Cordilleran Ice Sheet

The Cordilleran Ice Sheet formed when separate mountain glaciers coalesced across the North American Cordillera during repeated Pleistocene cold periods. Its ice occupied the Coast Mountains, the Columbia Mountains, the Rocky Mountains, and intervening plateaus, while outlet glaciers descended toward the Pacific coast and the interior plains.¹ During the Last Glacial Maximum, roughly 25,000 to 20,000 years ago, the ice covered much of British Columbia and Yukon and extended into southeastern Alaska, western Alberta, Washington, Idaho, and Montana. It was not a perfectly continuous blanket: rugged peaks could project above the ice as nunataks, and local topography divided the sheet into domes, corridors, and fast-flowing lobes.

The ice sheet left a distinctive landscape of scoured bedrock, U-shaped valleys, moraines, drumlins, eskers, and sediment-filled former lakes. Glacial erosion deepened mountain valleys and fjords, while meltwater redistributed gravel and sand across lowlands such as the Fraser, Thompson, and Columbia basins.¹ Its weight depressed the crust, and the land continued to rise after melting through isostatic rebound; raised shorelines around parts of the Pacific Northwest record this adjustment. Ice-dammed lakes were especially consequential: temporary lakes formed against retreating ice, redirected drainage, and sometimes released catastrophic floods. These events helped establish modern river courses and left extensive flood deposits in the interior Northwest.

Retreat began at different times in different regions and proceeded unevenly as climate warmed after the Last Glacial Maximum. Coastal and lowland sectors generally opened before some interior mountain valleys, while advancing and retreating margins repeatedly created and blocked routes.² The ice-free corridor between the Cordilleran and Laurentide ice sheets was not continuously passable: geological evidence indicates that its opening, ecological productivity, and suitability for human movement changed through time. Archaeological interpretations therefore treat it as one possible route rather than a simple explanation for the first settlement of the Americas. Deglaciation also exposed new coastal and interior habitats, affecting vegetation, animals, and later Indigenous land use.

Several details complicate the familiar image of a single western ice blanket. Ice-free refugia may have persisted on some offshore islands, coastal margins, and high ground, although their precise locations and biological importance remain subjects of research. Glacial lakes and outburst floods could transform drainage in a matter of days, leaving deposits far from the former ice margin. The sheet also interacted with the Laurentide Ice Sheet east of the Rocky Mountains, so its chronology cannot be reconstructed from one regional record alone. Scientists combine cosmogenic-nuclide exposure dating, radiocarbon ages, lake sediments, volcanic ash, and mapped landforms to establish retreat sequences. Modern ice-sheet models use these geological constraints to test how mountain ice responds to climate forcing.