The Shingle Beach as a Plant Habitat. 93 
the hooks. The former are of course largely mobile, the latter 
quiescent. 
The Main Bank in practically all cases exhibits many features 
in common. It has a sea face and a land face separated by the 
Crest (Text-fig. 7, C); the sea face as a rule is the steeper of the 
two faces, and rises in a series of more or less well-marked steps 
or platforms corresponding with the various categories of tide- 
marks. Apart from the topmost of these platforms (not reached 
by ordinary tides) which may be termed the storm shelf (S), the 
rest of this face bears no vegetation on account of its mobility. 
The vegetation of the storm shelf is usually very sparse indeed, and 
largely consists, when present, of prostrate species of A triplex and 
sometimes Beta maritima. The plants of the actual crest have 
much in common with those of the storm shelf, and it is evident 
that exposure to the winds (and doubtless spray) presents a serious 
check to plant establishment in either position. The crest is 
sometimes distinguished, as at Blakeney, by being the last strong¬ 
hold of several plants endowed with the capacity of creeping up 
the shingle from the landward side (cf. p. 92, footnote), but they 
are the merest vestiges of their former selves and shortly succumb. 
The Back of the bank (B) slopes away from the crest, usually 
at a low angle; sometimes, as on the Chesil, the slope may reach 
10—15 n . On banks which do not exceed six or eight feet in height 
above high-water-mark, the bank slopes gradually down to the 
marshes, but on high banks, like the Chesil, it falls away steeply in 
a series of ravines (R) which give on to the horizontal ground or 
Terrace (T) forming the landward fringe of the bank. Between the 
ravines are dormant buttresses, essentially downward continuations 
of the back, from which they differ in no material respect. 
The back is the principal region of vegetation and it is here 
that the greater number of “ shingle plants” occur. In respect of 
mobility a wide range is shown, and to some extent the mobile and 
resting parts are distinguished by their vegetations. Perhaps the 
crustaceous lichens are to be reckoned the most sensitive indicators 
of dormancy, though their behaviour on different banks stands in 
need of elucidation. Thus, whilst on the main bank at Blakeney 
lichens are hardly met with at all, on the Chesil extensive areas 
become covered with lichens and it is evident their settlement is 
quickly accomplished as soon as the conditions become favourable. 
Difference in climate may account for much, but the question needs 
comparative experimental investigation before it will be possible to 
