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Coastal Landforms

Waves are caused by wind, tides are caused by gravity from the moon and the sun, and currents are caused by tides, winds & temperature & density differences in different areas of the oceans.

Marine Erosion

  • The most powerful agents of marine erosion are waves, which originate due to sweeping of winds over the water surface, setting up a series of undulating swells surging forward
  • On approaching shallow water near the shores, their speed is reduced & the waves are curved or refracted against the alignment of the coast
  • The shallow water, when is less than the height of the waves, check their forward movement, the wave crest curl over & breaks into the shore
  • Water that finally rushes up the beach & hurls rock debris against the land is termed as swash, with the water that retreats or sucked back called backwash

Swash and Backwash

  • Another element in offshore drift is undertow, which flows near the bottom away from the shore
  • This current exerts the pulling effect which can be dangerous for sea bathers
  • Marine agent of erosion operates as corrasion, attrition, hydraulic action & solution to transform the coastal landscape

 

Corrasion
  • Waves armed with rock debris of all sizes & shapes charge against the base of the cliffs, & wear them back by corrasion
  • On-coming currents & tides complete the work by sweeping the eroded material into the sea
Attrition
  • The constantly moving waves that transport beach materials such as boulders, pebbles, shingle & fine sand, also hurl these materials against each other until they are broken down into very small pieces.
  • The grinding & polishing of such fragmented materials against the  cliff faces & against each other is largely responsible for the fine sand which forms the beaches
Hydraulic action
  • In their forward surge, waves splashing against the coast may enter joints & crevices in the rocks.
  • The air imprisoned inside is immediately compressed but when the waves retreat, the compress air expands with explosive violence.
  • Such action repeated again & again soon enlarges the cracks & rock fragments are prised apart
Solvent action
  • On limestone coasts, the solvent action of the sea water on calcium carbonate sets up chemical changes in the rocks & disintegration takes place

 

Coastal Features of Erosion

Capes & Bays

  • On exposed coasts, the continual action of waves on the rocks of varying resistance causes the coastline to be eroded irregularly.
  • This is particularly pronounced where hard rocks occur in alternate band with softer rocks.
  • The softer rocks are worn back into inlets or bays & the harder ones persist as headlands or capes.
  • Even where the coastline consists of one rock type, irregularities will be caused by variation within the rock.

Capes & Bays

Cliffs & Wave cut platforms

  • Generally, any steep rock platform adjoining the coast forms a cliff, whose rate of recession will depend on its geological structure
  • Means the stratification & jointing of the rocks & their resistance to wave attack.

Wave cut platforms

  • If the bed dips seaward, large blocks of rock will be dislodged & fall into the sea & cliff will rise in a series of steps.
  • On the other hand, if the beds dip landward, the cliff will be more resistant to wave erosion.
cliffs bending seaward
cliffs bending landwards
  • At the base of the cliff the sea cuts a notch, which gradually undermines the cliff, so that it collapses.
  • As the cliff recedes landwards under the pounding of waves, an eroded base is left behind called a wave cut platform.
  • The platform, upper part of which is exposed at low tides, slopes gently seawards, with its surface strewn with rock debris from the receding cliff.
  • Further the abrasion continues until the pebbles are swept away in the sea with eroded material deposited on off shore terrace.

Cliffs & Wave cut platforms

Cave, Arch, Stack & Stump

  • Prolonged waves attack on the base of the cliff & excavate holes in regions of local weakness called called
  • When 2 caves approach each other from either side of headland & unite, they form an
  • Further erosion by waves will lead to total collapse of the arch.
  • The seaward portion of the headland will remain as a pillar of rock known as
  • With the course of time, these stubborn stacks will gradually be eroded, leaving behind the stumps, which are only just visible above the sea level.

Cave, Arch, Stack, Stump, blowhole

Geos & Gloups (blow-holes)

  • The occasional splashing of the waves against the roof of a cave may enlarge the joints when air is compressed & released repeatedly inside them.
  • A natural shaft is thus formed which may eventually pierce through the surface.
  • Waves breaking into the cave may force water or air out of this hole. Such a shaft is termed as Gloup or blow hole.
Gloups
Geos & gloups

 

  • The enlargement of blow-holes & continual action of waves weakens the cave roof.
  • When the cave roof collapses, a long, narrow creek may develop known as Geos

 

Coastal Features of Deposition

Beaches

  • Sands & gravel loosened from the land are moved by waves to be deposited along the shore as beaches.
  • The eroded material is transported along the shore in several distinct ways.
  • Long shore drifts which comes obliquely to the coast carries material along the shore in the direction of the dominant wind.
  • At the same time, backwash removes part of the material seawards, along the bed of the sea, & deposits it on the off-shore terrace & even beyond.
  • The constant action of the waves automatically sorts out the shoreline deposits in a graded manner.
  • The coarser materials are dropped by the waves at the top of the beaches & the finer materials, carried down the beach by the backwash, are dropped closer to the sea.

Coastal Features of Deposition

Spits & Bars

  • The debris eroded by waves is continually moved by long shore drift & where there is indentation in the coast, such as at the mouth of the river or a bay; material may continue to be deposited across the inlet
  • As more materials are added, they will pile up into a ridge or embankment of shingle forming a spit, with one end attached to the land & other projecting into the sea
  • When a ridge of shingle is formed across the mouth of a river or the entrance to a bay, it is called a bar
  • Such a connecting bar that joins two land masses is known as Tombolo

Spits, bars, tombolo

Marine Dunes & Dune Belts

  • With the force of on-shore winds, a large amount of coastal sand is driven landwards forming extensive marine dunes that stretches into dune belts
  • Their advance inland may engulf farms, roads & even the entire villages;
  • Hence to arrest the migration of dunes, sand binding species of grass & shrubs, such as marram grass & pines are planted.
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