ICE AGE GEOLOGY
Excerpt from Illinois and Michigan Canal National Heritage Corridor
National Park Service
U.S. Department of the Interior


Glacial ridge along Salt Creek
Photo by Marcia Faye

"Continental glaciers invaded Illinois repeatedly during the Ice Age, a span of time from about 2.4 million years to 10,000 years ago. The modern landscape you now see records the retreat of the last major ice sheet that extended into Illinois from 25,000 to 14,000 years ago. This invasion took place during the most recent or Wisconsinan Glaciation, which geologists estimated extended from 75,000 to 10,000 years B.P. (before present time). During that time, the Laurentide Ice Sheet covered much of Canada and the northern United States. Nurtured by a continental climate colder than today's, the ice sheet grew as snow accumulated and the pressure of its own weight caused it to change to ice and spread outward from its Canadian center.

The tongue of the ice sheet that flowed into Illinois came from the north. It became known as the Lake Michigan Lobe because it flowed as a river of ice through the Lake Michigan Basin before it spread out into central Illinois. When the glacier reached its southernmost limit about 20,000 years ago, the ice was a mile thick at Chicago - an enormous weight that depressed the land beneath. Later as the ice retreated and the glacier's weight was released, the Earth's crust began to rebound. The crust is still rebounding today, especially from Milwaukee northward.

When the glaciers extended into Illinois, the climate was much different from today. At the ice margin, long-haired mastodons browsed among spruce forests. The average yearly temperatures were near or just about freezing, and most of the year's precipitation was in the form of snow. In the summer, enormous volumes of meltwater and sediment flowed away from the glacier, but during winter, river volumes were reduced to a relative trickle.

Retreat of the glaciers was caused by a gradual warming of climate in northern North America. Between 20,000 to 10,000 years ago, this climatic trend was frequently interrupted (and even reversed for short periods) which meant that the ice margin sometimes stopped melting back and at times re-advanced."

Evidence of the last Ice Age can still be found in the Salt Creek watershed in the form of glacial erratics. These erratics are boulders or rocks transported great distances from their source and carried along as "hitchhikers" within the glacier until they were dropped as the ice receded where they remain today. Other evidence of the glacier can been seen as scratches or striations left on the bedrock and unsorted ice-deposited sediment which is a mixture of clay, silt, sand and gravel called till or drift.

Another geological feature left by the glacier is the moraine. The melting at the end of the glacier caused it to deposit ground-up rock materials it was transporting, forming the moraines. DuPage County is home to three end moraines. You can see a portion of one at the Fullersburg Woods Forest Preserve located west of York Road and north of Spring Road along the banks and uplands of Salt Creek. Fullersburg Forest Preserve is owned by the Forest Preserve District of DuPage County.

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