IOWA ACADEMY OP SCIENCES. 
121 
be wholly independent of the process of accumulation. It is 
now pretty generally conceded that the loess is genetically 
related to the Iowan drift, — perhaps the over wash from that 
sheet. It is also well known that the Iowan carried the 
largest and freshest bowlders of any sheet and it is reasonable 
to suppose that the finer materials were equally fresh at the 
time they were deposited. This is evidenced by the Iowan 
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, wfind 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, where 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 
Iowan 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, whose habitat must have been very similar to 
that which prevails in the lowa-Nebraska region of to-day. 
* The exhaustive memoirs which embody the results of this keen, conscientious 
observer and conservative writer may be found in the recent volumes of these pro- 
ceedings. 
