Coastal Landscapes » Coastal Landforms of Erosion

Rock type and structure

Waves do not attack every part of the coast in the same way. The shape of a coastline depends on two things: the rock type and the geological structure. Both decide how quickly the sea can wear the land away.

Key terms:

• Hard rock: a rock that resists erosion well, such as chalk, limestone or granite. It wears away slowly.
• Soft rock: a rock that is easily eroded, such as clay or sand. It wears away quickly.
• Geological structure: how the layers of rock are arranged, for example in bands that run along the coast or out to sea.

Remember that waves erode by hydraulic power, abrasion and attrition. Soft rock gives way to these processes much faster than hard rock. That difference is the key to every landform in this lesson. Where the rock is soft, the land retreats quickly. Where it is hard, the land stands out and the coast changes slowly.

Concordant and discordant coasts

Rock layers can run in bands. The way these bands meet the sea gives two kinds of coastline.

Headlands and bays

Headlands and bays form on a discordant coast, where bands of hard and soft rock lie side by side.

• Headland: a piece of land that sticks out into the sea. It is made of harder rock that resists erosion.
• Bay: a curved, sheltered area of coast where the sea has cut back into the land. It is made of softer rock.

How they form:

1. The sea attacks the whole coast, but the soft rock erodes faster than the hard rock.
2. The soft rock is worn back to form a bay.
3. The hard rock is left sticking out as a headland.
4. The headland sticks out, so wave energy is concentrated on it and it is attacked from the sides. The bay is more sheltered, so waves there have less energy.

Flamborough Head in Yorkshire is a headland made of chalk. Even a headland is not safe forever. Over time the sea wears it away, which leads us to the next landforms.

Cliffs and wave cut platforms

Cliffs and wave cut platforms form where waves break against the foot of a steep coast.

Key terms:

• Cliff: a steep or vertical rock face at the coast.
• Wave cut notch: a groove cut into the bottom of a cliff by erosion.
• Wave cut platform: a flat, rocky area at the foot of a cliff, covered at high tide and uncovered at low tide.

How they form:

1. Waves attack the bottom of the cliff, between the high and low tide marks. Hydraulic power and abrasion cut a wave cut notch.
2. The notch gets deeper. The rock above is left without support.
3. Eventually the rock above the notch collapses.
4. The collapsed rock is broken up and washed away, and the cliff face moves back. We say the cliff retreats.
5. This repeats over many years. The flat area left behind is a gently sloping wave cut platform.

Cliffs of chalk, such as Beachy Head in East Sussex, are tall and steep because chalk is a hard rock. Soft rock cliffs are usually lower and more rounded, because they collapse and wear back more quickly.

Caves, arches and stacks

These landforms form on a headland, where a crack or weak point in the rock is attacked by waves.

Key terms:

• Cave: a hollow in the cliff, opened up from a crack.
• Arch: a gap in a headland where a cave has been eroded all the way through.
• Stack: a tall column of rock standing alone in the sea.
• Stump: what is left when a stack is worn down to a low rock.

The Needles off the Isle of Wight are a row of chalk stacks. Remember that the order is always: crack, cave, arch, stack, stump.

Putting it together

Rock type and structure set the scene. Soft rock wears back fast and hard rock stands out. On a discordant coast the result is headlands and bays. Cliffs retreat and leave wave cut platforms. Caves, arches and stacks form on the headlands. Each landform is a stage in the same story of the sea wearing the land away.

Exam-style question

(a) Explain how the geological structure of a coast can affect its shape. [2 marks]

(b) Describe and explain how a stack forms. [4 marks]