Guide to the Geologic Map of Illinois

Illinois State Geological Survey Educational Series 7
STATE of ILLINOIS Otto Kerner, Governor
DEPARTMENT of REGISTRATION and EDUCATION William Sylvester White, Director
1961
ILLINOIS STATE GEOLOGICAL SURVEY John C. Frye, Chief URBANA, ILLINOIS
PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS 42517-20M 2 (78783)
Although the age of the Earth is measured in billions of years, the face of Illinois is young—a mere 15,000 years old.
During the Ice Age, most of Illinois was repeatedly invaded by huge glaciers, sometimes towering a mile or more high, that carried embedded in them ground up rock materials they had gouged out of the bedrock to the north as they ponderously pushed south.
When the last of the glaciers melted from Illinois, about 15,000 years ago, the country that emerged looked far different from the preglacial land. Old hills and valleys had vanished, new ones had formed, and a mantle of unconsolidated rock material, the burden carried by the ice and dropped as the ice melted, lay over most of the region.
Most of this material, called glacial drift, was brought in by the ice during the last two of the four major periods of glaciation—the Illinoian period 100,000 to 150,000 years ago and the Wisconsinan 5,000 to 50,000 years ago. The older drift introduced during the Kansan and Nebraskan glacial periods is almost entirely buried beneath the later drifts.
Figure 1—A mantle of glacial drift covers the bedrock in much of Illinois.
WISCONSINAN GLACIAL DRIFT KANSAN GLACIAL DRIFT ILLINOIAN GLACIAL DRIFT