Coastal Landforms » Discordant and Concordant Coastlines

Rock layers and the coast

Rocks are not the same everywhere. Along a stretch of land you can find bands of hard rock (such as limestone or chalk) and bands of soft rock (such as clay or sand). These bands run in a certain direction. The way those bands meet the sea decides what the coastline looks like.

There are two main patterns. Either the bands run at right angles to the coast, or they run parallel to it. Think of a stack of paper strips: you can lay them end on to the sea, or side on to the sea.

Key terms:

• Discordant coastline: a coastline where bands of hard and soft rock run at right angles to the coast.
• Concordant coastline: a coastline where bands of hard and soft rock run parallel to the coast.
• Cove: a small, sheltered, almost circular bay with a narrow entrance.
• Drowned coast: a coast where the sea has flooded low land because the sea level rose.

Discordant coastlines

Remember from Headlands, Bays, Caves, Arches and Stacks that soft rock erodes faster than hard rock. On a discordant coast this is easy to see, because both rock types face the waves at the same time.

How a discordant coastline forms (steps to write in an exam):

1. Bands of hard and soft rock lie at right angles to the coast.
2. Waves attack the whole coast, but the soft rock is eroded faster.
3. The soft rock is worn back to form bays.
4. The hard rock is eroded more slowly, so it is left sticking out as headlands.
5. Waves then refract round the headlands, so wave energy is focused on the headlands and spread out in the bays (see Waves: Constructive, Destructive and Refraction).

Example: Swanage Bay on the Dorset coast, in southern England. Soft clay and sand have been eroded back to make the bay. Harder chalk and limestone stand out as headlands on either side.

Concordant coastlines

The hard rock at the front acts like a shield. It is eroded slowly, so the coast changes little for a long time. If the barrier is broken, the soft rock behind is attacked and wears away quickly.

How a cove forms on a concordant coastline:

1. A band of hard rock runs parallel to the coast and protects the soft rock behind it.
2. Waves attack a weak point, such as a crack or a place where a stream flows through.
3. Over time the waves breach (break through) the hard rock.
4. The waves reach the soft rock behind and erode it quickly, widening the gap sideways.
5. Waves enter through the narrow opening and spread out, so a rounded cove forms.
6. Further back, another band of harder rock slows the erosion down, so the cove stops growing.

Example: Lulworth Cove, Dorset. A band of hard Portland limestone forms the barrier. The sea broke through and eroded the softer clay and sands behind. Further back, chalk is harder again, so the cove stopped growing. The result is a small, almost circular bay with a narrow mouth.

A drowned concordant coast

Concordant coasts can also be changed by the sea level rising. After the last ice age, ice melted and the sea level rose. Low valleys running parallel to the coast were flooded. The ridges between them were left as islands.

Example: the Dalmatian coast of Croatia. Ridges and valleys run parallel to the coast. The flooded valleys became channels of sea, and the ridges became long, thin islands that run in the same direction as the coast. The result is a drowned concordant coast with many long islands.

Comparing the two

Feature	Discordant	Concordant
Rock bands	At right angles to the coast	Parallel to the coast
Shape of coast	Headlands and bays in turn	Fairly straight, with coves where breached
Erosion rate	Fast on soft rock, slow on hard rock, at the same time	Slow on the hard barrier, then fast on the soft rock once it is breached
Examples	Swanage Bay	Lulworth Cove, Dalmatian coast

Recognising them on a map or photo

• Discordant: look for a coastline that goes in and out again and again. Headlands and bays alternate. On a geology map, the colour bands cross the coast.
• Concordant: look for a smooth coast with a narrow gap leading into a round bay, or long islands lying side by side along the coast. On a geology map, the bands run along the coast.

Exam-style question

Fig. 1.1 is a made-up sketch map of two coasts. Coast A has bands of hard and soft rock at right angles to the sea, with three headlands and three bays. Coast B has a band of hard limestone parallel to the sea, with a small round bay where the limestone has a gap.

(a) Identify which coast is a concordant coastline. [1 mark]

(b) Describe the shape of the coastline at Coast A. [2 marks]

(c) Explain why Coast A has headlands and bays. [3 marks]

(d) Explain how the small round bay on Coast B formed. [6 marks]