Erosional landforms
Nature of eroded coastlines determined by two factors:
direction, strength and frequency of waves and geology of
coastline
Cliffs: produced by coastal erosion. wave action undermines land leading to
slumping/rock falls. produces a steep ‘edge’ (cliff). rapid and severe erosion occurs
when large waves break at cliff foot
Wave-cut notches: cliff foot eroded which forms wave-cut notch, undercutting the
cliff face as notch gets larger it collapses due to lack of support and weakening.
process of undercutting and collapse repeats, cliff will retreat and slowly increase
in height
Wave-cut platform: gently sloping area at foot of cliff. gets wider as cliff slowly
retreats inland
Effect of rock type along a discordant coastline, headlands and bays
headlands and bays forms along coastlines with alternating resistant and less
resistant rock. also, where trend of rock strata is at right angles to direction of the
coastline.
hard rock: slow erosion, headland
soft rock: fast erosion, bay
headland sticks out and exposed to full force of wind-waves and wave refraction
will concentrate erosion onto headlands.
cliffs/arches/stacks form due to increased erosion of headland.
wave refraction reduces energy of waves entering bays and deposition will produce
bay beaches as a result.
Effect of rock structure within a headland, caves
arches and stacks
headlands = resistant rock
wave refraction concentrates wave energy onto headland formation of caves, arches and stacks…
joint/fault in resistant rock
corrasion and hydraulic action widen the joint to form caves on either side of the
headland
caves are eroded until they cut completely through the headland and meet to form
arch
arch is eroded, and roof is too heavy so collapses
leaves a tall stack
stack is eroded and collapses to form a stump
Effect of rock structure, caves and blowholes
in cave the line of weakness continues upwards into body of the cliff. cavitation
and wave quarrying can erode crack to form an upwards tube. effective erosion at
high tide when cave mouth his submerged, so air is trapped at the top of the cave.
air compressed into cracks and eventually breaks through and forms a blowhole.
Cliff profiles
Factor Effect on the cliff profile
rock type – hard
rock
vertical cliff face. rock falls main type of mass movement. slow cliff retreat
rock type – soft
rock
gently-sloping. rotational slumping main type of mass movement and mudflows
at base of cliff. rapid retreat
rock structure –
layers of hard rock
dipping towards
sea
cliff face slopes towards sea. large slabs of rock slide off easily
rock structure –
layers of hard rock
dipping away from
sea
vertical cliff face but rugged and uneven. small rocks fall off when weathered.
slow retreat
rock structure – soft
rocks in horizontal
layers. porous at
cliff top.
impermeable at
cliff base
porous layers absorb water, heavy and unstable. lower layers form slide plane
lubricated by the water collecting from the layers above. conditions produce very
unstable cliffs with slumping and material flowing onto beach
erosional history –
raised beaches
higher sea level in past. cliff lines formed above current sea level. lower sea level
now so cliff line has degraded cliff line above.
Beaches
made of sand/shingle. they form between HWM (high water
mark) and LWM (low water mark). in the inter-tidal zone.
sediment is moved by LSD. beaches are a dynamic landform
with sediment moving along it.
Backshore, above influence of waves…
consists of storm beach made up of large sediment
material pushed to beach top by storm waves, back wash is too weak
berms form fortnightly by spring tide, and 2/3 slightly lower high tides that follow it
foreshore, inter tidal/surf zone…
zone between high tide mark and low tide mark
made of sand
range of features produced by swash and backwash
width of foreshore = depends on; beach gradient and tidal range
fulls: low ridges running parallel to coastline. formed by constructive waves at
different tide stages. ridges separated by runnels/swales
foreshore is a dynamic environment + feature positions change with every tide
flatter beach areas, small sand ripples form as backwash
amount of sand = depends of different sediment sources and LSD effect
nearshore, breaker zone…
zone between low tide level and level of spring side
longshore bars formed here. ridges of sand, run parallel to coastline, permanent
low tidal range beaches form at point that waves break. breakpoint bars
offshore, beyond influence of waves…
always under water
longshore bars are here. produced when circular movement of water touches sea
bed
friction slows wave and sediment swept up to form bars
gently sloping beaches + low tidal range. bars can form barrier islands/beaches
Role of tides in formation of the beach profile
ebb and flow of tides that makes the beach a transitional zone between land + sea.
ebb is the tide going in. flow is the tide going out
width + nature of foreshore/nearshore zones = tidal range and amount/type of
beach sediment
spring tides shapes berms
high tidal range beaches reveals new beach environment
Role of waves in formation of the beach profile
waves have energy to change beach
storm beach created by large waves during high spring tide
constructive waves = move material up beach. contribute to berms + increase
beach gradient
destructive waves = comb material down beach. contribute to longshore bars +
reducing beach gradient
type of waves that break change all the time
local conditions can influence wave type + nature/position of beaches. e.g.
summer = constructive waves. winter = destructive waves
angle at which waves approach is determined by wind. influences LSD. major
impact on beach nature
Role of sediment in formation of the beach profile
larger particle size, steeper the beach
water percolates into shingle so backwash has less energy
shingle is at tip of beach as backs is too weak to pull it down
sand less permeable than shingle so more backwash
longshore bar produced at low watermark by backwash producing sand ripples
sand smoother than shingle and causes less wave friction. beach is gentler so
wider. wave energy is dissipated over a wide area.
Beach cusp
semi-circular, scalloped depressions cut into lower edge of
storm beach.
self-sustaining.
swash is diverted around horn of each cusp. back wash runs
down middle of cusp, removing sediment and slightly deepening cusp.
Swash and drift aligned beaches
swash: aligned parallel to crests of prevailing waves. swash and backwash run
straight up and down beach. LSD doesn’t operate here. net movement of sediment
= zero. beach cusps form. tombolo’s form as waves are refracted around off shore
island.
Drift: aligned at angle to crests of prevailing waves. strong LSD. swash moves up at
angle, backwash runs straight. LSD control movement of sediment and spits, bays,
tombolo’s and cuspate forelands can be formed.
Simple and compound spits
occur when LSD extends beach part of way across an
estuary, bay or inlet
produced by constructive waves = sand spits
produced by destructive waves = shingle spits
height increased by storm waves
at top of spit, sand dunes form; raises level above high-water mark
behind spit, salt marshes form in slack water because of alluvial sedimentation in
estuary
can’t reach other side of estuary as river current + depth
simple spit: usually straight but has curved ends because of wave refraction. e.g.
spurn point, Yorkshire. waves carry material along the spit to the tip which extends
it
compound spit: narrow base attaching it to mainland. widens into broad re-curved
end
Bars
develop when a spit extends all the way across an inlet.
where there’s no powerful river current to stop spit growing.
lagoon forms between bar and coastline.
features also forms when offshore bar migrates landwards.
Tombolo’s
long, low, narrow features of sand/shingle that join mainland
to island
produced where shallow sea bed, allowing sediment
accumulation
formed by extended spit growth by LSD as sediment
accumulation driven onshore by rising sea-levels forms a
barrier beach.
no universally accepted theory for explanation of its formation
e.g. Chesil beach, Dorset. connect isle of Portland to mainland. continues
westwards for several miles. largest tombolo in UK. forms large lagoon; the fleet,
on shoreward side.
when two beaches extend to same island = cuspate tombolo produced. produced
as wave refraction causes LSD to operate in opposite directions either side of
offshore island where waves approach coastline at angles
Offshore bars, barrier beaches and barriers islands
barrier beaches are long, sandy beaches detached from the coastline that run
parallel. tidal lagoon forms in-between. salt marshes/mangrove swamps develop in
lagoon or strong currents clear the lagoon. form where there’s a low tidal range
and where sea bed and coastline are gently sloping with no cliffs.
e.g. south-eastern coast of USA, along coastline of Gulf of Mexico.
began as offshore bars and have been built up above low tide level by action of
storm wave.
or formed far out at sea when level was lower in last glacial period. sea level rose,
migrated landwards, sand dunes form on them and vegetation built them up above
high tide level. called barrier islands. 200-400m
wide and very long.
Sand dunes
sand dunes = ridges of sand that form at the back of a beach and also on spits and
barrier islands.
Importance: important depositional landform and distinctive coastal ecosystem
(habitat for plants and animals). protects land from flooding and erosion. easily
damaged so protected as conservation areas.
Needs following for formation: wide sandy beach that dries out at low tide. strong
prevailing onshore wind to carry sand up beach. obstacle at top of beach (e.g.
driftwood/plant) to cause wind to deposit sand.
Development: develop slowly over time, building land out into the sea. well-defined
sequence of dune ridges develops, each one with distinctive size and plant
community. example of plant succession
embryo dunes: first to develop. very harsh environment for plants. very dry with
high pH. very few nutrients in ever-shifting sand. only very hardy plants survive but
help more sand to build up, forming fore-dunes.
fore-dunes: hardy plants grow and die, add organic material to soil. more water can
be retained. less harsh environment. madam grass dominates ecosystem. madam
grass traps more sand but can grow quickly to keep up with dune growth.
mobile yellow dunes: high ridge develops. more plants grow/die so soil is richer and
moister. range of herbs and shrubs grow, e.g. dandelions and brambles.
semi-fixed grey dunes: extra organic content means they develop. sand is
sometimes exposed to wind and blows away, lowering surface.
dune-slack: if surface is lowered to water table. halophytic plants will grow (willow
shrubs/natter jack toad)
grey fixed dunes: trees grow, and they predominate.
Salt marshes
in sheltered river estuaries, behind a growing spit, silt/mud frequently deposited to
form inter-tidal mudflats
they’re intertidal so covered with salt water for much of the day but have a short
period each day where they’re uncovered.
blue/green algae and zoster survive in wet/salty conditions and trap more mud,
raising mudflat level.
mud surface level is higher, so period of inundation is reduced and plants
(glasswort) begin to colonise mudflats and trap more mud
level of mud builds leading to shorter tidal inundations and plants (cordgrass)
colonise
sward zone develops which is only covered by sea for 1hour a day
halophytic plants (grasses sea aster and lavender) grow in sward zone
high tide retreats so creeks form along which the receding seawater drains
eventually land is built up until its always above level of highest tides, shrubs and
woodland start to grow.
development is example of plant succession. humans drain salt marsh areas to
produce low lying grazing, so climax vegetation is rarely found these days.
valuable ecosystem as habitat and feeding ground for migrating birds.
UK e.g. north Norfolk coast near Blakeney, Essex coast between south end and
Felixstowe, west of spurn point in Humber estuary.
Sea level change
eustatic: world-wide. produced by climate change. glacial periods when ice caps
melt so levels rise, or ice sheets build up removing water, so levels dropped.
Isostatic: local. meting of local ice cap, sediment deposition or tectonic changes.
depression or elevation of crust so levels will change accordingly.
changes can work together or against each other.
isostatic rebound is the rise of land masses that were depressed by the huge
weight of ice sheets during the last ice age.
Landforms produced by rising sea level
a sea level rise drowns lowlands near old coastline and drowns lower parts of the
valley. due to rapid eustatic sea level rise in last glacial period.
rias: V-shaped with smooth concave long section, reflecting the fact they’re
drowned river valleys. form deep sheltered harbours with many branching arms
(drowned tributaries). e.g. Milford haven in Pembrokeshire, UK
fjords: drowned glaciated valleys. U-shaped cross section and get deeper as you
move further from sea. very long and narrow. e.g. Norway, New Zealand, south
Chile
Dalmatian coast: rivers flowed parallel to coast producing series of ridges and
valleys, sea level rose, and valleys were flooded, and ridges became long narrow
islands. e.g. Croatia
Landforms resulting from emergence
fall in sea level leaves features being left high and dry, often some way from
coastline. main landforms produced: raised beaches, emerged coastal plains.
raised beaches: relict features such as wave-cut platforms, cliffs, caves, arches and
stacks are found well above present day sea level. features are result of sea level
rise at end of last glacial period, followed by rise in land due to isostatic rebound.
e.g. isle of arran, Scotland
emerged coastline: in areas with a wide shallow continental shelf, a sea level fall
can produce a wide coastal plain, backed by a relict line of cliffs which represent
the old coastline. e.g. south east coast of USA
Future
sea level is still rising
due to climate change, melting ice at the poles and thermally expanding sea water
east coast of UK is high risk of being flooded and eroded.
too expensive to protect all coastlines
Essex marshes managed retreat strategies have been used, giving land back to
sea, allowing salt marshes to develop to protect land.
Thames barrier and embankments have been built to protect the upper Thames
estuary