362 ILLINOIS STATE ACADEMY OF SCIENCE 



leys. Since the tributaries of the Embarrass, Muddy, 

 Indian, and Raccoon creeks were not receiving supplies 

 of gravel, sand, and silt, from the ice edge, they could not 

 build up their valleys as fast as did the larger streams. 

 The result was that they were damned at their mouths 

 and ponded. Lakes covered their valleys which later 

 were filled with lake deposits. On such a basis may be 

 furnished the explanation for the extremely level surface 

 of these tributary valleys. 



When the ice edge retreated north of the source of the 

 Embarrass, its supply of gravel, sand and silt were, for 

 the most part, shut off, but the Wabash continued for a 

 long time to build up its valley. The result was that the 

 Embarrass was ponded for a distance up its valley. 

 Under such conditions, it is natural for the drainage 

 ditches, which for the most part follow the lines of old 

 channels, to drain into the Embarrass rather than into 

 the Wabash. This ponded condition also gives a good 

 explanation for the meanders of the lower Embarrass. 

 When the ice edge was near its southern position, the ma- 

 terial carried by the Wabash was coarse and the gravel 

 deposits were then laid down. As the ice edge retreated 

 farther to the north, the deposits became finer, and first 

 the sand and then the silt were laid down. When it re- 

 treated so far to the north that it was beyond the source 

 of the Wabash and its water found a lower outlet, the 

 river began to lower its bed by cutting through its de- 

 posits. This has proceeded only a short distance, but it 

 left broad terraces standing only a little higher than the 

 other portions of the lowland, and it permitted the Em- 

 barrass and its tributaries to lower their courses enough 

 to drain the ponds in their valleys, or if they had become 

 filled by lake deposits, it permitted these streams to lower 

 their channels slightly below the level of the surface of 

 those deposits. 



