PAPERS OX GEOGRAPHY AND GEOLOGY 1S5 



exposed. The fractured character of the bedrock form- 

 ing the floor of this col is a factor favoring rapid erosion. 

 The col, however, is only about an eighth of a mile wide 

 and does not itself seem likely to have carried a great 

 volume of water, nor does Bradshaw Creek, into which 

 the waters entered after crossing the col, show evidence 

 of having carried an abnormally large volume of water. 



Most of the materials which were presumably de- 

 posited in Little Grassy Creek lake have been eroded 

 away, but in places in the valley a deposit of gray clay 

 containing much rotted limestone boulders is found 

 which was probablv formed during the existence of the 

 lake. 



Very little outwash seems to have been developed in 

 front of th-e margin of the ice after it had reached its 

 maximum southern extent. Igneous boulders are found 

 in places for distances of two miles or more south of the 

 margin of the glacial deposits, but they are merely loose 

 in the beds of the creeks and were not seen in any defin- 

 ite arrangement that could be considered characteristic 

 of outwash deposits. The absence of these deposits is 

 probably due to the rapid and pronounced erosion which 

 has taken place since glacial times, and also to the ten- 

 dency of the present drainage to transport the debris in 

 the valleys to the north. Outwash boulders transported 

 in this direction soon become mixed with other glacial 

 material and are indistinguishable from it. 



The retreat of the Illinoian ice seems to have been in 

 a measure a replica of its advance in so far as the de- 

 posits which the ice left are concerned. Local lakes were 

 formed in valleys, and Drury and Little Grassy lakes 

 extended to the north until the ice ceased to obstruct the 

 drainage in that direction. In these lakes more silt and 

 sand accumulated, and elsewhere where water sorting 

 was not active, a deposit of more or less heterogeneous 

 materials. 



To summarize, then, the outstanding features of the 

 glaciation of the Oarbondale quadrangle are as follows: 



1. The advance of the ice to its maximum southern 

 limit was accompanied by the formation of lakes in the 

 vallpys of many streams draining toward the ice. In the 

 case of two lakes water rose high enough to cross the 



