How did glaciers form the basins of the Great Lakes?

The basins of the Great Lakes were primarily formed through the mechanical action of massive ice sheets during the Pleistocene Epoch, a process that began approximately one million years ago.12

Before the onset of the Ice Age, the region featured a system of river valleys formed by water erosion. As massive glaciers moved through the mid-continent, they scoured these preexisting valleys, significantly widening and deepening them. The Wisconsin Glacial Stage, the final phase of this period, was particularly instrumental in shaping the current configuration of the lake basins as the ice sheet melted and receded starting about 14,000 years ago.21

The formation was further influenced by the immense weight of the ice, which depressed the Earth’s crust. Following the retreat of the ice, the land underwent glacial rebound—a gradual uplift that tilted the surface and altered drainage patterns, eventually establishing the modern lake system.12