THE WORK OF SNOW AND ICE. 



269 



the great obliquity of the rays sometimes allows them to strike under 

 isolated bowlders. In this case, they are warmed from below, and thus 

 aid rather than hinder the melting of the ice. 



The same principles apply to the moraines. A thin bowlder mo- 

 raine in high latitudes is sometimes sunk below the surface (Fig. 242). 

 Usually, however, a medial moraine protects the ice beneath from 

 melting, and occasions the development of a ridge of ice beneath itself. 

 As the ice on either side is then lowered by ablation, the moraine mat- 

 ter of the medial belt tends to sHde down on either hand. The same 



Fig. 243. — A Swiss glacier, showing surface moraines, characteristic profile, etc. 



is true of the lateral moraines. So far does this spreading go, that in 

 some cases the lower end of a glacier is completely covered vAXh. the 

 debris which has spread from the medial and lateral moraines. Ex- 

 amples of this may be seen in almost any region of abundant, long, alpine 

 glaciers. 



Dust-wells. — The wind-blown dust sometimes gives rise to peculiar 

 topographic features of small size. The dust is not distributed by the 



