PAPERS ON GEOLOGY 



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picked up, together with fine material from other sources and 

 carried forward for some distance and again dropped, and the 

 process was repeated again and again until the fine silt was 

 spread widely over the interstream areas, and much of it is 

 now a long distance from its original source. 



SOURCE OF THE PEBBLES IN THE LOESS 



Pebbles are rare in the loess, but a few occur in the lower 2 

 to 5 feet of the deposit over practically all of the loess area, be- 

 ing more frequent where the loess is thin than where it is best 

 developed. The presence of occasional pebbles in the lower 

 part of the loess has been considered by some geologists as evi- 

 dence of the aqueous origin of at least this portion of the de- 

 posit, but it is just as difficult to explain how occasional 

 pebbles could be included in the midst of an otherwise homo- 

 geneous, fine grained, unstratified deposit laid down by water 

 as in such a deposit made by wind. 



A study of the process of accumulation of the loess now 

 forming over the surface of the early Wisconsin till has fur- 

 nished valuable information concerning the probable source 

 and manner of inclusion of the pebbles in the lower part of 

 the loess in other regions. The higher portions of the surface 

 of the early Wisconsin till in Champaign county are in places 

 covered with 1 to 3 feet of porous, fine grained, loess-like 

 material which, like the loess in other places in the State, is 

 largely composed of minute, fresh, angular fragments of 

 quartz, feldspar, hornblende, and other minerals derived from 

 igneous rocks. In this recent loess small pebbles are some- 

 what more numerous than in the lower part of thicker loess 

 deposits over the Illinoian and Kansan drift sheets, probably 

 on account of the slower rate of accumulation of the loess now 

 forming on the Wisconsin drift, compared with the rate at 

 which the loess accumulated in early Peorian time. 



Over the more dry, uncultivated areas of this surface bur- 

 rows of ground squirrels and other animals are common. 

 Many of these burrows pass through the thin loess mantle into 

 the unlerlying till, and the dirt thrown out around the tops of 

 the holes often contains a few small pebbles. In poorly drained 

 areas crayfish holes are in places almost as numerous as the 

 burrows on the higher lands. In the craters built up around 

 the tops of these holes a few pebbles ranging in size up to one 

 inch in diameter were collected. Pebbles are also occasionally 

 carried from areas where no loess is present to areas where 

 loess is accumulating in the mud on the feet of hoofed animals. 



