408 THE ILLINOIS GLACIAL LOBE. 



numerous. The well drillers usually distinguish the hard till from the over- 

 lying- softer till and apply to it the name "hardpan," while the soft till is 

 called clay. 



Although the great body of the drift is till, there are found numerous 

 thin beds of sand or gravel in which water collects in sufficient quantities 

 to supply the wells. There are also small pockets of dry sand or gravel 

 occupying but a few cubic feet each. Such pockets were found in the exca- 

 vation of the main lake tunnel, and have been described by Dr. Edmund 

 Andrews in a paper published in the American Journal of Science. 1 They 

 were in some cases completely inclosed by till. 



On the surface of the plain both above aud below the upper beach 

 there is quite generally present in Lake and Cook counties a clay in which 

 pebbles are far less numerous than in the till; it, however, carries occasional 

 bowlders. It rang-es in thickness from a few inches up to several feet. This 

 deposit is perhaps a subaqueous till dropped in a body of water held 

 between the ice front and the higher parts of the morainic system while 

 the ice sheet was still overhanging the inner slope. 



The depth of leaching and oxidation is markedly less on the plain 

 covered by Lake Chicag-o than on the till ridges or the Valparaiso morainic 

 system. Numerous acid tests show the leaching on the plain to extend 

 only to a depth of a few inches, seldom more than 2 feet. On the till ridges 

 the leaching is usually thorough to a depth of 3 or 4 feet, while on the 

 "Valparaiso system it is rare to obtain a response with acid within 5 or 6 feet 

 of the surface. On the plain and also on the till ridg-es the surface oxida- 

 tion is usually but 3 to 6 feet, while on the Valparaiso system it is 6 to 10 

 feet or more. This difference in the amount of oxidation and leaching may 

 be attributable in part to the more compact nature of the till ridges and 

 plains, but it is probably in part due to the later date at which they became 

 exposed to atmospheric action. 



There have been several deep lines of excavation made in Chicago and 

 vicinity which have afforded excellent opportunities for studying- the struc- 

 ture of the drift. The longest line is the Drainage Canal, now under con- 

 struction, which opens a channel 25 to 40 feet in depth from the Chicago 

 River at Bridgeport to the Des Plaines River at Summit. Along the Des 



! Ami. .lour. Sci., 2i) series, vol. 43, 1867, pp. 75-77. 



