406 THE ILLINOIS GLACIAL LOBE. 



sand were removed the variations would be still greater, since the sand is 

 thicker on the borders of the valleys than on the divides. 



Covert Ridge follows the shore of Lake Michigan so closely that a 

 plain is present on its inner border along only a small part of its course from 

 the State line northward to the Kalamazoo River. Where present it shows 

 a descent of 20 or 30 feet per mile toward the lake. In places where the 

 ridge is distant 2 or 3 miles and its inner border stands 60 to 70 feet above 

 the lake the till surface drops down nearly to lake level at the lake shore. 

 Sand deposits have considerable depth on the lower parts of this plain and 

 bring the surface up to a height of 35 to 40 feet or more above the lake. 



• THICKNESS OF DRIFT. 



On the Illinois portion of these jdains there is much difference in the 

 thickness of the drift. The difference in thickness is due almost entirely to 

 the variation in the surface of the underlying rock strata, since the surface 

 of the plains has only a slight variation in altitude. Within the city of 

 Chicago, where the surface is especially flat, several rocky prominences, 

 come to the surface, or are concealed but slightly by drift, while among 

 them the drift accumulations extend to depths of 100 to 125 feet or more. 

 Mr. Samuel G. Artingstall, formerly city engineer, has prepared a map of 

 the city showing the distance to rock in many places. This indicates that 

 a filled valley with rock floor 100 to 125 feet below lake level passed 

 through the north central part of the city, entering the lake south of Lin- 

 coln Park. West from the city the rock rises over quite extensive areas 

 nearly to the surface of the plain or to a height of 30 to 60 feet above the 

 level of Lake Michigan. 



In northern Cook and in Lake County, between the till ridges as well 

 as beneath them, the rock surface has an average altitude somewhat lower 

 than in the low plain in the vicinity of Chicago, for the majority of the 

 wells reach a level about 50 feet below the surface of Lake Michigan 

 before entering- rock. It is estimated that the thickness of drift in this 

 northern portion will average nearly 150 feet, while in the low plain the 

 average will scarcely exceed 50 feet. 



The thickness of drift in the Indiana and Michigan portions is usually 

 o-reat, as has been indicated above. (See pages 392-393.) 



