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.
The glaciers covered all of Illinois except the northwestern corner, the southwestern edge along the Mississippi River, and extreme southern Illinois, as shown in [figure 1]. In those areas the land is much as it was before the glaciers came. In the glaciated portion of the state, however, the bedrock generally is covered by the rock debris the ice carried from as far away as Canada. As the fringes of the ice melted, these loads of rock material were, in some places, dumped as ridges (moraines) which are the hills and mounds on the flat prairies of the present landscape. Such material also filled ancient river valleys, but new valleys were cut by torrents of water released by the melting ice.
Figure 1—A mantle of glacial drift covers the bedrock in much of Illinois.
WISCONSINAN GLACIAL DRIFT KANSAN GLACIAL DRIFT ILLINOIAN GLACIAL DRIFT
The glacial drift belongs to the youngest (topmost layer) of the major divisions of our rocks, which geologists have named the Pleistocene (scientific name for Ice Age deposits).