IOWA ACADEMY OF SCIENCES. 121 



be wholly independent of the process of accumulation. It is 

 now pretty generally conceded that the loess is genetically 

 related to the lowan drift, — perhaps the over wash from that 

 sheet. It is also well known that the lowan carried the 

 largest and freshest bowlders of any sheet and it is reasonable 

 to suppose that the liner materials were equally fresh at the 

 time they were deposited. This is evidenced by the lowan 

 drift itself, the surface, only, showing any signs of weathering. 

 The mud flats were, doubtless, much more important then 

 than now, and if atmospheric circulation was equally as vigor- 

 ous as at the present time, wind erosion and deposition would 

 be much more widespread and important, and the rate of 

 accumulation might be so much accelerated that oxidation and 

 leaching of the rock meal would be imperfect or almost wholly 

 wanting. The loess deposits, which have been protected by 

 the Wisconsin drift, lend credence to this view. The 

 exposures near Kelly and Ames are not only unoxidized and 

 unleached, but still retain their original blue color, which is 

 so characteristic of unaltered secondary deposits. These 

 deposits also emphasize the extremely short time interval 

 between the deposition of the loess and the Wisconsin advance. 

 The loess, whore unprotected, is a straw to gray-brown 

 throughout, and the lime concretions sufficiently attest that 

 incipient leaching has begun. In places where the deposit has 

 neither lost by erosion nor gained by deposition, the leaching 

 zone varies from two to four feet in thickness and is identical 

 with the wind accumulations along the streams of to-day. The 

 former, in all probability, originated through the rapid 

 accumulation of perfectly fresh materials from the extensive 

 mud flats and overwash plains, which formed an apron to the 

 lowan till sheet, while the latter represents the much slower 

 assembling of the leached and oxidized materials from the 

 alluvial plains of to-day. 



While the processes which obtained during the two sets of 

 deposits cannot be demonstrated to have been identical, their 

 inherent resemblances and environments are certainly very 

 striking. Aside from the comparisons already made, they are 

 very closely related faunally. Professor Shimek" has shown, 

 that with few unimportant exceptions, the loess molluscs were 

 all air-breathers, wiiose habitat must have been very similar to 

 that which prevails in the Iowa-Nebraska region of to-day. 



*The exhaustive memoirs which embody the results of this Iteen, conscientious 

 observer and conservative wiiter may be found in the recent volumes of these pro- 

 ceeding's. 



