AND OTHER WATER WAVES 189 



On the Causes which give rise to an Arrangement 

 of Pebbles according to their Size along the 

 Chesil Beach and certain other Beaches 



When considering on and off-shore action of 

 waves, we have seen that a sorting of materials 

 takes place. It is assisted by the seaward slope 

 of the shore. In the arrangement of pebbles along 

 a shore there is no such gravitational assistance 

 in sorting, except such as is given after the arrange- 

 ment is effected e.g., by the greater steepness of 

 a coarser beach. The origin of a longitudinal 

 arrangement of pebbles on a tidal shore is there- 

 fore to be sought in the 'longshore action of the 

 wash from the breakers during all the phases of 

 the tide. 



The effect of the action of the waves to determine 

 the size of the shingle is, of course, best studied 

 upon those parts of the shore where the beach is 

 not supplied with new stones by wastage of cliffs 

 at the back of the beach, or by a torrent entering 

 the sea. In beaches replenished by 'longshore drift 

 the replenishing material consists mainly of stones 

 not easily broken into smaller fragments. The 

 approximate permanence of such a beach, as a 

 whole, is apt to mislead the observer into sup- 

 posing the rate of 'longshore travel of individual 



