136 B. SHIMEK 



a shorter dry period during each year), and hence less dust; 

 that the winds were less violent, and that there were greater 

 areas completely covered with vegetation, this resulting in the 

 necessity of transporting dust much greater distances, which 

 would therefore be finer. 1 



It should be borne in mind that the above-noted differences 

 between the regions in question actually exist today. There is 

 more rain, there are larger areas closely covered with vegetation, 

 and less violent winds prevail, in eastern Iowa, and eastward, and 

 considering the position of mountain chains and seas, the same 

 differences must have existed for a long time. That they did 

 exist during the deposition of the loess is also indicated by the 

 proportionately somewhat larger number of species in the eastern 

 loess, which prefer or require moist habitats. But the fauna of 

 the eastern, or Mississippi River loess is essentially a terrestrial 

 fauna. The great fluviatile groups now everywhere common in 

 the streams of eastern Iowa are wanting in the loess, and the few 

 fossil aquatic species are such as today prefer ponds, and are 

 often found even in those which dry up during the summer. 



It may again be emphasized that the fossils show no greater 

 difference between the surface conditions which existed during 

 the deposition of the loess of the eastern and the western parts 

 of Iowa, than exists today between the surface conditions of the 

 same regions. This fact is irrefutable, and must not be over- 

 looked in any discussion of the conditions under which loess 

 was deposited. 



NOTES AND EXPLANATION OF MAP 

 [Scale, 8 in. to 1 mile] 

 The exposures are represented by heavy lines. 



EXPOSURES A, B, and C 

 These were cut out of the same ridge in street grading. The shell-bearing 

 stratum shows well on the east, south and west sides of C. It is about 12-15 

 feet thick. Above it there is a layer of clay about fifty feet thick and almost 

 entirely devoid of fossils. 



1 See Udden, loc. cit., pp. 56, 57 and 67. 



