Insight into the character of palaeo-ice-flow in upland regions of mountain valleys during the last major advance (Vashon Stade) of the Cordilleran Ice Sheet, southwest British Columbia, Canada

Lian, O. B. & Hicock, S. R. 2009: Insight into the character of palaeo-ice-flow in upland regions of mountain valleys during the last major advance (Vashon Stade) of the Cordilleran Ice Sheet, southwest British Columbia, Canada. Boreas, 10.1111/j.1502-3885.2009.00123.x. ISSN 0300-9483. A detailed glacial geological study was done on Vashon till, formed during the last (Fraser) glaciation, in upland areas of two relatively short and narrow mountain valleys which open onto the Fraser Lowland in southwest British Columbia. The orientation and association of glaciotectonic structures in till and bedrock, a-axis fabrics of stones in till and abrasion features, indicate that Vashon till formed initially by lodgement and that brittle deformation processes dominated at least during the latter stages of glaciation. The presence of local glacigenic bedrock quarrying suggests that ice flow experienced localized enhanced compressive flow along valley sides. These observations indicate that ice flow was relatively slow and they contrast with a previous study of bedrock geomorphology undertaken in some larger south Coast Mountains valleys and a model of ice-flow velocity in the Puget Lowland that suggest rapid ice flow. This indicates that either ice-flow conditions in the larger valleys were different from those in the valleys studied here, or that the observations from our study reflect subglacial conditions following the Last Glacial Maximum (LGM), but immediately prior to deglaciation when ice had thinned and slowed. If the latter scenario is correct, and if processes inferred from this study were also common along the upland parts of other southwest Coast Mountains valleys after the LGM, then the rate at which ice was supplied to lowland piedmont glaciers would have been reduced, and this may have accelerated decay of the southwest margin of the last Cordilleran Ice Sheet.