THE TIME OF GLACIAL LOESS ACCUMULATION 473 



years ago when the origin of the loess was under heated discussion. 

 Furthermore, the emphasis in the past has been on the agencies 

 of deposition rather than on the time of deposition. Indeed the 

 latter question does not seem to have received much consideration, 

 in spite of its importance in the interpreting of the climatic condi- 

 tions during a part of the past. 



These four hypotheses as to the time of origin of loess imply 

 differences in its climatic relations. If loess was chiefly formed 

 during t)rpical interglacial epochs, or toward the close of such epochs, 

 profound general aridity must seemingly have prevailed in order 

 to kill the vegetation and thus enable the wind to pick up sufficient 

 dust. If the loess was chiefly formed during times of extreme 

 glaciation when the glaciers were supplying large quantities of fine 

 material to out-flowing streams, less aridity would be required, but 

 seemingly there must have been sharp contrasts between wet 

 seasons in summer when the snow was melting and dry seasons in 

 winter. Alternate floods and droughts would thus affect broad 

 areas along the streams. Hence arises the hypothesis that the wind 

 obtained the loess from the flood plains of streams at times of 

 maximum glaciation. If the loess was chiefly formed during the 

 rapid retreat of the ice, alternate summer floods and winter droughts 

 would still prevail, but much material could also be obtained by the 

 winds, not only from flood plains, but also from the deposits exposed 

 by the melting of the ice and not yet covered by vegetation. 



In support of the hypothesis of the interglacial origin of loess, 

 Shimek and others state that the glacial drift which lies beneath 

 the loess commonly gives evidence that some time elapsed between 

 the disappearance of the ice and the deposition of the loess. For 

 example, most of the locally abundant shells of snails in the loess 

 are not of the sort now found in colder regions, but resemble those 

 found in the drier regions. It is probable, Shimek concludes, that 

 if they represented a glacial epoch all would be dwarfed by the 

 cold as are the snails of far northern regions. The gravel pavements, 

 discussed below, are pointed out by Shimek as strong evidence of 

 erosion between the retreat of the ice and the deposition of the loess. 



Turning to the second hypothesis, namely that the loess accumu- 

 lated near the close of the interglacial epoch rather than in the 



