246 ECONOMIC GEOLOGY 
Shingle beaches similarly protect the shore. The shingle 
travels along the coast at a rate that can be measured by 
placing a load of bricks upon the shore and observing their 
drift month by month. If the shingle of a beach is renewed 
it affords a permanent defence. Coast protection in many 
places depends on prevention of the migration of shingle. 
The simplest method is the use of groynes, or ‘ horses,” 
which are barriers of timber, masonry, or cement, built 
across the beach. Well-placed groynes hold the beach 
material and protect the coast beside them ; but by retaining 
the shingle that would renew the beaches further along the 
coast they lead to more rapid erosion elsewhere. It was 
reported to the Coast Erosion Commission (Rep. iii, 1911, 
p. 110) that groyning the English coast would, in 1911, have 
cost £300 a mile, with the result that for every acre saved 
another would be lost elsewhere. Groynes are useful in 
protecting important positions: but they endanger adjacent 
land. 
The design and size of groynes should be adjusted to the 
local conditions. The system designed by E. Case for 
Dymchurch in Romney Marsh on the coast of Kent, is based 
on the principle that the main drift is between mean sea- 
level and low tide; hence low groynes rising two or at most 
3 feet above the lower part of the beach are adequate, as 
they stop nearly all the drift and secure a slope at the natural 
angle of repose! Groynes are usually placed at right angles 
to the beach; but they may point leeward. If placed too 
close together groynes raise the water-level, and increase 
the backwash and thus hasten the transport of material 
down the beach. 
Harbour design is affected by some of the factors which 
control the efficiency of groynes. The most convenient site 
for a harbour is usually an estuary, where the river current 
or tide can be used to keep the channel clear. Some former 
British ports have been closed by silt. Chester was the 
chief port of the Western Midlands until the silting of the 
Dee estuary diverted the trade to Liverpool. London has 
maintained its position as a port. owing to the well-designed 
1 Ed. Case, Rep. Brit. Assoc., 1899, p. 859; J. 8S. Owens and G. O 
Case, Coast Erosion and Foreshore Protection, 1008.