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examBoard: Cambridge
examType: IGCSE
lessonTitle: Discordant and Concordant Coastlines
Coastlines come in many shapes and sizes, but their appearance is largely determined by the underlying rock structure. Two important types of coastlines are concordant and discordant coastlines, which form distinctive landscapes that affect both natural processes and human activities.
Key Definitions:
The type of coastline that forms depends on how the rock structure meets the sea. This is determined by geological processes that happened millions of years ago. The angle at which rock bands meet the sea creates either concordant or discordant coastlines, each with distinctive features and landforms.
Different rocks have different resistance to erosion. Hard rocks like granite and limestone erode slowly, while soft rocks like clay and shale erode quickly. This difference in erosion rates is crucial in shaping coastal landscapes, especially in discordant coastlines where bands of different rock types are exposed to wave action.
Concordant coastlines form when bands of rock run parallel to the coastline. This creates a relatively straight coastline with few major headlands or bays. Let's explore the key features and examples of concordant coastlines.
When rock layers run parallel to the shore, the coastline tends to be straighter and more uniform. This happens because waves hit similar rock types along the length of the shore, resulting in more even erosion patterns.
Concordant coastlines tend to be straighter with fewer major inlets because the same rock type faces the sea along the coast.
These coastlines often feature long, uninterrupted beaches where sediment can accumulate along the shore.
If the rock is resistant, concordant coastlines may form steep, uniform cliffs that run for long distances.
The Holderness Coast in East Yorkshire is a good example of a concordant coastline. It consists mainly of soft boulder clay (till) that runs parallel to the shore. This has created a relatively straight coastline that's experiencing some of the fastest erosion rates in Europe, retreating at about 1.5-2.5 metres per year. The uniform rock structure means erosion happens fairly evenly along the coast, though there are variations due to human interventions like sea walls and groynes.
Discordant coastlines occur when bands of rock run perpendicular (at right angles) to the coastline. This creates a much more varied and irregular coastline with distinctive headlands and bays. These coastlines are more common in the UK and create some of our most dramatic coastal scenery.
When rock bands run at right angles to the shore, waves encounter alternating bands of hard and soft rock. This leads to differential erosion, where soft rock is eroded faster than hard rock, creating a jagged, irregular coastline.
Hard rock forms protruding headlands that resist erosion, while soft rock forms bays where erosion happens more quickly.
Headlands may be eroded to form caves, arches, stacks and stumps as erosion processes continue over time.
These flat areas of rock are exposed at low tide and form as cliffs retreat due to wave erosion at their base.
The Jurassic Coast in Dorset is a UNESCO World Heritage Site and a classic example of a discordant coastline. Here, bands of resistant limestone and less resistant clay run perpendicular to the coastline. This has created famous features like Durdle Door (a limestone arch), Lulworth Cove (a near-circular bay) and the headlands of Portland Bill and Purbeck. The alternating bands of hard and soft rock have created a coastline of exceptional beauty and scientific importance, showcasing 185 million years of Earth's history.
The formation of headlands and bays is a perfect example of differential erosion in action:
Several erosion processes shape discordant coastlines:
Discordant and concordant coastlines affect human activities differently:
The type of coastline affects how we manage and protect coastal areas:
Different coastline types present different challenges for coastal management:
Concordant coastlines often need more continuous protection if they're made of soft rock. The Holderness Coast, for example, requires extensive management with sea walls, groynes and rock armour in populated areas. However, managed retreat is also used in some places where protection isn't economically viable.
Discordant coastlines require more targeted management. Bays might need protection if they contain settlements, while headlands may need less intervention. The varied nature of these coastlines means that different strategies are often used in different sections of the same stretch of coast.
In your exam, you might be asked to compare concordant and discordant coastlines. Remember these key points:
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