Lake Chicago - Origin

Origin

The city of Chicago lies in a broad plain which, hundreds of millions of years ago, was a great interior basin covered by warm, shallow seas. These seas covered portions of North America from the Arctic Ocean to the Gulf of Mexico. Evidence of these seas are found in the fossils of coral, such as those unearthed in Illinois quarries at Stony Island Avenue, Thornton and McCook Avenues, or at 18th Street and Damen Avenue, all in Chicago. Evidence may also be found in the fossils in the Niagara limestone bedrock found throughout the Chicago area and extending all the way to Niagara, New York.

Much later, the polar ice cap crept four times down across the continent, covering the region with ice to a depth of a mile (1500 m) or more. As the climate changed, the ice melted; and the last great ice flow (the Wisconsin Glacier of the Pleistocene period, which covered much of northern half of North America) retreated, and an outlet for the melting water developed through the Sag River and the Des Plaines River Valley around Mt. Forest, Illinois, in the area known as the Palos. Mighty torrents of water poured through those valleys, eventually leaving behind them the prehistoric Lake Chicago, the ancestor of Lake Michigan.