DRIFT SHEETS OF NORTHWESTERN ILLINOIS 267 



ity of these exposures occur on slopes where slopewash and deposi- 

 tion make their value debatable and uncertain. Many of the road 

 cuts are also too shallow to show what is vital. It was the working 

 principle of the present study to accept only such data in regard to 

 depth of weathering as could be obtained on the uplands, where 

 there is a minimum of wash, and to secure enough data, if possible, 

 to carry conviction. In addition to examining aU known exposures, 

 borings were made with a sectional auger, i\ inches in diameter, to 

 depths ranging from 2 feet to 17^^ feet. More than 500 such 

 borings were made. The work required somewhat more than two 

 field seasons with the aid of an assistant and an automobile. The 

 distribution of the borings and exposures, which serve as a part of 

 the basis of this paper, is shown in Figure i . 



GENERAL CONCLUSIONS 



As a result of the studies made the following conclusions were 

 reached. 



1. There are at least two distinct drift sheets in northwestern 

 Illinois, west of the Marengo Ridge and north and west of the 

 Bloomington moraine. The uppermost drift in southern Boone, 

 northern DeKalb, eastern Ogle, and southeastern Winnebago 

 counties was deposited at a much later time than the drift in the 

 area to the west. This area (see Fig. 2) has been caUed the Belvidere 

 lobe after the town of Belvidere, immediately south of which the 

 drift is well displayed. 



2. The drift west of the Belvidere lobe is truly Illinoian in age, 

 while that of the Belvidere area is Early Wisconsin in age. The 

 boundary between the two is radically different from the old lowan- 

 lUinoian boundary. 



3. A post-Illinoian ice sheet invaded the Green River basin, 

 approximately to Rock Island County, depositing a thin drift which 

 was subsequently largely buried by the outwash from the Early 

 Wisconsin ice during the building of the Bloomington moraine and 

 by the Rock River Valley train of Late Wisconsin age. The evidence 

 is insufficient to determine whether this ice lobe, now called the 

 Green River lobe, is lowan in age or a correlative of the ShelbyviUe 

 drift of Early Wisconsin age. 



