Shingle on the East Coasts of New Zealand. 67 



more being moved downwards at another time, and vice 

 versa. The less slope of the sandy shore destroys much of 

 the force of the incoming waves a long distance out, 

 and causes the waves to break before reaching the 

 shore. Several broken waves, one behind the other, may 

 be observed in almost calm weather rolling in at one 

 time on to a sandy shore. Each incoming wave having 

 to meet several return waves, the power of each to carry 

 material one way or the other is, in consequence, to a great 

 extent neutralised. With a similar sea coming on to a 

 steeper beach of shingle, the waves break much closer to the 

 shore ; but the shingle is better able than the sand to with- 

 stand the greater shock. A heavier sea, in both cases, 

 would break further out, and the incoming waves having to 

 meet a greater number of return waves, the excessive 

 removal of the material of the beach would be checked, as 

 above explained, notwithstanding the greater power of the 

 waves. Doubtless all waves coming into shallow water 

 lose much of their force, before they break, in causing 

 oscillation of the material forming the bed. 



36. It will be interesting, and will serve to illustrate this 

 point, to inquire what would happen supposing the waves 

 to break with too great force on a shore of sand or shingle, 

 or when the slope of either of these materials might be too 

 great. It has been already shown that some of the material 

 stirred up from the beach would be carried up the slope 

 and some down it. The steeper the slope the less would be 

 the quantity carried up the slope and the more that 

 carried down it. That carried downwards would be more 

 in the case of fine material than with coarse material. In 

 this way the slope would gradually be reduced to that best 

 suited to the material, or to that which would enable the 

 material just to withstand the waves. The force of the 

 waves would at the same time be lessened by the reduced 

 slope. An equilibrium would thus be gradually established. 



37. The reduction of the slope may be seen on any steep 

 slope of easily yielding material exposed to wave-action. At 

 the water-line the bank is cut into, and the material drawn 

 down the slope. Of course, the case of a slope too steep to 

 permit of the material when wet standing by itself must 

 not be selected. Where the slope is not so steep as to cause 

 the material to slip from its own weight, the material can 

 only be carried down by the action of the waves. The 

 slope is thus gradually flattened, and this goes on till the 



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