198 THE ILLINOIS GLACIAL LOBE. 



yellow and the blue till local beds of sand or gravel similar to those at 

 their junction. It seems on the whole probable that the two clays consti- 

 tute but a single deposit, and that the color distinction has been larg-elv 

 acquired since deposition. This view seems supported by the fact that the 

 deposits are similar in structure, even in the number of striated stones and 

 in the calcareous rock flour which they contain. The surface portion of 

 the yellow clay to a depth of 4 to 6 feet is more highly oxidized and shows 

 greater discoloration than the lower portion, and contains but little cal- 

 careous material, but these features seem to be due in the main to weather- 

 ing subsequent to deposition and not to original differences. ' 



Surface bowlders are irregularly distributed over the moraine. They 

 are nowhere rare, and occasionally become so numerous as to be trouble- 

 some in cultivating the land. In eastern Illinois they are most conspicuous 

 in the southwestern part of Edgar County. The bowlders consist almost 

 entirely of Canadian rocks, and few of them exhibit stria? or glacial planing-. 

 Some are sharply angular, a feature which indicates that but slight surface 

 change has been produced since they were deposited. This being the case 

 the absence of striae is due not so much to weathering - and removal after 

 striation had occurred as to an escape from striatiug agencies. It is thought 

 that their transportation may have been at a level considerably above the 

 base of the ice sheet. 1 



The portion of the Shelbyville moraine north of Shelbyville carries a 

 larger amount of surface silt than the portion east of that city. Not only 

 the moraine but the district to the east, for a distance of perhaps 20 miles, 

 has a coating of silt so thick that bowlders are completely concealed, for it 

 not infrequently reaches a depth of 5 or 6 feet. On the portion east from 

 Shelbyville the silt is usually but 1 to 2 feet in depth, so that bowlders are 

 frequently seen at the surface. In the vicinity of the Wabash Valley, how- 

 ever, the thickness is 4 to G feet. The silt is usually of a brownish-yellow 

 color, much like that of the oxidized till underneath it, though slightly paler 

 than the till. Where thickest it is somewhat calcareous in the lower portion 

 and carries small molluscan shells of land and water species, similar to 

 those found in the Iowan loess. The silt, however, is usually leached to a 

 depth of 3 or 4 feet. This silt appears to have been deposited very soon 



1 Conip. Cbamberlin, Jour. Geol., Vol. I, pp. 47-60. 



