Ridges of unassorted drift are found in many places in Illinois, in some cases forming long, broad, continuous ridges extending for hundreds of miles across Illinois and adjoining states. These are the "terminal moraines" of the North American ice sheet. The Shelbyville, Bloomington, and Valparaiso moraines are the most conspicuous ridges of this character in Illinois. Each is named from a city located on its crest. No agent except glaciers is known to produce such ridges as these moraines.
Numerous bowlders in the drift, and the "bowider belts" or "bowlder trains" many miles in length in Kankakee, Will, Grundy, Kendall, and Cook counties, tell that these great rocks have been plucked from the granite areas of the Lake Superior region and carried for more than 500 miles from the rock formations of which they were once a part. The thickness of the mantle rock of the gla ciated region is much greater on the average than that of the ungla dated districts. The bedrock beneath the drift is smoothed and polished by the grinding power of glacial action, while the mantle rock of the unglaciated areas grades gradually and with increasing coarse ness of form into the bedrock beneath. The bedrock exposed at Stony Island in South Chicago shows numer ous "chatter marks" characteristic of glacial action. At the Hawthorne Stone Quarry, just west of Chicago, the mantle rock has been removed for quarrying purposes, and the exposed surface of the bedrock is shown smoothed, striated, and polished, exhibiting the work of powerful tools in the grip of the massive ice sheet. The St. Peter sandstone at the Federal Plate Glass Factory at Ottawa presents a rock surface deeply grooved by glacial action.
Kames, short ridges of sand and gravel formed near the edge of the ice, are found in the Kaskaskia Basin and elsewhere among the terminal moraines of the state. Eskers, ridges of sand and gravel accumulated by the wash of streams in tunnels under the ice, are well developed in the pre-Iowan drift of Ogle and Stephenson counties.
Outwash plains, belts of debris deposited along the outer margin of the larger terminal moraines, and valley trains, long lines of debris deposited in valleys leading from terminal moraines, are numerous and well developed especially in the Wisconsin glaciation of the state.
Unglaciated areas.—Three regions of limited area within Illinois were untouched by the ice sheet. The largest of these
includes the seven southernmost counties of the state and the southern edge of the next four counties. The ice sheet pushed southward to the Ozark Ridge and up its northern slope, depositing drift 20 to 25 feet thick, but the ice did not override the crest of the ridge. This is the most southern latitude reached by the North American ice sheet. A second unglaci ated region within the state lies between the Mississippi and Illinois rivers in Calhoun and Pike counties. The Kansan ice sheet approached this region from the west and the Illinoisan ice sheet from the east, hut neither crossed the narrow rugged area.
The third unglaciated area of Illinois occupies nearly all of Jo Daviess County and small portions of Stephenson and Carroll counties. It is only a part of a much larger unglaciated district known as the "Driftless Area" which occupies portions of the four states, Illinois, Wisconsin, Minnesota, and Iowa. The Driftless Area includes 8,000 to 10,000 square miles, about 600 square miles of which is in Illinois.
This unglaciated region is entirely surrounded by thick deposits of glacial drift. Why it escaped glaciation is not well understood. The value per acre of its farm lands is very much less than that of the adjoining glaciated regions. The three unglaciated districts of Illinois have a total area of approximately 4,000 square miles.
For the unglaciated areas of the state see the soil map facing page 152, the red areas marked No. 1, in Joe Daviess, Calhoun, and the southernmost group of counties.
Early ice invasions.—The oldest drift sheet, the sub Aftonian of the first ice invasion, did not, so far as known, reach Illinois. It lies buried beneath the later drift in Iowa, where it has been exposed by erosion. The Kansan, or second ice invasion, produced a drift sheet which lies at the surface over a large area in Kansas, Missouri, Iowa, and Nebraska. The Kansan glacial lobe, which radiated from the Keewatin center of glaciation, seems to have crossed into Illinois, and it probably forced the Mississippi River, for the time being, into a channel farther east than its present course. Any Kansan drift laid down in Illinois has been deeply covered by the deposits of later invasions, and it is not found at the surface within the state.