THE TIME OF GLACIAL LOESS ACCUMULATION 477 



call for a gradual retreat of the ice scarcely faster than the vegetation 

 could advance into the abandoned area. Under Huntington's 

 solar-cyclonic hypothesis/ on the other hand, the climatic changes 

 may have been sudden and hence the retreat of the ice may have 

 been much more rapid than the advance of vegetation. Now- 

 wind-blown materials are derived from places where vegetation 

 is scanty. Scanty vegetation on good soil, it is true, is usually 

 due to aridity, but may also result because the time since the soil 

 was exposed has not been long enough so that it may be covered 

 with vegetation. Sand bars, mud flats, and flood plains are common 

 examples. Moreover, violent winds and low temperatures may 

 prevent the spread of vegetation. Thus it appears that unless the 

 retreat of the ice were as slow as the advance of vegetation, a barren 

 area of more or less width must have bordered the retreating ice 

 and formed an ideal source of loess. 



Several other lines of evidence seemingly support the conclusion 

 that the loess was chiefly formed during the retreat of the ice. 

 For example, Shimek, who has made almost a life-long study of 

 the lowan loess, emphasizes the fact that there is often an accumu- 

 lation of stones and pebbles at its base. This suggests that the 

 underlying till was eroded before the loess was deposited upon it. 

 The first reaction of most students is to assume that of course 

 this was due to running water. That is possible in many cases, 

 but by no means in all. So widespread a sheet of gravel could not 

 be deposited by streams without destroying the irregular basins 

 and hollows of which we have seen evidence where the loess Hes 

 on glacial deposits. On the other hand, the wind is competent to 

 produce a similar gravel pavement without destroying the old 

 topography. "Desert pavements" are a notable feature in most 

 deserts. The commonest winds are outward near the edge of an 

 ice sheet, as Hobbs has made us reahze.^ They often attain a 

 velocity of eighty miles an hour in Antarctica and Greenland. 



^Ellsworth Huntington, Earth and Sun, Yale Press, New Haven, 1922; and 

 Huntington and Visher, Climatic Changes, Their Nature and Causes, Yale Press, 1922. 



^W. H. Hobbs, Characteristics of Existing Glaciers, 191 1; "The Role of the 

 Glacial Anti-Cyclone in the Air Circulation of the Globe," Proceed. Am. Phil. Soc, 

 Vol. LIV (1915), PP- 185-225. 



