The Shingle Beach as a Plant Habitat. 81 
attrition of the travelling shingle will increase—a fact finding 
frequent expression in the progressive diminution in size of the 
pebbles as the apex is approached. In the absence of quantitative 
data bearing on the “life” of the various ingredients of beach 
material, no quantitative value can be assigned to this factor. 
Curvature of Spit. 
Though hooks do not tend to arise, at any rate as normal 
features, till a spit is approaching the close of its phase of longitudinal 
extension, there is another characteristic of maritime spits that is 
probably related to tidal causes. Shingle spits show a distinct 
tendency towards curvature, and when this is the case the convexity 
is directed to the sea. In the case of lake-shore spits Gilbert 
remarks that the spit, when not straight, presents its concavity 
towards the lake. 1 If this difference in the sense of the direction of 
curvature be really of diagnostic value, it is reasonable to suppose 
that it depends in some way or another upon the tidal currents 
which form the outstanding feature that differentiates the agencies 
at work on the sea-coast and lake-shore, respectively. The influence 
of the tidal current may directly determine the contour of the spit, 
or it may act indirectly by modifying the littoral shelf on which the 
spit rests and so determine its form. The matter is an intricate 
one however, and no useful purpose would be served by discussing 
it further. 
Multiple Hook-Formation. 
There are several features associated with hook-formation to 
which attention may now be directed. Not infrequently a succession 
of hooks is formed ( e.g ., Hurst Castle, Hamstead Dover, Text-fig. 4). 
Often these hooks are so closely approximated that the system 
which they form essentially resembles the apposition type of bank¬ 
building shown at Dungeness (Text-fig. 1). Nowhere is this 
approximation better illustrated than in the terminal series of hooks 
at Blakeney Point (Text-fig 3, C), where their numbers can hardly 
be determined by mere inspection on account of the dunes which 
mask them. 
Noteworthy too is the reversion to the juvenile, straight-growing 
phase which is occasionally found. This exceptional condition is 
illustrated on the grand scale by the Blakeney bank, which bears on 
its distal stretch of two miles no fewer than three distinct groups or 
systems of hooks—the intervening regions being of the type of the 
1 G. K. Gilbert, Lake Bonneville, p. 52. 
