THE BLOOMINGTON MORAINIO SYSTEM. 283 



well records, which show a change from soft till to hard at the base of this 

 deposit. It is much less than on the morainic ridges of the Bloomington 

 system, and probably does not average more than 50 feet. In southern 

 Kane County and thence southwest along the border of Fox River it 

 appears to be but 20 to 40 feet, but at the inner border of the main moraine 

 in Dekalb, Lasalle, and Bureau counties it is in places 100 feet or more. 

 In the counties bordering the Illinois River it is 50 to 100 feet or more, but 

 in the Vermilion Basin it scarcely reaches 50 feet. In Iroquois County it 

 is 50 to 100 feet. 



The older drift deposits are thin in the vicinity of Fox River and 

 Vermilion River and the west-flowing portion of the Illinois River, but 

 elsewhere within the tracts under discussion they are generally present in 

 large amount, and there is probably an average thickness as great as that 

 of the Wisconsin drift sheet. In portions of the Iroquois River basin 200 

 feet or more of the older drift is present, but in the remainder of the tract 

 a thickness exceeding 100 feet is rare. By reference to the detailed dis- 

 cussion of the wells in the several counties embraced within these inter- 

 morainic tracts the variations in thickness may be seen. 



STRUCTURE OP THE DRIFT. 



Throughout the greater part of the area embraced in these inter- 

 morainic tracts the Wisconsin drift consists mainly of a moderately stony, 

 soft, blue till, very similar to that found in the moraines. This till is 

 replaced by sand or gravel deposits along the line of the eskers which occur 

 in Kane and Dekalb counties, and also along many of the tributaries of 

 Fox River. There is also considerable sand associated with the till in a 

 belt several miles in width along the west side of Fox River in Kane, Ken- 

 dall, Dekalb, and Lasalle counties. On the borders of the Vermilion River 

 in Livingston County and southern Lasalle County sand and gravel deposits 

 predominate over the till. With these exceptions the till greatly predomi- 

 nates over the sand and gravel. In man}' localities it forms so solid a sheet 

 that strong wells are not obtained in it, whereas in the districts in which 

 sand and gravel predominate over the till abundance of water is obtained 

 at moderate depths. 



The drift of these intermorainic tracts is capped only by thin deposits 

 of silt,- seldom exceeding 3 feet in depth, and in places too thin to conceal 

 the surface bowlders. 



