Part I: Glacial Geology and General Characters of the Illi- 

 nois Sand Areas, especially those of Western Illinois. By Charles 

 A. Hart. 



Glacial Geology. 



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

 principal glacial invasions from the direction of Lake Michigan, 

 the limits of these being roughly concentric with the lower end 

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

 the work of Leverett ('99), whose statements, supplemented 

 by my personal observations, form the principal basis of this 

 account. 



The earlier of these ice invasions, the Illinoian, 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 filled up and often en- 

 tirely obliterated 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 about 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 vast 

 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 below 

 the bend at Hennepin are still in ancient preglacial valleys. 



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