Monday, 11 April 2011

Coastal landforms

There are many different landforms that are present along our coastline and they are all formed in slightly different ways and, as there are quite a few different ones, I am going to try and summarise the key points for each.

- They form in areas of alternating resistant and less resistant rocks. The less resistant rocks, such as sand and clay, erode a lot faster than the more resistant rocks, like chalk, and  this differential erosion often results in sections of land jutting out into the sea, known as headlands. Due to refraction, the headlands recieve the highest energy waves and are most vulnerable to the forces of erosion. The shallower areas around the headland provoke the waves to start to break earlier and so, as they refract around the headland, lower energy waves are experienced in the bays which leads to the accumulation of sediment.

- Coastlines can either be described as being concordant or discordant. In concordant coastlines the alternating stratas of soft and hard rock run parallel to the coastline whereas in discordant coastlines they run perpendicular to the coastline. Where the rocks run parallel with the coast (concordant) it is possible for continued erosion to break through the more resistant rocks on the coast and begin to attack the less resistant rock strata behind. When this occurs a cave is formed which is then only enlarged by further erosion - Lulworth Cove is possibly the best local example of this.
- When high and steep waves break at the foot of a cliff they concentrate their erosive capabilities into only a smaal area of the rock face, This concentration eventually leads to the cliff being undercut, forming a wave cut notch. Contiuned activity at this point increases the stress on the cliff and after some time, it collapses. This causes the cliff to retreat and when the overhang is undercut a platform forms. The platform continues to grow and, as it does, the waves break further out to sea and have to travel across more of the platform before reaching the cliff. This leads to greater dissipatation of wave energy which reduces the rate of erosion on the headland and thereby slowing down the growth of the platform.
- Hydraulic action and abraision, accompanied by water layer weathering and some of the other weathering processes, lead to the formation of wave cut notches and platforms.

So in short......
1) Weathering weakens the top of the cliff
2)The sea attacks the base of the cliff, forming a wave cut notch
3) The notch increases in size, causing the cliff to retreat
4) The backwash carries the rubble towards the sea, forming a wave cut platform
5) The process repeats and the cliff continues to retreat

- If the cliff is undercut a cave can be formed, usually from a combination of marine processes including hydraulic action, abraison and solution. If erosion continues upwards, it is possible for the cave to be extended to the top of the cliff where a blowhole will form. If the cave extends backwards, to meet another, an arch will form. As the dliff retreats and a wave cut platform forms, the arch will collapse and create a stack. In time, the sea will exploit the wave cut notch at the base of the stack and cause it to collapse, forming an stump. It is important to remember that caves, arches, stacks and stumps occur in a sequence, one after another

- A geo is a narrow steep sided inlet which is created along a joint that is exposed to the sea. A combination of hydraulic action and the other marine processes widens joints to form geos.

- Barrier islands are a series of sandy islands totally detached from, but running almost parallel to, the mainland. Between the idlands and the mainland is a tidal lagoon. Although they are relatively uncommon in the UK, they are widespread globally (lots in the USA, west of Africa, northern Netherlands and in Australia) and account for 13% of the worlds coastline. While their orgin is unknown, they do tend to develop on coasts with relatively high energy waves but a low tidal range. One theory suggests that they formed, below the low tide mark, as offshore bars of sand that have moved progressively landwards. Another thoery is that rises in post-glacial sea level have partly submerged older beach ridges.

- A tombolo is a depositional feature that connects an island to the mainland. They are often temporary features and so should not be built on.
- waves refract around the island and as they meet, before meeting the coastline, they cancel each other out and so sediment is deposited.

- Spits are another depositional feature that are predominantly caused by longshore drift and they form whenever you get a change in the direction of the coastline.
The spit curves at the end (into a hook) due to winds that blow in a different direction to the prevailing wind.
- Mudflats, brackish lagoons and salt marshes form behind the spit and these areas are important for migratory birds. Spits are also important as they protect the area behind from the most powerful waves and so create the perfect conditions for the development of a port.
- The rivers flow often prevents the spit from connecting to the other side of the estuary but, if there is no significant flow from the river and it does connect to the opposite side, a bar is formed.

1) Longshore drift moves material along the coastline
2) A spit forms when material is deposited
3) Over time the spit grows and develops a hook, if the wind direction changes
4) Waves cannot get past the spit, which creates a sheltered area where silt is deposited and mud flats or salt marshes form

The last landform is a beach but, when I find the time, I am going to write a seperate post on them and try to explain in a bit of detail the beach profile as there are a few key words to learn and I sometimes seem to get a bit mixed up when trying to use them to describe the profile of a beach. Hopefully I will managed to write this in the next few days but if not definetly over half term.

1 comment:

  1. Thanks for this! A great blog with helpful revision info