CHAPTER VII. 



THE IOWAN DRIFT SHEET AND ASSOCIATED DEPOSITS. 



THE IOWAN SHEET OF THE ILLINOIS LOBE. 



The sheet of drift to which the name Iowan is here applied is referred 

 to the Iowan stage of glaciation, not because of direct connection with the 

 Iowan drift of eastern Iowa, but because of an apparent similarity with the 

 Iowan drift of eastern Iowa in its connection with the great sheet of loess 

 in the Mississippi Basin. As shown below, the loess overlaps this drift sheet 

 only a short distance, and was deposited apparently while the ice sheet was 

 meltirjg away, there being no clear evidence of an exposure of its till to 

 protracted atmospheric action prior to the deposition of the loess. 



DISTRIBUTION. 



The Iowan sheet of the Illinois lobe was formed by a southwestward 

 ice movement over northern Illinois. Its western border has been traced 

 in some detail by Mr. Oscar Hershey, through Winnebago, Ogle, Lee, and 

 Whiteside counties. The writer had previously noted the occurrence of 

 this drift in Winnebago, Ogle, and Lee counties, but had not attempted a 

 precise mapping- of its western border. 



The border is found to enter Illinois from Wisconsin at the valley of 

 Sugar River, about 12 miles west of Beloit. It follows this valley south- 

 ward to the Pecatonica Valley and thence, as shown in PI. XII, passes up 

 the Pecatonica about to the line of Winnebago and Stephenson counties, 

 where it crosses to the south side of the river and returns eastward to the 

 city of Rockford, thus forming a narrow lobe at the Pecatonica Basin, hav- 

 ing a protrusion of about 12 miles and a width no greater than its length. 

 This lobe is called the Pecatonica lobe, since it occupies a low district or 

 basin drained by the Pecatonica River. 



