IOWA ACADEMY OF SCIENCE 
227 
4. It may be suggested that the deposit is of aeolian origin, and that 
the materials were caught up from the flood plains of nearby streams 
and deposited on slightly higher ground. To this it must: be said that 
the streams of this region are small, their flood-plains are narrow, and 
could not possibly yield such a mass of material. It must be remembered 
in this connection that this material, if it be indeed younger than the 
Kaman, wa's deposited after the erosion of the Kansan was complete, 
and the time has been comparatively short. In so short a time, and 
with so inadequate a source of supply, such an extensive deposit could 
not originate. A further objection is that, if this originated from local 
flood-plains, it would be thicker near the flood-plains and thinner on the 
crest of the hills. This is not the case. The theory of a local aeolian 
origin is untenable 1 . 
5. One geologist, in conversation with the writer, has suggested that 
the material may have blown in from the plains of the southwest, and 
may be the accumulations of all the ages since the retreat, of the ice- 
sheet. There are two objections to this theory. First, if this be in- 
deed <a! post-Kansan deposit, it is not the accumulation of all the ages 
since the retreat of the ice-sheet, but was deposited after the Kansan 
had reached its present stage of erosion. The time is too short for the 
accumulation of so much material. Second: only the finer dust would 
travel so far, and the resulting deposit would be a 1 very fine-grained 
loess. This deposit is not a fine-grained loess, but a coarse-grained joint 
clay. This theory therefore must be abandoned. 
So far as the writer can see, these five theories exhaust every possi- 
bility as to the post-Kansan, aeolian origin of this deposit. And a close 
acquaintance with conditions in the field shows every one of the: five to 
be entirely untenable. 
ADDITIONAL EVIDENCE AS TO THE NATURE OP THE SURFACE CLAY. 
The current theory as to the aeolian nature of the surface clay in the 
southwestern counties is based chiefly if not exclusively upon the fact 
that it is nearly devoid of pebbles and boulders. It is taken for granted 
that the latter must be present in all Kansan drift. They are not present 
(or are present sparingly) in the surface clay. Therefore it is pre- 
sumed that the surface clay is post-Kansan in its origin. 
In considering this question it occurred to the writer to question the 
universality of the presence of boulders and pebbles in the Kansan. In 
the very nature of things, it would seem strange that an ice-sheet should 
so arrange or assort its materials that the coarser would be distributed 
