TRAFSPOETATION BY WIND. 1 1 



been caused by the heating of particles of crushed rock. The friction of 

 the loosened mass upon the underlying rock, as well as the mutual friction 

 of the moving fragments, must produce more or less polishing and scratch- 

 ing of the stones. It is probable that it would be difficult to distinguish 

 such stones from those scratched beneath a glacier. 



On hillsides in Maine the slow, imperceptible sliding characteristic of 

 the soil-cap movement has often given an imperfect stratification to fine, 

 clayey till. The till becomes softened and somewhat plastic when saturated 

 by the rains or upheaved and loosened by the frost. When the ground 

 settles, the flat fragments tend to a horizontal position, and on hillsides the 

 shearing force caused by the slow downward movement causes the laminae 

 of clay and plastic materials to become arranged parallel with the slopes. 

 In such situations the till often weathers in layers as regular as those of 

 clay deposited in water. Part of this quasi stratification is doubtless due to 

 the pressure and shear to which the particles of the ground moraine of the 

 ice-sheet were subjected as the ice dragged its vast bulk over them. 



In the modes of drift transportation above mentioned gravity acts 

 directly as the impelling force. Another class of drift agencies comprises 

 those cases where the transportation is effected by moving liquids or gases, 

 including plastic solids, such as ice. In such cases gravity acting directly 

 on the transported matter often does not aid the movement; instead, the 

 weight of the transported body often has to be overcome by the mo\ang 

 fluid. 



TRANSPORTATION BY WIND. 



Where the winds are in general moderate, as they are in Maine, and 

 where rains or snows fall at frequent intervals, the climate is not well 

 adapted to wind transportation. Yet there are in the State large areas of 

 sand now drifting, besides multitudes of dunes long since overgrown with 

 vegetation. Thus the wind is seen to be an important drift agency. 



Most of the drifting sands were originally assorted and deposited by 

 water. The process of drifting generally begins at some small depression 

 in the sand, such as the burrow of an animal. By degrees the depression 

 enlarges, and the sand taken out of the hole goes to make up a low ridge 

 in the direction in which it is blown by the prevaihng Avinds. It is the dry 

 wind that transports sand, rather than even higher winds accompanied by 

 rain. The sand grains on the windward side of the ridge, being exposed 



