Erosional and Depositional Landforms

How coastal processes produce distinctive landforms

Coastal landforms record the balance between material being removed and material being deposited. Erosion wears away the coastline, creating features such as cliffs and caves. Deposition occurs when moving water or wind loses the energy needed to carry sediment, building features such as beaches and dunes.

Rock resistance and weaknesses help determine where erosion happens fastest. Hydraulic power is the force of waves acting on rock and compressing air in cracks. Abrasion occurs when waves throw or scrape sediment against the coast. These processes gradually enlarge weaknesses and remove material.

Headlands and bays: erosion at different rates

Where alternating bands of resistant and less resistant rock meet the coast, they erode at different rates. Less resistant rock, such as clay, is worn back more quickly, producing an inlet called a bay. Resistant rock remains projecting into the sea as a headland.

A headland commonly has steep cliffs along its sides. A bay curves inland and may contain a beach, although a beach is not essential to its definition. Sheltered water within a bay provides conditions in which sediment can be deposited.

As waves approach shallow water, they slow and change direction: this is wave refraction. Wave energy becomes concentrated around projecting headlands, helping explain why headlands develop further erosional features.

Cliffs retreat and leave wave-cut platforms

A cliff is a steep coastal rock face. Its shape depends partly on the rock: resistant rock commonly supports steep cliffs, while less resistant material may produce lower, less steep faces with evidence of slumping.

Waves attack the cliff base between the high and low water levels. Hydraulic power and abrasion erode an indentation called a wave-cut notch. As the notch deepens, the rock above loses support. Weathering also weakens the cliff, and the overhanging section eventually collapses.

Waves remove the fallen material, allowing them to attack the newly exposed cliff base. Repeated undercutting and collapse cause the cliff to retreat inland. A gently sloping rock surface remains in front of it: the wave-cut platform. This is exposed bedrock, not a beach made from deposited sand or shingle.

From a crack to a cave, arch and stack

A crack or other weakness in a headland provides a starting point for erosion. Hydraulic power widens it, while abrasion wears away the rock, eventually producing a cave.

Continued erosion can extend the cave through the headland, creating an arch: an opening with a rock roof above it. Waves erode the sides and base of the opening, while weathering weakens the roof from above. When the roof collapses, a separate column of rock remains offshore. This is a stack.

Further erosion undercuts the stack and weathering weakens it. Eventually it collapses into a much lower remnant called a stump. The sequence represents progressive erosion, but different stages can exist together along one coastline.

Beaches: sediment accumulates along the shore

A beach is an accumulation of sand or shingle deposited along the shore. Beaches often develop in sheltered locations, including bays, where wave energy is lower.

Constructive waves help build beaches because their swash carries sediment up the shore more effectively than their backwash removes it. Some water soaks into the beach, reducing the amount returning down the slope and helping sediment remain deposited.

Beaches are not fixed features. Calmer conditions can encourage material to accumulate, while storms and destructive waves can remove or redistribute it. Wave action also sorts sediment, so different sizes of material can occur in different parts of the beach.

Sand dunes: wind builds beyond the beach

Sand dunes are mounds or ridges of sand, usually found behind sandy beaches. Their formation needs a supply of dry, exposed sand and onshore winds capable of moving it inland.

When wind-blown sand meets an obstacle, such as driftwood or a plant, it is trapped and deposited. A small embryo dune begins to grow. As it catches more sand, the mound increases in size.

Pioneer plants colonise the sand. Vegetation traps further grains, while roots help hold the dune together. Marram grass is particularly important in stabilising developing dunes. Over time, plant remains add organic matter, making conditions more suitable for other vegetation. This gradual change in the plant community is called succession.

Dunes therefore depend on both wind deposition and vegetation. They can still change shape as wind moves exposed sand.

Spits and bars: deposition extends the coastline

A spit is a narrow ridge of sand or shingle attached to the coast at one end and extending into the sea. Longshore drift supplies its sediment. Waves approach the beach at an angle, so swash moves material diagonally up the shore; backwash carries it down the beach slope. Repeated movement transports sediment along the coast.

Where the coastline changes direction, sediment can continue beyond the bend. In shallow, sheltered water, material is deposited and gradually builds a ridge extending from the shore. Continued deposition lengthens the spit.

Changes in wind and wave direction can curve its end into a hook. Water behind the spit is sheltered, allowing fine sediment to settle and form mudflats or salt marshes. At an estuary, river flow can prevent the spit from extending completely across the mouth.

If a spit extends across a bay and joins the opposite side, it forms a bar. Unlike a spit, a bar connects land at both ends. It can enclose water behind it, creating a lagoon. Here, “bar” means this bay-crossing coastal landform, rather than an offshore sandbank.

The Dorset coast: identifying real examples

The Dorset coast in southern England provides an example of both erosional and depositional landforms. Its contrasting bands of chalk, limestone, clay and sandstone help explain why different features occur along the same section of coastline.

Around Swanage and Studland, less resistant sandstone and clay have been eroded to form bays containing beaches. Between them, the more resistant chalk of The Foreland projects into the sea as a headland. At its end, Old Harry is a chalk stack, while Old Harry’s Wife is a stump. These show the later stages of headland erosion.

Further west, Durdle Door is an arch in limestone. Wave erosion enlarged a weakness into a cave and then an opening through the headland. At Lulworth Cove, waves breached an outer band of resistant limestone and eroded the softer clay behind it. The result is a bay with a narrow entrance: the resistant rock restricts the entrance while the less resistant material behind has been worn back more widely.

A major depositional feature is Chesil Beach, an approximately 18-mile-long pebble ridge linking the Isle of Portland to the mainland. This connection is known as a tombolo. The shallow Fleet Lagoon lies behind it. Chesil Beach illustrates sediment accumulation, in contrast to the rock removal responsible for Dorset’s arches, stacks and bays.

When identifying features on a photograph or map, begin with shape and material. An isolated rock column suggests a stack; an opening beneath a rock roof indicates an arch; a gently sloping bedrock surface below a cliff suggests a wave-cut platform. For depositional ridges, check their connections to land: one attached end indicates a spit, both sides of a bay indicate a bar, and an island-to-mainland connection indicates a tombolo.

Erosional sequences

• Headlands and bays: less resistant rock erodes faster into bays; resistant rock remains as headlands.
• Cliffs and wave-cut platforms: waves erode a notch → unsupported rock collapses → debris is removed → repeated retreat leaves a gently sloping rock platform.