GEOLOGICAL AND NATURAL HISTORY SURVEY. 45 



water. These, however, form but a small part of the drift. The 

 bulk of the bluff is a compact till, but moderately pebbly and ex- 

 posing only an occasional imbedded bowlder. It was noted that 

 tlie pebbles and bowlders are usually glaciated. 



Neither the east ridge nor west ridge nor the northern 

 portion of the middle ridge has gravel knolls of any consequence, 

 but the southern portion of the middle ridge lying in Cook County 

 has many such knolls. Indeed, nearly every prominent knoll 

 contains gravel. It seldom exceeds 15 feet in depth and ap- 

 pears to be confined to the knolls, for they are situated on 

 a basement ridge of till similar to the till exposed along the lake 

 bluff. But one complete reliable section of the drift could be 

 obtained, which, though valuable, needs to be supplemented by 

 other records to furnish a satisfactory knowledge of the lower 

 portion of the drift. The well is located at Ravinia. Its section 

 was furnished by William McWendle, of Oak Glen. 



1. Pebbly, yellow clay • • • • 11 feet 



2. Grayish-blue, pebbly clay 60 " 



3. Gray clay, very pebbly. 10 to 12 



4. Grayish-blue, pebbly clay, lighter color than No. 2 70 



5. Clay, resembling putty 4 to 5 " 



6. Limestone ■ • 22 " 



Total drift ■ ■ 164 feet 



SUPPOSED COREEIiATIONS. 



None of these moraines have been found to have connection 

 at their southern end with the massive Valparaiso moraine, nor do 

 they admit of continuous tracing around the southern end of the 

 lake within (north of) that moraine. The weak development in 

 that district seems the more remarkable since there is on the east 

 side of Lake Michigan, northward from Porter County, Indiana, 

 a series of ridges of similar size and complexity to that under dis- 

 cussion and wdiich are probably its continuation. The conditions 

 which affected the southern end of the ice lobe at the time these 

 belts were forming are so poorly known that it may be difficult to 

 ascertain ' what caused this wide gap. The question naturally 

 arises whether the expanded lake and its old outlet may not have 

 removed the ridges. 



In the case of the western ridge this suggestion is inapplica- 

 ble, since the terminus at Mont Clare is outside the well-defined 

 beaches and above their level. The ice sheet here, however, may 



