90 SHINGLE BEACHES AND THEIR FIXATION
consequence of the battering of the waves move landwards; apposition beaches, except, of course, the outmost and last-formed rib, are inaccessible to the waves, and their shingle is perfectly stable. The problem of their reclamation is quite different from the other categories of shingle.
   The special conditions of Spits and Bars may now be set forth briefly.
   Spits and Bars. — Topography. — Commonly these banks, which follow a course more or less parallel to the shore, act
The datum levels to which they are referred are in the case of the Chesil the water-level of the Fleet, and in that of the Blakeney Bank the surface of the salt marshes. The figure also shows the positions of the Suarda fruticosa belts in both cases—marked I, and I, 11, m, respectively.
as the bulwark protecting a backwater, tidal flats, or reclaimed marshes from the sea. Spits, as a rule, are backed by salt marshes, whilst bars protect backwaters (Slapton Beach), chains of lagoons (Audierne shingle beach, Finistère), or marsh and meadow land (Pevensey Beach). In the case of the Chesil Bank the backwater or Fleet connects with the sea between Portland Island and the mainland, and is thus subject to slight tidal influence. Such beaches rest on the edge of a coastal shelf and are slowly driven landwards.
   In profile or section these beaches are comparable, showing many features in common. Two such profiles of the Blakeney and Chesil Banks, respectively, are illustrated in the accompanying fig. 20.
   An average section of the Blakeney Bank is some 350 feet