Erosional Landforms: Reading the Story in the Rock

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🏔️ Erosional Landforms: Reading the Story in the Rock

Diagram showing the cave-arch-stack-stump erosion sequence at a headland — showing how hydraulic action and abrasion gradually destroy a headland over time

Figure 2: Cave to Stack — the erosion sequence that produces some of Britain's most dramatic coastal scenery

Every cliff, headland, arch and stack tells a story about the processes that shaped it. The key to exam success is being able to tell that story — not just name the landform, but explain the sequence of events that created it.

Wave-Cut Platforms: The Retreating Cliff's Footprint

Stand on the beach at low tide on many British coasts and you will see a wide, flat rocky surface exposed in front of the cliffs. This is a wave-cut platform — and it is the evidence of how far the cliff has retreated. Here is the process as a cause-chain:

Step 1: Notch formation — Waves, loaded with sediment, attack the cliff at the high-tide waterline. Hydraulic action and abrasion concentrate their energy here, slowly cutting a horizontal groove called a wave-cut notch into the cliff base.
Step 2: Overhang and collapse — As the notch deepens, the rock above it is left unsupported — like cutting the legs off a table. Eventually, the weight of the overhanging cliff becomes too great and it collapses, either in a sudden rockfall or as a gradual slumping of material. This adds to the debris at the cliff base.
Step 3: Platform formation — The collapsed material is gradually removed by waves and longshore drift. The base of the old cliff — now exposed as flat or gently sloping rock — forms the wave-cut platform. The process begins again at the new cliff face.
Result: a wide, gently sloping platform — the wider it is, the further the cliff has retreated. On some UK coasts, platforms extend 500 metres or more seaward from the current cliff face. Ironically, a wide platform eventually slows erosion — waves lose energy travelling across it before they reach the cliff.

Headlands and Bays: Geology Writes the Coastline

Look at a map of the Dorset coast or the Pembrokeshire coast and you will see a deeply indented shoreline — bays tucked between prominent headlands. This pattern is not random. It is written by geology.

Where alternating bands of hard rock (resistant to erosion) and soft rock (easily eroded) meet the coastline at right angles, waves attack the soft rock first and fastest. The soft rock is eroded back to form a bay — broad, shelving, sheltered. The resistant hard rock on either side is left protruding into the sea as a headland. The key exam point is what happens next: once the headland is formed, it sticks out further into the sea than the bays on either side. Waves wrap around it — a process called wave refraction — concentrating their energy on the headland from multiple angles simultaneously. The headland now receives more erosion than the bay, which is somewhat sheltered. The initial difference in rock type is amplified by the shape the coastline has taken.

Caves, Arches, Stacks and Stumps: The Headland's Slow Destruction

A headland is not permanent. Once formed, waves attack any line of weakness — a joint, a bedding plane, a fault — with hydraulic action and abrasion. The sequence that follows produces some of the most dramatic coastal scenery in Britain:

Cave: Hydraulic action exploits a crack or joint in the headland cliff face. Compressed air forces the crack open with every wave. Over years, a small hollow deepens into a cave — often visible as a dark slot at the base of the headland, accessible at low tide. Caves can also form at wave-cut notch level, where abrasion concentrates its grinding action.
Arch: Where two caves form on opposite sides of a headland and eventually break through to meet each other — or where a single cave erodes through — the result is an arch: a natural rock bridge spanning open sea. Durdle Door in Dorset is one of Britain's most photographed examples — a narrow arch of Portland limestone projecting into the English Channel. The arch is maintained by the rock above it; the key question is how long before the roof becomes too thin to support its own weight.
Stack: When the arch roof becomes too thin and collapses, the seaward section of the headland is left as an isolated pillar of rock surrounded by water — a stack. Old Harry Rocks in Dorset and The Needles on the Isle of Wight are chalk stacks. The stack continues to be attacked by all four erosion processes, undermined at its base by hydraulic action and abrasion.
Stump: As the stack is eroded further, it is reduced to a low, wave-washed platform barely visible above the sea at high tide — a stump. The whole sequence from headland to stump may take thousands of years — but it is visible on any stretch of coastline where headlands have formed. You can often see cave, arch and stack in close proximity on the same headland, at different stages of the same process.

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