The Great Lakes that we see today have a storied history tied to the growth and collapse of the great continental glaciers that covered a significant part of North America.  Evidence of ancestral phases of the Great Lakes, with names such as Glacial Lake Chicago, Lake Algonquin, and Lake Algoma can be observed as ancient beaches, lake plains, and wave cut bluffs.  Many of these features lie high above the level of modern Lake Michigan, as a testament to the dynamic nature of ancestral lakes that were trapped between retreating glacier ice margins, and landforms of the glacial landscape.

Long, narrow ridges called moraines form as sediments are deposited at the edges of melting glaciers.  Around the rim of the southern Lake Michigan, a number of moraines shaped as arcuate ridges encircle the lake, somewhat parallel to the shoreline.  While the glacier still occupied ancestral Lake Michigan, meltwater was trapped between a number of these moraines and the edge of the glacier, creating an ancestral form of Lake Michigan named Glacial Lake Chicago.  Glacial Lake Chicago existed during the end of the Wisconsin Glacial Episode, approximately 14,000 years before present and it occupied a number of levels higher than the level of Lake Michigan that we see today. The levels of Glacial Lake Chicago are recorded on the landscape as relict beach ridges, spits and ancient wave-cut bluffs, such as the steep notch on the west side of the park.   Because Lake Chicago was higher than Lake Michigan, it extended farther west and south, including areas west of Sheridan Road and much of Chicago.  Glacial Lake Chicago initially drained through the area known as the Chicago Outlet, located where the Des Plaines River cuts through a moraine, but eventually was re-directed through the St. Lawrence Seaway as the last glaciers melted away and left an open course to the Atlantic Ocean

 

AncestralLakeMichigan.jpgClick to enlarge

For more information about Geology and the Ice Ages, visit the Illinois State Geological Survey's Geology Resources page

For more information on the Geologic history and the Ice Ages in Illinois, visit the Illinois State Geological Survey