Erosional landforms (Edexcel GCSE Geography A): Revision Notes
Erosional landforms
Coastal erosion creates several distinctive landforms along our shorelines. Understanding how waves, weathering, and different rock types work together helps explain why our coasts look the way they do. These landforms develop over long periods as the sea continuously attacks the land, wearing it away through various processes.
Time Scale Perspective
Coastal erosional landforms develop over hundreds to thousands of years. What appears permanent to us is actually part of an ongoing geological process where the coastline is constantly changing, albeit very slowly from a human perspective.
Headlands and bays
When waves attack coastlines made up of alternating hard and soft rocks, they create a distinctive pattern of headlands and bays. This happens because different rock types resist erosion at different rates through a process called differential erosion.
Formation process: The process begins when waves encounter mixed geology along a coastline. Softer rocks like sandstone and clay are much easier for waves to erode than harder rocks such as chalk or limestone. Over time, the waves wear away the softer rocks more quickly, creating curved indentations called bays. Meanwhile, the harder, more resistant rocks are left jutting out into the sea as prominent headlands.
Rock Resistance is Key
The rate of erosion depends entirely on rock type. Hard rocks like granite and limestone can resist wave attack for centuries, while soft rocks like clay and sand may be eroded within decades. This fundamental difference in resistance creates the characteristic headland and bay coastlines.
Fault lines and joints in the rock make this process even more effective. These natural weaknesses allow waves to penetrate deeper into the rock, speeding up the erosion process. The contrast between different rock types becomes more pronounced over time, creating the characteristic pattern we see along many coastlines.
Worked Example: Dorset Coast Formation
Step 1: Initial coastline has alternating bands of hard limestone and soft clay Step 2: Wave attack begins - clay erodes 10x faster than limestone Step 3: After 1000 years, clay areas become bays (like Lulworth Cove) Step 4: Limestone areas remain as headlands (like Durdle Door) Step 5: Pattern becomes more pronounced over time
Cliffs
Cliffs are steep rock faces that form where the land meets the sea. They're shaped by both weathering (the breakdown of rock) and erosion (the wearing away and transport of rock material by waves).
Types and formation: The type of cliff that forms depends largely on the rock type present. Soft rocks like clay and sand create gently sloping cliffs because they're easily worn away by weathering and wave action. The loose material slides down regularly, preventing steep faces from developing.
In contrast, hard rocks such as chalk and limestone form much steeper, more dramatic cliffs. These resistant rocks can maintain near-vertical faces because they're strong enough to withstand the constant battering from waves. However, even hard rock cliffs gradually retreat inland as waves continue their relentless attack at the base.
Weathering vs Erosion
While often used together, these are different processes:
- Weathering breaks down rock in place (like freeze-thaw action in cliff cracks)
- Erosion involves the removal and transport of weathered material (like waves carrying away broken rock)
Both work together to shape cliff profiles.
Caves, arches and stacks
These three landforms represent different stages in the same erosional process, typically occurring along headlands where waves can attack the rock from multiple angles.
The formation sequence: The process begins when waves repeatedly crash against weak points in the rock, such as joints or fault lines. Over many years, this constant hydraulic action (the force of water being thrown against rock) gradually opens up cracks and enlarges them into caves.
When caves form on both sides of a narrow headland, they may eventually break through to meet each other, creating a natural arch. The roof of this arch spans across the gap, supported by the rock on either side.
As erosion continues, the arch becomes increasingly unstable. Eventually, the roof can no longer support its own weight and collapses, leaving behind an isolated pillar of rock called a stack. This stack will itself gradually be worn away by continued wave action.
Worked Example: The Cave → Arch → Stack Sequence
Step 1: Wave Attack
- Waves target weak points (joints/faults) in headland
- Hydraulic action forces water into cracks
Step 2: Cave Formation
- Repeated pressure enlarges cracks
- Cave forms on one or both sides of headland
Step 3: Arch Creation
- Caves from both sides meet through the headland
- Natural arch spans the gap
Step 4: Stack Formation
- Continued erosion weakens arch roof
- Roof collapses, leaving isolated stack
Step 5: Stack Destruction
- Stack gradually eroded away completely
- Cycle may begin again on adjacent weak points
Wave-cut platforms
These are areas of flat rock that appear at the base of cliffs, visible especially at low tide. They form through a process called undercutting, where waves repeatedly attack the bottom of cliffs.
Formation process: Waves are most powerful at high tide when they can reach the base of cliffs directly. The constant pounding creates a wave-cut notch - a horizontal groove carved into the cliff face at high tide level. This notch gradually deepens and widens as erosion continues.
Eventually, the rock above the notch becomes unstable because it lacks support underneath. The overhanging rock face collapses, and the cliff retreats inland. The flat rock surface left behind at the base becomes the wave-cut platform. This process repeats continuously, causing cliffs to gradually move inland while leaving behind these distinctive flat rock shelves.
Tidal Influence on Platform Formation
Wave-cut platforms are most effectively carved at high tide level because this is where wave energy is consistently concentrated. The platform's height corresponds to the average high tide level, creating a distinctive horizontal feature that's exposed at low tide.
At high tide, these platforms are usually underwater, but at low tide they're exposed, creating rocky shores that are home to many coastal wildlife species.
Identifying features on maps
When working with Ordnance Survey maps, certain clues help identify these erosional landforms. Headlands often have the word 'Point' in their names, such as 'Reveril Point'. Cliffs are shown by closely spaced contour lines near the coast, and wave-cut platforms may appear as areas of flat ground between high and low tide marks.
Map Reading Tips for Coastal Features
- Headlands: Look for 'Point', 'Head', or 'Ness' in place names
- Cliffs: Closely spaced contour lines running parallel to coast
- Bays: Contour lines curve inland, often with place names ending in 'Bay' or 'Cove'
- Wave-cut platforms: Flat areas between high and low tide marks on detailed maps
- Stacks: Small isolated features just offshore from headlands
Key Points to Remember:
- Differential erosion creates headlands (hard rock) and bays (soft rock) due to varying resistance to wave attack
- Cliff steepness depends on rock type - soft rocks create gentle slopes while hard rocks form steep faces
- Cave → Arch → Stack represents a sequence of erosional landforms developing over time through continued wave attack
- Wave-cut platforms form through undercutting at the cliff base, causing cliff retreat and leaving flat rock surfaces
- All these landforms result from the ongoing battle between waves and coastal rocks, with geology determining the final shapes