Forest Preserve District of Cook County, Illinois
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Lake Chicago
Nature Bulletin No. 57   March 16, 1946
Forest Preserve District of Cook County
Clayton F. Smith, President
Roberts Mann, Superintendent of Conservation

Chicago lies in a broad plain which, hundreds of millions of years ago, was a great interior basin covered by shallow seas that divided North America from the Arctic Ocean to the Gulf of Mexico. Evidences of that are the coral reefs in quarries such as those at Stoney Island, Thornton and McCook, or at 18th Street and Damen; also the fossils in me Niagara limestone bedrock.

Later, four times, the polar ice-cap crept down across the continent, covering this region with ice to a depth of a mile or more. As the climate changed they melted back. The last one, named the Wisconsin glacier, had an outlet through the Sag Valley and the DesPlaines River Valley around Mt. Forest, now the Palos. Mighty torrents of water poured through those valleys. As it retreated, it created Lake Chicago, ancestor of our Lake Michigan, then extending west to LaGrange and south beyond Homewood and Lansing.

As the glacier retreated it found new outlets, finally at Niagara Falls and through the St. Lawrence River. Each new outlet caused Lake Chicago to drop -- first 20 feet, then 15 feet, finally another 20 feet. The outlet to the southwest dried up and the DesPlaines River, when in flood, overflowed into Lake Michigan.

At each of its three levels, Lake Chicago built up sand spits in its bays, beach lines and sand dunes. Our earliest trails and many of our modern roads follow these beach lines or the ridges of the sand spits. Ridge Road from Homewood through Thornton and Lansing is one; Michigan City Road through Riverdale, Dolton and Calumet City is another; LaGrange Road is another; Riverside Drive in Riverside, Grosse Point Road, Carpenter Road and Ridge Avenue through Evanston are some others. Notice the drop from Michigan Avenue at Roseland and Kensington toward the Pullman plant. Notice the sharp rise on Washington Blvd. at Central Ave., or on Addison at Narragansett. Blue Island and Stoney Island were actual islands at successive levels of Lake Chicago.

Chicagoland is the inevitable consequence of events happening thousands, millions and hundreds of millions of years ago. Walk and learn.

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