We are currently in the Quarternary geological period, which began about 2 .6 million years ago. This period is characterized by repeated cold glacial episodes (ice ages) alternating with warm interglacial episodes, each lasting tens of thousands of years.
Northeastern Illinois was covered at least six times by continental ice sheets that came down from Canada, cutting and deepening the basins of the Great Lakes along the way. The last sheet was the Wisconsin Glacier that retreated from Illinois into Lake Chicago (the precursor of today’s Lake Michigan) about 12,500 years ago, leaving behind 10 or more feet of sediment known as glacier drift, including a few large rocks called “glacier erratics” that can be found scattered over Wolf Road Prairie.
At its maximum, the glacier was over 3000 feet thick in Chicago and extended as far south as Peoria and Shelbyville, smoothing and flattening the bedrock topography of Illinois. The front or leading edge of the glacier advanced and retreated many times, leaving behind a series of long elevated ridges or hills known as moraines. Wolf Road Prairie is currently located on part of the large Valparaiso Moraine that extended around the bottom of Lake Michigan from Wisconsin through Illinois and into Indiana. The shoreline of Lake Chicago was as far as LaGrange Road, making Wolf Road Prairie almost “beach front property.” Obviously the lake was much higher then, and as recently as 3,000 to 4,000 years ago water from Lake Chicago flowed out south and west via the “Chicago Outlet” through low parts of the Valparaiso Moraine, forming the Des Plaines and Sag River valleys and connecting Lake Chicago with the Mississippi River system.
Much of the current Palos Forest Preserve, including the Little Red School House nature center, was located on Mount Forest Island between the Des Plaines and Sag rivers. Today thanks to the construction of the Chicago Sanitary and Ship Canal over 100 years ago in 1900, Lake Michigan again flows into the Des Plaines River, but that may not last due to fears that the Asian Carp may use this restored ancient water route to enter the Great Lakes.
Next time you’re at the Prairie keep an eye out for those glacial erratics and try to imagine a sheet of ice as high as two Willis Towers above you!
Source: “Geology of Illinois,” edited by Dennis R . Kolata and Cheryl K. Nimz, University of Illinois at Urbana-Champaign, 2010.