Two massive ice sheets dominated North America during the last glacial maximum, separate from Greenland's own ice mass. The Laurentide ice sheet, centered over Hudson Bay, was comparable in size to Antarctica's present-day ice cover. It extended all the way to the flanks of the Rocky Mountains in western Canada. Meanwhile, the Cordilleran ice sheet blanketed the Rockies themselves, stretching from northern Idaho and Washington up through Alaska, roughly the size of today's Greenland ice sheet. These weren't minor glacial advances. They were continent-scale formations that fundamentally reshaped the landscape, redirected rivers, carved valleys, and set the stage for catastrophic meltwater events when warming began.
Understanding the scale and configuration of these ice masses is essential for interpreting the evidence we see across North America today. The geology doesn't lie. When you walk the scablands, the erratics, the drumlin fields, you're reading the story these ice sheets left behind.