18 



SOIL PHYSICS AND MANAGEMENT 



(e) Waves. Wave action is confined to the shores of seas and 

 the larger lakes. In many places this agency breaks down solid cliffs 

 into masses of rock that become broken and worn into rounded 

 boulders, then to }>ebl)les, and finally into fine material that is car- 

 ried away and deposited in deeper water or in sheltered inlets to 

 form bars. On the Atlantic coast of Britain waves sometimes exert 

 a pressure of three tons per square foot. The average force is Gil 

 pounds per square foot in summer and 208(> pounds in winter. 

 Each wave results in the movement of more or less material, and 



Fro. 11. Wind-carved granite. The tools were grains of sand. Camps Bay, S. Africa. 

 (Chamberlain and Salisbury, Courtesy Henry Holt & Co.) 



this movement is accompanied by attrition producing fine material. 

 Shaler has observed that at Cape Ann, Mass., granitic paving blocks, 

 weighing about twenty pounds, when, exposed to the action of the 

 surf for a year, were worn into spheroidal boulders that would indi- 

 cate a loss of more than an inch. 



(f ) Wind. The movement of wind is universal, but its effect 

 is destroyed or greatly reduced, at least, at certain seasons of the 

 year over large areas of the land surface by the covering of vegeta- 

 tion. Along the coasts, in the arid interiors of continents, and 

 during winter and spring in many areas, a large amount of work is 



