Friday, January 20, 2012

COASTAL PROCESSES: Erosional landforms

Along a coastline there are features created by erosion. These include cliffs, wave-cut platforms and wave-cut notches. There are also headlands and bays, caves, arches, stacks and stumps.

Cliffs, wave-cut platforms and notches
One of the most common features of a coastline is a cliff. Cliffs are shaped through a combination of erosion and weathering - the breakdown of rocks caused by weather conditions. Soft rock, eg sand and clay, erodes easily to create gently sloping cliffs. Hard rock, eg chalk, is more resistant and erodes slowlyto create steep cliffs.

The erosion of cliffs

The process of cliff erosion
  1. Weather 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 collapse.
  4. The backwash carries the rubble towards the sea forming a wave-cut platform.
  5. The process repeats and the cliff continues to retreat.
Headlands and bays
Headlands are formed when the sea attacks a section of coast with alternating bands of hard and soft rock. The bands of soft rock, such as sand and clay, erode more quickly than those of more resistant rock, such as chalk. This leaves a section of land jutting out into the sea called a headland. The areas where the soft rock has eroded away, next to the headland, are called bays.

Geology is the study of the types of rocks that make up the Earth's crust. Coastlines where the geology alternates between strata (or bands) of hard rock and soft rock are called discordant coastlines. A concordant coastline has the same type of rock along its length. Concordant coastlines tend to have fewer bays and headlands.
Bay and headland in New Zealand

Discordant and concordant coasts in Dorset

Along the coastline of the Isle of Purbeck in Dorset, there are both discordant and concordant coastlines. The discordant coastline has been formed into Studland Bay (soft rock), Ballard Point (hard rock), Swanage Bay (soft rock) and Durlston Head (hard rock). After Durlston Head, the strata stop alternating and the coastline is made up of hard rock. This concordant coast has fewer features.

Caves, arches, stacks and stumps
Weathering and erosion can create caves, arches, stacks and stumps along a headland.
  • 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 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.
Platform, arch, cave and stack

Old Harry Rocks, Swanage, Dorset

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