GEOLOGICAL AND NATURAL HISTORY SURVEY. 47 



Middle Ridge the interval is but a few miles and is mainly 

 covered by heavy deposits of lake sand and gravel which would 

 obscure any bowlder connection which may have existed. There 

 seems, therefore, nothing to oppose the correlation of the bowlder 

 train and Blue Island ridge with the 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 may 

 however, be concealed in much of that district by the heavy depos- 

 its of lake sand. 



Continuation of East Ridge. — The East ridge apparently had 

 some continuation southward beneath the present lake. Prof. L. 

 E. Cooley, of the Chicago Drainage Commission, informs me 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 the terminus of the East ridge 

 at Winnetka. This bowldery belt is perhaps a residue from a ridge 

 of till which has been cut away by the lake. 



ASSOCIATED TILXi PliAINS. 



The ridges just discussed occupy but a small portion of the 

 area embraced between the Valparaiso moraine and the shore of 

 Lake Michigan. The greater part of the area is a plain underlain 

 by till deposits. On this plain, as is shown later, the lake has built 

 or carved its shore lines and made thin deposits of sand or gravel. 



Altitude and Slopes. — In the portion of the plain west and 

 south from Chicago the altitude at the border of the Valparaiso 

 moraine is 620 to 630 feet A; T., or 40 to 50 feet above the lake. 

 From this border there is a gradual descent toward the present 

 shore of Lake Michigan, where the altitude is about 590 feet A. T. 

 This portion of the plain was covered by the lake at the time the 

 upper beach was formed, the altitude of that beach being 635 to 640 

 feet A. T. 



Upon passing northward along the till plains just described 

 the narrow plains which separate the ridges soon rise above the 

 level of the Upper beach. The plain that lies between the West ridge 

 and the Valparaiso moraine slopes eastward at the rate of several 

 feet per mile. Its west border next the Valparaiso moraine rises from 

 635 feet at Ovington station, on the Omaha division of the C. & N. 

 W. railroad, to about 690 feet at the line of Cook and Lake Counties, 

 a distance of 18 miles, and about 725 feet in northern Lake County 

 at the line of Illinois and Wisconsin, a distance of 24 miles farther. 



