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Stages of great lake development.

Lake Chicago was a prehistoric lake that is the ancestor of what is now known as Lake Michigan, one of North America's five great lakes.

What is now 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 divided 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 Stoney 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 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 what is now the Sag River and the Des Plaines River Valley around Mt. Forest, Illinois, in the area now known as the Palos. Mighty torrents of water poured through those valleys, eventually leaving behind them the prehistoric Lake Chicago, the ancestor of what we now know as Lake Michigan.

Somewhat larger than Lake Michigan, Lake Chicago extended West to what is now La Grange, Illinois; and South beyond what is now Homewood and Lansing, Illinois; completely covering what is now Northwest Indiana, including the cities of Hammond and Gary.

As the Wisconsin Glacier continued to retreat, it created new outlets for the water in Lake Chicago, including at what is now Niagara Falls, and what is now the St. Lawrence River. As these outlets were developed, the water level in Lake Chicago began to drop more or less in three observable stages of from 15 to 20 feet each. Eventually even the outlet to the Southwest dried up and so what we now know as the DesPlaines River, when in flood, overflowed into what was fast becoming what we know today as Lake Michigan.

Vast amounts of sand in spits, dunes and beach lines -- particularly at what is now the Southern tip of Lake Michigan, in Indiana -- were left behind by each of the three stages of lake level drop. Today, evidence of these vast sand deposits are still clearly visible. In Northern Indiana, for example, are some of the most beautiful beaches found in any of the five great lakes; and many of the Chicago area's many trails and roads follow some of these ancient beach lines or ridges in the sand spits.

For example, Ridge Road from Homewood, Illinois through Thornton, and Lansing, Illinois is one; Michigan City Road through Riverdale, Dolton and Calumet City, Illinois is another; LaGrange Road is yet another; Riverside Drive in Riverside; Ridgeland Avenue in Oak Park, or, Grosse Point Road, Carpenter Road and Ridge Avenue through Evanston, Illinois are some others. What are today known as Blue Island, Illinois and Stoney Island Avenue were, literally, islands of sand left behind as Lake Chicago's water level fell.

• Killey, Myrna M. 1998. Illinois' Ice Age Legacy. Illinois State Geological Survey GeoScience Education Series 14. 66 pages.
• Willman, H.B. 1971. Summary of the Geology of the Chicago Area. Illinois State Geological Survey, Urbana, IL. Circular 460. 77 pages.
• Forest Preserve District of Cook County
• Oak Park - Ancient Lake Chicago & Continental Divide