The global coastline - Fundamentals of the Physical Environment - page 413

(a) (b) (c) (d)

0

(d)
8000 BP

(c)
10,000 BP

-35

BRITISH
ICE
SHEET

-95

(b) 18,000 BP

Figure 17.16 Evolution of the British coastline since the last glacial maximum: (a) approximate sea level below the present and
rate of rise at key intervals as global ice sheets melted; (b) extended continental shelf and British ice sheet at 18,000 BP . The
modern coast took shape from the start of the Holocene (c) and through separation from Europe (d). Exposed land surface shown
by mid-tone, recently abandoned shoreline by broken line.
Source: After Smith (1992)

Dune

High water mark,
ordinary tides

Storm beach

Berm

Beach
cusp

Rip channel

Ripples

Gully

Bar

Gully

Bar

Gravel bar Shingle Sand Mud

Figure 17.17 Beach-dune system morphology.
Source: Partly after Goudie (1984)

winter storms may also move large sediment volumes
inshore beyond the reach of lesser waves (see box, p. 397).
Coast-parallel nearshore bars form on dissipative
beaches and offshore at zones of either lower water
velocity or higher sediment concentration. They normally
form as single or multiple low ridges parallel to the coast,
or in crescentic form as linked beach-cusp bars, broken
by backwash or rip channels. Bars may be stable in the
energy environment at which they form, absorbing
80-100 per cent of wave energy, but unstable in any other
when they wash out or migrate. Bars accentuate as well as
respond to longshore currents and are capable of
extending across coastal embayments and estuaries as
spits in the direction of longshore drift. They also connect

islands to mainland with tombolos such as Chesil Beach,
linking the Isle of Portland to the Dorset coast of southern
England (see Plate 17.6 ).
Dune systems develop by deflation of dry sand from the
backshore landwards and, as with estuarine landforms,
develop in association with vegetation succession (see
Chapter 16). Biogenic processes assume even greater
importance on some tropical coasts. In addition to the role
of biogenic debris and bioherms in lagoons, described
earlier, reefs form more permanent wave-resistant and
biomorphological structures 10 1-2 m thick, 10 1 km wide
and 10 1-3 km long. Living reef corals, etc., grow on the
cemented debris of dead organisms and in that way can
contend with slow rates of sea-level change. Despite this,