Lake Chicago was a prehistoric proglacial lake that is the ancestor of what is now known as Lake Michigan, one of North America's five Great Lakes. Fed by retreating glaciers, it drained south through the Chicago Outlet River. Origin The city of Chicago lies in a broad plain that, 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 is found in the fossils of coral, such as those unearthed in Illinois quarries at Stony Island Avenue, Thornton Quarry, and McCook, Illinois, or at 18th Street and Damen Avenue 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, 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, in the area known as the Palos. The Kankakee Torrent poured through those valleys, eventually leaving behind the prehistoric Lake Chicago, the ancestor of Lake Michigan. Chronology 13,000 to 11,000 years before present The earliest formation of Lake Chicago occurred when the Michigan Lobe of the glacier retreated northward into the basin of modern Lake Michigan, ca 13,000 years ago. The edge of the retreating glacier formed moraines, the Park Moraine in present-day Illinois and the Lake Borders Moraine in Indiana and Michigan. The ice continued northward forming the next phase of Lake Chicago. The lake level remained at 640' above sea level, but the ice margin was a third of the way north, opening a channel across Michigan, draining the Lake Saginaw and Lake Whittlesey proglacial lakes in the Lake Huron and Lake Erie basins. When the ice retreated northward enough to open the Mohawk River valley, the waters in the Lake Huron and Lake Erie basin diverted from the Michigan outlet, leaving Lake Chicago as a headwaters lake. Continued northward migration of the glacier set the