Discordant Coastline Case Study

A discordant coastline occurs where bands of different rock type run perpendicular to the coast.

The differing resistance to erosion leads to the formation of headlands and bays. A hard rock type such as granite is resistant to erosion and creates a promontory whilst a softer rock type such as the clays of Bagshot Beds is easily eroded creating a bay.

Part of the Dorset coastline running north from the Portland limestone of Durlston Head is a clear example of a discordant coastline. The Portland limestone is resistant to erosion; then to the north there is a bay at Swanage where the rock type is a softer greensand. North of Swanage, the chalk outcrop creates the headland which includes Old Harry Rocks.

The converse of a discordant coastline is a concordant coastline.

Examples[edit]

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Durlston Head (limestone) to Handfast Point (chalk), with Peveril Point (limestone) dividing Durlston Bay from Swanage Bay

Discordant coastline occurs where bands of differing rock type run perpendicular to the coast.

The differing resistance to erosion leads to the formation of headlands and bays.

A hard rock type is resistant to erosion and creates a promontory whilst a softer rock type is easily eroded creating a bay.

Part of the Dorset coastline in the UK running north from the Portland limestone of Durlston Head is a clear example of a discordant coastline.

The Portland limestone is resistant to erosion; then to the north there is Swanage Bay where the rock type is a softer greensand.

North of Swanage, the chalk outcrop creates the headland which includes Old Harry Rocks.


A concordant coastline occurs where the bands of differing rock types run parallel to the coast.

The outer hard provides a protective barrier to erosion of the softer rocks further inland.

Sometimes the outer hard rock is punctured allowing the sea to erode the softer rocks behind.

This creates a cove which is a circular area of water with a relatively narrow entrance way from the sea.

Lulworth Cove has occurred on a concordant coastline.

The outer hard rock is Portland limestone.

The sea has broken through this barrier and easily eroded the clays behind it.

A chalk cliff face is at the back of the cove.

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