102 PHYSIOGRAPHY 



three or four feet high, are made in this way in a single winter. 

 Bowlders pushed up by the ice year after year sometimes make 

 "walls" around lakes; hence the name Wall Lake, which is not un- 

 common in the northern states (Fig. 3, PI. XXVII, p. 92). 



Ice in rivers. Rivers also freeze over in cold climates, and 

 when the ice breaks up in the spring, stones and bowlders to which 

 it was frozen in the banks, are sometimes floated miles down the 

 river. When the river ice breaks up in the spring, masses of it may be 

 floated down-stream, and may gather in vast "jams" behind dams 

 or bridges, and the dams or bridges may be swept away. The jams 

 themselves form a sort of obstruction, holding back the water and 

 causing floods above. When a jam breaks, the water above may 

 sweep down the valley with destructive violence. 



Ice on the sea. In high latitudes ice forms on shallow sea- 

 water, and in polar regions it becomes several feet deep, not only 

 along shores but on the open sea. The sea-ice is often broken up 

 in the summer, and the floating pieces are called floe-ice. When 

 the floes are crowded together, they make ice-packs, some of which 

 are hundreds of miles across. Ice-packs are one of the obstacles 

 to polar navigation. 



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

 of rock has been mentioned (p. 26). When it is remembered that 

 a freezing temperature occurs during some part of the year over 

 more than half the earth, it will be seen that the total effect of the 

 freezing of water in the pores and crevices of rock must be great in 

 long periods of time. Water freezing in the soil sometimes " heaves" 

 (displaces) walls if they do not go below the depth of freezing, and 

 it sometimes "works up" stones and bowlders through the soil in 

 cultivated fields. The frozen water in the soil has a protective 

 effect also. It makes the soil solid for the time being, and so retards 

 or prevents erosion by wind and water. 



Snow and snow-fields. Snow falls in high latitudes during 

 much of the year, and in middle latitudes during winter. Except 

 on high mountains, little snow falls in low latitudes, and the little 

 that does fall is soon melted. While snow lies on the surface, it 

 protects the vegetation beneath from great changes of temperature, 

 and especially from the repeated thawings (by day) and freezings 



