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Geography

Erosional landforms

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Caves, arches, stacks and stumps

Platform, arch, cave and stack

Platform, arch, cave and stack

Weathering [weathering: The gradual breakdown of rocks due to the effects of weather. ] and erosionerosion: Erosion is the process whereby rock or soil is worn away by the action of the wind, waves or water. can create caves, arches, stacks and stumps along a headland.

Old Harry Rocks, Swanage, Dorset

Old Harry Rocks, Swanage, Dorset

  • Caves occur when waves force their way into cracks in the cliff face. The water contains sand and other materials that grind away at the rock until the cracks become a cave. Hydraulic action [hydraulic action: Erosion caused by waves hitting the cracks on a cliff face. The air in the cracks becomes compressed and then explodes outwards, breaking off bits of rock. ] is the predominant process.
  • If the cave is formed in a headland, it may eventually break through to the other side forming an arch.
  • The arch will gradually become bigger until it can no longer support the top of the arch. When the arch collapses, it leaves the headland on one side and a stack (a tall column of rock) on the other.
  • The stack will be attacked at the base in the same way that a wave-cut notch is formed. This weakens the structure and it will eventually collapse to form a stump.
  • One of the best examples in Britain is Old Harry Rocks, a stack found off a headland in the Isle of Purbeck.

Watch

The formation of Old Harry Rocks in Dorset.

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