Coastal Deposition Landforms: Features and Formation

Material that is transported by the waves along a coastline is eventually deposited forming distinctive deposition features. There are four main deposition features that you need to learn the formation of. These are:

1. Beaches
2. Spits
3. Bars
4. Tombolos

Beaches Beaches are the main feature of deposition found at the coast, these consist of all the material (sand, shingle etc.) that has built up between the high and low tide mark. There are number of different sources of beach material - the main source being rivers, where fine muds and gravels are deposited at the river mouth. Other sources of beach material include longshore drift (bringing material from elsewhere along the coast); constructive waves (bringing material up the beach from the sea) and from cliff erosion.

As constructive waves build up beaches, they often form ridges in the beach known as berms. The berm highest up the beach represents the extent to which the water has reached during high tide.

Click on the diagram below to see the main sources of beach material


SPITS
Spits are long narrow ridges of sand and shingle which project from the coastline into the sea.

The formation of a spit begins due to a change in the direction of a coastline - the main source of material building up a spit is from longshore drift which brings material from further down the coast.

Where there is a break in the coastline and a slight drop in energy, longshore drift will deposit material at a faster rate than it can be removed and gradually a ridge is built up, projecting outwards into the sea - this continues to grow by the process of longshore drift and the deposition of material.

A change in prevailing wind direction often causes the end of spits to become hooked (also known as a recurved lateral).
On the spit itself, sand dunes often form and vegetation colonises (for example Blakeney Point - North Norfolk)
Water is trapped behind the spit, creating a low energy zone, as the water begins to stagnate, mud and marshland begins to develop behind the spit;
Spits may continue to grow until deposition can no longer occur, for example due to increased depth, or the spit begins to cross the mouth of a river and the water removes the material faster than it can deposited - preventing further build up.

Examples of Spits
- Spurn Head - Holderness Coast
- Orford Ness - Suffolk

Click below for an annotated diagram of spit formation:


BARS

These form in the same way as a spit initially but bars are created where a spit grows across a bay, joining two headlands. Behind the bar, a lagoon is created, where water has been trapped and the lagoon may gradually be infilled as a salt marsh develops due to it being a low energy zone, which encourages deposition.

Example of a Bar: Slapton Sands - Devon.



TOMBOLOS

Tombolos are formed where a spit continues to grow outwards joining land to an offshore island.

Example of a Tombolo: - Chesil Beach - which joins the South Dorset coast to the Isle of Portland.


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REVISING COASTAL DEPOSITION FEATURES

Remember - as well as being able to describe the formation of each feature of coastal deposition, you should be able to give a named and located example e.g. a spit - Spurn Head (Holderness Coast). You should also try and learn a labelled diagram to show the formation of each feature.

- Beach - Dawlish Warren (Devon)
- Spit - Orford Ness (Suffolk) or Spurn Head (Holderness Coast)
- Bar - Slapton Sands (Devon)
- Tombolo - Chesil Beach (joining S Dorset Coast to Isle of Portland)

Having now learnt both erosion and deposition features you need to make sure that you can distinguish between them. Have a go at the dustbin game below - click on play to begin. Start by studying the two lists when you think you are ready to test yourself on whether a landform is a feature of erosion or deposition start the game by clicking proceed. Drag the feature to the correct dustbin to make your choice!

click here for full screen version

Follow Up Links:

Animation of spit formation (Wycombe High School Link)
Bar and Spit Animation (Wycombe High School Link)
Longshore Drift Animation
Simple Longshore Drift Animation

Key Terms Check
Spit - a ridge of sand and shingle projecting from the mainland into the sea
Bar - a ridge of sand and shingle which has joined two headlands, cutting off a bay
Tombolo - a ridge of sand and shingle joining the mainland to an island

Podcast: Coastal Deposition Features
You can listen to a podcast of this post below - to download a copy to listen to on your .mp3 player click here.

Coastal Processes: Erosion, Transport and Deposition

Remember, there are 3 main processes that cause a coastline to change:
1. Erosion
2. Transport
3. Deposition.

There are number of factors which affect each of these processes - we are going to start by exploring erosion processes and the factors that can affect the amount of erosion that may take place along a coastline.

COASTAL EROSION

Erosion Processes:
Erosion is the wearing away of rocks, at the coast there are 6 main types of erosion processes in action (see animations here):

1. ABRASION (this is also known as corrasion) - this is where rock fragments are hurled at cliffs by breaking waves, gradually scraping away at the cliff face;

2. HYDRAULIC ACTION - as waves break against the cliff face, the pressure of the breaking wave can compress air in cracks. This compressed air gradually forces open the crack in the rock - as this process continues, the rock becomes increasingly weakened.

3. SOLUTION (this is also known as corrosion) - this occurs where the salt water is able to dissolve some of the chemicals in rocks - for example, limestone cliffs are gradually weakened as the salt water dissolves the calcium carbonate in the limestone.

4. SCOURING - this occurs at the base of the cliff as the waves break and swirl around, gradually removing loose rock.

5. ATTRITION - this is where rock fragments carried by the waves hit against each other and gradually wear down to form sand and silt

6. WAVE POUNDING - the sheer force of waves hitting against the cliff face

These processes of erosion form a series of distinctive landforms at the coast.

Rates of Coastal Erosion

So what are the factors that determine how much erosion can take place at the coast?

1. The Resistance of the Rocks - e.g. limestone, chalk and granite are resistant rocks (often forming cliffs and headlands) and erode relatively slowly, whilst less resistant rocks such as clay are easily eroded.

2. The Strength of the waves - affected by the wind strength and duration and its fetch

3. The shape of the coastline (which is dependent on its geology) - on concordant coastlines, rocks are parallel to the wave front and therefore rates of erosion are similar along the coastline. On discordant coastlines, differential erosion may occur, where bands of hard and soft rock outcrop at right angles to the sea. Consequently headlands and bays form along discordant coastlines and whilst headlands remain exposed to the force of the waves, bays are sheltered.

Click on the diagram below for a summary of factors affecting coastal erosion




COASTAL TRANSPORT

The second process operating at the coast is transport. Material eroded by the sea is carried within the water in a number of ways, minerals dissolved from rocks are carried in solution, whilst small rock fragments, light enough to be held within the water, float in suspension. The largest rock fragments which are too heavy to be picked up by the waves, are transported by the process of traction, this is where they roll along the bed when the waves pick up enough energy. Finally, medium sized rock particles, which cannot be carried by the waves all the time, are moved by saltation. This is where during times of higher wave energy the particles are picked up and then dropped again as the wave looses its energy.

The main form of transport operating at the coast is that of LONGSHORE DRIFT.

Longshore drift is the process by which sand and pebbles are moved along a beach by the movement of the waves.



COASTAL DEPOSITION

Material is moved up the beach by the swash at an angle which is controlled by the prevailing wind. The backwash then carries material back down the beach at right angles to the coastline under the influence of gravity. Gradually the material is moved along the coastline, its direction being controlled by the prevailing wind direction.

The final process operating at the coast is that of deposition - this is where material that is too heavy to be transported any more is left behind, building up the beach. Due to the importance of energy in transporting sand and shingle, it is the largest material that is deposited first. A number of distinctive features may form due to coastal deposition.

Follow up links:
Animations of Coastal Erosion Processes (BBC Bitesize)
Transport and Deposition (BBC Bitesize)

Key Terms Check:
Erosion - the wearing away and removal of material
Deposition - the dropping of material
Abrasion - the wearing of rock due to rock fragments being hurled against cliffs
Attrition - the breakdown of rocks as they hit against each other
Hydraulic Action - the force of waves causing rocks to split apart as waves compress air in cracks in the rocks
Wave Pounding - sheer force of water hitting rocks
Solution - where minerals in rocks are dissolved by the action of sea water
Scouring - occurs where water and broken rock fragments swirl around at the base of cliffs gradually wearing rock away.
Longshore Drift - the movement of material along a coastline

Podcast: Coastal Processes
You can listen to a podcast of this post below - to download a copy to listen to on your .mp3 player click here.