The Laurentian Great Lakes were formed nearly 20,000 years ago
when the earth's climate warmed and the last glacial continental
ice sheet retreated. The glacier, up to 2 miles thick, was so heavy
and powerful it gouged out the earth's surface to create the lake
basins. Meltwater from the retreating glacier filled the newly created
basins. Approximately 3,500-4,000 years ago, the Great Lakes attained
their modern levels and area.
Above three images from "Living
with the Lakes."
Published by the U.S. Army Corps
of Engineers and the Great Lakes Commission, 1999.