Coastal Processes and Landform Formation

A coastline is shaped by connected processes

The coast is where land and sea meet. Its cliffs, beaches and other landforms are shaped by geomorphological processes: natural actions that change the Earth's surface. On UK coasts, processes acting on land work alongside those acting in the sea.

Weathering weakens rock in place. Mass movement carries loosened material downhill. Marine erosion removes material, transportation moves it elsewhere, and deposition lays it down. These are connected: a cliff can supply the sediment that later becomes part of a beach or spit.

Weathering weakens cliffs before material moves

Weathering breaks down rock where it is. It does not itself transport the material away.

Mechanical weathering physically breaks rock without changing its chemical composition. In freeze–thaw weathering, water enters cracks. When it freezes, it expands and puts pressure on the sides. Repeated freezing and thawing widens the cracks until pieces break off.

Chemical weathering changes or dissolves minerals. Rainwater absorbs carbon dioxide and becomes weakly acidic. It reacts with susceptible rocks such as limestone, gradually weakening them.

Biological weathering involves living organisms. Plant roots grow into cracks and push them wider as the roots thicken. Burrowing animals can also loosen material.

These land-based processes help weaken the upper parts of cliffs, while waves attack their bases.

Gravity moves unstable cliff material

Mass movement is the downhill movement of rock or soil under gravity. Water can make a slope less stable by adding weight and reducing friction within the material. Waves can also remove support by undercutting the cliff base.

In a rockfall, loosened pieces fall from a steep cliff and collect at its foot. In a landslide, a mass of material slides down a slope along a slip surface. In rotational slumping, material moves down a curved slip surface, rotating as it moves. This is common where weaker material such as clay becomes saturated.

Mass movement changes the cliff's shape and supplies debris to the shore. Waves may then break up and remove that debris, allowing further erosion of the cliff base.

Waves erode, transport and deposit

Wind blowing over the sea transfers energy to the water and creates waves. When waves break, swash moves water up the beach; backwash returns it down the beach under gravity.

Constructive waves have relatively strong swash and weaker backwash, so they tend to deposit sediment and build beaches. Destructive waves have stronger backwash relative to swash, so they tend to remove beach material. The balance between sediment arriving and leaving determines whether a beach grows or shrinks.

Marine erosion involves four important processes:

• Hydraulic action: waves force water and air into cracks. Air is compressed as a wave arrives and pressure is released as it retreats. Repetition widens cracks and breaks off rock.
• Abrasion: sand and stones carried by waves scrape against the cliff, wearing it away like sandpaper.
• Attrition: moving stones collide with one another, becoming smaller, smoother and rounder. This changes the sediment rather than directly wearing away the cliff.
• Solution: water dissolves soluble minerals, including those in limestone and chalk. Moving water carries the dissolved material away.

Retreating cliffs leave wave-cut platforms

Hydraulic action and abrasion attack the foot of a cliff, cutting a wave-cut notch. As the notch deepens, the rock above becomes unsupported. Weathering weakens it further, and it eventually collapses under gravity.

Waves remove the fallen material and begin attacking the newly exposed cliff face. Repeated undercutting, collapse and removal cause the cliff to retreat inland. A gently sloping rocky surface remains in front of it: the wave-cut platform, which may be exposed at low tide.

Headlands, bays, arches and stacks

Where bands of rock with different resistance meet the sea, less resistant rock is eroded faster. It retreats to form a bay, while more resistant rock remains projecting into the sea as a headland. This produces an irregular coastline.

Waves bend as they enter shallow water, a process called refraction. This concentrates wave energy on headlands, encouraging erosion, while bays are more sheltered and can accumulate beach sediment.

On a headland, hydraulic action and abrasion enlarge cracks into caves. Continued erosion may cut through the headland to form an arch. Further erosion and weathering weaken the arch until its roof collapses. An isolated pillar of rock remains offshore: a stack.

These landforms are stages in continuing change, not permanent end points.

How the sea transports sediment

Coastal sediment comes from sources including eroded cliffs, rivers and offshore deposits. How it moves depends on particle size and the energy of the water:

• Traction: large stones roll or are dragged along the seabed.
• Saltation: smaller particles, such as sand, bounce along the seabed.
• Suspension: fine particles, such as silt and clay, are carried within the water.
• Solution: dissolved minerals travel in the water and cannot be seen as separate particles.

Sediment can also move along the shore through longshore drift. When waves approach at an angle, their swash carries sediment diagonally up the beach. Gravity pulls the backwash straight down the slope, approximately at right angles to the shoreline. Repeated swash and backwash move sediment along the coast in a zig-zag path.

For example, imagine following one pebble: each swash moves it upwards and sideways; each backwash brings it down the beach. After several waves, it has moved along the shore even though each individual movement was short.

Deposition builds beaches and spits

Deposition happens when water loses enough energy that it can no longer carry sediment. Larger particles generally require more energy to transport than finer particles.

A beach is an accumulation of sand or shingle along the shore. Constructive waves bring sediment onto it, and sheltered bays provide places where deposition can exceed removal.

A spit forms where longshore drift carries sediment towards a change in the direction of the coastline, such as a river mouth. Sediment continues beyond the bend and is deposited in the water. Continued deposition builds a narrow ridge attached to the coast at one end.

Changes in wave or wind direction can curve the end of the spit. Sheltered water behind it allows fine sediment to settle, potentially forming mudflats and salt marsh. At a river mouth, flowing water can keep a channel open and prevent the spit from completely closing the entrance.

Estuaries bring river and marine processes together

An estuary is the tidal mouth of a river, where freshwater mixes with seawater. It is shaped by the interaction of river flow, tidal currents, erosion, transportation and deposition.

Rivers deliver sediment to the coast, while tides move water and sediment into and out of the estuary. Moving water maintains channels, but in sheltered areas where flow slows, fine sediment settles. Repeated deposition builds mudflats; salt-tolerant plants can colonise suitable areas to form salt marsh.

An estuary therefore contains both moving-water channels and depositional surfaces. Its landscape shows why coastal landforms are best understood through interacting processes rather than one process acting alone.

Land-based processes

• Weathering: rock breaks down in place — mechanical, chemical or biological.
• Freeze–thaw: water enters cracks → freezes and expands → repeated cycles break rock.
• Mass movement: gravity moves material downhill — rockfall, landslide or rotational slump.
• Weathering weakens cliffs; saturation and undercutting can trigger collapse.

Marine erosion

• Hydraulic action: wave pressure and compressed air widen cracks.