64 Shingle on the East Coasts of New Zealand. 



lasting ; the arrangement of the shingle on a beach is, as a 

 rule, perpetually changing. Occasionally, a permanent 

 arrangement may be met with, as, for instance, when a heavy 

 sea, especially during a high spring tide, has thrown pebbles 

 up to a position from which the ordinary waves may be 

 unable to displace them. 



(c.) Action on the Bed of the Sea near the Shore. 



29. Water falling from the crest of a wave on to a cushion 

 of water would, in stirring up the bed, cause some of the 

 material to rise in the water. The deeper the water-cushion 

 the greater the force necessary to disturb the bed, the 

 material being the same. The particles, on approaching 

 the surface, would be carried towards the shore by the 

 waves (after breaking), or, in some cases, by the wind 

 if blowing in that direction. The greater the force of 

 the waves, as a rule, the coarser and heavier the material 

 liable to disturbance. With a heavy sea and a gale blowing 

 on shore, large stones might easily be transported from a 

 considerable depth in the bed to the beach. 



30. The greatest depth at which this action could take 

 place is uncertain. It is often said that loose rubble is safe 

 from disturbance at a depth of 15 feet ; some say at any 

 depth over 12 feet. The rubble foundation of the Alderney 

 Breakwater was, however, disturbed at a depth of 20 feet 

 below low- water level.* Much must depend on the size 

 of the stones used for rubble. It is on record that " drift 

 stones of large dimensions,, measuring upwards of 30 cubic 

 feet, or more than two tons in weight, have, during storms, 

 been thrown upon the [Bell] Rock from deep water. "f For 

 shingle the above limits of depth are far too little. Sir 

 John Coode remarked, after an examination of pebbles 

 in the bed of the sea in front of the Chesil Bank : — " These 

 facts demonstrate clearly that at a depth of 6 and 8 

 fathoms there must have been a considerable amount of 

 motion during heavy gales."J The late Astronomer Royal 

 concluded that in some places the stones on the beach " had 

 been torn up by the violence of the surf from the bottom of 

 the sea."§ 



31. The opinion of the late Astronomer Royal did not 

 meet with general acceptance. It was controverted, as 



* Proceedings of the Institution of Civil Engineers, vol. xxxvii., pp. 74 and 108. 

 t Ibid., vol. vii.,p. 333. J Ibid., vol. xii., p. 535. § Ibid., vol. xxiii. p. 228. 



