146 



THE OPAL SEA 



Wave mo- 

 tion alon-g 

 shores. 



Bowlders at 

 the cliff 

 base. 



in the stones and sands it can move, drive be- 

 fore it, drag about from point to point, push 

 into gravel pens, and whirl around and out and 

 along the smooth ledges again. 



This is all helped on by the manner in which 

 the incoming waves usually strike the shore. 

 It is seldom that they come " head on." More 

 often they advance at an angle with a side 

 thrust, a diagonal rub, for some length along 

 the rock bases. Pebbles, shells, and round 

 bowlders are swept along in swift procession, 

 or are dropped momentarily into shallow beds 

 by the loss of wave motion, only to be caught 

 up again by the wave following after. In any 

 event the grating process goes on, and in time 

 both rock base and battering bowlder are the 

 losers. 



When the bowlder blocks first fall into the 

 sea at the base of the cliff they perhaps lie there 

 for years and, in measure, protect the cliff by 

 warding off the waves. Gradually the rough 

 edges are worn away so that they are more easily 

 rolled. Sea weeds gather about them — weeds 

 having bladders like pea pods that hold air and 

 buoy up the stones, making them more trans- 

 portable. Barnacles and limpets grow in among 

 the weeds and make an outer armor that partly 

 protects the stones themselves. When a win- 



