140 B. SHIMEK AFTONIAN FOSSILIFEROUS GRAVEL AND SAND BEDS 



However, the excellent preservation of the fossils, especially the deli- 

 cate molliisks, their distribution and abundance, the occurrence of parts 

 of the same skeleton (as in the Akron and Wilkenson sections) in such 

 position that some of the ligaments must have been present when deposi- 

 tion took place, and the fact that the same fossils have not found their 

 way into the drifts, and moreover are not known from any clearly older 

 horizon, all give strong testimony that the fossils are really of Aftonian 

 age. 



Conclusions 



The following conclusions are based on the field studies here recorded : 



1. The silts, sands, and gravels under discussion are interglacial and 

 Aftonian. 



2. The Aftonian is widely distributed in Iowa and adjacent territory 

 and is especially prominent in western Iowa and eastern Nebraska. 



3. The fauna of the Aftonian, both mammalian and molluscan; terres- 

 trial and aquatic, indicates a comparatively mild climate during that 

 period. 



4. The presence of both the large land mammals and the terrestrial 

 mollusks shows that large land areas were exposed in the vicinity of the 

 flooded streams, which evidently transported and deposited the Aftonian 

 sands and gravels. 



5. While the Aftonian mammalian fauna has become extinct, the mol- 

 luscan fauna, so far as it has been observed, has remained unchanged to 

 the present day. 



