136 THE ILLINOIS GLACIAL LOBE. 



In the southeastern part of Winnebago County and in the portion of 

 Ogle Count v between the outer moraine of the Wisconsin series and the 

 present valley of Rock River, and also in southwestern Boone County, 

 gravel knolls of considerable prominence are found. These in most cases 

 appear to be a product of the Iowan invasion, for the gravel has a freshness 

 corresponding to that of the Iowan till. In one instance (near King's 

 Station, in Ogle County) a gravel knoll, opened for railway ballast, shows 

 what appears To be a more aged gravelly deposit at the base than that 

 constituting the body of the knoll. It had a height of 45 feet before the 

 excavation was made, and several others in that vicinity are equally large, 

 though the majority are 20 feet or less. A chain of sharp gravel knolls in 

 southwestern Boone County, near Irene, trends north to south and has a 

 length of over a mile. At the north it merges into a till ridge with gentle 

 slope and with a smoother contour than the gravelly belt. This ridge, with 

 the gravelly knolls, is probably a marginal deposit formed at a slight halt 

 made by the ice. The knolls are only 20 to 30 feet high, but rise promptly 

 from the bordering upland plain. Near Stillman Valley a few esker-like 

 ridges of gravel are developed which trend northeast to southwest. These 

 are closely associated with gravelly knolls, and the longest are only a mile 

 or less in length. An esker-like ridge was also observed by Hershey near 

 Sterling, Illinois. 



The greater part of the surface of this sheet of drift, like that of the 

 Illinoian sheet, is plane or but very gentlv undulating and devoid of 

 notable topographic features. The principal ridg-es and knolls associated 

 with the drift sheet are represented on PI. XII, a reference to which will 

 serve to show the small percentage of the area occupied by them. 



THICKNESS OF THE DRIFT. 



In eastern Winnebago and northern Boone counties, where the drum- 

 linoid drift ridges and associated knolls abound, the drift of Iowan age is 

 known to have considerable thickness, possibly an average of 40 feet or 

 more. Aside from this small district, the drift referable to the Iowan 

 invasion appears to form generally only a thin and somewhat patchy 

 deposit. In the vicinity of the border the drift is often confined to the 

 small knolls, 10 or 15 feet in height, the low ground among the knolls 



