50 CHICAGO ACADEMY OF SCIENCES. 



in thickness from a few inches up to several feet, in which pebbles 

 are far less numerous than in the till. It however carries occa- 

 sional bowlders. This deposit seems attributable to the combined 

 agency of water and ice. It was probably a deposit made in a 

 body of water held between the ice front and the Valparaiso 

 moraine while the ice sheet was still in the immediate neighborhood 

 of the moraine. 



In the district within the limits of the upper beach there are 

 deposits of sand and gravel marking the shore lines of the lake. 

 These deposits form a less regular coating than the clay just men- 

 tioned. Over much of the plain the deposition was so light as to 

 leave scarcely a trace of sand in the soil, but in places the deposits 

 cover the till of the old lake bottom to a depth of 20 to 25 feet. The 

 heavy deposits are found chiefly along the present lake border 

 from Evanston southward, where there is a continuous belt of sand 

 ranging in width from one-half mile to three or four miles, and 

 having an average depth of not less than 10 feet. The southwest- 

 ward lake outlet appears to have carried away much of the sand 

 which was brought into the southern end of the lake while that 

 outlet was open. 



The depth of leaching and oxidation is markedly less on the 

 plain covered by the lake than on the till ridges or the Valparaiso 

 moraine. Numerous tests with hydrochloric acid show the leach- 

 ing on the plain to extend only to a depth of a few inches, seldom 

 more than two feet, while on the ridges it is rare to obtain a re- 

 sponse at less than five or six feet from the surface. On the plain 

 the surface oxidation of the till is only two to six feet, while on the 

 ridge it is six to ten feet or more. This difiference in the amount 

 of oxidation and leaching is probably attributable in part to the flat- 

 ness of the plain and in part to the later date at which the plain 

 was exposed to atmospheric action. 



There have been several deep lines of excavation made in 

 Chicago and vicinity which have afforded excellent oportunities 

 for studying the structure of the drift. The longest line is the 

 drainage canal, now under construction, which opens a channel 25 

 CO 40 feet in depth from the Chicago River at Bridgeport to the 

 Des Plaines River at Summit, that is entirely in drift. Along the 

 Des Plaines also the excavation is largely in drift to the vicinity 

 of Lemont, where the canal becomes a rock channel. From 

 Bridgeport to Summit there is little besides, till, but from Summit 

 to Lemont gravel, sand and tlie coarser material deposited along 



