THE WORK OF THE OCEAN 301 



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 depends on their strength and on the 

 concentration of their blows. 1 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 determined 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. The average force of waves on the Atlantic coast of 

 Britain has been found to be 611 Ibs. per square foot in summer, 

 and 2,086 Ibs. in winter, but winter breakers which exert a pressure 

 of three tons per square foot are riot infrequent. Exceptional 

 storm-waves are known to have moved blocks of rock exceeding 

 100 tons in weight. Waves are most efficient on bold coasts 

 bordered by broad expanses of deep water, for here 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. 



The less familiar phases of wave-work are accomplished by 

 hydraulic pressure, compressed air, shore ice, etc. When the water 

 of a wave is driven into an open joint or a cave, the hydraulic 

 pressure may be so great as to break the rock if it is weak. When 

 water is driven with force into a cave, the compression of the air 

 may be great if the wave is high enough to close the entrance. 

 When the water runs out of a cave, the air within may be greatly 

 rarefied, while that above exerts its normal pressure. In either 

 case the roof of the cave, if it is weak, may be broken. At certain 

 seasons of the year, especially during the spring, waves make 

 destructive use of the ice which is then breaking up, but it is only 

 in high latitudes that sea-ice is of importance in this way. In 



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



