THE CORDILLERAN ICE SHEET IN SPACE AND TIME (Invited Presentation)

The plateaus of interior British Columbia (BC) and southern Yukon remained ice free throughout most Pleistocene glacials. On occasion, however, these areas were covered by an ice sheet that extended onto the Pacific continental shelf on the west, into southern Yukon on the north, and into the northwestern conterminous United States on the south. Typically, 10,000 years or more elapsed between the time when glaciers in high mountains began to grow and the time of full ice-sheet development. During the Last Glaciation (MIS 2), for example, alpine glaciers began to thicken and advance in British Columbia and northern Washington State before 30,000 years ago, but it was not until about 17,000 years ago that the Cordilleran Ice Sheet achieved its maximum size. Limited evidence indicates that much of this ice sheet was warm-based and supported many fast-moving, topographically confined ice streams that flowed westward from major accumulation centres in the Coast Mountains and southward into northern Washington, Idaho, and western Montana. At the local Last Glacial Maximum, the ice sheet isostatically depressed the lithosphere on which it rested by up to several hundred metres. The last Cordilleran Ice Sheet decayed much faster than it grew. By 11,000 years ago, only 6000 years after the local Last Glacial Maximum, the ice sheet had disappeared, a victim of a warmer climate, eustatic sea-level rise along its western margin, and perhaps a reduction in precipitation. Deglaciation proceeded by frontal retreat at the periphery of the ice sheet and by downwasting, complex frontal retreat, and localized stagnation in interior areas. The chronology of the last glaciation is constrained, albeit with inherent dating errors, by AMS radiocarbon and ¹⁰Be surface exposure ages. High-elevation sites at the western margin of the BC Interior Plateau, east of the Coast Mountains, became ice-free between about 15,000 and 12,000 years ago, even as the land surface rose several hundred metres. Ice cover in the southern Coast Mountains was sufficiently extensive during the Younger Dryas Chronozone (12,900-11,700 years ago) that glaciers advanced into low-lying areas north and east of Vancouver. At the same time, however, tongues of dead or dying glacier ice covered interior valleys.