This article is about the prehistoric lake, For other geographic features with this name, see Chicago
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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 covered portions of 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 Stony 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 (1500 m) 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 the Sag River and the Des Plaines River Valley around Mt. Forest, Illinois, in the area known as the Palos. Mighty torrents of water poured through those valleys, eventually leaving behind them the prehistoric Lake Chicago, the ancestor of Lake Michigan.
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. As these outlets were developed, a partial lake capture occurred and the water level in Lake Chicago began to drop in three observable stages of 15 to 20 feet (5-6 m) each. Eventually even the outlet to the southwest dried up, and the Des Plaines River overflowed into the basin that became Lake Michigan.
Vast amounts of sand in spits, dunes and beach lines—particularly at the southern tip of Lake Michigan—were left behind by each of the three stages of lake level drop. Today, evidence of these vast sand deposits are still clearly visible. Northern Indiana, for example, contains some of the most beautiful beaches found in any of the five Great Lakes; and many of the Chicago area's 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, and then crossing the state border into Munster and Highland, Indiana, is one; Michigan City Road through Riverdale, Dolton, and Calumet City, Illinois, is another; LaGrange Road is another; Riverside Drive in Riverside; Ridgeland Avenue in Oak Park, or Grosse Point Road, Carpenter Road, and Ridge Avenue (The Rosehill Spit) in the Rogers Park/West Ridge neighborhood of Chicago, north of Devon Avenue and continuing north through Evanston, Illinois, are some others. Blue Island, Illinois, and Stony Island were, literally, islands left behind as Lake Chicago's water level fell.
- Bretz, J.H. 1939. Geology of the Chicago Region, Part 1 - General. Illinois State Geological Survey, Bulletin 65. 118 pages.
- Killey, Myrna M. 1998. Illinois' Ice Age Legacy. Illinois State Geological Survey GeoScience Education Series 14. 66 pages.
- Willman, W.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