Advanced Coastal Landforms

At A-Level, AQA requires detailed knowledge of how erosional and relict coastal landforms develop over time. This lesson examines the formation sequences of key landforms and the evidence for sea-level change preserved in the coastal landscape.

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Erosional Landform Sequences

Many coastal erosional landforms represent stages in a progressive sequence of development. Understanding this sequence is crucial for A-Level success.

The Headland Erosion Sequence

The classic sequence of landform development on a headland proceeds as follows:

Stage 1: Crack and Joint Exploitation

• Waves identify lines of weakness: joints, faults, bedding planes
• Hydraulic action forces compressed air into these weaknesses
• Chemical weathering (corrosion) widens them from within

Stage 2: Cave Formation

• Prolonged erosion along a line of weakness creates a cave (also called a geo or zawn if open to the sky)
• Caves typically form at the base of a cliff between high and low tide marks
• The back of the cave is eroded more rapidly than the entrance due to wave compression

Stage 3: Arch Formation

• If caves on opposite sides of a narrow headland erode through to meet, an arch is formed
• The roof of the arch is supported by the remaining rock above
• The arch widens at the base through continued erosion while the roof is weathered from above
• Example: Durdle Door, Dorset — a natural limestone arch on the Jurassic Coast

Stage 4: Stack Formation

• The roof of the arch eventually collapses under its own weight (gravity, weathering)
• This leaves an isolated pillar of rock called a stack
• The stack is separated from the mainland and is attacked by erosion on all sides
• Example: Old Harry Rocks, Studland, Dorset — chalk stacks

Stage 5: Stump Formation

• Continued erosion at the base of the stack undermines it
• Marine erosion and weathering reduce the stack to a stump — a low rock platform visible at low tide
• Eventually the stump may be reduced to a submarine reef
• Example: Old Harry's Wife — the stump adjacent to Old Harry stack

Geos and Zawns

A geo (from Old Norse, meaning inlet) or zawn (from Cornish) is a narrow, steep-sided inlet formed by the erosion of a fault, joint or zone of weakness.

Formation:

1. Waves exploit a major joint or fault in a resistant rock such as granite, limestone or basalt
2. Hydraulic action and abrasion gradually widen the fissure
3. The result is a deep, narrow cleft open to the sea
4. If the roof of a cave collapses inland, this can also create a geo

Examples:

• Huntsman's Leap, Pembrokeshire — a narrow geo cut into Carboniferous limestone
• Geo at Yesnaby, Orkney — eroded into Old Red Sandstone

Blowholes

A blowhole forms when a cave roof is weakened by erosion and weathering until a vertical shaft opens to the cliff top.

Formation sequence:

1. A cave develops along a weakness at the base of the cliff
2. Hydraulic action forces air and water upwards through a joint or fault in the cave roof
3. Repeated pressure weakens the rock above
4. A vertical shaft breaks through to the surface
5. During storms, water and spray are forced upwards through the blowhole with considerable force

Example: The Blowhole at Trevose Head, Cornwall

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Shore Platforms

Shore platforms (wave-cut platforms) are gently sloping rock surfaces extending seaward from the base of a cliff. They provide evidence of cliff retreat over time.

Types of Shore Platform

Type	Gradient	Formation	Typical Location
Type A (sloping)	1–5°	Formed mainly by wave action; gradient reflects the balance between erosion and wave energy	Exposed coasts with resistant rock
Type B (horizontal)	Near horizontal (< 1°)	Formed mainly by weathering processes; surface lowered uniformly	Sheltered coasts, tropical regions
Plunging cliff	No platform	Waves reflect off the cliff base rather than breaking; no undercutting occurs	Very deep water at cliff base

Controls on Platform Development

• Rock resistance — harder rocks produce wider, more durable platforms
• Wave energy — higher energy produces wider platforms (up to 500 m) but beyond a certain width, waves lose energy crossing the platform (negative feedback)
• Tidal range — macrotidal coasts tend to have wider platforms because erosion is spread across a greater vertical range
• Biological activity — organisms (e.g., limpets grazing algae, piddock boring) contribute to platform lowering

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Emerged and Submerged Coastlines

Sea levels have changed significantly over geological time due to eustatic (global) and isostatic (local) changes. These changes produce distinctive coastal landforms.

Eustatic Change (Global)

Eustatic sea-level change affects the entire ocean and is caused by: