Erosional Landforms

Erosion creates some of the most dramatic and recognisable features of the coastline. At A-Level, you need to understand not only the characteristics of each landform but also the precise combination of processes and conditions that produce them. AQA expects detailed knowledge of the formation sequence, with reference to specific examples and case studies.

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Cliffs and Wave-Cut Platforms

Cliffs are the most widespread erosional landform on the coast. Their form depends on the interplay between marine erosion at the base and sub-aerial processes acting on the cliff face.

Cliff Formation and Retreat

The process of cliff formation follows a well-established sequence, first described in detail by Sunamura (1992):

1. Waves attack the base of a coastal slope, concentrating erosion at the high tide level through hydraulic action and abrasion
2. A wave-cut notch develops — an indentation at the base of the cliff, typically 1-3 m deep
3. The rock above the notch becomes increasingly unsupported and eventually collapses through mass movement
4. The debris is removed by wave action, exposing a fresh rock face for further attack
5. The cliff retreats landward while maintaining its steep profile
6. The flat or gently sloping rock surface left behind as the cliff retreats is the wave-cut platform (also called a shore platform)

graph LR
    subgraph "Cliff Retreat Sequence"
    A["1. Wave attack at cliff base"] --> B["2. Wave-cut notch forms"]
    B --> C["3. Overhang collapses"]
    C --> D["4. Debris removed by waves"]
    D --> E["5. Cliff retreats, platform extends"]
    E --> A
    end

Wave-Cut Platforms

As the cliff retreats, it leaves behind a gently sloping rock surface — the wave-cut platform. Key characteristics include:

• Typically slopes seaward at an angle of 1-3°
• Width is limited to approximately 500 m — beyond this, waves lose too much energy crossing the platform to erode the cliff base effectively (this was demonstrated by Trenhaile, 1987)
• Surface is often smoothed by abrasion and pitted by solution
• Rock pools form in depressions and joints, supporting rich intertidal ecosystems
• They are exposed at low tide and submerged at high tide

Key Definition: A wave-cut platform (shore platform) is a gently sloping rock surface extending seaward from the base of a retreating cliff, formed by the progressive removal of rock through erosion.

Cliff Profiles

The shape of a cliff profile depends on the balance between marine erosion at the base and sub-aerial weathering and mass movement on the face:

Cliff Type	Profile	Dominant Processes	Example
Vertical/near-vertical	Steep, sheer face	Strong marine erosion; resistant rock	Beachy Head, East Sussex (chalk, 162 m high)
Sloping	Angled face, 40-70°	Sub-aerial processes dominate; softer rock	Barton-on-Sea, Hampshire (clay)
Terraced/benched	Stepped profile with horizontal ledges	Alternating resistant and weak rock layers	Lulworth, Dorset (Portland stone over Purbeck beds)
Composite	Complex profile with multiple angles	Variable geology; multiple processes	Robin Hood's Bay, Yorkshire

Factors Influencing Cliff Profile

The profile of a cliff is determined by several interacting factors:

Rock resistance: Hard rocks (granite, basalt, limestone) maintain steep profiles because marine erosion at the base outpaces sub-aerial weathering on the face. Soft rocks (clay, sand, glacial till) develop more gentle slopes because weathering and mass movement are rapid relative to marine erosion.

Geological structure: The orientation of bedding planes, joints and faults has a profound effect:

Bed Orientation	Effect	Resulting Profile
Horizontal beds	Uniform resistance across cliff face	Vertical cliff with horizontal ledges
Beds dipping inland	Stable — rock layers lean into the cliff	Steep, stable cliff
Beds dipping seaward	Unstable — rock layers slide towards the sea	Prone to landslides; gentler profile
Beds dipping steeply (near vertical)	Erosion follows weaker beds	Irregular, often castellated profile

Vegetation: Plant cover on the cliff face and top stabilises soil, intercepts rainfall and reduces surface runoff. Removal of vegetation (by erosion, human activity or grazing) accelerates mass movement.

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Headlands and Bays

Where a coastline is composed of alternating bands of resistant and less resistant rock, differential erosion creates a pattern of headlands and bays.

Formation

1. Where rocks of different resistance are exposed at the coast, waves erode the weaker rock more rapidly
2. The weaker rock is eroded back to form a bay — a curved indentation in the coastline
3. The more resistant rock projects seaward as a headland — a promontory exposed to wave attack on three sides
4. Wave refraction then concentrates energy on the headlands and disperses it in the bays (see Lesson 2)
5. Beaches develop in the bays (lower energy, deposition dominant) while the headlands continue to be eroded

Examples

• Swanage Bay and The Foreland, Dorset: Bands of resistant Portland limestone and Purbeck limestone form headlands (The Foreland, Ballard Point), while softer Wealden clays have been eroded to form Swanage Bay
• Flamborough Head, Yorkshire: A massive chalk headland projecting into the North Sea, flanked by bays cut into softer boulder clay to the north and south
• Lulworth, Dorset: A complex pattern where resistant Portland limestone forms a protective outer barrier, behind which softer clays have been eroded into Lulworth Cove

Key Definition: Differential erosion is the selective erosion of rocks of varying resistance, producing an irregular coastline of headlands (resistant rock) and bays (less resistant rock).

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Caves, Arches, Stacks and Stumps

This is one of the most frequently examined landform sequences at A-Level. It represents the progressive erosion of a headland through a predictable series of stages.

Formation Sequence

graph TD
    A["Headland exposed to wave attack on three sides"] --> B["Weakness (joint/fault/bedding plane) exploited by hydraulic action and abrasion"]
    B --> C["CAVE: deep recess eroded into cliff face"]
    C --> D["Caves eroded on both sides of headland meet"]
    D --> E["ARCH: opening right through the headland"]
    E --> F["Arch roof widened and weakened by weathering and erosion"]
    F --> G["Roof collapses — seaward portion isolated"]
    G --> H["STACK: isolated pillar of rock"]
    H --> I["Stack eroded at base; undermined"]
    I --> J["STUMP: low remnant visible at low tide"]

Detailed Process Analysis