390 



GEOLOGY, 



sand, gravel, and bowlders at the bottom. If the adjacent land is low, 

 the ice in expanding may shove up over it, carrying the debris frozen 

 in its bottom. It may even push up loose gravel and sand in front of 

 its edge if they be present on the shore. Where bowlders are frozen 

 to the bottom of the ice, the shore- ward thrust in expanding has the 

 effect of shifting them in the same direction, and even of lifting them 



Fig. 333. — Calcareous tufa domes. Pyramid Lake, Nev. (Russell.) 



a little above the normal water-level. This constant process of con- 

 centrating bowlders at the shore-line gives rise to the ^'walled" lakes, 

 which are not uncommon in the northern part of the United States. 

 The ''wall" does not commonly extend entirely around a lake, though 

 it exists at various points on the shores of many lakes. In making 

 the walls, the ice shoved up by winds, especially in the spring when the 

 ice is breaking up, cooperates. 



