PLEISTOCENE.
Glacial Deposits.—After the close of the Tertiary the entire surface of the State north of the Ozarks except small portions at the northwestern corner, and in Calhoun county, was covered with a sheet of ice of great thickness. This ice sheet came to us from the regions southwest and southeast of Hudson's Bay, and on its way picked up the soils and loosened fragments of rocks which had been accumulating over that re gion for hundreds of thousands of years. When the ice melted these ma4 terials were deposited over the surface of Illinois, making a layer which varies greatly in thickness but will probably average 50 to 100 feet. This is known as boulder or glacial clay. Three ice sheets separated by long intervals are supposed to have covered portions of Illinois. The first Illinois Glacier extended as far south as the latitude of Carbondale, and when it disappeared left a thick deposit of glacial clay. The sec
ond, or Iowan Glacier, extended but a short distance below the northern boundary. We do not yet know the exact limits of the territory which it covered. The third, or Wisconsin Glacier, covered an area in the north eastern portion of the State which averages about 90 miles east and west by about 200 north and south. Its boundaries are marked by a prom inent ridge or moraine 'which passes near Charleston, Shelbyville, De catur, Peoria, Princeton, Rochelle, Woodstock, and Harvard.
Each of these ice sheets left its deposit of boulder clay, and as we di& through the deposits the upper surface of each is marked by one or more of the following characteristics,,yellow clay, black earth, pieces of wood, gas, or large water-bearing gravel beds.