Monday, August 20, 2007

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 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:


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 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.


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 s
and 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.

1 comment:

d3n1sE said...

wow this is really a good blog. i'm not a st ivo student but i think this is really useful. GEOGRAPHY ROCKS!