Coastal Landforms: Erosional and Depositional Features

These landforms develop through differential erosion:

1. Destructive waves attack the discordant coastline using hydraulic action and corrasion.
2. The less resistant rock (e.g., clay or sand) erodes backwards much faster, creating a curved, sheltered inlet called a bay.
3. The resistant rock (e.g., chalk or limestone) erodes slower and is left protruding out into the sea as a steep-sided headland.
4. As waves enter the shallower water of the newly formed bay, they slow down and bend towards the headlands. This process, called wave refraction, concentrates erosional energy on the headlands and allows deposition of sandy beaches in the sheltered bays.

For an OCR case study, look at the Isle of Purbeck in Dorset. Less resistant Wealden Clay has eroded rapidly to form Swanage Bay, while resistant Chalk to the north remains as The Foreland headland.

Erosional Landforms: The Cave, Arch, Stack, and Stump Sequence

Some of the most famous coastal landmarks in the UK, like Old Harry Rocks, started as tiny cracks in a cliff face. This sequence occurs on resistant headlands where wave refraction concentrates wave energy on the sides of the cliff.

1. Waves exploit natural weaknesses, such as faults or bedding planes, in the headland.
2. Hydraulic action and corrasion widen the crack into a cave.
3. Continued erosion deepens the cave until it breaks completely through the headland, forming an arch (e.g., Durdle Door).
4. The roof of the arch is continuously widened from below by wave action and weakened from above by sub-aerial weathering (like freeze-thaw weathering and salt crystallisation).
5. Eventually, gravity causes the unsupported roof to collapse, leaving an isolated pillar of rock called a stack (e.g., Old Harry).
6. The base of the stack is undercut by waves to form a wave-cut notch. The stack eventually collapses, leaving a small, flat stump that is often only visible at low tide.

Depositional Landforms: Beaches

Beaches form in sheltered areas, like bays, where waves lose their energy and drop their sediment. This process follows a specific sequence:

1. Constructive waves reach the shore in low-energy environments.
2. The powerful swash carries and pushes sediment (sand or shingle) up the beach.
3. Because the backwash is weak, the water lacks the energy to pull the material back out to sea, leading to accumulation.
4. On shingle beaches, water quickly percolates (soaks) through the gaps between pebbles, further weakening the backwash and allowing ridges called berms to form.
5. Sandy beaches develop gentle gradients as small grains are washed far up the shore, whereas shingle beaches develop steeper gradients due to the rapid loss of backwash energy.

Depositional Landforms: Spits

A spit is an elongated ridge of sand and shingle extending from the land into the sea. Spits are created through a step-by-step sequence:

1. Longshore drift transports sediment continuously along the coastline.
2. The coastline abruptly changes direction (for example, at a river estuary like Spurn Head on the Holderness Coast).
3. Waves lose energy in the deeper, sheltered water of the estuary, causing sediment deposition in the same direction as the original coastline.
4. Over time, this deposited material builds up above sea level, extending outwards.
5. Temporary changes in the prevailing wind direction, or wave refraction around the tip, push the sediment inward to form a hooked recurved lateral at the distal end of the spit.
6. The water trapped behind the spit becomes a very low-energy zone. Fine mud and silts are deposited here, eventually forming a salt marsh.