154 WAVES OF THE SEA 



shore there are two additions to its mode of shore- 

 ward action upon pebbles. In the first place, 

 and obviously, the cataract which is discharged 

 obliquely upon the ground drives pebbles forwards. 

 In the second place, if we look at what happens 

 just behind the fallen front, we see that, instead 

 of subsidence and backward flow, there occur up- 

 welling and shoreward flow. The reason is evi- 

 dently that the head of water .in front has been 

 destroyed, the pressure is now from behind, and 

 the breaking wave has, in fact, rolled over. 



It is evident that this predominantly forward, 

 or shoreward, action of nearly, and quite, breaking 

 waves upon pebbles could be neutralised by set- 

 ting the waves to roll the pebbles up a slope. 

 There would be a slope of some particular steep- 

 ness on which gravity would just neutralise the 

 forward action of the waves. 1 



Moreover, since the advantage of the forward 

 action over the backward action so much depends 

 upon quick subsidence of the particles, it appears 

 a proper inference that upon a moderate seaward 

 slope there will be a travel of large material shore- 

 ward, while material of a somewhat smaller size 

 will merely oscillate. 



1 This is a reason against the construction of steep groynes. 



