GENERAL ASPECTS OF ILLINOIAN DRIFT SHEET. 27 



15 to 30 feet above the bordering plains, and the breadth of the main ridge 

 one-half mile, more or less. 



In the northwestern part of Illinois, in Stephenson, Ogle, and Carroll 

 counties, there are gravelly ridges, some of which are of esker type, and 

 are discussed in. detail below. 



Aside from the ridged belts just mentioned, there have been found 

 only occasional knolls, either isolated or in small clusters, and these seldom 

 rise 50 feet above the bordering plains. The knolls are more abundant in 

 western Illinois than in southeastern Illinois and southwestern Indiana. In 

 the latter districts entire counties have been traversed without the discovery 

 of a knoll so much as 10 feet in height. 



Thickness of the drift. — The well records obtained in the region occupied by the 

 Ulinoian drift indicate that the average distance to rock on the preglacial or 

 original uplands is not far from 50 feet. In this 50 feet is included the loess, 

 which is several feet in depth, and possibly also pre-Illinoian drift. Along 

 original valleys the thickness is 100 to 200 feet or more. There are extensive 

 areas in the northwestern counties of Illinois and in the counties bordering 

 the Wabash River, both in Illinois and in Indiana, where the ridges carry 

 scarcely any drift, while neighboring valleys may be filled to a depth of 

 100 feet or more. In western and south-central Illinois the ridges usually 

 carry 30 to 50. feet of drift, the general amount of drift being greater than 

 in northwestern Illinois or southeastern Illinois and southwestern Indiana. 

 Throughout the area occupied by the Ulinoian drift the main preglacial 

 valleys are usually but partially concealed, though the small A^alleys and 

 tributaries are often filled so completely that their position is revealed only 

 by borings. In this respect the Ulinoian drift is in striking contrast with 

 the Wisconsin, for where that sheet is present the main valleys are as com- 

 pletely concealed as the lesser ones. 



Records of a large number of wells which have been sunk in the 

 Ulinoian drift area are presented in the portion of this report dealing with 

 the wells (Chapter XIV). 



structure of the drift. — Throughout the area ocCTrpied by the Ulinoian drift 

 till predominates, there being but a small amount of sand or gravel except 

 in deeply filled valleys. The ridges above mentioned are usually com- 

 posed of till, though pockets or thin beds of sand or gravel have been 

 discovered in some of them. Upon passing toward the glacial boundary 



