Geological Structure
Geography: Edexcel A Level, topic 3.1.2
Concordant vs Discordant Coastlines
The shape of a coastline is continually changing due to the surrounding geology which contributes to its morphology. Coasts are either concordant or discordant.
Concordant coastlines
- Concordant coastlines have alternating bands of hard and soft rock parallel to the coast.
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Discordant coastlines
- Discordant coastlines have alternating bands of hard and soft rock at 90o to the coast.
- Erosional landforms are more common on discordant coastlines because erosion happens at different rates along their length.
Example of a discordant coast
- In Dorset, the coastline consists of Portland limestone and Greensands.
- The Greensands is less resistant to erosional processes, and so retreats at a faster rate than the Portland limestone.
- This has resulted in the creation of a bay and headlands.
Coastal Morphology
The shape of a coastline is continually changing because of the surrounding geology, which contributes to its morphology.
Pacific coasts
- Concordant coastlines, also referred to as Pacific coasts, lead to the formation of Dalmatian coasts.
- Dalmatian coasts are made up of offshore islands and coastal inlets running parallel to the coastline.
- E.g. Lulworth Cove, located on the south-west Dorset coastline. Here, the more-resistant rock (e.g. limestone) acts as a barrier to protect the less-resistant rock.
Atlantic coasts
- Headlands and bays often form on discordant coastlines (also referred to as Atlantic coasts).
- As the faster eroding, less-resistant rock retreats (e.g. the clays at Swanage Bay), this leaves behind the more-resistant rock as headlands.
Bays and headlands
- On discordant coastlines, the retreating, less resistant rock and the exposed resistant rocks cause a change in the shape of the coastline. This leads to wave refraction.
- This change in the way in which waves approach the coastline can cause an increase in the rate of erosion on the headlands.
- This leads to the formation of headland features like caves, arches, stacks and stumps.
Geological Structure
The shape of a coastline is continually changing because of the surrounding geology, which contributes to its morphology.
Joints and faults
- The geological structure of rocks (the formation of joints, the angle of the dip, faulting and folding) is caused by the deformation and stress placed on them from tectonic processes.
- Rocks with the presence of more joints and faults, like sedimentary rocks, are more susceptible to the processes of erosion and weathering. This is because these processes exert forces on the weaknesses found in the layers of the rocks.
Dip
- The angle the rock layer forms with the horizontal bedding plane is referred to as the dip. Rocks can dip either towards land or towards the sea.
- Bedding planes that dip towards the sea create a gentler cliff profile. But these cliffs are vulnerable to mass movement processes, like rockfalls.
- Bedding planes that dip towards land tend to create a steeper cliff profile. But these cliffs are more vulnerable to erosion processes, like hydraulic action and abrasion.
Test yourself on Geological Structure
Which of the following rock types in Dorset is most resistant to erosion?
On a concordant coastline, how are the rock layers (strata) arranged in relation to the coastline?
What are concordant coastlines also referred to as?
Which of the following form on discordant coastlines?
What kind of cliff profile do bedding planes that dip towards the sea create?