CHAPTER V 

 THE WORK OF SNOW AND ICE 



Ice beneath the surface. The wedge-work of ice in the crevices 

 of rock has already been mentioned (p. 25). When the great 

 areas where water freezes during some part of the year are con- 

 sidered, it is clear that the aggregate effect of its freezing in the 

 pores and crevices of rock must be great in long periods of time. 

 Even the freezing of water in the soil is not without effect. This 

 is shown by the disturbance of the walls of buildings if their founda- 

 tions are not below the depth of freezing, and by the working up of 

 stones and bowlders through the soil of the fields, as freezing and 

 thawing succeed each other. Frozen water in the soil makes it 

 solid, and temporarily retards or prevents surface erosion. 



Ice on lakes and ponds. Since fresh water is densest at 39 

 . Fahr., ice does not commonly form on the surface of a lake until 

 the temperature from top to bottom is reduced to this point. Cooled 

 below 39, the surface water fails to sink, and cooled to 32, it freezes. 

 If the lake is small and shallow, it will freeze over completely where 

 the temperature is notably below 32 for any considerable period 

 of time. It is under these circumstances that lake ice becomes 

 most effective. 



Let us suppose a lake in temperate latitudes, where the range 

 of winter temperature is considerable, to be frozen over when the 

 temperature is 25 Fahr. If now the temperature is lowered to 

 10, and such a temperature is not uncommon in the northern part 

 of the United States, the ice contracts. 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 from which the ice is withdrawn quickly freezes; if the latter, water 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 25 

 the ice expands, and the solid cover becomes too large for the lake, 

 and must either crowd up on the shores, or arch up (wrinkle) 



I elsewhere. 

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

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