Multiphase flow of the late Wisconsinan Cordilleran ice sheet in western Canada

In central British Columbia, ice flow during the late Wisconsinan Fraser gl aciation (ca. 25-10 ka) occurred in three phases. The ice expansion phase o ccurred during an extended period when glaciers flowed westward to the Paci fic Ocean and east-southeastward onto the Nechako Plateau from ice centers in the Skeena, Hazelton, Coast, and Omineca Mountains. Initially, glacier h ow was confined by topography along major valleys, but eventually piedmont and montane glaciers coalesced to form an integrated glacier system, the Co rdilleran ice sheet. In the maximum phase, a Cordilleran ice divide develop ed over the Nechako Plateau to 300 km inland from the Pacific coast. At thi s time, the surface of the ice sheet extended well above 2500 m above sea l evel, and flowed westward over the Skeena, Hazelton, and Coast Mountains on to the continental shelf and eastward across the Rocky Mountains into Alber ta. In the late glacial phase, a rapid rise of the equilibrium line caused ice lobes to stagnate in valleys, and restricted accumulation centers to hi gh mountains. Discordant directions in ice how are attributed to fluctuations of the ice divide representing changes in the location of accumulation centers and ice thickness. Ice centers probably shifted in response to climate, irregular growth in the ice sheet, rapid calving, ice streaming, and drainage of prog lacial and subglacial mater bodies. Crosscutting ice-flow indicators and pr eservation of early (valley parallel) flow features in areas exposed to lat er (cross-valley) glacier erosion indicate that the ice expansion phase was the most erosive and protracted event.