Caeruthees. — On the Formation of detached Shingle Beaches. 475 



Art. LXXIII. — On the Formation of detached Shingle Beaohes. 

 By John Careuthers, M. Inst. C.E. 

 [Read before the Wellington Philosopldcal Society, nth November, 1877.] 

 The travel of sand and shingle along a sea coast is not due to any currents 

 wliicli may obtain, but to the breaking of the waves on the beach. There is 

 very seldom a littoral current strong enough to carry shingle or sand along 

 with it, but where even a small wave breaks it has sufficient impetus to 

 move shingle. 



If the wave breaks square to the coast the shingle is thrown directly 

 forward ; when the wave recedes it carries the shingle back with it to the 

 place it started from. In this case there is no travel of the shuigle ; it is 

 thrown ashore and pulled back by the waves, time after time, over the same 

 course until it is ground down into sand or mud. If, however, the waves 

 break obliquely on the shore, the shingle is carried forward by the wave, 

 but when the latter recedes it does not travel on the same track by which it 

 rolled forward, but takes the line of quickest descent, which is at right angles 

 to the beach. The shingle thus travels in a zig-zag path moving gradually 

 along the shore in the direction of the waves. This causes a travelling 

 beach. 



The traveUing shingle will often refuse to follow a sudden indenta- 

 tion of the coast and goes straight across the bay, forming a detached 

 beach with a lagoon behind it. This may, I think, be explained in 

 this way : Waves travel less quickly in shallow water than in deep ; the 

 inner end of a long oblique wave being in shallower water than the outer 

 end is, therefore, checked as it approaches the shore and the wave takes a 

 curved form. At the point forming the beginning of the bay this action is of 

 course intensified, and the waves roll round the point in long curves which 

 tend more and more to become perpendicular to the coast line. The vary- 

 ing speed of the waves, due to the varying depths and the different distances 

 they have to travel, destroys the uniformity which obtained along the 

 straight part of the coast; the waves cease to be continuous billows; those 

 rolling on the outside of the point strike those breaking on the inside on 

 the flank, and are tripped up and broken exactly as they would have been 

 on a solid shore. The shingle is then deposited, and a spit is formed 

 stretching out into the bay. When the spit reaches deep water the process 

 becomes more simple ; the waves rolling into the bay beyond the spit being 

 in deep water do not break, but those striking the spit are broken and carry 

 on the shingle exactly as was done on the straight coast line. In this 

 manner the spit stretches at last quite across the bay, and becomes daily 



