When was North America last covered in over one mile of ice? The last glacial maximum occurred between 26,500 years and 19–20,000 years ago. The glacial retreat lasted until about 11,650 years ago. The ice sheet was around two miles thick at its thickest in the area which is currently Quebec. At the maximum extent of the ice sheets, which occurred about 21,000 years ago, the Laurentide Ice Sheet covered most of Canada and parts of the northern United States, and was estimated to have an average thickness of about 2,400 meters (7,900 feet). In some areas, such as the region around Hudson Bay, the ice sheet may have been up to 4,000 meters (13,000 feet) thick.

In addition to the Laurentide Ice Sheet, other smaller ice sheets and glaciers covered parts of Alaska, the Rocky Mountains, and other areas of North America during the last glaciation period. Overall, the ice sheets and glaciers of the last glaciation period had a profound impact on the landscape and ecology of North America, shaping the geography and leaving behind a legacy that can still be seen today.