Illinoisan glaciation.—The third ice invasion radiated from the Labrador center of the North American ice sheet and over spread so much of Illinois that the glacial lobe and the glacial drift of this ice advance is known as the Illinoisan. The move ment of the Illinois glacial lobe was southwestward. The ice at this stage reached a lower latitude, 40', than elsewhere in North America, and a point 1,600 miles from the center of accumulation, a distance somewhat greater than the movement of any other ice invasion from its center.
The ice sheet of the Illinoisan glaciation crossed the Mis sissippi River between Rock Island, Illinois, and Fort Madison, Iowa, and forced the Mississippi about twenty miles farther west than its present course. The Mississippi River thus suf fered important changes of position in portions of its course by both the Kansan and the Illinoisan glacial invasions. The Mississippi channel which had been established by the Kansan lobe was completely obliterated and deeply covered by the drift of the Illinoisan glaciation.
The Illinoisan drift sheet extends northeastward under the later glacial deposits far back from the southern margin. The surface exposures of the Illinoisan glaciation forms a great crescent-shaped area extending southward beyond the more recent drift sheets through Wisconsin, Illinois, Indiana, and Ohio. This drift area is widest and best developed in western and southern Illinois, hence the name.
Terminal moraines were formed in the Illinoisan glaciation as ridges or mounds which now form low but conspicuous irregularities in the landscape. One group of these morainal elevations is readily traced on the soil map from Jackson County northward and northeastward along the Kaskaskia River, thence northwestward to Logan and Mason counties. Another group is found extending from Pike County north ward along the Mississippi and Illinois rivers. On the map No. 2 stands for the terminal moraines of the Illinoisan glaci ation.
Divisions of the Illinoisan glaciation.—An examination of the map shows that the Illinoisan drift sheet is divided into three parts: the Lower Illinoisan glaciation (No. 3), mainly between the Wabash and Kaskaskia rivers; the Middle Illinoisan glaciation (No. 4), between the Kaskaskia and Illinois rivers; and the Upper Illinoisan glaciation (No. 5),
between the Illinois and Mississippi rivers. This division is based on differences in the agricultural values and in the properties of the soils in these regions.
The pre-Iowan glaciation, No. 6 of the map, in the north western part of the state is sometimes classified with the Illinoisan, but it may be a drift sheet intermediate between the Illinoisan and Iowan glaciations.
Iowan glaciation.—In the Rock River Basin of northern Illinois is a portion of a drift sheet known as Iowan. It is represented by No. 7 on the map.
The loess.—Loess is a variety of silt, intermediate in the size of its particles, between clay and sand. The loess of Illinois is associated with the Iowan stage of glaciation. The loess covers the areas of Illinoisan drift, and it is covered by the Wisconsin drift. It seems to be a wind-blown product, and has a very wide distribution in the state. Over the uplands it commonly has a depth of 3 to 10 feet. Along the valleys, especially the Mississippi, Illinois, and Wabash, the deposits of loess are much thicker, a depth of 30 to 40 feet being com mon, with a maximum of nearly 100 feet. These are known as deep loess areas, and are represented on the map by No. S.
Wisconsin glaciation.—The fifth and last ice invasion of the United States covered the northeast quarter of Illinois, extending southward as far as Clark and Cumberland counties. The Early Wisconsin glaciation is represented by Nos. 9 and 11 of the map and the Late Wisconsin glaciation by Nos. 10 and 12. Wherever the Wisconsin drift sheet is found in the state, it lies at the surface, covering the older drift sheets of northeastern Illinois. The Wisconsin drift covers a large area in Wisconsin and swings in a broad curve across north western Indiana and far into Michigan.
While the terminal moraines of the Illinoisan drift are of very moderate elevation and length, and those of the Iowan but slightly developed, the moraines of the Wisconsin are among the largest and longest of the world. There are three conspicuous and very extensive moraines of the Wisconsin drift.
1. The Shelbyville moraine marks the outer edge of the Wisconsin drift, and extends from Indiana across Illinois to Peoria County.