The Shingle Beach as a Plant Habitat. S5 
There are three principal ways, quite distinct from one another, 
in which shingle is kept mobile; though all are operative on nearly 
every bank, their relative importance will vary in different cases. 
A.—WAVE IMPACT. 
.The first of these methods comes into play under the influence 
of the more violent on-shore gales, and consists of the scattering of 
the shingle over the back of the bank as the waves break over 
the crest. In this way shingle already in place is kept on the move, 
whilst new shingle is contributed from the tideway. Its results are 
manifest on the Biakeney Bank in the finger-like points of shingle 
which stretch out over the marshes on the leeward side at 
numerous places (cf. PI. 4, Fig. 3). 
The same phenomenon is unusually well shown on parts of the 
Hamstead Dover—a shingle spit of small dimensions on which 
mobility-effects, in addition to other features, are displayed under 
exceptionally favourable conditions for study. Here the spit is 
flanked on the landward side by a series of pointed tongues of 
shingle which lie diagrammatically in the direction of principal wave- 
impact. The cumulative effect of these displacements is very 
manifest on this spit, as the parts most subject to this buffeting 
show a distinct landward sag (just visible on the chart, Text-fig. 4). 
The evidence of direct wave-action, so manifest on these and 
other banks, fully confirms local testimony that when a heavy on¬ 
shore gale coincides with a high spring tide, the crest and back of 
the Biakeney Bank are awash. Under these conditions the whole 
structure must be so permeated with water as to render the shingle 
practically fluid. 
Mobility induced by wave-impact will in the long run bring 
about a landward creep of the bank—not equally everywhere, but 
now here, now there. The statement occurs in one of the Reports 
of the Coast Erosion Commission 1 that the Biakeney Bank is 
drifting landwards at the rate of two yards a year. Though the 
precise data on which this estimate is based are not cited, I can well 
believe it to be no exaggeration. 
An outstanding example of this type of mobility is the Northam 
pebble ridge which defends the famous golf course of Westward 
Ho! against the sea. The matter has reached a critical phase in 
recent times, and is now causing serious local anxiety. Whilst 
moderate wave-impact tends to the maintenance of the crest at a 
1 Roy. Com. on Coast Erosion, Min. of Evidence, Vol. I, pt. 2, 
appendix XXIII, p. 240, 1907. 
