THE LAKE-BOEDER MOEAINIO SYSTEM. 



411 



the following analyses of samples of these pebbleless clays, made by Prof. 

 W. A. Noyes. of Terre Haute, Indiana. 1 



A nalyses of clays of Pleistocene age. 



In explanation of the contrast in the amount of coarse material in the 

 clays bordering- the lake in northwestern Indiana, compared with those in 

 Cook County, Illinois, it may be remarked that it is probable that the char- 

 acter of the underlying rocks will prove an important factor. In Cook 

 County, Illinois, the underlying rocks are the somewhat resistant Lockport 

 (Niagara) limestone, while in the neighboring portion of Indiana the rocks 

 immediately beneath the drift are the Devonian shales, which are easily com- 

 minuted and ground into a clayey material. It is probable, however, that 

 the conditions of deposition were slightly different in the two districts. In 

 Cook County the southwestward line of discharge may have been open 

 sufficiently to allow a considerable part of the fine material to be carried 

 down the Des Plaines Valley, while in the Indiana district it may all have 

 been deposited without much transportation or sifting out of the finer 

 material. The localities where the pebbleless clay are best exposed are 

 beneath the bay-like extensions of Lake Chicago, and may possiblv be 



>22d Ann. Eept. Indiana Geol. Survey, 1898, pp. 128, 134, 137, and 139. 



