Saturday, June 16, 2018

The History of Lake Chicago; Today's Lake Michigan.

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 or more. As the climate changed, the ice melted, the last great ice flow, the Wisconsin Glacier of the Pleistocene period, which covered much of the 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, in the area known as the Palos.

The Kankakee Torrent poured through those valleys, eventually leaving behind the prehistoric Lake Chicago or Glacial Lake Chicago, the term used by geologists for a lake that preceded Lake Michigan when the Wisconsin Glacier retreated from the Chicago area, beginning about 14,000 years ago.
Lake Chicago's level, at its highest, was almost 60 feet higher than the level of present Lake Michigan and the lake completely covered the area now occupied by Chicago. Its northern outlet into the St. Lawrence River was still blocked by remnants of the glacier and it drained through the so-called Chicago outlet, a notch in the Valparaiso moraine[1], into the Mississippi system. Its western shores reached to where Oak Park and LaGrange now exist.
As the glacier shrank in stages, the major three of which are often referred to as the Glenwood phase (50 feet above the level of Lake Michigan; circa 12,000 years ago), the Calumet phase (35 feet; circa 10,000 years ago), and the Tolleston phase (20 feet; less than 8,000 years ago). After each stage, the next barrier remained solid, holding the lake stable and creating distinct sandy beaches. If the outlet was formed by a steady erosion of the barrier, it would have been less likely that the well defined beaches would have been created.
This undated marker is located in the southern portion of Lincoln Park, on the foot path paralleling the east side of Stockton Drive. A second identical marker is located on the same ancient beach ridge 485 feet East-North-East from the first one.
The lake's southern shores were dammed by the hills of the Tinley-Valparaiso terminal moraine systems. As the glacier retreated farther and cleared the northern outlet, the lake level fell further and Lake Chicago became Lake Michigan. Along the eastern shore of Lake Michigan, the beaches of Lake Chicago were destroyed by erosion, except the highest beach. Much of this beach was also destroyed. The best remaining segments are along the southern tip of Lake Michigan, now known as the Indiana Dunes National Lakeshore.

Compiled by Neil Gale, Ph.D. 

[1] Moraines are accumulations of dirt and rocks that have fallen onto the glacier surface or have been pushed along by the glacier as it moves.

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