Geology

The last major glaciations in North America during the Pleistocene covered all of Canada and the northern third of the United States. The thickest, central portion of the ice sheet covered Hudson Bay. The ice sheet stripped Canada of its topsoil, scoured and polished bedrock, and gouged out numerous future lake basins. The till and outwash were deposited to the south, forming the fertile farmlands of the United States. The ice carved out the Great Lakes basins, which are rimmed by end moraines. Glacial lakes were abundant in the Canadian prairies, North Dakota, and Minnesota. Alpine glaciers remained in California, the Rocky Mountains, and the northern Appalachians after the main ice sheet retreated.

The formation of the vast ice sheet lowered the sea level by about 130 meters (430 feet). Evidence for this includes terrestrial fossils, such as mammoths, that have been found beneath the ocean on the continental shelves. As glaciers receded, more water was contributed to the oceans and sea levels rose. A fiord is a steep‐walled, fingerlike coastal inlet that was carved by glacial action and later flooded by the rising sea.

Pluvial lakes formed during the wetter climates that existed during and after glacial retreat. For example, the Great Salt Lake in Utah is a remnant of a much larger pluvial lake. Most pluvial lakes have shrunk because of the more arid conditions that have prevailed since they formed.

The tremendous weight of glacial ice may depress the earth's crust more than 200 meters. After the glacier retreats, the crust begins to move back to its previous level. This crustal rebound is still in progress around the Great Lakes in the United States.