Shingle on the East Coasts of JS T eiv Zealand. 71 



of which it is composed, the waves have a tendency to draw 

 material down below the water-line — that is, to flatten the 

 slope. In consequence of this the force of the waves is 

 reduced. 



(11.) If the slope be too flat, material on being thrown up 

 by the waves will be likely to remain on the beach, and thus 

 the slope will be gradually increased. The power of the 

 waves will be increased in consequence. 



IY. — Movement of Shingle along the Shore. 



44. The way in which the waves cause the shingle to 

 travel along the shore having been explained, the effect of 

 the varying directions of the wind, and of the waves resulting 

 therefrom, on a coast of very irregular outline may be first 

 noticed under this head. In connection with this matter, it 

 must not be overlooked that waves on coming into shallow 

 water have a tendency to shift round a little towards the 

 coast, and thus strike more directly on a shore than the 

 direction in which they are moving in the open sea would 

 seem to indicate. Shingle will not only move at varying 

 rates at different times, but on many, perhaps most, parts 

 of the coast it will move sometimes in one direction and 

 sometimes in the opposite direction, though, as a rule, there 

 is decidedly a greater movement one way than the other. 

 Waves will impinge at different angles on different parts of 

 the coast, and will sometimes drive the shingle entirely 

 away from an exposed point. In this way some cliffs, which 

 are at times protected from the sea by a beach of shingle, are 

 at other times much cut away. But even where a beach is 

 never thus entirely denuded, very great differences in the 

 quantity of shingle are generally noticeable at different times. 



45. Another point not to be overlooked is that, although 

 the quantity of shingle in motion may be constant, it may, 

 nevertheless, appear to be different at different points. 

 Where the trend of the coast favours a rapid movement one 

 way, the width of beach is likely to be less than where the 

 movement is less constantly in one direction, or the waves 

 strike with less force. In other words, the sectional area 

 would be in inverse ratio to the mean velocity. 



46. As shingle is carried along it is gradually reduced in 

 size ; large stones are reduced to small pebbles, and small 

 pebbles to sand. The softer the material the more rapidly 

 the reduction takes place. This reduction is not, however, 



