224 GEOLOGY 



temperature is 20 Fahr. If now the temperature is lowered to 

 -10, and such a change of temperature is not uncommon in the 

 northern part of the United States, the ice contracts notably. In 

 contracting, it either pulls away from the shores, or cracks. If 

 the former, the water from which the ice is withdrawn quickly 

 freezes; if the latter, water rises in the cracks and freezes there. 

 In either cases, the ice-cover of the lake is again complete. If the 

 temperature now rises to 20 the ice expands, and the solid cover 



Fig. 182. Shore of Wall Lake, Iowa. (Photo, by Calvin.) 



becomes too large for the lake, and must either crowd up on the 

 shores or arch up (wrinkle) elsewhere. 



If the water near the shore is very shallow, the ice freezes to the 

 sand, gravel, and bowlders at the bottom. If the land at the shore 

 is very 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 are 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 a little above the normal water- 



