Landforms of coastal erosion

In this section you will learn about:

• the origin and development of erosional coastal landforms
• cliffs and wave-cut platforms
• caves, arches, stacks and stumps
• the role of geology, lithology and wave energy in shaping coastal landscapes
• examples of erosional coastal landscapes from the UK and beyond

Coastal landscapes and landforms

Coastal landscapes are shaped by the interaction of:

• geology and lithology
• marine processes
• subaerial processes
• climate and sea-level change

A landscape is the overall coastal environment, while landforms are the individual features found within it, such as cliffs, arches or wave-cut platforms.

No single coastal landscape is typical. Around the world, coastlines vary according to:

• rock type and geological structure
• wave energy and fetch
• sediment supply
• tectonic activity
• rates of erosion and deposition

High-energy coastlines dominated by marine erosion often develop dramatic rocky landscapes containing cliffs, caves, arches and stacks.

Headlands and bays

Many erosional coastlines contain alternating headlands and bays formed through differential erosion.

This occurs where bands of resistant and less resistant rock lie alongside one another.

• Softer rock erodes more rapidly to form bays
• More resistant rock remains protruding into the sea as headlands

Wave refraction then reinforces these differences:

• wave energy becomes concentrated on headlands
• wave energy is dissipated in bays

As a result:

• erosion dominates at headlands
• deposition is more common in bays

This creates distinctive coastal landscapes, such as the Dorset coast in southern England.

Cliffs and wave-cut platforms

Wave-cut notches

Cliffs form where marine erosion attacks the base of the coastline.

Wave energy is concentrated around the high-tide zone where processes such as:

• hydraulic action
• abrasion
• cavitation
• solution (on soluble rocks)

erode the cliff foot.

Over time, erosion cuts a wave-cut notch into the cliff base.

As the notch deepens:

• the cliff above becomes unstable
• gravity causes collapse
• debris is removed by wave action

Repeated undercutting and collapse cause the cliff to retreat inland.

Wave-cut platforms

As cliffs retreat, they leave behind a gently sloping rocky surface called a wave-cut platform.

Wave-cut platforms:

• are usually exposed at low tide
• slope gently towards the sea
• widen as the cliff retreats

Marine erosion continues across the platform surface, while weathering enlarges joints and rock pools.

However, platforms rarely become extremely wide because waves begin to break further offshore as the platform expands. This reduces wave energy reaching the cliff and slows further erosion, an example of negative feedback.

Factors affecting cliff profile and retreat

The shape and rate of cliff retreat vary according to several interacting factors.

Rock type (lithology)

Rock resistance strongly influences cliff form.

Resistant rocks

Rocks such as:

• granite
• basalt
• limestone

usually produce:

• steep cliffs
• narrow wave-cut platforms
• slower retreat rates

Weak rocks

Less resistant rocks, such as:

• clay
• shale
• glacial till

often form:

• lower-angle cliffs
• wider platforms
• unstable slopes prone to slumping

The Holderness Coast in eastern England is a well-known example of rapid erosion affecting weak glacial till cliffs.

Geological structure

Geology affects how easily waves can exploit weaknesses within rock.

Important structural features include:

• joints
• faults
• bedding planes
• folds
• dip angle

Concordant and discordant coastlines

On discordant coastlines, alternating bands of resistant and weak rock often create headlands and bays.

On concordant coastlines, rock strata run parallel to the coastline. Breaches in resistant outer rock layers can expose softer rock behind, forming features such as coves.

Lulworth Cove in Dorset is a classic example.

Dip of rock strata

The angle of rock layers influences cliff shape and stability.

Seaward dip

Where rock strata dip towards the sea:

• bedding planes may act as slip surfaces
• landslides and rockfalls become more common

Landward dip

Where strata dip inland:

• cliffs are often steeper and more stable
• collapse is less frequent

Wave energy and fetch

High-energy waves increase rates of erosion and cliff retreat.

Wave energy depends on:

• fetch
• wind strength
• storm frequency
• coastal orientation

Exposed Atlantic coastlines, such as western Ireland and parts of Cornwall, experience particularly powerful wave attack because of long fetches across the Atlantic Ocean.

The role of weathering and mass movement

Subaerial processes are essential in cliff development.

Weathering weakens rock before marine erosion removes it.

Processes include:

• freeze–thaw weathering
• salt weathering
• biological weathering
• carbonation

Mass movement then transfers material downslope through:

• rockfalls
• rotational slumping
• landslides

This demonstrates that cliffs are shaped by the interaction of marine and terrestrial processes.

Caves, arches, stacks and stumps

These erosional landforms commonly develop on resistant headlands where lines of weakness are present.

They represent a sequence of landscape evolution.

Cave formation

Waves exploit weaknesses such as:

• joints
• cracks
• faults
• bedding planes

Hydraulic action, abrasion and cavitation enlarge these weaknesses to form caves.

Some caves may develop into narrow inlets called geos.

Where vertical shafts form to the surface, blowholes may develop.

Arch formation

As erosion continues:

• caves deepen further into the headland
• caves may eventually meet
• an arch is formed through the headland

Weathering and marine erosion continue to weaken the arch roof.

Stack formation

Eventually, the arch roof collapses due to:

• gravity
• weathering
• continued erosion

This leaves an isolated pillar of rock known as a stack.

Stacks are exposed to wave attack on all sides and continue to erode.

Stump formation

As marine erosion continues:

• the stack becomes smaller and weaker
• wave-cut notches form at its base
• collapse eventually occurs

A low remnant, called a stump, often remains, visible only at low tide.

Landscape evolution

Erosional coastal landscapes evolve over time.

A typical sequence may include:

• Differential erosion forms headlands and bays
• Wave-cut notches develop
• Cliffs retreat and wave-cut platforms form
• Caves enlarge into arches
• Arch collapse creates stacks
• Continued erosion leaves stumps

However, coastal evolution is rarely perfectly linear because:

• sea levels change
• storm frequency varies
• geology differs between locations
• sediment supply changes over time

Examples from the UK and beyond

Dorset Coast, England

The Dorset coastline contains many classic erosional landforms including:

• Durdle Door (arch)
• Old Harry Rocks (stacks and stumps)
• Lulworth Cove

Differential erosion and geological structure strongly influence the landscape.

Holderness Coast, England

This coastline is characterised by:

• rapid cliff retreat
• rotational slumping
• weak glacial till geology

Strong North Sea waves and soft sediments produce some of the fastest erosion rates in Europe.

The Twelve Apostles, Australia

Along the Great Ocean Road in Victoria, powerful Southern Ocean waves have eroded limestone cliffs to form:

• caves
• arches
• stacks

The remaining stacks continue to collapse as erosion progresses.

Dyrhólaey, Iceland

The south coast of Iceland contains dramatic erosional landforms shaped by:

• powerful Atlantic waves
• volcanic geology
• marine erosion

Arches, cliffs and stacks are common along this high-energy coastline.