THE SURFACE OF THE EARTH 167 



sand. They are not stationary, but migrate in the 

 direction toward which the prevailing wind blows. 

 The migration of the dunes may do great damage by 

 covering up farm land, forests, railroads, and some- 



FIG. 63. The pines and grasses shown at the left hold the sand 

 and prevent the migration of the sand dune. 



times buildings. Trees, shrubs, and grasses that will 

 grow in sand are sometimes planted on dunes to pre- 

 vent their migration. The roots of the plants hold 

 the sand. 



Effect of change of temperature on rocks. Changes 

 in the temperature of the atmosphere also have a 

 mechanical effect on the earth's surface. You have 

 already learned that heat expands and cold contracts 

 objects. Exposed rock surfaces are heated in the day 

 and cooled at night. On mountain tops this daily 

 change of temperature is very great. This continual 



