Location and Formation of a Coastal Landscape

Dorset: one coastline, contrasting landscapes

The Dorset coast, on the south coast of England, is a named distinctive UK coastal landscape. It forms part of the Jurassic Coast, which stretches for about 155 km from Exmouth in Devon to Poole in Dorset. Its distinctive features include Lulworth Cove, the arch at Durdle Door, the chalk stack of Old Harry and the shingle barrier of Chesil Beach.

Its location matters for more than finding it on a map. This coastal setting exposes the rocks to wave erosion and allows sediment to be transported and deposited. Within the same region, exposed cliffs and sheltered bays experience different conditions. Crucially, the coast cuts through bands of sedimentary rock with different resistance to erosion. Harder limestone and chalk generally resist erosion better than softer clay, helping to explain why neighbouring stretches have different shapes.

Ancient rocks, a much younger coastline

The rocks formed long before the present coastline. During the Jurassic Period, sediments accumulated in seas and eventually formed rocks including clay and limestone. Later, during the Cretaceous Period, further sedimentary layers formed, including chalk.

Following the last ice age, sea level rose. Wave erosion and sediment deposition then helped shape the modern coast. This separates two parts of the explanation: geological history supplied the rocks, while more recent coastal processes shaped them into cliffs, bays, arches and beaches.

Both the resistance of the rocks and the arrangement of their bands matter. A concordant coastline has rock bands parallel to the shore. A discordant coastline has bands running approximately at right angles to it. These arrangements give waves different access to softer and harder rocks.

Lulworth Cove: breaking through a resistant outer band

Lulworth Cove demonstrates formation on a concordant coastline. A resistant band of limestone runs along the coast, with softer clay behind it. Initially, the limestone limits the sea’s access to the weaker rock.

Wave erosion eventually opened a gap in the limestone. Waves could then reach the softer clay and erode it more rapidly, widening the sheltered area behind the opening into a bay. The limestone remained more resistant, so the entrance stayed narrow compared with the wider cove behind it.

The important connection is rock arrangement → access by waves → unequal erosion → coastal shape. Saying only that waves formed the cove misses why this particular shape developed.

Nearby, Durdle Door is an arch in resistant limestone. Wave erosion enlarged a crack into a cave and then an opening through the rock. Its arch lies unusually along, rather than across, the coastline.

Headlands, bays and the retreat of Old Harry

Around Swanage and Studland, contrasts in resistance produce headlands and bays. Softer sandstone and clay are eroded more readily, creating bays. The harder chalk of The Foreland, between Swanage Bay and Studland Bay, remains as a projecting headland. This illustrates the headland-and-bay pattern associated with differential erosion on a discordant coast.

Wave attack also changes the headland itself. At The Foreland, cracks have been enlarged into caves and arches. When an arch roof collapses, it leaves an isolated pillar called a stack. Old Harry is a chalk stack; further erosion and collapse have reduced Old Harry’s Wife to a stump.

Waves are not acting alone. Weathering weakens the chalk in place, including chemical weathering and biological weathering associated with vegetation. Erosion removes material, while gravity brings weakened rock down. Together, these processes cause coastal retreat.

Deposition creates a different landscape

Not every Dorset feature is produced by removing rock. Chesil Beach is an approximately 18-mile-long pebble and shingle barrier stretching north-west from Portland towards West Bay. The Fleet Lagoon separates much of the barrier from the mainland.

Chesil links Portland to the mainland and is often described as a tombolo. Its origin is debated, with glacially supplied sediment suggested as part of its formation. It should not be explained as though one simple, certain sequence accounts for the entire feature. The barrier has also moved landwards: a depositional landform can continue to change after it forms.

Sediment movement matters elsewhere too. At Swanage, longshore drift carries beach material from south to north. Moving sediment along the shore can alter the beach even without changing the underlying geology.

Which factors have the greatest influence on change?

Geology is the main control on the broad pattern of landforms: resistance and rock arrangement explain why coves, headlands and bays develop where they do. Wave energy and storms strongly influence how quickly that landscape changes, while weathering and mass movement weaken and remove cliff material.

A named example of rapid change is the storm of 14 February 2014. Winds reached up to 80 mph and waves exceeded 30 feet in height. Parts of Chesil Beach were lost, sea defences were breached, and collapsing cliffs at West Bay retreated by several metres. This shows that coastal change can occur in sudden events, not just through steady erosion.

Over longer timescales, rising sea level can allow waves to attack further inland and increase flooding risk. Heavy rainfall can increase cliff instability, encouraging slides and falls. Human defences can modify local change, but the 2014 breaches show that they do not remove the influence of powerful storms. Dorset’s landscape therefore reflects interacting controls, rather than a single cause.