Shingle on the East Coasts of New Zealand. 7T 



given as a summary. Many of these deductions may "be 

 of general application : — 



(1.) Shingle is caused to move up and down the slope of 

 any beach by the action of the waves, and is reduced in 

 size by constant attrition. 



(2.) Shingle is caused to travel along the coast by waves 

 striking obliquely on the shore. 



(3.) Shingle of any considerable size is rarely carried much 

 below the water-line by wave-action alone. On the contrary, 

 waves tend to throw up on to the beach material from the 

 bed near the shore. At times very heavy stones have been 

 so thrown up. The finer material is carried down again by 

 the return waves. 



(4.) There is a certain slope, peculiar to each size of shingle, 

 which may be called its normal slope, and which enables a 

 shingle beach best to withstand the action of the waves. 

 The larger the shingle the steeper its normal slope, and 

 vice versa. 



(5.) The tendency of wave-action is to arrange the shingle 

 in regular order on a beach, as follows : — 



(a) The largest pebbles to collect near the water-line, 



or between the levels of high and low water. The 

 size should diminish slightly from high-water level 

 upwards, and -very rapidly from low- water level 

 downwards. 



(b) The slope will be steepest between the levels of 



high- water and low- water. Above high- water level 

 the beach will assume a convex shape ; below low- 

 water level the profile of the bed will be concave. 



(c) With a slope flatter than the normal, the largest 



shingle ought to be at, or above, high- water level. 

 With a slope steeper than the normal, the large 

 shingle should be looked for near low- water level. 



(d) If the slope differ from the normal, the wave-action 



tends to restore the slope, reducing it if too steep, 

 and increasing it if too flat. 



The theoretical results are seldom attained, owing to the 

 perpetual changes going on. The longer the forces at 

 work remain constant, the nearer the theoretical condition 

 is approached. 



(6.) When the shore runs in one, or nearly one, direction 

 for several miles, without any projecting points or bays to 

 break the uniformity of the coast-line, shingle will be 

 uniform in its character. 



