Coastal Landforms » Headlands, Bays, Caves, Arches and Stacks

Headlands and bays: hard rock against soft rock

Look at a map of many coastlines and you will see a wavy edge: points of land sticking out, with curves of sea cutting in between. The cause is differential erosion. This means different rocks are worn away at different speeds.

Along some coasts, bands of hard rock (such as chalk or granite) sit next to bands of soft rock (such as clay or sand). The waves attack them all, but the soft rock is worn back much faster. Here is the sequence:

1. The coast is made of hard rock and soft rock side by side.
2. Waves erode the soft rock quickly, so it retreats.
3. The hard rock erodes slowly, so it is left sticking out into the sea as a headland.
4. The eroded soft rock forms a curved bay between the headlands.

Remember, wave refraction then sends wave energy to the headlands, so they keep being attacked. How the bands of rock lie along a coast is covered in the lesson on discordant and concordant coastlines.

Key terms:

• Differential erosion: different rocks eroding at different speeds, so the harder rock is left standing out.
• Cliff: a steep rock face at the edge of the land, next to the sea.
• Wave-cut notch: a groove cut into the bottom of a cliff by waves.
• Wave-cut platform: a flat or gently sloping area of rock at the foot of a cliff, left behind as the cliff retreats.
• Cave: a hollow in a cliff or headland, made by waves.
• Arch: a natural opening through a headland, with rock still across the top.
• Stack: a tall column of rock standing alone in the sea.
• Stump: the low remains of a stack, often covered at high tide.

Cliffs, notches and wave-cut platforms

A cliff is not just a rock wall. It is a landform that is being worn back all the time. Waves break against the base of the cliff, where hydraulic action and abrasion (also called corrasion) do most of their work. Because the waves only reach so high, the damage is concentrated in one narrow band, between the low and high tide lines.

The cliff retreats in these steps:

1. Waves attack the base of the cliff, between the low and high tide lines.
2. Erosion cuts a groove into the cliff. This is the wave-cut notch.
3. The notch gets deeper. The rock above it is left overhanging with nothing to hold it up.
4. The overhang becomes too heavy and collapses into the sea.
5. The cliff face is now further back. The fallen rock is broken up and carried away by the waves.
6. This happens again and again, so the cliff keeps moving inland. It retreats.

As the cliff retreats, it leaves behind a flat or gently sloping surface of rock that slopes down towards the sea. This is the wave-cut platform. You can often walk on it at low tide. It is rough and uneven, with rock pools and loose boulders.

Caves, arches, stacks and stumps

These four landforms usually form one after another on a headland. Headlands take the full force of the waves, and they often have weaknesses such as cracks. The sequence is:

1. Waves attack a crack in the headland. Hydraulic action forces air and water into it, and abrasion wears it wider.
2. The crack grows into a cave.
3. The cave is eroded deeper, until it breaks through to the other side of the headland to make an arch.
4. The arch is worn wider at the bottom, and weathering weakens the top. The roof becomes too weak and falls into the sea.
5. A tall column of rock is left standing alone. This is a stack.
6. The stack is attacked at the base, just like a cliff. It collapses and leaves a low stump, which is covered at high tide.

In an exam, put the steps in order and link them with words such as then and until. The order is always: crack, cave, arch, stack, stump.

Landform	What it looks like	What happens next
Cave	Hollow in the headland	Erodes through to form an arch
Arch	Opening through the headland with a rock roof	Roof collapses to leave a stack
Stack	Tall column of rock in the sea	Base erodes and it falls to a stump
Stump	Low rock, covered at high tide	Worn away over time

Real examples

You should be able to name real places for these landforms:

• Old Harry Rocks in Dorset, England: chalk stacks.
• Durdle Door in Dorset, England: a limestone arch.
• The Twelve Apostles in Victoria, Australia: limestone stacks standing off the cliffs.

Examiners like a named example, so learn these places.

Exam-style question

Fig. 1.1 is a photograph of a headland with a cave at its base and a stack standing in the sea just off the coast. It was taken at low tide.

(a) Identify the landform made when the roof of an arch collapses. [1 mark]

(b) Describe the shape of a wave-cut platform. [2 marks]

(c) Explain how a wave-cut platform is formed. [3 marks]

(d) Explain how the cave in Fig. 1.1 may later become a stack. [6 marks]