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Coastal Erosion Landforms

Distinctive coastal landforms are the result of rock type, structure and physical processes.

Erosion Landforms: Headlands and bays, cliffs and wave cut platforms, caves, arches and stacks
Coastlines are littered with the evidence of erosion and the power of the sea. Erosion makes the coastline varied and interesting, and often give the coastline its "wow" factor.

Key words;

  • Arch - A wave-eroded passage through a small headland. This begins as a cave formed in the headland, which is gradually widened and deepened until it cuts through.
  • Cave - A large hole in the cliff caused by waves forcing their way into cracks in the cliff face.
  • Cliff - A steep high rock face formed by weathering and erosion along the coastline.
  • Headlands and bays - A rocky coastal promontory made of rock that is resistant to erosion; headlands lie between bays of less resistant rock where the land has been eroded back by the sea.
  • Stack - An isolated pillar of rock left when the top of an arch has collapsed; over time further erosion reduces the stack to a smaller, lower stump.
  • Wave cut platform - A rocky, level shelf at or around sea level representing the base of old, retreated cliffs.

Wave cut platform

For each landform you need to understand how processes of weathering (freeze thaw, hydration, biological, salt crystallisation), mass movement (slumping, slides, rock falls) and erosion (Hydraulic Power, Corrasion, Corrosion) contributes to the formation.

Caves, arches stacks and stumps

These features are formed on cliffs or headlands. Waves attack vertical lines of weakness in the rock known as Faults. Processes such as hydraulic action and abrasion widen these faults into cracks and eventually the waves will penetrate deeply enough to create caves.
Over time, the cave will be eroded into an arch, accessible to the sea on both sides. Weathering will also play a role, with physical weathering processes such as freeze thaw and salt crystallisation and chemical processes such as carbonation weakening the rock surrounding the cave or arch making it more susceptible to mass movement and collapse.
Finally, the erosion and weathering continues and the arch collapses leaving behind a stack (a vertical column of rock). These stacks can be attacked further, and eventually the stack may collapse to leave a low-lying stump.

Caves, arches, stacks and stumps

Bays and headlands
In areas where the geology or rock type runs at right angles to the coastline, bays and headlands can be created. If there are alternating bands of harder and softer rock running at right angles to the sea, the sea will erode these bands at different rates (called differential erosion). Hydraulic action, abrasion and corrosion are more effective at eroding the softer rock, particularly during storms, and this will erode further inland than the harder rock. During calmer weather and no stormy periods, the hard rock will absorb a lot of the wave energy and refract or bend the waves into the area with the softer rock, allowing sediment to be deposited and accumulate as beaches. The net result of this over long periods of time is that the hard rock is left jutting out to sea as a headland, and the softer rock is eroded into curved sand filled bays.


Cliffs and cliff retreat

A cliff is a vertical, near vertical or sloping wall of rock or sediment that borders the sea. They generally differ in their angle of slope because of their rock structure and geology, but the processes involved in their formation are the same.
Marine erosion processes attack the foot of the cliff and cause the erosion at a wave cut notch. Waves can pound this area causing fragments to break off, and the water can also trap air in pore spaces, faults and crevices, compressing the air which in turn exerts pressure on the rock causing it to break off. This process is known as hydraulic action. Another process that occurs is corrasion, where sediment and rocks in the sea water are hurled against the cliff face. All three of these processes erode the wave cut notch at the base of the cliff undermining the whole structure of the cliff. These processes are variable and depend upon the fetch of the wave (the distance it travels over open water), wind speed and how many storms there a year, but they are more or less continuous over long periods of time.

Cliff retreat
At the same time that the base of the cliff is being eroded, the cliff face and its structure are being weakened by weathering processes. Oxidation and carbonation are some of the chemical processes that can weaken the structure of the rock, and depending upon the climate physical processes such as freeze thaw and water layer weathering can take effect. Over time this weakens the structure of the cliff face, and coupled with the erosion of the wave cut notch at a critical point this cliff face will succumb to the influence of gravity and collapse in a process of mass movement. This material will then be carried away by the sea in the process of long shore drift by the transportation process of solution, suspension, saltation and traction (depending upon the particle sizes).  The cliff retreats, leaving behind a flat wave cut platform.



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