CannuTHERS.—On the Formation of detached Shingle Beaches. 475 
Art. LXXIII.—On the Formation of detached Shingle Beaches. 
By Jonn Carrutuzrs, M. Inst. C.E. 
[Read before the Wellington Philosophical Society, 17th November, 1877.] 
Tue travel of sand and shingle along a sea coast is not due to any currents 
which 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 shingle; 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 travelling 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 lme. In this 
manner the spit stretches at last quite across the bay, and becomes daily 
