\ 
THE IOWAN GLACIATION 89 
from these reasonable grounds, there is good basis for think- 
ing that the noncalcareous zone of the loess is largely due to 
the leaching process. There are to be found on the surface 
today living snails, which, according to Professor Shimek, who 
has made an extensive study of them, are mostly of the same 
species as the snails whose shells are found as fossils in the 
loess. If this is true, it is quite evident that if the loess has 
suffered no leaching, snail shells should be found in the non- 
calcareous zone as well as in the calcareous zone beneath. As 
to where present snails secure their carbonate of lime, it is 
quite possible that the lime may be obtained from the feldspar 
particles of the clay, which then may be carbonated by ground 
water. Another evidence that the loess has in reality suffered 
leaching is the fact that lime concretions are found near the top 
of the calcareous zone, the material of these concretions being 
that which was dissolved from the zone above. 
These phenomena, therefore, lead to the conclusion that the 
mass of the loess, which is associated with , the Iowan drift, 
was once calcareous and gray, and that ' it accumulated at a 
time when the rate of deposition was greater than the rate of 
leaching. This suggests strongly a special episode of deposi- 
tion for this particular loess. 
2. The Source of the Loess . — The loess associated with the 
Iowan area is thickest in three situations : ( 1 ) along the river 
valleys leading from the Iowan drift; (2) around the border 
of the Iowan drift; and (3) in those elliptical hills which McGee 
termed paha. 
In some instances the buff loess is so thick along the larger 
valleys that the valley walls rise distinctly above the border- 
ing upland like low ridges. This is so noticeable along the 
Cedar river southwest of Marion and along the Wapsipinicon 
river west of Oxford Junction that one is reminded of the com- 
mon remark of the early geologists who studied these localities, 
that the streams left the plains to cut through the hills. 1 Of 
course, in some instances dune sand is associated. In many 
other places along valleys the loess is thick without forming 
distinct ridges. The accumulation of loess in such marked de- 
posits along valleys is probably due to nearness tq a source 
U t is evident in such cases that the ridges came into existence after 
the valleys were cut. 
