THE TIME OF GLACIAL LOESS ACCUMULATION 475 



the end of the interglacial epochs, and (3) at times of full glaciation, 

 each hypothesis is much weakened by evidence that supports the 

 others. The evidence of boreal animals seems to disprove the 

 h3q)othesis that the loess was formed in the middle of a mild inter- 

 glacial epoch. On the other hand, Penck's hypothesis as to loess 

 at the end of interglacial times fails to account for certain character- 

 istics of the lowest part of the loess deposits and of the underlying 

 drift. Instead of normal valleys and consequent prompt drainage, 

 such as ought to have developed before the end of a long interglacial 

 epoch, the surface on which the loess lies shows many undrained 

 depressions. Some of these can be seen in exposed banks, while 

 many more are inferred from the presence of shells of pond snails 

 here and there in the overlying loess. The pond snails presumably 

 hved in shallow pools occupying depressions in the uneven surface 

 left by the ice. Another reason for questioning whether the loess 

 was formed chiefly at the end of an interglacial epoch is that this 

 hypothesis does not provide a reasonable origin for the material 

 which composes the glacial loess deposits of important loess- 

 covered regions. Near the Alps, where the loess deposits are 

 small and where glaciers probably persisted in the interglacial 

 epochs and thus supplied flood material in large quantities, this 

 .shortcoming perhaps does not appear important. In the broad 

 Upper Mississippi basin, however, and also in the Black Earth 

 region of Russia there would seem to be, during an interglacial 

 epoch, no way to get the large body of material composing the loess, 

 except by assuming the existence of great deserts to windward. 

 But there is little or no evidence of such deserts where they could 

 be effective. The mineralogical character of the loess of lowan 

 age proves that the material came from granitic rocks, such as 

 formed a large part of the drift. The nearest extensive outcrops 

 of granite are in the southwestern part of the United States, nearly 

 a thousand miles from Iowa and Illinois. But the loess is thickest 

 near the ice margins and thins toward the southwest and in other 

 directions, whereas if its source was the southwestern desert its 

 maximum thickness would probably be near the margin of the 

 desert. Furthermore the similarity in calcareous content of the 

 loess and the underlying drift, reported by Chamberlin, points 



