WHEN AND HOW WAS THE LAND SURFACE OF ILLINOIS FORMED?
The ground surface over most of Illinois was formed during the ice age (Quaternary Period) which lasted from about one million years ago up to nearly 5,000 years ago. As much as 90 percent of the state was covered by one of several sheets of glacial ice ([fig. 3]). The glaciers smoothed down and covered much of the old, rocky, hilly land surface and filled old valleys with loose, unconsolidated deposits.
WHERE DID THE LOOSE, UNCONSOLIDATED DEPOSITS COME FROM AND WHAT ARE THEY MADE OF?
In Canada vast thicknesses of snow and ice accumulated until the weight of the ice finally caused it to flow slowly outward, mainly to the south ([fig. 4]). Rocks and surface materials of all types were picked up and carried by the glaciers for great distances before being deposited. Pieces of granite, quartzite, and native copper among the many local rocks and minerals are found in glacial deposits and indicate that at least part of these deposits came from the Great Lakes Region. The materials deposited range from clay-size minerals to large boulders.
Figure 3—Extent of the main glacial advances which began with the Nebraskan (a—the oldest) and ended with the late Wisconsinan (f—the youngest). Diagram “d” shows the major stream development during the time between the Illinoian and the earliest Wisconsinan glaciers. The heavy lines on the diagrams indicate major stream valleys that were present during these times.
a. Inferred limit of NEBRASKAN glaciation b. Inferred limit of KANSAN glaciation c. ILLINOIAN glacial advance d. SANGAMONIAN major drainage e. Maximum WISCONSINAN glacial advance f. Late WISCONSINAN Valparaiso front and Kankakee Flood
HOW DID ILLINOIS GET ITS SOILS?
Figure 4—Limits of ice age glaciers.