48 PRODUCTIVE FARMING 



and thawing of water in it. This force is especially active 

 in cold climates. 



EXERCISE. To Show the A dion of Freezing Water. Fill 

 a small bottle with water and place it where the water will 

 freeze. The bottle will probably be broken by the expansion 

 due to the change in temperature. 



Water also makes the rock particles finer. After rains 

 the flowing of water grinds the fragments together. The 

 force of running water is continually wearing them smaller 

 (Fig. 256) . The finest particles are first carried away by the 

 water to valleys below (Figs. 26 and 27). 



FIG. 27. A rocky hill changing into soil. The soil is gradually carried to the valley 

 by rains and irountain streams. (Plant Industry.) 



Water in the form of glacial ice, carrying sand and boul- 

 ders frozen into the under side, wears down the rocks over 

 which it passes and carries the material with it (Fig. 28). 



The effects of plants in the rotting of rocks are of two 

 kinds: (1) The roots grow into the crevices found in rocks. 

 This in time forces the rock particles apart (Fig. 29). 

 Roots of trees often lift large rocks and cause them to crack 

 by their own weight. This aids the rocks to crumble into 

 soil grains. (2) When the plants decay acids are formed, 



