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.
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 re curved lateral). On the spit itself, sand dunes often form and vegetation colonizes (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.
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.
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