132 THE ILLINOIS GLACIAL LOBE. 



Below Rockford the border is found to follow nearly the course of 

 Rock River to the vicinity of Oregon. Here Hershey makes a division of 

 the drift margin. The outer margin passes westward to Polo. The inner 

 margin passes southward along or near Rock River to the vicinity of Dixon. 

 It there crosses the river in a westward course and follows the north border 

 of a lowland tract which extends a few miles back from Rock River. North 

 £>f Sterling it joins the outer margin. Hershey's tracing of the outer margin 

 was carried no farther than Polo, but the writer has examined the district 

 south and west from Polo, and also the portion of this margin from Polo 

 eastward to Rock River. 



Eastward from Polo there is a definite border, characterized by low 

 swells of till, among which are saucer-like depressions, giving the surface a 

 much fresher appearance than that of the older sheet which occupies the 

 district to the north and west. At Polo the margin appears to swing south- 

 ward and follow the east border of the Elkhom Creek Basin into Whiteside 

 County. The border is not so definite here, however, as east of Polo, there 

 being only a few till swells and occasional basins in the outer part of the 

 drift sheet. This line connects in eastern Whiteside County, about 5 miles 

 northeast of Sterling, with the inner margin traced by Hershey. Hershey's 

 chief criterion in mapping the inner border is a change in the character of 

 the till, such as characterizes the margin farther north, that to the east of it 

 being more sandy than that to the west and displaying a pink tint not noted 

 to the west. The portion between the inner and outer margins has a more 

 compact texture than the remainder of the lowan drift sheet. The cause 

 for this difference is not yet apparent. 



The margin lies north of Rock River at least as far west as Rock Island 

 Junction. There is a small area on the north side of Rock River, west from 

 Rock Island Junction, in which a sheet of drift with fresher aspect than the 

 Illinoian is found. It is apparently confined to a lowland tract whose north 

 border is followed approximately by the Chicago and Northwestern Rail- 

 road from Round Grove to Morrison, and whose west border lies along Rock 

 Creek from Morrison south to the valley of Rock River. To the north and 

 west of this lowland there is a heavy deposit of loess which largely con- 

 ceals the underlying deposits, but the lowland has only a thin coating of 

 loess except in a few ridges resembling the paha of eastern Iowa. Such an 

 extension of the lobe as would be necessary to carry it to Morrison seems 



