premise: Cordilleran Ice Sheet covered a large portion of North America.
hypothesis: The Cordilleran ice sheet was a major ice sheet that periodically covered large parts of North America during glacial periods over the last ~ 2.6 million years . The ice sheet covered up to 2.5 million square kilometres at the Last Glacial Maximum and probably more than that in some previous periods , when it may have extended into the northeast extremity of Oregon and the Salmon River Mountains in Idaho .