Formed about 13,000 years ago and fed by retreating glaciers, it drained southwest through the Chicago Outlet River.

The city of Chicago lies in a broad plain that, hundreds of millions of years ago, was part of a great interior basin covered by warm, shallow seas.

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.

[4] Extending somewhat further south, west and east than Lake Michigan, Lake Chicago extended west to present day La Grange, Illinois and south beyond Homewood and Lansing, Illinois, completely covering what is now Northwest Indiana, including the cities of Hammond and Gary, Indiana.

As the Wisconsin Glacier continued to retreat, it created new outlets for the water in Lake Chicago, including Niagara Falls, and the St. Lawrence River.

Eventually even the outlet to the southwest dried up, and the Des Plaines River overflowed into the basin that became Lake Michigan.

As the Michigan Lobe of the Labradorean Ice Cap retreated northward, the area of Lake Chicago increased.

After each stage, the next barrier remained solid, holding the lake stable and creating distinct beaches.