Geological Structure and Erosional Landforms

Why geology matters at the coast

Coastal erosion wears away and removes rock and sediment. However, the same wave action does not produce the same landscape everywhere. The result depends partly on rock type—how resistant the material is—and partly on geological structure—how the rocks are arranged and where they contain weaknesses.

Harder, more resistant rock takes longer to erode. Softer, less resistant materials, such as sands and clays, are more easily removed. Here, ‘hard’ and ‘soft’ are useful comparisons: hard rock can still be eroded, but usually more slowly than neighbouring soft rock.

Discordant coasts: forming headlands and bays

On a discordant coastline, bands of rock meet the coastline at an angle, often approximately at right angles. Where alternating bands of harder and softer rock are exposed to the sea, they retreat at different rates. This is called differential erosion.

The softer rock is removed more quickly, making the coastline retreat inland to form a bay. The harder rock retreats more slowly and remains projecting into the sea as a headland. The contrast in erosion rates therefore produces an uneven coastline rather than a straight one.

Imagine alternating strips of resistant rock and softer clay reaching the shoreline. As erosion continues, the clay sections retreat farther inland while the resistant sections remain farther seaward. It is the combination of rock arrangement and rock resistance that creates the headland-and-bay pattern.

Concordant coasts: a resistant outer barrier

On a concordant coastline, rock bands run parallel to the coast. A resistant outer band can protect softer rocks behind it, because the sea initially reaches the outer rock rather than all the bands at once.

If erosion breaks through a weak point in this outer band, seawater can reach the less resistant rocks behind it. These rocks are then eroded more easily, allowing an inlet or cove to develop behind the opening.

Around Lulworth Cove in Dorset, resistant Portland Limestone runs parallel to the sea, with less resistant Purbeck Limestone and Wealden Clay behind it. Erosion through weaknesses in the outer limestone has allowed the sea to erode the rocks behind and form coves.

The diagram contrasts two arrangements of rock bands. It is an explanatory sketch, not a map of Dorset: use it to connect the direction of the bands with which rocks the sea can reach.

Joints and faults create weaknesses

Even a resistant rock mass is not necessarily solid and unbroken. A joint is a fracture without displacement; a fault is a fracture along which rock has moved. Both provide weaknesses that expose rock to erosion and weathering. Weathering breaks down rock in place, whereas erosion also removes material.

This helps explain why erosion can be concentrated at particular points rather than progressing evenly along a cliff. To explain a coastal shape, consider not only the rock’s resistance but also where fractures make it more vulnerable.

From a weakness to a cave, arch and stack

A resistant headland can remain standing while waves exploit joints and faults within it. Hydraulic action, in which waves force water into cracks, and abrasion, in which rock fragments carried by waves scrape and batter the rock, enlarge these weaknesses. Over time, a crack can widen and deepen into a cave.

Continued erosion may extend the cave right through the headland, creating an arch: an opening with rock still forming a roof above it. Further erosion and weathering weaken the arch until its roof can no longer support itself and collapses under gravity. An isolated pillar of resistant rock remains offshore as a stack.

The stack is not permanent. Waves can undermine its base until it collapses, leaving a shorter remnant called a stump. This sequence shows why resistant rock is not immune to erosion: its weaknesses help determine where change begins, while the surrounding stronger rock temporarily remains standing.

Rock type also influences cliff appearance

Hard-rock cliffs are typically high and steep, with bare, rugged rock faces. Rocks and boulders may collect at their foot. Their greater resistance allows steep faces to persist for longer.

Soft-rock cliffs are generally lower and less steep. Their faces may be smoother and show evidence of slumping, where material moves down the slope. At their foot there may be sand and mud, with fewer large rocks.

These are typical contrasts rather than absolute rules. Geological structure and physical processes act together, so rock type alone does not explain every feature of a cliff.

Cliff retreat leaves a wave-cut platform

Waves attack the base of a cliff through hydraulic action and abrasion, exploiting weaknesses in the rock. Erosion cuts an indentation called a wave-cut notch. As the notch deepens, the rock above becomes overhanging and unsupported. Eventually it collapses, and waves remove the fallen debris.

Repeated undercutting and collapse move the cliff face inland. This is cliff retreat. The relatively flat rock surface left in front of the retreating cliff is a wave-cut platform, often exposed at low tide and covered at high tide. It is the remaining bedrock, rather than a pile of material deposited by the collapse.

Compare the two sequences: erosion through a headland can leave an isolated stack, while repeated erosion at a cliff’s base moves the cliff inland and leaves a platform. In both, rock resistance and structural weaknesses influence how the rock responds to wave attack.