Looking at a map of North America, one's attention is immediately drawn to the Great Lakes, the most conspicuous geographic feature of the Continent. Yet, carved by the Pleistocene glaciers, they are also among the youngest landforms on the surface of our planet.
The Precambrian basement of our continents, like the visible topography of Earth, is a composite of ridges, basins and plains. One of these basins, centered over Lower Michigan, encompasses the Great Lakes region, having filled in with Paleozoic, Mesozoic and Tertiary sediments. Over the course of this deposition, a major river system developed within the Michigan Basin, likely draining toward the Atlantic. During the Pleistocene, which began 2 million years ago, four major continental glaciers plowed across that basin, altering the landscape and widening the river valleys; the last of these glaciers, the Wisconsin, began to form just 70,000 years ago and reached its peak advance about 18,000 years ago. As it began to melt back into Canada, its surging waters filled the remodeled lake basins, which initially drained into the Mississippi watershed via the Illinois, Kankakee and Wabash Rivers. Relieved of the glacial weight, the Upper Midwest began to rebound and some of the drainge shifted eastward, through the Mohawk River Valley, until the St. Lawrence Seaway opened and the Great Lakes assumed their current shape and hydrology; it has only been during the last 4-6,000 years that this massive lake system, which holds almost 25% of the liquid fresh water on Earth, reached an apparent steady state.
Lake plains and inland dunes, most evident across northern Ohio and Indiana, attest to the larger size of the Lakes prior to opening of the St. Lawrence River. Even today, the drainage pattern continues to change as southern Canada rebounds from the weight of the ice; as the land north of the Lakes rises, many geologists and hydrologists believe that the primary drainage will shift back to the southwest, threatening urban centers along the lakeshores and river corridors. Then again, since the Holocene could be just another warm, interglacial period, the ice may return to reclaim the Lakes.
