Erosional Landforms (Cliffs & Wave-Cut Platforms)

Formation of Cliffs

Cliffs are steep rock faces found along coastlines, formed mainly by coastal erosion. The key processes involved in cliff formation are hydraulic action and abrasion.

Hydraulic action occurs when waves crash against the cliff face, forcing water and air into cracks. The pressure causes the cracks to widen and pieces of rock to break off.

Abrasion happens when waves carry sand, pebbles, and larger rocks that grind against the cliff, wearing it away like sandpaper.

The type of rock affects how quickly cliffs erode. Harder rocks like granite erode slowly, forming steep, rugged cliffs. Softer rocks like clay erode faster, creating gentler slopes or cliffs that retreat quickly.

The amount of wave energy also influences cliff formation. High-energy waves, often from storms or strong prevailing winds, cause rapid erosion and steep cliffs. Low-energy waves cause slower erosion.

For example, the white chalk cliffs at Flamborough Head in Yorkshire are steep because chalk is relatively hard and the area experiences strong wave action from the North Sea.

Learning example: If waves hit a cliff with a force of $5000\,\mathrm{N}$ repeatedly, and hydraulic action causes cracks to widen by 1 mm each wave, over time the cliff face weakens and collapses, retreating inland. This gradual weakening can lead to measurable cliff retreat over years.

Wave-Cut Platforms

Wave-cut platforms are flat, often rocky areas found at the base of cliffs, visible at low tide. They form as cliffs retreat inland.

The process begins when waves erode the base of a cliff, creating a notch through hydraulic action and abrasion. Over time, the overhanging rock above the notch becomes unstable and collapses.

This cycle repeats, causing the cliff to retreat and leaving behind a gently sloping, flat platform where the cliff once stood. Weathering also helps break down rock on the cliff face, aiding erosion.

Wave-cut platforms are usually smooth and can be slippery due to algae and seaweed. They often have pools of seawater trapped in depressions.

For example, the wave-cut platforms at Hurst Castle in Hampshire show clear evidence of cliff retreat over thousands of years.

Coastal Erosion Processes

Hydraulic action is the force of waves compressing air in cracks, causing rock to break apart.

Abrasion involves waves throwing rock fragments against cliffs, grinding them down.

Attrition is when rock fragments carried by waves smash into each other and break into smaller, smoother pieces.

These processes work together to erode cliffs and shape coastal landforms.

Learning example: When waves carry pebbles that collide, attrition reduces their size, making sand that can be deposited elsewhere along the coast. This reduction in sediment size can be seen as a progression from larger rocks near cliffs to finer sand on beaches.

Examples in the UK

Famous cliff locations include:

• Seven Sisters in East Sussex – chalk cliffs formed by erosion of the South Downs.
• Durdle Door in Dorset – limestone cliffs shaped by erosion.
• Flamborough Head in Yorkshire – chalk cliffs exposed to strong wave action.

Wave-cut platforms can be seen at:

• Hurst Castle in Hampshire – a wide, flat platform formed by cliff retreat.
• St Bees Head in Cumbria – rocky platforms at the base of sandstone cliffs.

Human activity can affect erosional landforms. Coastal defences like sea walls reduce erosion but can disrupt natural cliff retreat and wave-cut platform formation.

In some areas, footpaths and tourism increase erosion by damaging vegetation that protects cliffs from weathering.