2B.4C: Coastal Landscapes Produced by Erosion

Erosion creates distinctive coastal landforms (wave-cut notches, wave cut platforms, cliffs, the cave-arch-stack-stump sequence)
Wave cut notches and platforms (e.g. Kimmeridge Bay)
A wave cut notch is a curved indentation of about 1-2 m high extending along the base of a cliff. It forms between the high tide and low tide marks, where destructive waves impact against the cliff.
  • It's eroded by hydraulic action and abrasion, and in some cases corrosion.
  • The depth of the notch varies depending upon the resistance of the rock at different points.  

A wave cut platform is a flat rock surface exposed at low tide, extending out to sea from the base of a cliff. 
Their formation:
  • Marine erosion between the high tide and low tide mark by abrasion and hydraulic action forms a wave-cut notch along the length of the cliff base. 
  • The notch deepens by further erosion until the overlying material collapses by mass movement due to gravity, forming a cliff. 
  • The process repeats, and the position of the cliff retreats (coastal recession)
  • The rock just below low tide level is always submerged, it's uneroded as it's never exposed to wave impact. 
  • As the overlying material is eroded, uneroded rock at low tide level is left as flat rock surface, the wave cut platform. 

Other stuff about wave-cut platforms:
  • They often slope seaward at 4 degrees. 
  • Weathering attacking weaknesses in the platform surface may produce indentations, creating rock pools at low tide. 
  • Platforms rarely extend for more than a few hundred metres. 
  • This is because the shallow water causes the wave to begin to break as soon as it reaches the platform  

Cliffs
Cliffs are steep slopes that are usually unvegetated. 
  • Marine erosion of land between the high tide and low tide mark by hydraulic action and abrasion forms a wave cut notch
  • The notch deepens until the overlying rock collapses by mass movement due to the force of gravity. 
  • The exposed face forms a cliff. 
 
Cave-arch-stack-stump sequence
  1. Rocks have  joints, faults or vertically dipping bedding planes in their geological structure. 
  2. These are eroded more rapidly (e.g. hydraulic action), and the deepening and widening of a weak point forms a sea cave.
    1. This is accentuated by how wave refraction concentrates energy on the sides of the headland, producing destructive waves with a very large wave height. 
  3. Where a line of weakness extends right through the headland caves form on both sides. 
  4. Marine erosion deepens the caves until they connect up, creating a complete tunnel through the headland and forming an arch.
  5. Hydraulic action and abrasion attack the sides of the arch between low tide and high tide, forming wave cut notches. 
  6. The undercutting of the sides leads to the collapse of some overlying material by mass movement, widening the arch. 
  7. Weathering and other sub-aerial processes attack the arch roof. 
  8. Eventually, the roof of the arch will collapse by blockfall leaving the seaward end of the headland detached from the land as a tall vertical column called a stack.
  9. Marine erosion at the base of the stack will form a notch on all sides until the stack collapses by blockfall. 
  10. Remnants of the stack base form a stump, a small projection of rock, exposed only at low tide. 

However, unconsolidated and soft sedimentary or metamorphic rock won't undergo this because it isn't competent enough. (e.g. boulder clay, clay, shale) 

(Probably) Superfluous Information

  • The cliff slope angle depends upon the dip of the rock strata. 
    • Horizontal, vertical or landward dip produces steep cliffs. 
    • Seaward dip produces a shallower slope angle - which can also be produced when the lithology is unconsolidated
  • ​On a wide platform wave will have broken and swash lost energy before it reaches the cliff, halting recession.