SUBGLACIAL DYNAMICS OF THE LAST CORDILLERAN ICE SHEET NEAR ITS TRANSITION FROM THE COAST MOUNTAINS TO ADJACENT LOWLANDS, SOUTH-WESTERN BRITISH COLUMBIA

During the Pleistocene the Canadian Cordillera were repeatedly covered by large ice sheets that had a profound effect on global sea level and climate. Although there has been considerable sedimentologic and stratigraphic work undertaken to understand the extent of the last Cordilleran Ice Sheet, and the timing of its advance and retreat, comparatively little work has been done to reconstruct subglacial conditions that can be used to understand the nature of glacier flow. Knowledge of this parameter is crucial to developing three-dimensional models of ice sheet geometry and behaviour.

One of the most informative regions of the last Cordilleran Ice Sheet was near its south-west margin, where ice from accumulation areas in the Coast Mountains drained into adjacent lowlands. Existing sedimentologic and glacial geologic evidence from the Fraser and Puget lowlands suggest that ice flow in these regions was enhanced by a soft (commonly wet and muddy) bed of unlithified sediment that provided little resistance to flow. This in turn suggests that ice in the lowlands was undergoing extending (rapid) flow, which would have resulted in relatively low ice surface profiles. In contrast, very little is known about the nature of ancient ice flow in the mountain valleys that open onto the lowlands. In this paper we present glacial geologic evidence from subglacial till in valleys in the Pacific Range of the Coast Mountains (immediately north of Vancouver) which suggests that ice flowing in the valleys was moving over a substrate (till) that was deforming mainly by brittle deformation (simple shear) and indicates effective transfer of shear stress from the ice to the substrate. This in turn suggests less extending flow, and that the mountain valleys acted as "bottlenecks" for ice draining accumulation areas in the back-ranges.