CHAPTER XI 



IOE AND WIND SCULPTURES 



165. Snow in Winter. When the temperature of the air 

 falls below the freezing point, its moisture congeals into 

 little flake-like crystals and falls as snow. Where the 

 cold is continuous for a con- 

 siderable time, the snow may 



accumulate in deep layers over 



the ground. If the heat of 



the summer is not sufficient 



to melt all the snow which SNOW CRYSTALS - 



falls in the winter, then the layers of snow will increase 



from year to year. 



To have this occur the temperature for the whole year 

 need not be below the freezing point, but the heat of the 

 summer must not be sufficient to melt the snow which fell 

 in the colder season. Lofty mountains, even in the trop- 

 ics, have their upper parts snow-covered. In the far north 

 and the far south the line of perpetual snow falls to sea 

 level, inclosing the mighty expanse of the Arctic and the 

 Antarctic snow fields. 



166. Glaciers. Wherever there is not enough heat in 

 the warm season to melt the snow which accumulates dur- 

 ing the cold season, a thick covering of snow and ice will 

 in time be formed. The ice is due to the pressure exerted 

 on the lower layers by the weight of the snow above and 

 to the freezing of the percolating water which comes from 

 the summer melting of the upper snow layers. > 



Although ice in small pieces is brittle, in great masses 



