Landforms of Coastal Erosion

Edexcel B GCSE Geography > The UK’s Evolving Physical Landscape > Landforms of Coastal Erosion

Why do some coastlines have steep cliffs and others gentle bays? It all comes down to a combination of erosional processes, rock type, and geological structure. In this section, we’ll explore how these factors work together to shape the UK’s coastal landscapes.

Key Coastal Erosion Processes

Before we look at the geology, it’s important to understand how the sea erodes the coast:

Hydraulic action: Waves crash against the coast, compressing air in cracks and forcing the rock apart.
Abrasion: Waves transport material which rubs against the cliffs, wearing them away.
Attrition: Rocks and pebbles carried by waves smash into each other, becoming smaller and rounder.
Solution (corrosion): Sea water dissolves certain types of rock like chalk and limestone.

These destructive processes work together to break down the coastline and form distinctive landforms, but how fast they work depends heavily on the geology.

Rock Type: Hard vs Soft Rock

Different rocks erode at different speeds:

Hard rock (e.g. chalk, limestone, granite) is more resistant to erosion. It forms steep cliffs, headlands, and features like stacks.
Soft rock (e.g. clay, sands, boulder clay) erodes more quickly. This creates bays, slumping cliffs, and wider areas of low-lying coastline.

This variation in erosion creates contrast along the coast, mainly when hard and soft rocks are found next to each other.

Geological Structure

It’s not just the rock type that matters; how the rock is arranged plays a significant role too. Coastlines can be discordant or concordant depending on how the layers of rock are arranged with the shoreline.

Concordant Coastlines
These have rock layers that run parallel to the coast.

If hard rock is on the outside, it protects the softer rock behind it.
However, if the outer hard rock is breached, the sea can erode the softer rock quickly, creating coves (like Lulworth Cove in Dorset).

Discordant Coastlines
Here, rock layers are at right angles to the coast.

The sea erodes soft rock faster, forming bays.
The more resistant rock sticks out as headlands (like at Swanage Bay, Dorset).
This creates a dramatic, jagged coastline with alternating landforms.

The map below shows a section of the Dorset Coast in southern England, which features both discordant and concordant coastlines.

A map showing the concordant and discordant stretches of the Dorset coastline

Dorset features both a discordant and a concordant stretch of coastline

Characteristics of Bays and Headlands

Feature	Bays	Headlands
Shape	Curved inward – forms a sheltered indentation	Area of more resistant rock extending into the sea. The landform is typically longer than it is wide.
Rock type	Made of softer, less resistant rock (e.g. clay, sands)	Made of harder, more resistant rock (e.g. chalk, limestone)
Erosion rate	Erodes quickly due to weaker rock	Erodes slowly, resistant to wave attack
Wave energy	Lower energy – wave refraction reduces impact	Higher energy – exposed headland takes full force of waves
Features found	Often has beaches formed by deposition	Steep cliffs, caves, arches, and stacks may form
Example	Swanage Bay, Dorset	Durlston Head, Dorset

The image below shows Swanage Bay on the Dorset Coast. The geology of the land behind the bay is soft clay and sand. Ballard Point is a headland formed from more resistant chalk.

Swanage Bay and Ballard Point

Joints and Faults

All rocks have natural weaknesses.

Joints are cracks in the rock.
Faults are larger fractures caused by the movement of the Earth’s crust.

Caves, Arches, and Stacks

Waves exploit joints and faults in headlands through hydraulic action and abrasion. Combined with weathering processes such as freeze-thaw and salt weathering, landforms such as caves, arches, and stacks are formed.

Erosion of a headland

Cracks to Caves

Coastal erosion begins at points of weakness, such as joints and faults in the headland.

Hydraulic action forces air into cracks, widening them.
Abrasion occurs as pebbles and sand thrown by waves grind against the rock surface.
Over time, the crack is enlarged into a sea cave.
These processes are strongest at the base of the cliff, where wave energy is concentrated.

Caves to Arches

As erosion continues:

Hydraulic action repeatedly forces water and air into the cave, further weakening the structure.
Abrasion deepens and enlarges the cave, especially when waves carry larger fragments.
If a cave grows deep enough or two caves meet on opposite sides of a narrow headland, it breaks through to form a natural arch.
Solution may also play a role if the rock contains calcium carbonate (e.g. chalk or limestone).

Arches to Stacks

The newly formed arch is exposed to both marine erosion and subaerial weathering:

Hydraulic action and abrasion continue to attack the base of the arch, weakening its support.
From above, weathering processes such as freeze–thaw, salt crystallisation, or biological weathering further weaken the roof.
Eventually, the arch collapses under its weight, leaving a stack – a tall, isolated pillar of rock just offshore.

Stacks to Stumps

Stacks remain exposed to the full force of the sea.
Abrasion continues to undercut the base of the stack.
Hydraulic action exploits cracks, causing more instability.
Weathering processes continue from above.
Eventually, the stack collapses, leaving behind a stump – a low, eroded remnant of rock that may only be visible at low tide.

Wave-Cut Platforms and Cliffs

Cliffs and wave-cut platforms are classic landforms found along coastlines exposed to strong wave energy. They form where the sea erodes the base of a rock face, causing the cliff to retreat over time, leaving behind a flat, rocky surface.

The formation of a wave-cut platform

Cliff Formation

Cliffs form where land meets the sea and is constantly attacked by waves.

If the rock is hard and resistant (like limestone or chalk), steep cliffs are formed.
Where the rock is softer (like clay or sand), the cliffs are usually gentler and more sloped, as erosion happens more quickly.
The face of the cliff is shaped by both marine erosion at the base and weathering (like freeze–thaw or salt weathering) higher up.

Wave-Cut Notch Development

At the base of the cliff, the sea concentrates its energy between the high and low tide marks.

Hydraulic action forces air and water into cracks, breaking pieces of rock apart.
Abrasion scrapes and grinds the cliff face as waves hurl pebbles and sediment against it.
In some places, solution (also called corrosion) gradually dissolves minerals in rocks like limestone.
These processes carve out a wave-cut notch – a small hollow at the base of the cliff.

Collapse and Cliff Retreat

As the wave-cut notch becomes deeper:

The rock above is undercut and loses support.
Gravity causes the overhanging rock to collapse.
The cliff face shifts further inland, a process known as coastal retreat.

Wave-Cut Platform

After each collapse:

The backwash of the waves removes the eroded material.
Over time, this reveals a flat, gently sloping surface of bare rock at the base of the cliff – this is the wave-cut platform.
The platform is usually covered at high tide but becomes visible when the tide goes out.
As the sea continues to erode the cliff, the platform becomes wider, and the cliff continues to retreat.

Summary

Erosional Processes
Waves erode the coast through hydraulic action, abrasion, attrition, and solution, each weakening and breaking down rock in different ways.
Rock Type and Erosion
Hard rocks like chalk and granite resist erosion, forming cliffs and headlands, while soft rocks like clay erode easily to create bays and low-lying land.
Concordant vs Discordant Coasts
On concordant coasts, rock layers run parallel to the sea, often protecting weaker rock behind. Discordant coasts have alternating hard and soft rocks forming headlands and bays.
Formation of Caves, Arches, and Stacks
Wave energy exploits cracks in headlands, forming caves. Continued erosion can turn these into arches, which collapse into stacks and eventually stumps.
Cliff and Wave-Cut Platform Formation
Cliffs retreat as wave action creates a wave-cut notch that collapses the rock above. Over time, this leaves behind a flat wave-cut platform.
Real-World Examples
Swanage Bay and Ballard Point on the Dorset coast illustrate discordant and concordant features, with clay bays and chalk headlands.