Page 95 - (DK) Ocean - The Definitive Visual Guide

COASTAL LANDSCAPES 93

Wave-erosion Coasts beach energy concentrated

Of all the different types of coastal landscape, perhaps the most familiar part of wave on headland as wave
front refracts
front opposite
are wave-eroded cliffed coasts, a type of secondary coast. Wave erosion beach continues
on these coasts occurs through two main mechanisms. First, waves hurl forward erosion eventually
beach material against the cliffs, which abrades the rock. Second, each divides headland
into stacks
wave compresses air within cracks in the rocks, and on reexpansion
the air shatters the rock. Where waves encounter headlands,
refraction (bending) of the wave fronts tends to focus their
erosive energy onto the headlands. At these headlands,
distinctive features tend to develop in a classic sequence.
First, deep notches and then sea caves form at the bases
of cliffs on each side of the headland. Wave action
gradually deepens and widens these caves until they
cut through the headland to form an arch.
Next, the roof of the arch collapses to leave part of wave
front opposite
an isolated rock pillar called a stack, and headland slows
finally the stack is eroded down to a stump. as it encounters
shallower water

lobe of
sediment

CONCENTRATION OF
WAVE ENERGY
When a wave front reaches
a shore consisting of bays and
wave front
(extended crest headlands, it refracts in such a
of wave) way that wave energy tends to be
concentrated onto the headlands.
UNDERCUT CLIFF SEA CAVE SEA STACKS
Wave action has eroded a notch, and This deep indentation and sea The Old Harry Rocks are chalk sea
an adjoining platform, at the base of cave have been eroded into cliffs stacks at a headland near Swanage movement of
this cliff in the Caribbean. in the Algarve, Portugal. in southern England. salt sand along
river marsh headland beach
current spit backwash swash
Marine-deposition Coasts
Marine depositional coasts are formed from sediment brought to a coast
by rivers, eroded from headlands, or moved from offshore by waves. An
important mechanism in their formation is longshore drift. When waves
strike a shore obliquely, the movement of surf (swash) propels water and
sediment up the shore at an angle, but backwash drags them back down
at a right angle to the shore. Over time, water and sediment are moved
along the shore. Where the water arrives at a lower-energy environment,
the sediment settles and builds up to form various depositional features,
including spits, baymouth bars,
second most common direction of prevailing wind
and barrier islands (long, thin wind and wave direction longshore drift and wave direction
islands parallel to the coast).
SPIT FORMATION
On this coastline, sand and
water is carried past the
headland by longshore drift,
but the sand settles at the
mouth of an estuary where
the waves are opposed by
the sluggish outflow from
a river. There it forms a
slowly growing spit—a
sandy peninsula with one
end attached to the land.

BAYMOUTH BAR
Where a spit extends most
or all of the way across the
mouth of a bay or estuary,
the result is called a OCEAN ENVIRONMENTS
baymouth bar. Here, a bar
across the mouth of an
estuary in Scotland, and an CLATSOP SPIT
older spit, have created a This aerial view shows the
sheltered coastal area of impressive Clatsop Spit, at the
sandflats and salt marshes. mouth of the Columbia River
in Oregon. The spit extends for
2½ miles (4 km) across the river
mouth and is still growing.