Coastal Landforms: Erosional and Depositional Features

Erosion and deposition shape the coast

Coastal landforms develop as waves wear away rock, move sediment and deposit it elsewhere. Erosion removes material from the coast; deposition builds features from transported material. These processes are connected: sediment released by erosion can later help build a beach or spit.

Rock type and structure influence where erosion happens fastest. Wave energy and the movement of sediment then help determine how the coastline develops.

Different erosion rates create headlands and bays

A headland is land that projects into the sea. A bay is an inlet where the coastline curves inland. They can form where alternating bands of more resistant and less resistant rock meet the shoreline at roughly right angles. This arrangement is called a discordant coastline.

Imagine a coastline with bands of clay beside bands of chalk. Waves erode the less resistant clay faster, causing those sections of coast to retreat inland and form bays. The more resistant chalk erodes more slowly, so it is left projecting into the sea as headlands. This difference in erosion rates is called differential erosion.

Headlands remain exposed to wave attack, while bays are more sheltered. In a sheltered bay, waves may lose enough energy to deposit sediment and build a beach. The bay itself is an erosional feature; a beach within it is depositional. Not every bay contains a beach.

Waves enlarge weaknesses into caves and arches

Even resistant rock contains weaknesses such as cracks. Waves attack these weaknesses through hydraulic action, the force of water striking rock and entering cracks, and abrasion, in which rock fragments carried by waves scrape and strike the rock face. Repeated erosion widens a crack into a cave, a hollow extending into the headland.

Wave action can be particularly concentrated around headlands. As waves enter shallower water, they slow and bend—a process called wave refraction. This directs wave energy towards the projecting headland and helps erosion continue around its sides.

As a cave becomes deeper and wider, it may eventually break through the headland. The opening now passes right through the rock, with a roof remaining above it: this is an arch.

Arch collapse leaves a stack

Waves continue to erode the sides and base of the arch, widening the opening and weakening its supports. At the same time, weathering breaks down the exposed rock above. Together, erosion below and weathering above make the roof increasingly unstable.

Eventually, the roof collapses. The seaward part of the former arch is left separated from the headland as an isolated column of rock called a stack. Further erosion and weathering can eventually reduce the stack to a low stump.

The sequence is weakness → cave → arch → roof collapse → stack. Each stage results from continued erosion and, especially during collapse, weathering; these are not separate features appearing without a connection.

Sediment builds coastal landforms

Not all coastal landforms are made by wearing rock away. Beaches and spits are built by deposition: sediment is dropped when water no longer has enough energy to transport it. Their formation therefore depends on both a supply of material and conditions that allow it to accumulate.

Coastal sediment can come from eroded cliffs, rivers or offshore sources. Waves move this material, sometimes bringing it onto a beach and sometimes transporting it further along the coast. A landform builds up where deposition adds material faster than it is removed.

How waves build a beach

A beach is an accumulation of sediment along the shoreline. It may consist of sand, shingle or a mixture of both. Beaches commonly develop in sheltered bays, where reduced wave energy allows sediment to be deposited.

When a wave breaks, water moves up the beach as swash. It then returns down the beach as backwash. Constructive waves have strong swash and weaker backwash: they carry sediment up the beach but remove relatively little as the water returns. Repeated waves therefore build up the beach.

This is a changing balance rather than a permanent one-way process. Calmer conditions favour accumulation, while waves can also remove and redistribute beach material. A beach's size depends on the continuing balance between sediment arriving and sediment leaving.

Longshore drift moves material along the coast

Waves often approach the coast at an angle because of the prevailing wind—the wind direction that is most common. The swash carries sediment diagonally up the beach in the direction of the approaching waves. Gravity then pulls the backwash straight down the beach, at right angles to the shoreline.

Repeated swash and backwash move sediment along the shore in a zigzag pattern. This movement is called longshore drift. Imagine following one pebble: it moves diagonally up the beach, straight back down, then diagonally up again a little further along the coast. Many repetitions produce a net movement along the shoreline.

Longshore drift links different parts of a coastline. Material supplied at one location can be transported elsewhere, helping to feed beaches and spits.

How a spit grows out from the coast

A spit is a long, narrow ridge of sand or shingle that projects into the sea while remaining attached to the land at one end. Spurn Point on the Holderness Coast is a UK example.

Spit formation begins with longshore drift transporting sediment along the shoreline. Where the coastline changes direction, or at the mouth of an estuary, sediment can be carried beyond the existing shoreline into more sheltered water. As the water loses energy, it deposits sediment. Continued transport and deposition extend the ridge out into the sea.

The tip may become curved or hooked. Changes in wind and wave direction can redirect material, while waves bending around the end can carry sediment towards more sheltered water.

A spit does not necessarily continue growing indefinitely. Growth slows or stops if water becomes too deep, or if material is removed faster than it is deposited. At an estuary mouth, river flow carries sediment seaward and can prevent the spit from extending right across the opening.

Shelter behind a spit encourages further deposition

The spit shelters the water on its landward side from wave action. In this lower-energy area, fine sediment can settle, allowing mudflats or salt marshes to develop.

The complete connection is therefore transport along the coast → deposition beyond a change in coastline → spit growth → sheltered water behind the spit → further deposition. The diagram illustrates this general sequence; it is not a map of Spurn Point or any other real coastline.