2B.3B Rock Strata and Complex Cliff Profiles
Differential erosion of alternating strata in cliffs (permeable/impermeable, resistant/less resistant) produces complex cliff profiles and influences recession rates.
Complex cliff profiles are produced where cliffs are composed of strata of differing lithology. Less resistant strata erode and weather quickly, being cut back rapidly, wave cut notches may be formed. Resistant strata erode and weather slowly, retreating less rapidly. They may form a 'bench' feature at the cliff base. Higher up, they form overhanging sections until they collapse by mass movement. However, generally the overall rate of cliff recession is determined by the resistance of its weakest rock layer.
Rocks show different levels of resistance to marine erosion in the foreshore zone.
Rocks show different levels of resistance to weathering or mass movement in the foreshore and backshore zones.
Complex cliff profiles can also be produced when there are alternating permeable and non-permeable strata.
Permeable rocks are those that allow water to flow through them. This may be because:
Permeable rocks tend to be less resistant to weathering because water percolating comes into contact with a large surface area that can be chemically weathered.
Impermeable rocks do not allow water to flow through them.
1) A spring creating erosion
2) Groundwater flow removing cement
Water flows through the permeable (sands) but can't flow through the impermable (clay), so flows along the interface. Groundwater flow through rock layers can weaken rock by removing the cement that binds them together. Weak, unconsolidated layers slump.
3) Pore water pressure leads to slumping and sliding
It also produces pore water pressure: (the internal force within cliffs exerted by a mass of groundwater within permeable rocks)
4) Saturation leads to slumping and sliding
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.
Complex cliff profiles are produced where cliffs are composed of strata of differing lithology. Less resistant strata erode and weather quickly, being cut back rapidly, wave cut notches may be formed. Resistant strata erode and weather slowly, retreating less rapidly. They may form a 'bench' feature at the cliff base. Higher up, they form overhanging sections until they collapse by mass movement. However, generally the overall rate of cliff recession is determined by the resistance of its weakest rock layer.
Rocks show different levels of resistance to marine erosion in the foreshore zone.
Rocks show different levels of resistance to weathering or mass movement in the foreshore and backshore zones.
Complex cliff profiles can also be produced when there are alternating permeable and non-permeable strata.
Permeable rocks are those that allow water to flow through them. This may be because:
- they're porous (e.g. chalk)
- these are rocks containing voids called pores, for example chalk and poorly cemented sandstones
- they have numerous joints (e.g. carboniferous limestone)
Permeable rocks tend to be 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 carbonation converting calcium carbonate to soluble calcium bicarbonate.
- Feldspar in granite weathered by hydrolysis into kaolin (china clay)
Impermeable rocks do not allow water to flow through them.
- Clays, mudstones, and most igneous and metamorphic rocks are impermeable.
1) A spring creating erosion
- Where a permeable rock overlays an impermeable stratum groundwater is unable to percolate down into the lower layer.
- Water accumulates in the permeable layer, producing a saturated layer where the pores are full of water.
- 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 off erosion( will attack the saturated permeable bed and lower impermeable stratum, reducing the angle of the cliff profile.
2) Groundwater flow removing cement
Water flows through the permeable (sands) but can't flow through the impermable (clay), so flows along the interface. Groundwater flow through rock layers can weaken rock by removing the cement that binds them together. Weak, unconsolidated layers slump.
3) Pore water pressure leads to slumping and sliding
It also produces pore water pressure: (the internal force within cliffs exerted by a mass of groundwater within permeable rocks)
- Pore water pressure in the saturated layer pushes rock particles apart.
- Reduces friction between grains in unconsolidated material
- Lubricates lines of weakness, e.g. joints and bedding planes.
4) Saturation leads to slumping and sliding
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.
Other Information from Class Notes
There are two main types of cliff profile:
- Steep, unvegetated cliffs
- produced where marine erosion dominates
- there is little or no debris at the base because it is broken up by attrition and transported offshore or along the coast.
- Shallow-angled, vegetated cliffs
- they have a convex profile (convex = curved like the interior of a circle)
- there is debris at the base
- because sub-aerial processes (weathering, mass movement and surface run-off erosion) slowly move sediment downslope, but marine erosion is unable to remove it from base
- produced where there is little active marine erosion
