THE DISTRIBUTION OF LOESS FOSSILS 133 



laminated loess clay, which is usually non-fossiliferous, and 

 which varies from a few to more than 30 feet in thickness. 

 When fossils occur in this upper stratum they are few in number 

 and widely scattered. 1 



The presence of this shell-bearing stratum suggests that, for 

 the period during which it formed the surface soil, and while it 

 was slowly accumulating, the conditions in this particular locality 

 were more favorable to the growth of land snails than now. 

 There was probably more vegetation, and hence the surface was 

 not so frequently storm-swept as at present. This does not 

 necessarily signify that general climatic conditions were differ- 

 ent, but that these particular banks or bluffs were more heavily 

 timbered, with the Missouri River probably flowing at its base, 

 its surface conditions being similar to those of many timbered 

 hills and knolls between Omaha and Nebraska City west of the 

 Missouri. 



It is interesting to note that between Iowa and Nebraska the 

 Missouri River now flows along the western side of its broad 

 valley, and that the adjacent western bluffs are more heavily 

 timbered and contain all the living species of molluscs herein 

 recorded, with the exception of Nos. 1 and 16, while the more 

 remote eastern bluffs are more barren and rugged. The shell- 

 bearing band may simply represent the period during which the 

 river in its shiftings occupied the eastern part of the valley. 



The foregoing facts lend support to the aeolian theory of the 

 origin of the loess, as is shown by the following considerations. 



I. The general manner of distribution of the modern and 

 fossi! molluscs is essentially the same, this fact indicating that 

 they were not carried by waters, but were quietly buried in dust. 

 Had they formed a part of river drift they would be more fre- 

 quently heaped together, not scattered as we find them in the 

 loess, and fluviatile shells would be more or less intermingled. 



1 At the base of the bluff, in exposure A", what seemed to be a second shell-bear- 

 ing layer was observed about 75 feet below the main fossiliferous band. The section, 

 however, was more or less obscured, and the mass may have slipped from the bluff 

 above. The fossils in column K in the table are from this stratum. It will be 

 observed that they are ordinary forms which are abundant in the main shell stratum. 



