THE GLACIAL PERIOD Preglacial Illinois.—During the long ages from the close of the Pennsylvanian period of the Paleozoic era to the opening of the Glacial period in the late Cenozoic era, Illinois was exposed to the constant activities of the ordinary processes of erosion in operation today. The surface rocks of the Penn sylvanian and earlier periods were weathered into fragments. Then, as now, a network of streams carried the rock waste along their courses across the state. Master-streams like the Mississippi, Ohio, Illinois, and Wabash received the debris and carried it on to the sea. Valleys were deeply carved, and they were widened to the stage of maturity. Much of the state possessed a rugged, mature topography similar to that now found in the Driftless Area of northwestern Illinois and southwestern Wisconsin. The smaller stream systems were wholly different in their details from those of today; even the vigorous Mississippi occupied, at places, a very different course from the one along which it now flows.
A mantle of soil, weathered from the underlying rocks, overspread the state; plants appropriate to the soil and the climate had become established; and animal life, adapted to the environment of the time, wandered over the region. We cannot know the exact conditions of the state in preglacial time, but by comparison with the adjoining unglaciated dis tricts we know that the topography, developed by long continued stream erosion, was more rugged than now, the soil thinner and not so fertile, and the conditions for the development of plant and animal life, especially with refer ence to human needs, not so favorable as at present.
The Glacial period.—Glaciation has been an important factor in shaping the present relief of Illinois. A change of climate produced conditions over Northern North America and Northwestern Europe such that more snow fell during the winters than could be melted during the succeeding summers.
These accumulations of snow resulted, after centuries of time, in great snow fields, which, under their own weight, became slowly moving continental glaciers. The North American
ice sheet, with centers of accumulation in Labrador, in Kee watin in Central Canada, and in the Canadian Cordillera, covered nearly all of Canada and much of Northern United States. It reached its farthest extension southward in Illinois.
where its southern edge rested on the northern flank of the Ozark Ridge in Jackson, Williamson, Saline, and Gallatin counties, 1,600 miles from the center of accumulation in the peninsula of Labrador.
During the Glacial period the ice sheet advanced into the United States five times. At least three of these invasions the Illinoisan, Iowan, and Wisconsin—reached far into Illinois, and each produced profound changes in the surface and the soils of the state. Extensive studies have been made by glacial geologists to determine the duration of the Glacial period. The estimates place the beginning of the period at more than 300,000 years, possibly as much as 1,000,000 years ago; the climax of the Illinoisan invasion at more than 140,000 years; the Iowan at more than 60,000 years; and the Wisconsin at more than 20,000 years ago. The Glacial period is only a small fraction of the total of geologic time.
Evidences of glaciation.—It,is fully demonstrated that at least nine-tenths of Illinois has been glaciated. The character of the mantle rock, the glacial "drift" or "till," and the appearance of the surface of the bed rock of the glaciated regions can be explained only by glacial action. The glacial drift varies in thickness from a thin veneer to more than 300 feet ; its depth for the entire glaciated region averages about 75 feet. This drift is made up of materials varying from fine particles of clay to large bowlders, the whole so thoroughly mixed together that the result could be accomplished only by glacial action. Among the drift are numerous pebbles and bowlders, subangular in form and strongly striated in the manner characteristic of glacial action.