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The character of a coastline is fundamentally determined by its geology — the type, structure and arrangement of rocks. Lithology and geological structure control rock resistance, permeability, jointing patterns and slope stability, which in turn determine how the coast responds to erosional and depositional processes. This lesson examines how geology shapes coastal landscapes at multiple scales.
Key Definition: Lithology refers to the physical and chemical characteristics of a rock, including its mineral composition, grain size, texture, colour and, crucially for coastal geomorphology, its resistance to erosion and permeability.
Different rock types exhibit vastly different resistance to coastal erosion:
| Rock Type | Hardness (Mohs scale approx.) | Resistance to Erosion | Permeability | Coastal Character |
|---|---|---|---|---|
| Granite | 6-7 | Very high | Very low (impermeable unless jointed) | Rugged, steep cliffs; headlands; slow retreat |
| Basalt | 5-6 | High | Low | Columnar cliffs (e.g., Giant's Causeway); resistant headlands |
| Limestone | 3-4 | Moderate-high (mechanical); vulnerable to solution | Variable (porous or pervious depending on type) | Vertical cliffs; caves; solution features |
| Chalk | 2-3 | Moderate | High (very porous — up to 40% porosity) | White vertical cliffs; wave-cut platforms; stacks |
| Sandstone | Variable (3-7) | Variable (depends on cement) | Variable | Cliffs with ledges; differential erosion along beds |
| Shale/mudstone | 2-3 | Low | Very low (impermeable) | Slumping; gentle cliff profiles; rapid retreat |
| Clay | 1-2 | Very low | Very low (impermeable; becomes unstable when wet) | Rapid cliff retreat; rotational slumping; mudflows |
| Glacial till | N/A (unconsolidated) | Very low | Variable | Fast retreat (1-5+ m/year); irregular profiles |
Permeability determines how water moves through rock, which has profound effects on cliff stability:
Porous rocks (e.g., chalk, some sandstones) have interconnected pore spaces that allow water to pass through. This can reduce surface runoff and overland flow, but saturated porous rock becomes heavy and weak.
Pervious rocks (e.g., well-jointed limestone) allow water to pass through along cracks, joints and bedding planes rather than through pore spaces.
Impermeable rocks (e.g., clay, granite, shale) do not allow water to pass through. Surface runoff is high, and water accumulates at the junction between permeable and impermeable layers, creating a spring line that can lubricate potential failure surfaces and trigger mass movement.
Exam Tip: Distinguish carefully between porous and pervious rocks in your exam answers. Chalk is porous (water passes through tiny pore spaces between grains); limestone is pervious (water passes through joints and bedding planes). Both are permeable, but the mechanism is different and has different geomorphological consequences. This distinction demonstrates advanced understanding.
The three-dimensional arrangement of rocks — their bedding, folding, faulting and jointing — is as important as their lithology in determining coastal form.
The angle at which rock beds meet the coastline has a critical effect on cliff stability:
| Orientation | Description | Stability | Resulting Cliff Profile | Example |
|---|---|---|---|---|
| Horizontal | Beds are flat-lying | Moderate | Stepped cliff with horizontal ledges | Whitby, North Yorkshire |
| Dipping inland | Beds slope away from the sea | High — beds lean into the cliff | Steep, stable cliff | Parts of the Dorset coast |
| Dipping seaward | Beds slope towards the sea | Low — beds slide towards the sea | Prone to landslides; gentler profile | Barton-on-Sea, Hampshire |
| Vertical | Beds are near-vertical | Variable | Erosion follows weaker beds; creates gullies and differential forms | Parts of the Hartland coast, Devon |
Joints are fractures in rock along which no displacement has occurred. Faults are fractures where displacement has occurred. Both create weaknesses that are exploited by erosion:
Research by Naylor and Stephenson (2010) at Staithes, North Yorkshire, demonstrated that erosion rates on the same rock type varied by a factor of five depending on joint density and orientation.
Where tectonic forces have folded rock layers, the resulting anticlines (upfolds) and synclines (downfolds) create patterns of differential resistance:
One of the most important structural controls on coastal form is whether the geological strata run parallel or perpendicular to the coastline.
On a concordant coastline, rock strata run parallel to the shore. This produces a relatively straight, uniform coastline because the same rock type is exposed along the entire coast.
Case Study: Lulworth Cove, Dorset
Lulworth Cove is the classic textbook example of erosion on a concordant coastline:
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