Geology Reference
In-Depth Information
they form when waves approach at right-angles to the
shore, although a few think that oblique waves cause
them. Their mode of formation is disputed, and they
have been variously regarded as depositional features, ero-
sional features, or features resulting from a combination
of erosion and deposition.
Inner
and
outer crescentic bars
are sometimes
called
rhythmic topography
. They have wavelengths
of 100-2,000 m, although the majority are somewhere
between 200 and 500 m. Inner bars are short-lived and
associated with rip currents and cell circulations. Their
horns often extend across surf-zone shoals into very large
shoreline cusps known as sand waves, which lie parallel
to the shore and have wavelengths of about 200-300 m.
Outer crescentic bars may be detached from the shore
and are more stable than inner crescentic bars.
Many coasts display an orderly sequence of
capes
and
bays
. The bays usually contain
bayhead
or
pocket
beaches
(Figure 13.9). In some places, including parts
of the east coast of Australia, asymmetrically curved bays
link each headland, with each beach section recessed
behind its neighbour. These are called
headland bay
beaches
,or
fish-hook beaches
,or
zetaform beaches
,
owing to their likeness in plan-view to the Greek letter
zeta,
ΞΆ
(Figure 13.10).
Spits, barriers, and related forms
Accumulation landforms occur where the deposition
of sediment is favoured (Figure 13.9). Suitable sites
include places where obstructions interrupt longshore
flow, where the coast abruptly changes direction, and
in sheltered zones ('wave shadows') between islands and
the mainland. Accumulation landforms are multifarious.
They may be simply classified by their degree of attach-
ment to the land (Table 13.1). Beaches attached to the
land at one end are
spits
of different types and
forelands
.
Spits are longer than they are wide, while forelands are
wider than they are long. Beaches that are attached to the
land at two ends are
looped barriers
and
cuspate bar-
riers
,
tombolos
, and
barrier beaches
. Beaches detached
from the land are
barrier islands
.
Beach
Paired
spits
Foreland
Arrow
Lagoon
Tombolo
Spit
Beach
barrier
Longshore
currents
Recurved
spit
Comet-tail
spit
Looped
barrier
Figure 13.9
Depositional coastal landforms, shown diagrammatically.
