546 THE ILLINOIS GLACIAL LOBE. 



rock. Of the 18,000 square miles covered by the Wisconsin drift, there 

 are about 4,000 square miles in which the thickness is much below the 

 average. This comparatively thin drift is found in five counties which 

 border the Kankakee River and the Chicago Outlet, viz, Kankakee, Cook, 

 Will, Grundy, and Lasalle counties. There are 254 borings in these 

 counties which enter rock at an average depth of only 68.8 feet. Of these 

 243 are on preglacial uplands and enter rock at an average depth of only 

 63. 8 feet, while 11 along preglacial valleys average 179 feet to rock. 



If now the Wisconsin drift area be corrected for the imperfect repre- 

 sentation of preglacial valleys, in accordance with the corrections applied 

 to the whole State, it is found that the average thickness, on a basis of 30 

 per cent being in valleys, would be 143 feet; on a basis of 40 per cent in 

 valleys the average will be 154 feet, and on a basis of 50 per cent in valleys 

 it will be 165 feet. The general average of the Wisconsin drift is thus 40 

 to 45 feet above that for the entire State. 



Turning to the Iowan drift, there is found a markedly greater amount 

 of drift in the counties lying east of Rock River than in those west of that 

 stream. The few borings which have been made indicate that the thickness 

 in the former district will average not less than 100 feet, while in the latter 

 it is but 50 to 75 feet. The thinness of the Iowan drift near its margin is 

 perhaps due in the main to the withdrawal of the loess which apparently 

 has been derived in large part from the Iowan ice sheet, but has been 

 scattered widely outside the limits of the Iowan till. 



In the portion of the State l}ang outside the limits both of the Iowan 

 and the Wisconsin till sheets — i. e., the portion occupied by Illinoian till and 

 loess — there is a marked variation in the thickness of the drift. The thinnest 

 drift of the State is found in the district lying east of the Kaskaskia, a 

 district having an area of nearly 11,000 square miles. Of the 138 borings 

 reported which reach rock in that district, 128 are found on preglacial 

 uplands, and enter rock at an average depth of only 20.4 feet. The 10 

 borings entering rock along- preglacial valleys show an average drift thick- 

 ness of 106 feet. In the remainder of the Illinoian drift area the borings 

 enter rock at an average depth of about 55 feet, including 52 valley 

 borings. The average thickness on the preglacial uplands of that region 

 scarcely exceeds 40 feet. 



