44 CHICAGO ACADEMY OF SCIENCES. 



about forty feet to the plain along the Chicago River, and of 20 

 to 25 feet to the marshy plain in Lake Comity. 



The 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. 



The 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 tmiform height above Lake Michigan throughout their 

 course in Illinois. 



Thickness of Drift. — In numerous borings, 75 to 100 feet in 

 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 is probably largely of earlier date. The follow- 

 ing 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. At Senator C. B. Farwell's artesian well, in Lake Forest, 

 rock is struck at 160 feet. The well mouth is 40 to 45 feet below 

 the crest of the east ridge at Lake Forest and about 75 feet above 

 Lake Michigan. 



2. At Highland Park rock is struck at 160 to 175 feet. The 

 elevation above the lake is 100 to 115 feet. 



3. At Lloyd's artesian well, in the north part of Winnetka, 

 rock is struck at 150 feet. The well mouth is by surveyors" level 

 78 feet above Lake Michigan. 



4. At Ravinia a well strikes rock at 164 feet. The surface 

 level has not been accurately determined, but it is probably about 

 100 feet above Lake Michigan. 



5. Near Schermerville rock is struck at 147 feet. The surface 

 elevation does not exceed 100 feet above Lake Michigan. 



6. On the crest of the west ridge in Sec. 14, Maine Township 

 (T. 41, R. 12 E.), rock is struck at 1 10 feet. The elevation is prob- 

 ably about the same amount above the lake. 



Structure of the Drift. — Along the lake shore the blufifs from 

 Winnetka to the vicinity of Waukegan rise abruptly 75 to 90 feet 

 and present many good exposures of the drift to this depth. There 

 is at the surface a pebbly yellow clay 8 to 13 feet in depth, which is 

 similar to that in the Valparaiso moraine. Beneath this clay is a 

 grayish blue till containing occasional sand pockets saturated with 



