Geology Reference
In-Depth Information
touching an opposite shore, another spit, or an island.
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
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
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).
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.
( ) Submarine bar
a
Bar
Lagoon
Barrier island
( ) Spit elongation
b
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
bodies, which are connected to the sea by channels or
tidal inlets between islands. They are also called
barrier
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
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,
with lagoons forming as the land behind the old beaches
was flooded. Barrier beaches may also have formed by
the accumulation of sediment carried landwards by wave
action as sea level rose.
Interestingly, tectonic plate margins strongly influence
the occurrence of barrier coasts (barrier spits, barrier
beaches, and barrier islands). Of all the world's barrier
coasts, 49 per cent occur on passive margins, 24 per cent
on collisional margins, and 27 per cent on marginal sea
coasts.
( ) Rise in sea level
c
Bank or dune ridge
Lagoon
Barrier island
Figure 13.11
Ways in which barrier islands may form.
(a) The growth of a submarine bar. (b) The elongation of a
spit. (c) The submergence of a beach ridge or dunes by a
rising sea-level.
Source:
Adapted from Hoyt (1967)
Beach ridges and cheniers
Sandy
beach ridges
mark the position of former shore-
lines, forming where sand or shingle have been stacked up
by wave action along a prograding coast. They may
