Coastal Landforms » Beaches, Spits, Bars and Sand Dunes

Depositional landforms: where the sea builds

Last lesson was about landforms made by erosion. This lesson is about landforms made by deposition, when the sea drops the material it is carrying. They form where waves lose energy: in sheltered bays, in shallow water, and where the coastline changes direction. Remember, constructive waves have a strong swash and a weak backwash, so they leave material behind.

Key terms:

• Beach: an area of sand or shingle deposited between the low tide line and the highest point reached by the waves.
• Shingle: small, rounded pebbles and stones, larger than sand grains.
• Berm: a ridge of sand or shingle at the top of a beach, left by constructive waves at high tide.
• Spit: a long, narrow ridge of sand or shingle, joined to the land at one end and sticking out into the sea or across a river mouth.
• Recurved end: the hooked tip of a spit, curved towards the land.
• Salt marsh: a flat, muddy area behind a spit that floods with salt water at high tide and has plants that tolerate salt.
• Bar: a ridge of sand or shingle that has grown right across a bay.
• Lagoon: a body of water cut off from the sea by a bar.
• Sand dune: a hill of wind-blown sand at the back of a beach.
• Embryo dune: a small, young dune that has just started to form.
• Marram grass: a tough grass with long roots that grows on dunes and holds the sand together.

Beaches

A beach is the most common depositional landform. Constructive waves build it up, because the swash carries material up the beach and the weak backwash cannot drag it all back. Over time the beach gets wider and higher.

Feature	Sand beach	Shingle beach
Material	Fine grains	Rounded pebbles and stones
Slope	Gentle and wide	Steeper and narrower
Where found	Sheltered bays, where waves have less energy	Places with stronger waves, often near eroding cliffs that supply stones

At the top of a beach you may see one or more berms. At each high tide, constructive waves push material to the highest point they reach and leave it there as a ridge. A beach with several berms shows the sea has reached different heights at different times, for example in a storm.

Spits

A spit forms where the coastline changes direction, for example at a bend in the coast or at a river mouth. Longshore drift keeps moving material along the coast. When the coast turns, the material carries on in a straight line into deeper water and is deposited there. Here is how a spit forms:

1. Waves hit the beach at an angle, so longshore drift moves sand and shingle along the coast.
2. The coastline changes direction, for example at a river mouth or bend.
3. Longshore drift carries on in the original direction, and material is deposited where the water loses energy.
4. The deposited material builds up into a long, narrow ridge, joined to the land at one end.
5. Strong winds or waves from a different direction curve the end of the spit towards the land. This is the recurved end.
6. The area behind the spit is sheltered, so fine mud is deposited. Salt marsh forms behind the spit.

If a river flows out at the mouth, its flow helps to keep a channel open, so the spit does not join the other side. Spurn Head in Yorkshire, England, is a spit across the mouth of the Humber estuary.

Bars and lagoons

A bar is a spit that has kept growing until it stretches all the way across a bay. This cuts the bay off from the open sea. The water trapped behind it is a lagoon. The steps are:

1. A spit starts to form across a bay, as above.
2. Longshore drift keeps adding material, so the spit grows longer.
3. The spit reaches the other side of the bay and joins it. This ridge is now a bar.
4. The water behind the bar is trapped and becomes a lagoon.
5. Streams may feed the lagoon with fresh water and silt, so over time it can slowly fill in.

Slapton Sands in Devon, England, is a bar of shingle. Behind it is a freshwater lagoon called Slapton Ley.

Sand dunes

Sand dunes form at the back of wide, sandy beaches. They need a lot of sand, a strong onshore wind (blowing from the sea to the land), and a large flat area behind the beach. When the tide goes out, the sand dries and the wind blows it up the beach. Dunes then form in stages:

1. Dry sand is blown up the beach by onshore wind until it hits an obstacle, such as seaweed, driftwood or a plant.
2. The sand piles up behind the obstacle and forms a small embryo dune.
3. Plants such as marram grass begin to grow. Their roots bind the sand and their leaves trap more sand, so the dune gets bigger.
4. Dead plants rot and add nutrients to the sand, so more types of plant can grow. The dune grows taller and becomes more stable.
5. Over many years, the dunes further back from the sea are older, taller, covered with plants and more stable. These are mature dunes.

The pattern is that dunes closest to the sea are smallest and youngest, and dunes furthest inland are tallest and oldest.

Stage	Where	What it is like
Embryo dunes	Closest to the sea	Small, bare or with a few grasses, easily blown away
Young dunes	Behind embryo dunes	Taller, marram grass growing, more stable
Mature dunes	Furthest from the sea	Tallest, covered in plants, soil forming

The Curonian Spit, a long sand spit on the Baltic Sea shared by Lithuania and Russia, is known for its very large dunes.

Exam-style question

Fig. 1.1 is a sketch map of a spit. Sand and shingle are joined to the land at one end and stretch across the mouth of a river. The free end is curved towards the land, and an area of salt marsh lies behind it.

(a) Identify the feature at the free end of the spit in Fig. 1.1. [1 mark]

(b) Give one difference between a sand beach and a shingle beach. [1 mark]

(c) Describe how a berm forms. [3 marks]

(d) Explain how a spit forms. [6 marks]