THE IOWAN GLACIATION 
91 
ner. Thus it is clear that the loess also is younger than the 
Illinoian drift by a considerable interval. 
In the Iowan area the loess is generally so thin that the 
leached zone extends down through the loess into the till, but 
it is a striking fact that in those areas where the loess averages 
four to six feet in thickness the leached zone passes into the 
underlying till scarcely more than a few inches to one and 
one-half feet. Evidently the leaching process has but recently 
reached the till. This view is supported also by the fact that 
the top of the till is practically the same color as the lower part 
of the loess. No soil or gumbo development was found any- 
where between the Iowan drift and the loess. Where the loess 
is absent or nearly so the drift is leached correspondingly more, 
but a little less than the loess where thick sections are exposed. 
All of these evidences bear out the interpretations made from 
the areal relations of the loess, its composition and character- 
istics, that the loess was deposited closely following the retreat 
of the Iowan ice-sheet. 
This conclusion may at first seem incompatible with the evi- 
dence of the fossil shells which, according to Shimek, indicate 
much the same climatic conditions as the region possesses today. 
But it should be recognized that the climate at the close of a 
glacial epoch must be decidedly different from that at the be- 
ginning. A glacier is the product of glacial conditions. Its 
development and advance are preceded first by the culmination 
of glacial temperatures and precipitations. Its retreat occurs 
only after the glacial climate ceases. Probably the climate of 
the zone in close proximity to a retreating continental ice- 
sheet’ is affected somewhat by the presence of the ice, but yet 
the conditions are probably much less severe than in the case 
of an advancing ice-sheet. In the first case, the climate opposes 
the existence of the ice mass, while in the second, it is respon- 
sible for if. Is not the close of any glacial epoch in reality 
that time when the climate becomes permanently effective in 
causing glacial retreat? Granted that present temperatures 
probably did not prevail in the immediate vicinity of the Iowan 
ice-edge, it does seem likely that after the earth’s climate had 
changed and the ice had melted back several hundred miles from 
its extreme limit, seasons approximating those of the present 
prevailed where the marginal loess occurs. Even though this 
