96 SHINGLE BEACHES AND THEIR FIXATION
important thing- is to prevent the surface layer of shingle from acquiring mobility.
   B.	Percolation.—This depends on the open texture of the material and the difference in level of the water on the face and back of the shingle beach. Most shingle beaches are liable to percolation at high water, or when breakers hurl themselves on the front. The Chesil Bank offers an extreme example. Owing to the slight effect of the tides on the water-level of the Fleet (which oscillates about mean sea-level), the difference in level within and without may reach a height of 6, 8, or even 10 feet. Moreover, as this bank consists of remarkably pure shingle with but slight admixture of finer materials, the conditions for percolation are unusually favourable. The result of this action is the production of numerous ravines or “cans” on the lee side, from the bottoms of which shingle is displaced and shot out in the form of talus fans into the Fleet (fig. 7 a, p. 42). It is the aggregate of these fans that constitutes the terrace. This displacement of shingle naturally communicates itself to the back and sides of the ravines, which are kept perpetually on the move and bare of vegetation.1 Plate IX, 1, shows a number of ravines seen over the Fleet; the black bushes on the terrace are Suceda fruticosa. A projecting fan is shown in Plate IX, 2.
   Shingle spits with a nearer approximation to equality of water-level on the two faces are usually affected only to a trifling extent—as at Blakeney Point2—with production of gullies at the foot of the bank, of slight vertical extension. In some cases the materials have become so completely consolidated that percolation does not take place, as for instance Slapton Beach, where the backwater (Slapton Lea, famous for its freshwater fish), though distant but a stone’s throw from the tide mark, is immune from contamination by salt water.
   C.	Scour.—Beaches sometimes suffer erosion on the landward side when in their advance they impinge on channels and tidal creeks. In this way a talus may be kept active for an indefinite period. Where estuaries are exposed and the land-
1 Phenomena associated with percolation on the Chesil Bank are described in F. W. Oliver’s
“The Shingle Beach as a Plant Habitat”, New Phytologixt Vol. XI, p. 86.
    3 Under certain conditions well-marked ravines occur here. Cf. pp. 233, 234.