302 THE ILLINOIS GLACIAL LOBE. 



to much disturbance after deposition. In this respect these knolls and 

 ridges differ from the bedding- usually displayed by eskers. 



Bowlders are found in moderate number over all parts of the moraine. 

 The}- are especially abundant in a belt extending from Gilbert station north- 

 ward to the vicinity of Crystal Lake. The belt is widest near the Kane- 

 McHenrv county line, having there a width of about 4 miles. There is 

 probably significance in the fact that this bowlder belt borders the head of 

 the Kishwaukee gravel plain, which was an avenue of discharge for the 

 glacial waters. 



The bowlders are usually crystalline rocks of Canadian derivation, but 

 there are also present large limestone blocks, which in some cases will 

 furnish several wagon loads of building - stone. These are especially 

 abundant west of Elgin, and are found occasionally farther north, in both 

 Kane and McHenry counties. They are apparently derived from the ledges 

 of Lockport (Niagara) limestone, which underlie the district immediately 

 to the east and probably extend into the district covered by this moraine. 

 In some cases these large masses of limestone have led the residents to 

 suppose that the rock is in situ and that extensive quarries might be opened 

 by the removal of a slight amount of drift. The error of this interpretation 

 has usually been discovered upon quarrying a few loads of stone. W ells 

 west of Elgin, in the neighborhood of these limestone blocks, indicate that 

 the drift there has an average thickness of more than 100 feet. Whether 

 the supposed limestone outcrop in eastern McHenry County, mentioned in 

 the Geology of Illinois, 1 is a ledge in situ or a transported block similar to 

 those just mentioned was not ascertained. 



CORRELATIONS. 



Satisfactory correlations of this portion of the composite belt with the 

 more clearly differentiated moraines in the districts to the south have not 

 yet been established. Two quite distinct interpretations have been suggested 

 in the course of the investigation. By the first interpretation this portion 

 of the composite belt is made to be the continuation of the inner part of the 

 Bloomington morainic system, which connects with it near Elburn. The 

 very strongly morainic tract immediately northeast of Elburn would, in this 

 case, be situated at a sharp bend or reentrant angle in the ice margin, and 



i Vol. IV, pp. 131, 132. 



