THE CORDILLERAN ICE SHEET

The Cordilleran Ice Sheet (CIS) was an interconnected mass of valley and piedmont glaciers and mountain ice sheets that repeatedly covered British Columbia, Yukon and northwestern United States. Most of our knowledge of the CIS comes from the last period of ice-sheet glaciation, coincident with Marine Isotope Stage (MIS) 2. Glacial events prior to this are much more difficult to resolve because of the fragmentary nature of the deposits and poor dating control. The most continuous record of Early and Middle Pleistocene glaciation is preserved in Yukon and Alaska, where drift sheets of different ages are exposed at the surface. Here the oldest CIS glaciation has been dated to ~2.6 Ma with volcanic rocks and tephras providing age constraint on other Early and Middle Pleistocene glaciations. The record in British Columbia and northwest Washington is more fragmental. However, taken together, these data indicates the Early and Middle Pleistocene record beginning to approach that of the Marine Isotope record.

Differentiation of MIS 6 and 4 glaciations in most areas is problematic due to a lack of dating control. An exception is in Alaska and Yukon where TCN boulder dating of penultimate glacial deposits indicate MIS 4, while in other areas tephrochronology indicates a MIS 6 age. A significant MIS 4 glaciation of the southern CIS has been confirmed in an ocean core as well as by stratigraphic studies in NW Washington.

The last CIS attained its maximum size in British Columbia where it was up to 900 km wide and was 2000-3000 m thick over the plateaus of the interior. A framework for conceptualizing the growth of the CIS has been developed where small alpine ice fields grew and advanced out of the mountains, across plateaus and lowlands, eventually coalescing to form an ice sheet. In some areas flow reversals occurred where the ice divide migrated from above the Coast Mountains to over the Interior Plateau. Where robust chronology exists, the maximum extent of this ice sheet is time transgressive: earliest in the north and latest in the south. Deglaciation was characterized by complex frontal retreat in peripheral glaciated areas and by downwasting accompanied by widespread stagnation throughout the Cordilleran interior; although in some interior areas, modification to the model have been suggested with more emphasis on frontal retreat.