2B.2c Geological structure

• cliff profiles are influenced by 2 main things:

  • the resistance of the rock to erosion
  • the dip in rock strata in relation to the sea
• many cliffed coastlines are composite so have different rock layers
• what is the dip of a cliff?

  the angle of strata
• horizontal dip:

  vertical or near-vertical profile with notches reflecting strata that are more easily eroded
• seaward dip:

  sloping low angle profile within one rock layer facing the sea: vulnerable to rock slides down the dip slope

  the rock layer facing the sea is vulnerable to rock slides down the dip slope when uppermost strata are attacked by sub-aerial processes
• seaward dip low angle:

  profiles may exceed 90°, producing areas of overhanging rock making it very vulnerable to rock falls

  it has many joints: vulnerable to freeze-thaw weathering

  example: steeply dipping chalk on Jurassic coast
• landward dip:

  steep profiles of 70-90° producing a very stable cliff with reduced rock falls

  less vulnerable to erosion due to its stabilised rock materials, leaving a very strong resistant surface
• faults:

  the result of pressure or stress causing it to fracture the rock strata, due to tectonic forces involving the displacement of rocks either side of a fault line.

  faults can make the rock less stable: movement along faults can create zones of weakness, making the rocks more susceptible to weathering and erosion
• joints:

  vertical cracks caused by tectonic forces e.g. uplift or contraction. they divide up the rock strata into block of regular shape. regular patterns of jointing is due to sedimentary rocks being folded together due to crustal compression. joints create areas that can be exploited by erosion or biological and freeze-thaw weathering
• fissures:

  smaller cracks in rocks normally only a few cm long but they provide weakness that erosion can exploit for example, micro-features such as caves begin to develop
• folds:

  the result of pressure during tectonic movement causing the rock strata to fold. Horizontal strata are squeezed together, creating a series of anticlines and sinclines
• microfeatures such as caves and notches are the result of faults with high amounts of joints are fissures being eroded
• example coastline: Glamorgan Heritage coast
• the rate of cliff recession is determined by the layer of strata with the least resistance:

  less resistant strata such as coal are eroded and weathered more quickly which can cause a wave cut notch to form. the more resistant overhanging sections are subject to gravity, leading to cliff collapse and further recession
• rocks can be permeable due to:

  • strata is porous (e.g. chalk or poorly cemented sandstones) these contain air spaces and empty voids called pores
  • there are many joints which enable water to travel through the rock e.g. carboniferous limestone and sandstones
• permeable rocks are less resistant to weathering because water percolating comes into contact with a large surface area that can be chemically weathered e.g. limestone weathered by chemical weathering ( carbonation) converting calcium carbonate to soluble calcium bicarbonate

  permeable rocks can hold more water so become saturated and so are more heavy and so more vulnerable to mass movement
• a spring creating erosion:

  • where a permeable rock overlays an impermeable strata groundwater is able to percolate down into the lower layer
  • water accumulates in the permeable layer producing a saturated layer
  • a spring will form on the cliff face at the top of the saturated layer
  • as the stream flows down the cliff, fluvial erosion (surface run) will attack the saturated permeable bed and lower impermeable strata, reducing the angle of the cliff profile
• where there is unconsolidated sediment present:

  • saturation promotes mass movement through lubrication and by adding weight. leads to slumping in unconsolidated material and sliding in consolidated strata - producing a complex cliff profile
  • groundwater flow can weaken rock layers by removing the cement that binds sediment together this can also lead to high pore water pressure (weight of water)