Pleistocene Glaciation of British Columbia
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See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/241070675 Pleistocene Glaciation of British Columbia Article in Developments in Quaternary Science · December 2011 DOI: 10.1016/B978-0-444-53447-7.00044-1 CITATIONS READS 28 3,142 2 authors: John J. Clague Brent C. Ward Simon Fraser University Simon Fraser University 458 PUBLICATIONS 11,310 CITATIONS 67 PUBLICATIONS 522 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Sensitive clay landslides View project Landslide-generated waves View project All content following this page was uploaded by John J. Clague on 23 December 2017. The user has requested enhancement of the downloaded file. Provided for non-commercial research and educational use only. Not for reproduction, distribution or commercial use. This chapter was originally published in the book Developments in Quaternary Science, Vol.15, published by Elsevier, and the attached copy is provided by Elsevier for the author's benefit and for the benefit of the author's institution, for non- commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who know you, and providing a copy to your institution’s administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial From: John J. Clague and Brent Ward, Pleistocene Glaciation of British Columbia. In J. Ehlers, P.L. Gibbard and P.D. Hughes, editors: Developments in Quaternary Science, Vol. 15, Amsterdam, The Netherlands, 2011, pp. 563-573. ISBN: 978-0-444-53447-7. © Copyright 2011 Elsevier B.V. Elsevier. Author's personal copy Chapter 44 Pleistocene Glaciation of British Columbia John J. Clague* and Brent Ward Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia, V5A 1S6 Canada *Correspondence and requests for materials should be addressed to John J. Clague. E-mail: [email protected] 44.1. INTRODUCTION for establishment of the ice sheet involves its inception in an extended alpine phase that, either progressively or more British Columbia lies within the mid-latitudes of the North likely episodically, leads to establishment of a continuous Hemisphere adjacent to the Pacific Ocean, which is the mois- ice cover over the interior of the province during an ice- ture source for present and past glaciers in the region. The sheet phase (see below). region is dominated by northwest-trending mountain ranges, rolling plateaus, and, on the west, coastal lowlands. The highest peaks in British Columbia are in the St. Elias Moun- 44.2. CHARACTER AND EXTENT OF tains (Mt. Fairweather, 4671 m a.s.l.), the southern Coastal CORDILLERAN ICE SHEET Mountains (Mount Waddington, 4016 m a.s.l.), and the southern Rocky Mountains (Mount Robson, 3954 m a.s.l.). The Cordillera of western Canada was repeatedly envel- The high mountains of British Columbia today support oped by a continental ice sheet, known as the Cordilleran valley and cirque glaciers and some ice caps. The total cur- ice sheet, during the Pleistocene and latest Pliocene (Flint, rent ice cover in British Columbia is about 26,000 km2, 1971; Clague, 1989; Jackson and Clague, 1991). At its max- which is probably close to the minimum value for the entire imum extent, the Cordilleran ice sheet and its satellite gla- Quaternary. ciers covered almost all of British Columbia, as well as Most of our knowledge of glaciation in British Colum- southern Yukon Territory and southern Alaska. It extended bia comes from the last period of ice-sheet glaciation south into the northwestern conterminous United States (the Late Wisconsinan; approximately coincident with (Fig. 44.2). The ice sheet, to a considerable extent, was con- Marine Isotope Stage (MIS) 2). Patchy stratigraphical evi- fined between the high mountain ranges bordering the dence exists for earlier Pleistocene glaciations, but much of Canadian Cordillera on the west and east, but large areas the record for these evidence was eroded by Late Wiscon- on the east flank of the Rocky Mountains and west of the sinan glaciers or buried beneath their deposits. Thus our Coast Mountains were also covered by ice. Glaciers in sev- understanding of Pleistocene glaciation in the region may eral bordering mountain ranges, such as the Olympics, Cas- be biased by temporal filtering of events. Nonetheless, cade, and Mackenzie Mountains and the Queen Charlotte the record of the last Cordilleran ice sheet is well preserved, Range, were more or less independent of the ice sheet, even within the range of radiocarbon dating, and readily inter- at times of maximum ice cover. pretable. From this record, we now understand that a true The last Cordilleran ice sheet attained its maximum size ice sheet, formed by coalescence of glaciers over the inte- in British Columbia where it was up to 900 km wide and rior of the province, was a rarely achieved during the Pleis- reached to 2000–3000 m elevation over the plateaus of tocene. Through nearly all the Pleistocene, the plateaus of the interior (Wilson et al., 1958). When fully formed, the interior British Columbia were not covered by an ice sheet; ice sheet in British Columbia probably had the shape of instead, glaciers were confined to high mountains and inter- an elongate dish, with gentle slopes in the interior region montane valleys. The style of glaciation over these long and steeper slopes at the periphery. It closely resembled periods was alpine; the signature of long-lasting alpine gla- the present-day Greenland ice sheet at such times. More ciation is strong in all high mountain ranges in British commonly, the interior of the ice sheet had an irregular, Columbia (Fig. 44.1). undulating surface, with several ice divides that shifted The development of the Cordilleran ice sheet is, of through time. These ice divides were subordinate to the course, rooted in alpine glaciation. The current paradigm main divide along the axis of the Coast Mountains. Developments in Quaternary Science. Vol. 15, doi: 10.1016/B978-0-444-53447-7.00044-1 ISSN: 1571-0866, # 2011 Elsevier B.V. All rights reserved. 563 Author's personal copy 564 Quaternary Glaciations - Extent and Chronology FIGURE 44.1 Glaciated landscape, southern Coast Mountains, British Columbia. The erosional landformsvisiblein thisphotographare typicalofa landscape sculpted by alpine glaciation and include cirques, areˆtes, horns, and U-shaped val- leys. Although this area was also covered by the Cordilleran ice sheet for up to a few thousand years during the Late Wisconsinan, alpine glaciers and mountain ice caps were much more important in shaping the landscape during the Quaternary (Province of British Columbia). In western British Columbia, ice streamed down fjords deteriorated early during each glaciation (Fig. 44.3B; Kerr, and valleys in the coastal mountains and covered large areas 1934; Davis and Mathews, 1944; Fulton, 1991). With con- of the Pacific continental shelf. At these times, parts of the tinued cooling and an increase in precipitation, glaciers British Columbia continental shelf were exposed due to expanded and coalesced to form a more extensive cover eustatic lowering of sea level driven by the growth of con- of ice in mountains (Fig. 44.4B). The glaciers advanced tinental ice sheets on land. Some lobes at the western mar- out of the mountains and across plateaus and lowlands gin of the ice sheet extended to the shelf edge where they (Fig. 44.4C), eventually coalescing to form an ice sheet that calved into deep water. Glaciers issuing from the southern covered most of British Columbia and adjacent areas Coast Mountains and Vancouver Island Ranges coalesced (Figs. 44.3C and 44.4D). During this period, which spanned over the Strait of Georgia to produce a great outlet glacier thousands of years, the major mountain ranges remained the that flowed far into Puget Lowland in Washington State principal sources of ice, and ice flow was controlled by (Waitt and Thorson, 1983). Glaciers streaming down val- topography. Ice thickened to such an extent during the final leys farther to the east likewise terminated as large lobes phase of glaciation that one or more domes became estab- in eastern Washington, Idaho, and Montana (Waitt and lished over the interior of British Columbia, with surface Thorson, 1983). flow radially away from their centres. This full continental On the east, ice flowed from the British Columbia interior ice sheet phase of glaciation was rarely achieved. The tran- and the Rocky Mountains and locally coalesced with ice orig- sition into this final phase was accompanied by a local inating in the Keewatin sector of the Laurentide ice sheet over reversal of ice flow in the Coast Mountains, as the ice divide the Rocky Mountain Foothills (Clague, 1989). These ice shifted from the mountain crest eastwards to a position over masses coalesced only rarely,at timesofmaximumglaciation, the British Columbia interior (Stumpf et al., 2000). A com- most recentlyabout 17,000 years ago (Jackson et al., 1999). At parable westward shift and reversal of flow may also have other times, an ice-free zone (the “Ice-free corridor”) existed occurred locally in the Rocky Mountains. The flow rever- between the Cordilleran and Laurentide ice sheets. sals resulted from the build-up of ice in the interior to levels higher than the main accumulation areas in the flanking mountains. 44.3. GROWTH AND DECAY OF The model outlined in the preceding paragraph provides a CORDILLERAN ICE SHEET framework for conceptualising the growth of the Cordilleran ice sheet, but the actual history of the ice sheet is more com- The Cordilleran ice sheet nucleated in the high mountains of plicated (Clague, 1989).