Coasts are made of rocks of different strengths. Waves wear the weaker ones back into curves and leave the tougher ones standing out as headlands. Dorset's coast shows it all within a few miles.
AI-generated image, not a photograph of a real person or place.
READ THE ROCK behind the beach
Weak rock becomes bay
Strong rock becomes headland
Waves bend to the points
One coast, many strengths
THE SHORT ANSWER
Bays are carved from the weak spots.
Where rock strength changes along a coast, the sea finds the difference.
ROCK
Weak rock wears back
At Swanage, the Geologists' Association notes that stronger rocks resist erosion while weaker ones are more easily eroded, giving a headland and bay coastline.
Lulworth Cove in Dorset cuts through four bands of rock. Pick one to see how it behaves. The bar shows how strongly it resists erosion compared with the others.
Based on the published geology of Lulworth Cove. Relative strengths only: the same rock can behave differently on another coast.
TWO NUMBERS WORTH KNOWING
A thin wall, a wide hollow.
One figure on how a cove opens up, one on when a bay becomes national water.
300–500 m
Soft clays and sands behind the limestone
At Lulworth, a band of Portland limestone under 30 metres wide guards 300 to 500 metres of much weaker clays and greensands. Once breached, the soft rock was scooped out.
LULWORTH COVE · GEOLOGY
24 nautical miles
Widest mouth that can be closed off
If the natural entrance of a bay is no wider than 24 nautical miles, a closing line can be drawn across it and the water inside counts as the coastal state's internal waters.
BAYS · UN LAW OF THE SEA
UNESCO lists the Dorset and East Devon Coast, which includes Lulworth Cove, as a World Heritage Site: 155 km of coast recording about 185 million years of Earth's history and classic coastal landforms.
FOUR THINGS TO LOOK FOR
Read the coast from the clifftop.
You do not need a geology degree. On your next coastal walk, these clues show why the shoreline curves where it does.
Find the headlands
Points that stick out are usually made of tougher rock. At Swanage, the chalk of Ballard Down forms the headland with Old Harry Rocks.
Look at the bay's back wall
Bays sit where weaker rock, such as clay or sand, has been worn back. These soft cliffs often slump after wet winters.
Watch how the waves arrive
Waves bend as they reach shallow water, wrapping towards the points and spreading out, weaker, into the bay.
Spot a cove in the making
Next to Lulworth Cove, Stair Hole is a small gap in the limestone where the sea has just begun to reach the clay.
AI-generated image, not a photograph of a real person or place.
STACKS, ARCHES AND SLIPS
Headlands do not last for ever.
AI-generated image, not a photograph of a real person or place.
The wave energy that spares the bays keeps working on the points. Old Harry Rocks, at the tip of a chalk headland in Dorset, are stacks the sea has cut off from the land.
Bays change too. In Swanage Bay, landslides in the weak cliffs follow wet winters, and sea walls and drainage now protect parts of the town while other stretches are left to erode naturally.
Questions
Questions people ask
What is the difference between a bay and a headland?
A bay is a curved inlet where the sea reaches into the land. A headland is the high point of land that sticks out into the sea beside it. They often form as a pair, where weak and strong rocks meet the coast.
Why are bays usually sandy?
Waves bend towards the shallower water off headlands, so most of their energy is spent on the points. Inside the bay the waves are weaker, so sand and shingle tend to settle rather than be carried away.
How did Lulworth Cove get its round shape?
The sea broke through a narrow band of hard Portland limestone and then wore away the much softer clays and sands behind it. Waves squeezing through the narrow entrance bend into arcs, which shapes the curve.
Why do cliffs in a bay slip?
The Geologists' Association lists four controls on erosion at Swanage: the geology, gravity, water and wave attack. A 2013 landslide in Swanage Bay came after a wet winter, and the sea was not the cause because a sea wall protects that stretch.
What counts as a bay in international law?
Under Article 10 of the UN Convention on the Law of the Sea, an indentation is a bay only if its area is at least that of a half-circle drawn across its mouth. If the mouth is 24 nautical miles or less, the water inside can be internal waters.
