384 "THE ILLINOIS GLACIAL LOBE. 



which may have existed. There seems, therefore, nothing to oppose the 

 correlation of the bowlder train and Blue Island Ridge with Middle Ridge. 



From the south end of Blue Island Ridge to the till ridges in Porter 

 County, Indiana, no line of bowlders or indication of the position of the 

 ice margin has been found. Such features as bowlders may, however, be 

 concealed in much of that district by the heavy deposits of lake sand. 



East Ridge apparently had some continuation southward beneath the 

 present lake. Prof. L. E. Cooley, of the Chicago Drainage Commission, 

 states 1 that in a series of dredgings in the south end of the lake, made a 

 few years since, under his direction, a bowldery belt was traced for several 

 miles southeastward from Winnetka, the present terminus of East Ridge. 

 This bowldery belt is probably a residue from a ridge of till which has been 

 cut away by the lake. 



RELIEF. 



West Ridge rises with a somewhat abrupt slope about 25 feet above 

 the plain along the Des Plaines River. On the inner (eastern) side there is 

 a gradual descent of about 40 feet to the plain along the Chicago River, 

 and of 20 to 25 feet to the marshy plain in Lake County. 



Middle Ridge has a relief of 20 to 25 feet above the marshy plain on 

 its outer border, and a gradual slope of 25 to 40 feet to the sag or slough, 

 which lies on its inner border. 



East Ridge has a relief of 20 feet in northern Lake County and 35 to 

 40 feet in southern Lake and northern Cook counties above the sag on its 

 outer border. The reliefs of all these ridges are lessened at the northern 

 end because of increase of elevation in the sags or plains which separate 

 them. The ridges each maintain a nearly uniform height of about 100 feet 

 above Lake Michigan throughout their course in Illinois. 



THICKNESS OF DRIFT. 



In numerous borings, 75 to 100 feet iii depth, no rock is struck and no 

 outcrops of rock occur along this portion of the lake shore. The drift 

 beneath the level of the base of these moraines was probably deposited by 

 earlier ice advances. The following list of borings which have struck rock 

 indicates that in several places at least the rock surface lies much below 

 the surface of Lake Michigan. 



1 Communication to the writer. 



