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

  • 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.

 

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.

  • 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.
 
 
  • 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.

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.

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.

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.

 

  • 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.

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 abar
  • Such a connecting bar that joins two land masses is known as 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.