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Quaternary Geology of Illinois


Image courtesy Illinois State Geological Survey (fair use policy)

Almost completely hiding the bedrock of Illinois are sand, clay, and gravel deposits that date from the ice ages of geologically recent times—the Pleistocene and Holocene epochs, together known as the Quaternary Epoch.

Two episodes of glaciation are abundantly represented in Illinois. The older one is named the Illinoisan because its deposits are so widespread in this state. The salmon-colored area is mostly composed of till, the ground-up rock of all grain sizes that is typically produced by glaciers and left behind when they melt.

The green area is covered with deposits from the most recent episode, the Wisconsinan, which ended only about 10,000 years ago. The sinuous dark-green landforms are end moraines, low ridges of till that build up at the edges of glaciers where they melt and drop their sediment load. The pattern of these moraines shows how the glacier retreated a bit at a time, leaving one moraine after another behind. The lighter green represents ground moraine, sediments that were being carried along the bottom of the glacier when it melted. The light-blue areas also date from this episode; they are clayey beds that formed in lakes that once existed on the cold, barren plains around the glacier's edge.

There are two older episodes of glaciation in the North American Quaternary record, called the Kansan and Nebraskan. Deposits of those ages were almost totally wiped away in Illinois. Only the Wolf Creek Formation along the western border remains from those times.

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