50 SOILS AND FERTILIZERS 



results in breaking up the rock masses, forming smaller 

 bodies. The force exerted by water when it freezes is 

 sufficient to rend large rocks. 



56. Physical Action of Water. Water acts upon soils 

 both chemically and physically. It is the most impor- 



FlG. 19. Boulder split by Frost. 

 (Minnesota Geological and Natural History Survey.) 



tant agent that takes a part in soil formation. The sur- 

 face of rocks has been worn away by moving water and 

 in many cases deep ravines and canons have been 

 formed. This is called erosion. The pulverized rock, 

 being carried along by the water and deposited under 

 favorable conditions, forms alluvial soil. This physical 



