26 PHYSIOGRAPHY 



freezes, and the soil becomes solid. While frozen it cannot be 

 blown or washed away. In low temperatures, too, the moisture 

 which falls from the atmosphere falls as snow instead of rain, and 

 does not have the same effect on the land as rain. When the snow 

 melts, the water runs over the surface much as rain-water would. 



Where the soil is thin, the waters which sink may freeze in the 

 cracks of the rocks beneath. Since water expands about one-tenth 

 on freezing, the ice which forms in a crack acts like a wedge, prying 

 the rock apart. The effect of expansion during freezing is illus- 

 trated by the breaking of a bottle in which water is allowed to freeze. 

 This process of rock-breaking is most important when there is abun- 

 dant moisture, and where the changes of temperature above and 

 below the freezing-point of water are frequent. 



Expansion and contraction of rock ; rock-breaking. When solid 

 rock has no covering of loose material, as is often the case on steep 

 slopes, it is heated by day and cooled by night. At high altitudes, 

 and especially on slopes and cliffs exposed to the noonday sun, the 

 daily changes of temperature of the surface of the rock are great. 

 In such places, the surface of the rock may become very hot while 

 the sun shines. Heat expands rock, and as the heated part expands, 

 it is likely to scale off from the part beneath (PL IX, p. 16). As 

 the sun goes down, the surface cools and contracts. The outermost 

 film of rock cools first and most, and tends to break. The breaking 

 of cool glass when touched with hot water, or of hot glass when 

 touched with cold water, involves the same principle. 



The breaking of rock by heating and cooling, even when ice is 

 not formed, is very common. On hot days in summer, the blocks 



Fig. 17. A cement walk broken under expansion by sun-heat. 



of cement in cement walks sometimes expand so much that they 

 arch up where they meet (Fig. 17). This results in the breaking of 

 the cement. The heat of the sun sometimes so expands the rock in 



