72 Shingle on the East Coasts of New Zealand. 



regular along the coast. Where the movement along that 

 portion of the coast fully exposed to the prevailing seas is 

 rapid, the shingle might appear to be of one size for a long 

 distance ; where the trend of the coast is different — is partly 

 sheltered from the prevailing seas, or is much exposed to 

 seas from other directions, giving rise to a contrary move- 

 ment at times — the movement of the shingle in the prevailing 

 direction being less rapid, it is likely to be reduced in size 

 in a comparatively short length of coast. 



47, Shingle may disappear at certain points of the coast- 

 line notwithstanding the supply is continuous. It will be 

 well to consider how this takes place. Wave -action does 

 not ordinarily carry shingle of any size much below low- 

 water level, except where, from some cause or other, a steeper 

 slope than the normal exists. A current flowing round a 

 point, or a current sweeping round a bay, may remove the 

 finer material forming the base of the shingle, and may thus 

 increase the slope. In such case some shingle will probably 

 be carried below low- water level by the action of certain 

 waves, as, for instance, waves breaking at long intervals., and 

 allowing the return wave to act with the greatest effect. 

 But, when the beach has its normal slope, shingle may 

 disappear under the following circumstances : — 



(1.) The fresh water, or tidal, flow at the mouth of a river 

 or lagoon must carry below the water-line any shingle that 

 may be thrown into the stream by wave-action or other- 

 wise,, especially at the point of each tongue of shingle, which 

 is generally formed where a break in the beach occurs from 

 the above cause. 



(2.) As the shingle is worn and reduced in size, the finer 

 particles are carried under the water-line, while the shingle 

 remains on the beach. 



(3.) Where a shingle beach appears to die out gradually, 

 running into sand, the slope becomes natter and flatter as the 

 shingle changes to sand. Then, the sand, if not accumulat- 

 ing, notwithstanding the continued travel of the shingle 

 towards the spot, must be drawn under the water-line, and 

 tend to raise the sea-bed. It goes, in fact, as is very com- 

 monly the case, to silt up the head of the bay. 



48. It may have been noticed in a former part of this 

 paper that the formation of a lagoon by a strip of shingle 

 beach is not unfrecjuent on these coasts. This occurs at a 

 point where, before the shingle beach was formed, a deep 

 indentation in the coast-line existed. Owing to the shore- 



