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

****:LAKE CHICAGO

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.