Erosional Coastal Landforms: Headlands, Cliffs, and Stacks

Erosional coastal landforms are features created by wave erosion, including cliffs, wave-cut platforms, headlands and bays, caves, arches, stacks and stumps. In brief: where the coast has bands of hard and soft rock, the soft rock erodes faster and forms bays, while the harder rock stands out as headlands. Waves attack a headland and form a cave, then an arch, then a stack, and finally a stump. A cliff retreating by erosion leaves a gently sloping wave-cut platform at its base.

These landforms are among the best-known scenic features of coasts, from the chalk stacks of southern England to the sandstone pillars of Australia. This guide explains the formation of each feature in order, compares them in a table, works through calculations of retreat and landform size, outlines the history of their study and corrects common mistakes.

How do cliffs and wave-cut platforms form?

Waves concentrate their energy at the base of a cliff, between high and low tide. Hydraulic action and abrasion cut a wave-cut notch at that level. As the notch deepens, the rock above is left unsupported, and it eventually collapses, often helped by weathering and mass movement. The cliff therefore retreats inland. The rocky surface left at the foot of the cliff, gently sloping toward the sea and covered at high tide, is called a wave-cut platform. It grows wider as the cliff retreats, and the broken rock from the collapse is carried away or used as tools by the waves to cut again.

The shape of a cliff depends on the rock. Hard, horizontal rock may form steep or vertical cliffs, whereas soft clays often form gentler, slumped slopes. Rock layers (strata) that dip toward the sea tend to give slope failures, and layers that dip inland give steeper cliffs.

How do headlands and bays form?

On a discordant coastline, bands of hard and soft rock run at right angles to the coast. The waves erode the soft rock, such as clay or sand, more quickly and create sheltered bays. The resistant rock, such as chalk or limestone, erodes slowly and sticks out into the sea as a headland. Waves refract (bend) as they approach, so their energy is concentrated on the headland and spread out in the bay, where deposition may form a beach. Over time, the headlands are eroded more strongly and the coast tends to become straighter. On a concordant coastline, bands run parallel to the coast, and a resistant outer layer protects the softer rock behind until the waves find a gap, as at Lulworth Cove in Dorset, where the sea broke through limestone and eroded the clay behind it.

How do caves, arches, stacks and stumps form?

These features develop on headlands in a sequence of stages.

1. Crack: waves attack a weakness, such as a joint or fault, in the headland.
2. Cave: hydraulic action and abrasion enlarge the crack into a cave.
3. Arch: the cave grows until it cuts through the headland and forms an arch, with the sea flowing through.
4. Stack: weathering and erosion of the arch roof eventually cause it to collapse and leave a tall, isolated column of rock, the stack.
5. Stump: the stack is undercut and falls, leaving a low stump that is covered at high tide.

Blowholes can form when a cave extends upward to the cliff top and spray is forced out through the opening.

What are well-known examples?

Durdle Door in Dorset is a natural limestone arch, and Old Harry Rocks near Swanage are chalk stacks and stumps. The Needles off the Isle of Wight are a row of chalk stacks. The Twelve Apostles in Victoria, Australia, are limestone stacks, and some of them have collapsed in the past, which shows that the process continues. Cliffs of this type are also found at Étretat in Normandy and in many other places.

Erosional landforms compared

Worked example: retreat and platform width

1. Cliff retreat and platform width: a chalk cliff retreats by 0.5 m a year. Over 1,000 years it moves back 0.5 × 1,000 = 500 m. In a simplified picture, the wave-cut platform left behind is about 500 m wide, because the platform is the surface the cliff has retreated across.
2. Platform slope: a platform that drops 2 m over a distance of 100 m has a gradient of 2 ÷ 100 = 0.02, or about 1.1 degrees (arctan 0.02).
3. Stack lifetime: a stack of 30 m height has a base eroding by 2 cm per year on each side. If the base is 10 m across, it takes about 5 m ÷ 0.02 m = 250 years for the two sides to meet and the stack to collapse (very approximate).
4. Notch volume: a notch cut 2 m deep, 1.5 m high and 400 m long along a cliff removes 2 × 1.5 × 400 = 1,200 m³ of rock before the rock above collapses.
5. Headland retreat: if the soft rock in a bay retreats 1.2 m per year and the hard headland only 0.2 m per year, after 100 years the bay is 120 m inland and the headland 20 m, so the difference is 100 m.

How did we come to understand these landforms?

The idea that the sea cuts into land at its margins was already discussed in the 18th and 19th centuries. William Morris Davis and later geomorphologists proposed cycles of landform change that included the sequence from cliffs to stacks. Modern research uses repeated surveys, aerial photographs, laser scanning (LiDAR) and drones to measure retreat very accurately, and shows that most cliff retreat occurs in episodes, with long periods of little change followed by sudden falls after storms and heavy rain. Events such as the collapse of one of the Twelve Apostles in 2005 remind people that the process continues.

Which mistakes are common?

Mistake 1: Thinking that stacks are formed by waves alone from a single event

Stacks result from a long sequence of stages (crack, cave, arch, stack, stump) lasting hundreds or thousands of years.

Mistake 2: Saying that hard rock always forms cliffs and soft rock never does

Soft rock can form cliffs too, although they retreat faster and often have a gentler or slumped shape.

Mistake 3: Confusing wave-cut notches and wave-cut platforms

The notch is a groove at the base of the cliff. The platform is the sloping rock surface left behind as the cliff retreats.

Mistake 4: Assuming that erosion is smooth and constant

Cliff erosion is usually sudden and episodic. Average rates hide the fact that large falls may occur after long calm periods.