48 CHICAGO ACADEMY OF SCIENCES. 



This rise of i-| to 3 feet per mile is, of course, not perceptible to 

 the eye. 



The plain between the West and Middle ridges rises from 630 

 feet at Oak Glen to 680 feet at Deerfield, a distance of six miles. 

 From Deerfield north to the State line, a distance of 24 miles, it is 

 shown by the C, M. & St. P. railway survey to stand at 670 to 685 

 feet A. T. Russell station, near the State line, is 673 feet. 



The plain between the Middle and East ridges rises from 630 

 feet opposite Winnetka to about 670 feet at the point where the 

 ridges unite, 20 miles north from Winnetka. 



Thickness of Drift. — On this plain there is much difference in 

 the thickness of the drift and since the surface, aside from the till 

 ridges just described, and the low beach lines, has very little varia- 

 tion in altitude, the difference in depth is due almost entirely to 

 the variation in surface of the underlying rock strata. 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 (i). 



Mr. Samud G. Artingstall, while City Engineer, prepared a 

 map of the city giving the distance to rock at many places. This 

 indicates that a filled valley with rock floor 100 to 125 feet below 

 lake level passes through the north central part of the city, enter- 

 ing the lake south of Lincoln Park. 



West from the city the rock rises over quite extensive areas 

 very nearly to the surface of the plain, or to a height of 30 to 60 

 feet above the level of Lake Michigan. On the border of the Chi- 

 cago outlet there is a nearly continuous exposure of rock from near 

 Summit to the head of the Illinois. 



In the northern portion of the district, between as well as be- 

 neath the till ridges, the rock surface has an average altitude slight- 

 ly lower than in the low plain in the vicinity of Chicago, for the 

 majority of the wells go about 50 feet below the level 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. 



Strueture of Drift. — The drift beneath this plain, like that 

 of the moraine, consists largely of a soft blue till beneath which 

 are remnants of a hard till of earlier age. The tills appear to be of 



\l\ A large amount of material has been collected by the Geological and Natural 

 History Board of The Chicago Academy of Sciences, with a view to preparing a 

 topographic map of the rock surface underlying the citv and border tracts, and 

 which is designed to be published as a bulletin of the Academy. 



