treated from them. Four relict drainage systems have been identified in the Bobrowsky (1989) presented arguments against extensive Peace River area in northeast British Columbia. The two Late Wisconsinan Cordilleran glaciation in the Finlay River youngest relict drainages are buried valleys. Each of the

# 2008 NRC Canada Hartman and Clague 563 two valley fills comprises a basal fluvial gravel unit and an Columbia Ministry of Energy, Mines and Petroleum Resources, overlying, thicker glaciolacustrine mud unit. The glacio- Paper 1992-1, pp. 363–374 lacustrine sediments are overlain across an erosional contact Campbell, D.D. 1959. Portage Mountain Dam (sire 3A) drill logs. by till or postglacial fluvial deposits. In Peace River Hydro-Electric Project — Geology. Peace River The valley-fill sediments of the study area provide evi- Power Development Company Limited, Vancouver, BC. dence for three Laurentide and two Cordilleran glaciations. Catto, N.R. 1984. Glacigenic deposits at the Edmonton Convention The glaciations are separated by interglaciations or lengthy Centre, Edmonton, Alberta. Canadian Journal of Earth Sciences, interstades, during which the east-flowing drainage was 21: 1473–1482. re-established and rivers achieved maturity. The earliest Catto, N.R. 1991. Quaternary geology and landforms of the eastern Peace River region, British Columbia NTS 94A/1,2,7,8. British Laurentide advance was less extensive than the penultimate Columbia Ministry of Energy, Mines and Petroleum Resources, advance, and the penultimate advance, in turn, was less Open File Report 1991-11. extensive than the Late Wisconsinan advance. An advance Catto, N.R., Liverman, D.G.E., Bobrowsky, P.T., Rutter, N., and of the Cordilleran ice sheet reached beyond the Rocky Mandryk, C.A.S. 1996. Laurentide, Cordilleran, and Montane Mountain front, but the extent of the Cordilleran glacier in glaciation in the western Peace River – Grande Prairie region, Peace River valley during the Late Wisconsinan remains Alberta and British Columbia, Canada. Quaternary International, uncertain. 32: 21–32. Dethier, D.P. 2001. Pleistocene incision rates in the western United Acknowledgements States calibrated using Lava Creek B Tephra. Geology, 29: 783– Financial support for this study was provided by the Natural 786. doi:10.1130/0091-7613(2001)029<0783:PIRITW>2.0.CO;2. Sciences and Engineering Research Council of Canada Dyke, A.S. 2004. An outline of North American deglaciation with (NSERC) through a Discovery Grant to John Clague and by emphasis on central and northern Canada. In Quaternary glacia- the Geological Survey of Canada courtesy of Stephen Evans. tions: extent and chronology. Edited by J. Ethers and P.L. Virginia Pow, Bryan Hartman, John Orwin, Kenna Wilkie, Gibbard. Elsevier, Amsterdam, The Netherlands, pp. 373–424. Dyke, A.S., Andrews, J.T., Clark, P.U., England, J.H., Miller, G.H., Britta Jensen, and Tracy Arsenault assisted in the field. Shaw, J., Veillette, J.J., and Mix, A.C. 2002. The Laurentide and Jonathan Driver identified the dated interstadial bison verte- Innuitian ice sheets during the last glacial maximum. Quaternary bra. Reviews of the paper by Norm Catto, Duane Froese, Rob- Science Reviews, 21: 9–31. doi:10.1016/S0277-3791(01)00095-6. ert Gilbert, and Vic Levson contributed to its improvement. Eyles, N., Eyles, C.H., and Miall, A.D. 1983. Lithofacies types and vertical profile models; an alternative approach to the descrip- References tion and environmental interpretation of glacial diamict and Ashley, G.M. 1988. Classification of glaciolacustrine sediments. In diamictite sequences. Sedimentology, 30: 393–410. doi:10.1111/ Genetic classification of glacigenic deposits. A.A. Balkema, j.1365-3091.1983.tb00679.x. Rotterdam, The Netherlands, pp. 243–260. Fladmark, K.R., Driver, J.C., and Alexander, D. 1988. The Paleo- Barendregt, R.W., and Irving, E. 1998. Changes in the extent of North indian Component at Charlie Lake Cave (HbRf-39), British American ice sheets during the late Cenozoic. Canadian Journal of Columbia. American Antiquity, 53: 371–384. doi:10.2307/ Earth Sciences, 35: 504–509. doi:10.1139/cjes-35-5-504. 281025. Beach, H.H., and Spivak, J. 1943. The origin of the Peace River Geertsema, M.E., and Jull, A.J.T. 2002. Charcoal and sedimen- Canyon, British Columbia. American Journal of Science, 241: tation record from a fan at Bear Flat, northeastern British 366–376. Columbia. Geological Society of America, Abstracts with Pro- Bednarski, J.M. 1999. Preliminary report on mapping surficial grams, 34(6): 319. geology of Trutch map area, northeastern British Columbia. In Goddard, E.N., Trask, P.D., DeFord, R.K., Rove, O.N., Singewald, Current research. Geological Survey of Canada, Paper 1999-A, J.T., Jr., and Overbeck, R.M. 1963. Rock Color Chart. Geologi- pp. 35–43. cal Society of America, New York, NY. Bednarski, J.M. 2001. Drift composition and surficial geology of Gustavson, T.C., Ashley, G.M., and Boothroyd, J.C. 1975. Deposi- the Trutch map area (94G), northeastern British Columbia. 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