344 GEOLOGY, 



driven back and forth over them, and in time reduced to movable 

 dimensions (Fig. 302). They then become the tools of the waves, and 

 in use, are reduced to smaller and smaller size. Thus bowlders are 

 reduced to cobbles, cobbles to pebbles, pebbles to sand, and sand to silt. 

 The silt is readily held in suspension in agitated water, and thus is 

 carried out beyond the range of breakers, and settles in water so deep 

 as not to be effectively agitated to its bottom. Thus one generation of 

 bowlders after another is worn out, and the comminuted products are 

 carried out from the immediate shore and deposited in deeper water. 



The effectiveness of waves, whether they work by impact of water 

 alone, or by impact of water and detritus, is dependent on their strength 

 and on the concentration of their blows. ^ The strength of waves is 

 dependent on the strength of the winds (or other generating cause) and 

 the depth and expanse of the water, and the concentration of their 

 blows is conditioned by the slope against which they break. On exposed 

 ocean-coasts the fetch of the waves is always great. The winds are 

 variable. For a given coast they have an average strength, but the 

 effectiveness of wave-erosion is determined less by the average strength 

 of waves than by the strength of the storm-waves. This is often very 

 great. On the Atlantic and North Sea coasts of Britain, winter breakers 

 which exert a pressure of three tons per square foot are not infrequent.^ 

 So great is the force of exceptional storm-waves that blocks of rock 

 exceeding 100 tons in weight are known to have been moved by them. 

 Ground-swells, ''even when no wind is blowing, often cover the cliffs 

 of north Scotland with sheets of water and foam up to heights of 100 or 

 even nearly 200 feet. During northeasterly gales the windows of the 

 Dunnet Head lighthouse, at a height of upwards of 300 feet above high- 

 water mark, are said to be sometimes broken by stones swept up the 

 cliffs by sheets of sea-water." ^ The average force of waves on the Atlan- 

 tic coast of Britain has been found to be 611 lbs. per square foot in sum- 

 mer, and 2086 lbs. in winter. ^ 



Where deep water extends up to the shore, the force of the wave is 

 almost wholly expended near the water line ; where shallow water 

 borders the land, the force of the waves is expended over a greater area. 



1 Willis. Jour, of Geol., Vol. I, p. 481. 



2 Stevenson. Trans. Roy. Soc. Edin., Vol. XVI, p. 25. Treatise on Harbors, 

 p. 42. Quoted by Geikie, Text-book of Geology, p. 437. 



3 Geikie. Text-book of Geology, 3d ed., p. 437. 

 * Brit. Assoc. Rept., 1850, p. 26. 



