Coastal Landscapes » Coastal Deposition Landforms and the Dorset Coast

Landforms of deposition

Last lesson was about landforms the sea wears away. This lesson is about landforms the sea builds. They form where the sea drops its load of sediment, for example in sheltered bays and where the coastline changes direction. Remember, the dropping of sediment is called deposition.

Key terms:

• Beach: a gently sloping strip of sand or shingle between the low tide line and the highest point the waves reach.
• Shingle: small, rounded pebbles, bigger than sand grains.
• Sand dune: a hill or ridge of wind-blown sand found at the back of a beach.
• Spit: a long, narrow finger of sand or shingle joined to the land at one end, sticking out into the sea or across a river mouth.
• Bar: a ridge of sand or shingle that stretches right across a bay and joins two headlands.
• Lagoon: a shallow body of water cut off from the sea by a bar.

Beaches

A beach forms when waves deposit sand and shingle along the shore. Constructive waves are the main builders. Their strong swash carries sediment up the beach and their weak backwash drags very little of it back.

Beaches are never still. Waves and longshore drift keep moving the sediment, so a beach can grow in calm weather and shrink in a storm.

Sand dunes

Sand dunes form behind wide sandy beaches. At low tide the sand dries out and the wind blows it up the beach. They form in stages:

1. Wind-blown sand is trapped by an obstacle, such as seaweed or driftwood on the high tide line.
2. The sand piles up into a small mound called an embryo dune.
3. Tough plants such as marram grass begin to grow. These are pioneer plants, the first to settle on bare sand.
4. Marram grass roots bind the sand together and stop it blowing away. The grass keeps growing upwards as sand buries it, so the dune gets higher.
5. Dead plants rot and add nutrients, so more plants can grow and the dune becomes more stable.

The newest dunes are nearest the sea. Dunes further back are older, taller and covered with more plants.

Spits

A spit forms where the coastline suddenly changes direction, such as at a bend in the coast or the mouth of a river. Longshore drift keeps moving sediment along the beach. At the bend, the sediment carries straight on and is deposited in the water beyond the corner.

• Sediment builds up in a long, narrow ridge that sticks out into the sea.
• Changes in wind direction and waves can bend the end of the spit into a curve, called a recurved end.
• Behind the spit the water is calm and sheltered. Mud builds up there and plants grow, forming a salt marsh.

A famous example is Spurn Head in Yorkshire, which curves across the mouth of the Humber estuary.

Bars

A bar forms in the same way as a spit. The difference is that it keeps growing across a bay until it joins two headlands. The bay behind is then sealed off from the sea and becomes a lagoon. Chesil Beach in Dorset is a shingle bar about 29 km long, with the Fleet Lagoon behind it.

Example: the Dorset coast around Swanage

The coast near Swanage in Dorset is a great place to see landforms of both erosion and deposition. Rocks of different strength run in bands that meet the coast at an angle, so this is a discordant coast. The harder rocks (chalk and limestone) are left sticking out and the soft clays and sands have been worn back into a bay.

The chalk cliffs and stacks show erosion. The beaches and dunes show deposition. Put them together and one short stretch of coast shows both processes at work.

Exam-style question

(a) Name one landform of deposition found on the Dorset coast. [1 mark]

(b) Explain how a spit is formed. [4 marks]