Page 349 - Fundamentals of Geomorphology

332 PROCESS AND FORM

touching an opposite shore, another spit, or an island. ( ) Submarine bar
a
Looped barriers grow in the lee of an island when two
comet-tail spits join. Cuspate barriers (cuspate fore-
lands) resemble forelands except that they have been
enlarged by the building of beach ridges parallel to their Bar
shores and contain lagoons or swampy areas. An exam-
ple is Dungeness in Kent, England, which is backed by
marshland. If the lagoons or swamps drain and fill with Lagoon Barrier island
sediment, cuspate barriers become forelands. Cuspate
barriers form by a spit curving back to the land (a looped
spit), or else by two spits or tombolos becoming joined
to an island, which then vanishes (double-fringing spit). ( ) Spit elongation
b
Barrier beaches seal off or almost seal off the fronts,
middles, or heads of bays and inlets. They are the prod-
uct of single spits growing across bays or from pairs of
converging spits built out by opposing longshore cur-
rents. They may also possibly form by sediment carried
into bays by wave action independently of longshore
movement.

Barrier islands
Barrier islands are elongated offshore ridges of sand par-
alleling the mainland coast and separated for almost their
entire length by lagoons, swamps, or other shallow-water ( ) Rise in sea level
c
bodies, which are connected to the sea by channels or
tidal inlets between islands. They are also called barrier Bank or dune ridge
beaches, barrier bars, and offshore bars. Sections of
long barrier-island chains may be large spits or barrier
beaches still attached to land at one end. As to their for-
mation, some barriers are sections of long spits that have Lagoon Barrier island
become detached, while some may simply be ‘overgrown’
bars (Figure 13.11). Others may have been formed by the
rising sea levels over the last 10,000 years and perhaps
have grown on former dunes, storm ridges, and berms, Figure 13.11 Ways in which barrier islands may form.
with lagoons forming as the land behind the old beaches (a) The growth of a submarine bar. (b) The elongation of a
was flooded. Barrier beaches may also have formed by spit. (c) The submergence of a beach ridge or dunes by a
the accumulation of sediment carried landwards by wave rising sea-level.
action as sea level rose. Source: Adapted from Hoyt (1967)
Interestingly, tectonic plate margins strongly influence
the occurrence of barrier coasts (barrier spits, barrier Beach ridges and cheniers
beaches, and barrier islands). Of all the world’s barrier
coasts, 49 per cent occur on passive margins, 24 per cent Sandy beach ridges mark the position of former shore-
on collisional margins, and 27 per cent on marginal sea lines, forming where sand or shingle have been stacked up
coasts. by wave action along a prograding coast. They may