Marine landforms

The Coastal Landforms are formed by the constant action of the waves, tides, and currents. The coastline under the influence of these denudational agents changes the coastal landforms and gives shapes to various types of marine landform features.

Agents of Erosion


  • Waves accomplish most of the changes along the coasts.
  • Constant impact of breaking waves drastically affects the coasts.
  • When waves break, the water is thrown with great force onto the shore, and simultaneously, there is a great churning of sediments on the sea bottom.
  • Storm waves and tsunami waves can cause far-reaching changes in a short period of time than normal breaking waves.
  • On calm days, when winds are slight, waves do little damage to the shoreline and may instead help to build up beaches and other depositional features.

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Tides and Currents

  • Tides and Currents, on contact with the shores, make very little direct attack on the coastline
  • Tides affect marine erosion mainly by extending a line of erosion into a zone of erosion. This zone correspond to the area between the low water level and the high water level
  • Currents help to move eroded debris and deposit it as silt, sand and gravel along the coasts

The mechanism of marine erosion

The rate of marine erosion depends on the nature of rocks, the amount of rock exposed to the sea, the effects of tides and currents, and human interference. Marine agents of erosion operate in the following ways to transform the coastal landscape

Corrasion Corrasion is a process of mechanical erosion. Waves armed with rock debris break on cliff faces and slowly erode it. On-coming currents and tides complete the work by sweeping the eroded material into the sea.

Attrition - Attrition occurs when waves cause loose pieces of rock debris such as boulders, pebbles, shingle and fine sand, to collide with each other. Under attrition, these materials are broken down into finer, smaller and rounder particles which are largely responsible for the fine sand that forms the beaches.

Hydraulic action in their forward surge, waves splashing against the coast may enter joints and crevices in the rocks. The air trapped inside is immediately compressed. When the waves retreat, the compressed air expands with explosive violence. Such repeated action causes enlargements of the cracks and rock fragments are prised apart.

Solvent action this refers to chemical erosion of rocks. This process is limited to limestone coasts. On limestone coasts, the solvent action of seawater on calcium carbonate sets up chemical changes in the rocks and disintegration takes place.

Types of Coasts

Other than the action of waves, the coastal landforms depend upon the configuration of land and sea floor and whether the coast is advancing (emerging) seaward or retreating (submerging) landward.

There are different types of coastlines based on a great variety of coastal features. However, it is important to discuss, two types of coasts (assuming sea level to be constant) to explain the concept of evolution of coastal landforms:

  1. Submerged coasts (high, rocky coasts)
  2. Emerged coasts (low, smooth and gently sloping sedimentary coasts)

Submerged coasts

  • Submerged coasts are found either because of the sinking of the land or due to the rise of the sea
  • The coasts are rocky and river appears to have drowned in the sea creating estuaries.
  • Erosional landforms dominate coastal landform and depositional landforms are absent
  • Along high rocky coasts, waves break with great force against the land shaping the hill-sides into cliffs, which further develops a wave-cut platform, caves etc.
  • As the erosion along the coast takes place a good supply material becomes available to longshore currents and waves to deposit them as beaches, bars, spits etc.

Emerged Coasts

  • Emerged coasts are found due to either uplift of the land or fall in the sea level
  • They are less common
  • Along, the low sedimentary coasts the rivers appear to extend their length by building coastal plains and deltas.
  • The coastline appears smooth with occasional incursions of water in the form of lagoons and tidal creeks.
  • The land slopes gently into the water.
  • Marshes and swamps may abound along the coasts.
  • Depositional features dominate.
  • When waves break over a gently sloping sedimentary coast, the bottom sediments get churned and move readily building bars, barrier bars, spits and lagoons.
  • The maintenance of these depositional features depends upon the steady supply of materials.
  • Large rivers which bring lots of sediments build deltas along low sedimentary coasts.

Marine Landforms

Erosional landforms

Sea cliffs

  • The most widespread landforms of erosional coasts are sea cliffs.
  • Generally, any very steep rock face adjoining the coast forms a cliff
  • Almost all sea cliffs are steep and may range from a few m to 30 m or even more.
  • Their steep nature is the result of wave-induced erosion near sea level and the subsequent collapse of rocks at a higher elevation.
  • At the base of the cliff, the sea cuts a notch, which gradually undermines the cliff so that it collapses
  • The best-known cliffs are the Chalk cliffs of the English channel and the White Cliffs of Dover


Wave-cut platforms

  • When the sea waves strike against a cliff, the cliff gets eroded gradually and retreats.
  • With constant pounding by waves, as the cliffs recede, an eroded base is left behind, called a wave-cut platform.
  • The waves level out these platforms to create a flat surface
  • Such surfaces may measure from a few metres to hundreds of metres wide and extend to the base of the adjacent cliff.


Sea caves

  • Prolonged attack of waves against the base of the cliff and the rock debris that gets smashed against the cliff along with lashing waves create holes in regions of weakness and
  • These holes get further widened and deepened to form sea caves.
  • Example Flamborough head, England


Sea arches

  • When two caves approach one another from either side of a headland and unite, they form a bridge like structure, known as arch
  • These archways may have an arcuate or rectangular shape, with the opening extending below water level.
  • The height of an arch can be up to tens of metres above sea level.
  • It is common for sea arches to form when the waves attack a rock- form from two opposite sides, the differential erosion
  • Example the Neddle Eye near Wick, Scotland


Sea Stack

  • Continued erosion, under the attack of the wave, can result in the total collapse of an arch
  • The seaward portion of headland will remain as an isolated pillar of rock known as stack
  • Like all other features, sea stacks is also temporary and eventually, the stack will also disappear



  • The stack is gradually eroded, leaving behind only the stump
  • Stumps are only just visible above the sea level

Blow holes

  • The occasional splashing of the waves against the roof of a cave may enlarge the joints when compressed air is trapped inside
  • A natural shaft is thus formed which may eventually pierce through the surface
  • Waves breaking into the cave may force blasts of water from the top
  • Such shaft is termed as Blow-hole or Gloup .
  • Example Holborn Head, Scotland


  • The enlargement of blow-holes and the continued action of waves weaken the cave roof.
  • When the roof collapses a long, narrow inlet or creek develops.
  • Such long and deep clefts are called Geos
  • Example the Wife Geo, Scotland

Depositional landforms


  • Beaches are characteristic of shorelines that are dominated by deposition but may occur as patches along even the rugged shores.
  • Sands and gravels loosened from the land are moved by waves to be deposited along the shore as beaches
  • Most of the sediment making up the beaches comes from land carried by the streams and rivers or from wave erosion.
  • Most of the beaches are made up of sand sized materials. Beaches called shingle beaches contain excessively small pebbles and even cobbles.
  • Beaches are temporary features.




  • Just behind the beach, the coastal sands lifted and winnowed from over the beach surfaces will be deposited as sand dunes.
  • On shore, winds play a major part in the formation of these dunes
  • Sand dunes forming long ridges parallel to the coastline are very common along low sedimentary coasts.
  • Sand dunes are common in the coasts of Belgium, Denmark and the Netherlands



  • When a ridge of sand and shingle formed in the sea in the off-shore zone (from the position of low tide waterline to seaward), it is called a bar
  • The off-shore bars and barriers commonly form across the mouth of a river or at the entrance of a bay.
  • Bars are submerged features and when bars show up above water, they are called barrier bars.
  • Generally, bars are approximately parallel to the coast


  • Tombolo joins two landmasses by a connecting bar
  • The tombolo is a deposition landform in which an island is attached to the mainland by a narrow piece of lands such as a spit or bar.
  • A tombolo is a sandy isthmus.
  • An example of Tombolo can be found in Chesil beach in England which links the Isle of Portland with mainland


  • An off-shore bar which is exposed due to further addition of sand is termed a barrier bar.
  • The off-shore bars and barriers commonly form across the mouth of a river or at the entrance of a bay.
  • They usually occur in chains
  • They are subject to change during storms and other action, but absorb energy and protect the coastlines and create areas of protected waters where wetlands may flourish.


  • Barrier bar which gets keyed up to the headland of a bay is called a spit.
  • Spits are projected depositional landforms with one end attached to the land and the other end projecting into the sea
  • Spits may also develop attached to headlands/hills.
  • The mode of formation of spit is similar to a bar or barrier.
  • A shorter spit with one end curved towards the land is called a hook.
  • When barrier bars and spits form at the mouth of a bay and block it, a lagoon forms.
  • The lagoons would gradually get filled up by sediments from the land giving rise to a coastal plain.

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