76 Shingle on the East Coasts of New Zealand. 



for the information of Sir John Coode, which is dated 20th 

 March, 1879. In it he remarked : — " The depth at which 

 the shingle travels can be judged to some extent from the 

 difference in the soundings north-west of the Bluff. The 

 cavities in the bottom, 15 feet below water, are found 

 emptied of shingle after easterly weather." Around the 

 Bluff the sea has " a very uneven bottom, which traps the 

 shingle in its journey round the Bluff. Soundings have 

 proved that, in this very exposed locality, shingle travels to 

 the depth of fully 18 feet."* It may be open to question 

 whether the movement of shingle at this depth is due 

 entirely to wave-action, or is partly due to a current flowing 

 round the Bluff ;f also, whether much of this shingle is 

 thrown up again from a depth of 18 feet on to the beach or 

 not. Much depends on the size of the waves at the point. 



56. The important point to consider here is, whether the 

 fact of shingle shifting at a depth of 18 feet at the Bluff 

 can be taken as evidence that it would cross the bar at the 

 entrance to the present harbour at such a depth or not. In 

 face of the other facts before referred toj — that during 

 westerly weather (when the shingle would be driven back- 

 wards) the depth increases, but easterly weather (which 

 drives the shingle forward) reduces the depth again — the 

 author cannot admit that any such depth on the bar could 

 be maintained so long as the supply of shingle is unchecked. 

 If this depth were obtained in any way, the author is of 

 opinion that the shingle would not cross the entrance ; it 

 would accumulate till the bar became nearly as at present, 

 and then it would begin to travel once more.§ 



57. A depth of 8 to 10 feet would therefore appear to be 

 the maximum at which the shingle can travel freely in this 

 case. Under altered conditions a different depth would 

 perhaps result. Further observations are necessary before 

 anything more definite can be stated on this point. One 

 thing, however, is scarcely open to question : no harbour suited 

 to vessels of any size can be secured where there is travelling 

 shingle, unless means be adopted to keep the shingle from 

 the entrance. 



Concluding Remakks. 



58. The following deductions from the foregoing remarks 

 regarding shingle on the beaches visited by the author are 



* See note on p. 79. f See par. 31. J See par. 54. 



§ See Appendix II., at p. 84. 



