Coastal Landforms
Landforms created by Erosion
- Cliffs are a common feature of coasts formed by the action of waves and weathering. Resistant rock tends to make high cliffs.
- Wave-cut platforms are flat surfaces left behind when a cliff is eroded back.
- Bands of alternating soft and hard rock form alternating bays and headlands. When weaker rock is uncovered, it erodes quickly, to form a bay. Harder rock is left as headland.
- Erosion processes take advantage of weak spots in the rock to form caves.
- Caves on the opposite sides of a narrow headline will eventually join up to form arches.
- When an arch collaspes it forms a stack.
- Stacks become worn down to form stumps, these are only visible at low tide.
Landforms created by Deposition
- Beaches are an obvious landform created by deposition. They form when constructive waves dump sediment on the shore.
- Bars are formed by waves breaking offshore. The waves gradually deposit sediment which was picked up further out to sea. Bars can build up until they're visible above sea level. An example of a bar is Scolt Head Island off the Norfolk coast.
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- A spit is a bank of sand or shingle that sticks out into the sea, an example of which is Spern Head. A spit is created by the process of Longshore Drift.
- A spit or bar that connects an island to the mainland is called a tombolo. Chesil Beach in Dorest is an example of a tombolo.
- Cuspate forelands are triangular shaped bits of land sticking out into the sea. They form by longshore drift, by waves coming from different directions. Dungeness in Kent is a cuspate foreland.
How Geology affects Coastal Landforms
Different coastal rocks result is the formation of varying landforms. There are two main types of rocks which affect the landform of a coast. These are coherent and incoherent rocks.
Different coastal rocks result is the formation of varying landforms. There are two main types of rocks which affect the landform of a coast. These are coherent and incoherent rocks.
- Rocks that have strong crystalline bonds, for example most metamorphic and igneous rocks, are resistant to erosion. This is also true for strongly bonded sedimentary rocks, for example chalk, carboniferous limestone and old red sandstone. These rock types are coherent and tend to form high steeply angled cliffs.
- Rocks with weak bonds and weak structures such as conglomerates, sands and clays are incoherent and easy to erode. This causes their cliffs to easily become undercut making them unstable. As a result they are susceptable to slumping and rotational slip which causes stepped cliffs.
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How Waves affect Coastal Formation
- Swash aligned beaches are built when waves break parallel to the shore. Sediment moves up and down the beach, but not along it. This results in cresent shaped beaches in bays and linear beaches on straight coasts.
- Drift aligned beaches are built when waves break at an angle to the shore. They often have spits with recurved laterals (This indicates that the end of the spit is curved back in on itself).
- Sediment size also affects wave action and beach formation. Shingle beaches are steep and narrow, the larger particles tend to pile up at steep angles. Sand beaches are longer and flatter, with a stronger swash.
- Wave refraction affects the erosion and deposition processes of a coast. When the coast is curved or indented, wave fronts curve and distort as they approach the coast.
- Where waves diverge they lose power. This power loss means that they drop their load of sediment, resulting in deposition.
- Where waves converge they increase in power. This increase in power increases their friction forces and ability to hold sediment, resulting in erosion.
Summary of Terms
Wave fetch: The distance of open water over which a wave has passed. Maximum fetch is the distance from one coastline to the next landmass, it often coincides with prevailing wind direction (South West in the UK).
Wave crest: Highest point of a wave.
Wave trough: Lowest point of a wave.
Wave height: Distance between trough and crest.
Wave length: Distance between one crest/trough and the next.
Swash: Water movement up a beach.
Backwash: Water movement down a beach.
Wave fetch: The distance of open water over which a wave has passed. Maximum fetch is the distance from one coastline to the next landmass, it often coincides with prevailing wind direction (South West in the UK).
Wave crest: Highest point of a wave.
Wave trough: Lowest point of a wave.
Wave height: Distance between trough and crest.
Wave length: Distance between one crest/trough and the next.
Swash: Water movement up a beach.
Backwash: Water movement down a beach.