CHAPTER XXI 

 GLACIERS AND LAKES 



263. The Snow Line. Whenever the temperature of 

 the air is below the freezing point, if much water vapor is 

 present, crystals of ice form and fall singly or in groups, 

 called snowflakes. These sometimes melt in passing through 

 warm air near the earth and then fall as rain, but in cold 

 climates they reach the earth as snow. At very high alti- 

 tudes, even in the tropics, the mountains and plateaus may 

 be perpetually covered with snow. The summer limit of the 

 snow is called the snow line. In winter, of course, the snow 

 line is lower down the mountain than at other seasons. In 

 summer numerous waterfalls on the mountains testify to 

 the melting of the snow back to a higher line. 



264. The Beginning of a Glacier. Above the summer 

 snow line, the snow by its own weight becomes more and 

 more densely packed. As the air between the flakes is 

 pressed out, the snow loses its white look and becomes 

 almost like ice. Where this takes place on a slope, the ice 

 begins to move slowly downward in a broad or narrow sheet, 

 according to the form of the land beneath. Such moving 

 ice is a glacier. It may extend many miles down the valley, 

 as is the case with several glaciers in Alaska, Norway, and 

 Switzerland. 



A high plateau may be covered with an ice sheet which 

 moves very slowly in all directions from its highest part. 

 A large part of a continent may be covered with moving 

 ice, as happened in the northern part of North America 

 many thousand years ago and as is the case in Greenland 

 to-day. 



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