SURFACE OF THE EARTH 



193 



216. Work by freezing water. When water falls upon rock 

 surfaces, some of it may enter crevices in the rock. When 

 water freezes, it expands, and its expansive force is very 

 great. Alternate freezing and thawing may break the rock 

 into smaller and smaller pieces. An interesting experiment 

 upon the effect of freezing water may be made by filling a 



FIG. 94. The results of frost action 



A mountain field strewn with rock which has been hroken, principally by frost. 

 (Alleghany Mountains in Pennsylvania) 



strong metal cylinder with water, plugging the ends of the 

 cylinder tightly, then placing the cylinder where the water 

 will freeze and carefully noting the results. 



Along rocky cliffs where water runs into crevices and 

 freezes, one may often observe its effects in breaking up the 

 rock (fig. 94). Tons of rock may be pushed aside a short dis- 

 tance 'by the expansive force exerted through the freezing of 

 a small amount of water. When we think of all the small 

 crevices in rock into which water may run and afterwards 



