262 THE ILLINOIS GLACIAL LOBE. 



THICKNESS OF THE DRIFT. 



The thickness of the drift in the Bloomington morainic system is prob- 

 ably about equal to the measure of the relief above the outer-border 

 district, which has a maximum of nearly 200 feet and which averages 75 

 to 100 feet along the ridges. The thickness is 50 feet or less between the 

 ridges and on plane tracts north and east of them. Small valleys had been 

 formed in the Shelbyville sheet prior to the Bloomington invasion, and 

 these valleys were filled with the drift of the Bloomington morainic system. 

 They appear, however, to have been usually but 30 to 50 feet or less in 

 in depth, so that the thicknessof the Bloomington sheet is not greatly 

 increased at these lines. 



The drift extends to some depth below the base of the Bloomington 

 drift sheet. It is found that the earlier sheets of the Wisconsin series are 

 present in considerable strength, as well as the Illinoian drift. The Iowan 

 drift is present in northern Illinois, but its border, as already noted, passes 

 under the Wisconsin in Bureau County. It is not known to be present 

 beneath the Bloomington system south from Bureau County, unless it be 

 on the inner border of the system in Iroquois and neighboring counties. 

 This matter is discussed above in connection Avith the Iowan drift sheet. 



In determining the lower limits of the Wisconsin drift, two conspicuous 

 lines of evidence are drawn upon. One is an abrupt change in the texture 

 of the drift, the Wisconsin drift being fresh and soft, while the underlying- 

 sheets are harder and more aged in appearance. The other is the occur- 

 rence of a black soil, beds of peat, or other decisive evidence of atmospheric . 

 action, produced at the surface of the lower or buried sheet of drift prior to 

 the deposition of the later drift. In the portion of the Wisconsin drift 

 lying outside the limits of the Iowan it is often an easy matter to decide upon 

 the line of contact between the Wisconsin and the Illinoian by the change 

 in texture alone. It is so marked that the majority of well drillers have 

 recognized the two sheets even where no soil or peat has been preserved at 

 their junction. Where the two lines of evidence are combined, it becomes 

 an easy matter to decide upon the line of contact. It is not so easy a 

 matter to decide upon the limits of the Wisconsin drift where it is underlain 

 by the Iowan, for the contrast in texture is not so great as between the 

 Wisconsin and Illinoian, though the Iowan is seldom so fresh in appearance 



