582 THE ILLINOIS GLACIAL LOBE. 



midst of the city of Chicago. Calumet River enters the county from 

 Indiana, and after running westward to Blue Island, a distance of about 

 12 miles, it turns abruptly eastward. As previously noted, it formerly 

 returned into Indiana and discharged at the head of the lake, but the 

 present mouth of the river is at South Chicago, in Illinois. The Des 

 Plaines River flows southward from the north line of the county to Summit, 

 having an average distance of about 10 miles from the lake. It there turns 

 southwestward, leaving the county near the village of Lemont. The 

 extreme northwestern portion of the county is tributary to Fox River. 

 Drainage lines are poorly developed in both the elevated and the low-lying 

 portions of the county, a feature which is characteristic of several counties 

 in northeastern Illinois. 



The eastern portion of the county is a plain rising gradually westward 

 to the borders of the Valparaiso morainic system. A large part of this plain 

 stands only 10 or 15 feet above Lake Michigan, but on the western border 

 its elevation is 40 to 60 feet or more. The plain is interrupted by a small 

 drift ridge leading north a few miles from Blue Island. There are also 

 drift ridges near the border of Lake Michigan, in the north part of the 

 county, which rise to a height of 75 or 100 feet above the lake. The por- 

 tion of the plain standing within 60 feet of lake level, as already noted, 

 has been occupied by Lake Chicago, whose discharge was southwestward 

 through the "Chicago Outlet." 



The Valparaiso morainic system passes across the northwestern part of 

 Cook County in a southward course, and, after crossing Dupage Count} 7 , 

 again enters Cook, occupying the southwestern borders of the county. 

 This system stands in its higher parts fully 200 feet above the lake, and a 

 small area in the extreme northwest part of the county reaches an elevation 

 more than 300 feet above the lake. 



The drift is comparatively thin on much of the plain in Chicago and 

 to the south and also along the Chicago Outlet, rock quarries being numer- 

 ous and many instances of wells encountering rock at slight depth being 

 found. There are, however, occasional wells which reach a level 100 feet 

 or more below Lake Michigan before encountering rock. The available' 

 data seem to indicate that a buried valley enters the lake near Lincoln 

 Park, whose course can be followed for several miles back from the lake 



