478 STEPHEN SARGENT VISHER 



Such winds, however, usually decHne rapidly in velocity only a few 

 score miles from the ice. Thus their effect would be to produce 

 rapid erosion of the freshly bared surface near the retreating ice. 

 The pebbles would be left behind as a pavement, while sand and 

 then loess would be deposited farther from the ice where the winds 

 were weaker and where vegetation was beginning to take root. 

 Such a decrease in wind velocity may explain the occasional vertical 

 gradation from gravel through sand to coarse loess and then to 

 normal fine loess. As the ice sheet retreated, the wind in any given 

 place would gradually become less violent. As the ice continued 

 to retreat, the area where loess was deposited would follow at a 

 distance, and thus each part of the gravel pavement would in turn 

 be covered with loess. 



The hypothesis that loess is deposited while the ice is retreating 

 is in accord with many other lines of evidence. For example, it 

 accords with the boreal character of the mammal remains as de- 

 scribed above and of the depauperated snail fauna found in the zone 

 nearest the ancient ice sheets. Again, the advance of vegetation 

 into the barren zone along the front of the ice would be delayed by 

 the strong out-blowing winds. The common pioneer plants depend 

 largely on the wind for the distribution of their seeds, but the 

 glacial winds would carry them away from the ice rather than toward 

 it. The glacial winds discourage the advance of vegetation in 

 another way, for they are drying winds, as are almost all winds 

 blowing from a colder to a warmer region. Such winds, however, 

 would interfere less with the northward spread of grasses propagated 

 by root shoots and by abundant seeds than it would interfere 

 with the spread of trees. The fact that remains of trees sometimes 

 occur at the bottom of the loess probably means that the deposition 

 of loess extended into the forests which almost certainly persisted 

 not far from the ice at its maximum advance. This seems more 

 hkely than that a period of severe aridity before the coming of the 

 glacier killed the trees and made a widespread steppe or desert. 

 Penck's chief argument in favor of the formation of loess before the 

 advance of the ice rather than after appears to be that since loess 

 is lacking upon the youngest drift sheet in Europe it must have 

 been formed before rather than after the last or Wiirm advance of 



