IOWAN DRIFT SHEET AND ASSOCIATED DEPOSITS. 143 



Milford and vicinity. 



Feet. 



Upper or Wisconsin drift sheet 50 -(- 



Middle drift sheet 40 or 50 



Lower drift sheet 25 



Donovan and vicinity. 



Feet. 



Upper or Wisconsin drift sheet 100-|- 



Middle drift sheet 40 



Lower drift sheet 30 



Iii counties farther north a' series is found similar to that displayed in 

 Iroquois County. It is best shown in localities where the drift is very 

 thick, as is the case in much of Kane, Dekalb, and McHenry counties. 

 As these counties are adjacent to the exposed portion of the Iowan drift 

 sheet and equally far to the north, there is scarcely a doubt that the middle 

 sheet should be referred to the Iowan. Numerous sections of wells in 

 these counties are presented in the portion of this report dealing with the 

 wells of Illinois (Chapter XIV), together with a discussion of the probable 

 age of the sheets, where more than one was penetrated. From these it may 

 be seen that the Middle or Iowan drift, as in the exposed portion, is much 

 thinner than the Wisconsin and has a depth about the same as the middle 

 sheet in Iroquois County. These points of resemblance, although not 

 demonstrative of contemporaneity, at least suggest the possibilit}^ that the 

 Iowan drift extends into Iroquois County. 



A loessdike silt, as shown below, covers the Illinoiau drift of southern 

 Illinois outside the limits of the Wisconsin and is traceable northward 

 several miles beneath the Wisconsin. This appears to be a deposit of 

 Iowan age like the loess of western Illinois, and, like the loess, it may be 

 derived from the ice sheet. Its presence in southeastern Illinois and also 

 in districts farther east, for it is found as far east as central Ohio, is thought 

 to bring strong support to the view that the ice sheet, at the Iowan stage, 

 did not fall short many miles of reaching the line occupied at a later date 

 by the Wisconsin ice invasion. This silty outwash is of sufficient volume 

 to mantle the region, as far south as the Ohio River, with a deposit having 

 an average depth of probably 5 feet. The volume of this deposit would 

 seem to indicate that the southern limits of the ice sheet from which it was 

 derived were at least within 100 miles, and possibly much nearer, the south- 

 ern limits of the Wisconsin drift sheet, not only in Illinois, but in Indiana 

 and Ohio. 



