476 STEPHEN SARGENT VISHER 



against Penck's hypothesis, for if the loess did not accumulate 

 until near the close of an interglacial epoch, it is probable that the 

 calcareous matter would have been largely leached from the upper 

 layers of the underlying drift. The interglacial epochs are now 

 known to have been sufficiently long for much weathering to take 

 place. Thus considerable evidence seems inconsistent with the 

 hypothesis that the loess was formed chiefly toward the close of an 

 interglacial epoch. 



There is much less evidence against the hypothesis that the great 

 loess deposits accumulated chiefly during the maximum extension 

 of the ice. Indeed it remains as a worthy working hypothesis. 

 However, the question may be raised as to whether or not flood 

 plains of streams would provide adequate supplies of materials for 

 such widespread, rather uniform deposits as those of Russia and 

 of Iowa and Illinois. A further question comes to mind: Would 

 the type of vegetation which would probably occur along the ice 

 front at its maximum extension be that of which the loess gives 

 evidence ? Indeed it seems probable that when the ice advanced, 

 its front lay close to areas where the vegetation was not much thinner 

 than that which today prevails under similar chmatic conditions. 

 If the average temperature of glacial maxima was only about 6°C. 

 lower than that of today, as many authorities consider likely, the 

 conditions just beyond the ice front when it was in the loess region 

 from southern Indiana to Nebraska would probably have been 

 Kke those now prevailing in Canada from New Brunswick to 

 Winnipeg. The vegetation there is quite different from the grassy 

 vegetation of which evidence is found in the loess. The roots and 

 stalks of such grassy vegetation are generally agreed to have helped 

 produce the columnar structure which enables the loess to stand 

 with almost vertical surface. Thus it seems appropriate to add a 

 supplementary hypothesis to suggest that certain phenomena would 

 be readily explainable in case the chief accumulation was during 

 glacial retreat, rather than at the time of maximum extension of ice. 



We are now ready to consider the probability that loess accumu- 

 lated mainly during the retreat of the ice. Such a retreat exposed 

 a zone of drift to the out-blowing glacial winds. Most glacial 

 hypotheses, such as that of upKft, or depleted carbon dioxide. 



