Pari 1. (rldciiil Geolo(jij a>i</ Goicrul Clniracfers of the lUi- 

 nois ^(tnd Areas, especially those of Western Illinois. By Charles 

 A. Hakt. 



Glacial ("teology. 



Illinois as a land area has been subjected to at least two 

 principal glacial invasions from the direction of Lake ^lichigan. 

 the limits of these Iteiug roughl}- concentric with the lower end 

 of this lake. Much of our knowledge of them is derived from 

 the work of Leverett ("91)1. whose statements, supplemented 

 by ray personal observations, form the principal basis of this 

 account. 



The earlier of these ice invasions, thelllinoian, apparently 

 much more remote from the later one than this is from the 

 present time, reached about as far west as the Mississippi 

 River, and covered nearly all of Illinois except the northwest 

 corner and the hill country in the extreme south. The later 

 invasion, the Wisconsin, reached about half as far across the 

 state. The heavy deposits of gravel, sand, and clay brought 

 down by these ice-sheets have largely tilled up and often en- 

 tirely ol)literated the previous lines of drainage. 



The contour of the rock surface beneath these deposits is 

 very imperfectly known, and deserves specific investigation. 

 Records of wells, borings, etc.. show, however, that it is deeply 

 carved by immense river valleys of which there are often no 

 surface indications whatever. The great depth of these val- 

 leys, usually aliout one hundred feet below present river levels, 

 does not necessarily indicate a subsidence of this part of the 

 earth, but merely the undisturbed action of water for a va.st 

 period of time, or of a greater volume for a lesser period, deep- 

 ening them to a low gradient nearing base-level, after which 

 the energy of the stream was mostly occupied in widening 

 them. 



The Mississippi from Keokuk down and the Illinois Itelow 

 the bend at Hennepin are still in ancient preglacial vallej's. 



