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)
Guide to the
GEOLOGIC MAP OF ILLINOIS
GLACIAL GEOLOGY
Although the age of the Earth is measured inbillions of years, the face of Illinois is young—amere 15,000 years old.
During the Ice Age, most of Illinois was repeatedlyinvaded by huge glaciers, sometimes toweringa mile or more high, that carried embedded in them groundup rock materials they had gouged out of the bedrock tothe north as they ponderously pushed south.
When the last of the glaciers melted from Illinois,about 15,000 years ago, the country that emerged lookedfar different from the preglacial land. Old hills and valleyshad vanished, new ones had formed, and a mantleof unconsolidated rock material, the burden carried bythe ice and dropped as the ice melted, lay over most ofthe region.
Most of this material, called glacial drift, wasbrought in by the ice during the last two of the four majorperiods of glaciation—the Illinoian period 100,000 to150,000 years ago and the Wisconsinan 5,000 to 50,000years ago. The older drift introduced during the Kansanand Nebraskan glacial periods is almost entirely buriedbeneath the later drifts.
The glaciers covered all of Illinois except the northwesterncorner, the southwestern edge along the MississippiRiver, and extreme southern Illinois, as shown infigure 1. In those areas the land is much as it was beforethe glaciers came. In the glaciated portion of thestate, however, the bedrock generally is covered by therock debris the ice carried from as far away as Canada.4As the fringes of the icemelted, these loads of rockmaterial were, in someplaces, dumped as ridges(moraines) which are thehills and mounds on theflat prairies of the presentlandscape. Such materialalso filled ancientriver valleys, but newvalleys were cut by torrentsof water releasedby the melting ice.
Figure 1—A mantle of glacialdrift covers the bedrock inmuch of Illinois.
- WISCONSINAN GLACIAL DRIFT
- KANSAN GLACIAL DRIFT
- ILLINOIAN GLACIAL DRIFT
The glacial drift belongsto the youngest(topmost layer) of the majordivisions of our rocks,which geologists havenamed the Pleistocene(scientific name for IceAge dep