Cliffs

When you walk on a cliff top you are walking over hundreds of thousands of years of landscape evolution.

Cliffs can form where a 'fault' in the Earth's crust allows a section of rock to be pushed upwards. They can also form where sea levels have suddenly dropped, due to a cold climate, which results in water being locked up as ice.

The local geology then determines how steep the cliffs will become and how fast they will erode over time from the force of the sea.

Different rock types can form different features dependent on their location, the processes that formed the rock and how long the sea has been trying to break them down.

Sedimentary rocks

Sedimentary rock can be composed of different materials such as sand and mud, which over time have been compressed into rock. Sometimes these rocks can be quite soft, for example:

Burton Bradstock, Dorset

At Burton Bradstock, the thin shelly limestone and clays form crumbling cliffs that erode and slump very easily. Part of the World Heritage Site, the Jurassic Coast, this area of Dorset is also famous for fossils. Further east, at Dancing Ledge, the limestone is harder and was once quarried for building stone.

The White Cliffs of Dover

The famous White Cliffs of Dover are composed of layers of chalk which, though high, are still soft and are regularly eroded by the action of the sea. This process keeps the cliffs the white colour of the chalk.

Limestones

Limestones are a harder type of sedimentary rock composed of microscopic animals that inhabited shallow oceans millions of years ago:

Stackpole, Pembrokeshire

At Stackpole, the limestone has formed long expanses of cliffs with many features of caves, arches and stacks.

Marsden Rocks, Durham Coast

On the Durham Coast another type of limestone, called magnesian limestone, has formed high cliff areas at Marsden Rocks including the stack, which is an important breeding colony for sea birds.

Metamorphic rocks

Metamorphic rocks are produced by the alteration of pre-formed rocks by pressure and temperature, often deep in the Earth’s crust.

Bolt Head, Devon

Bolt Head, Devon
© National Trust

At Bolt Head, the 9km of large cliffs have been formed from schists, thin layers of rock.

The Lizard, Cornwall

On the Lizard, many different types of hard volcanic rock, like granites and basalts, have been turned into gneiss (leaflike layers of rock), schists and serpentine (a multi-coloured rock which is used ornamentally). This change has led to a complex geology in a small area.

Volcanic rocks

Volcanic rocks form where the Earth's crust cracks and large amounts of molten rock or lava can pour through and form new rock. As lava can force its way through the cracks of sedimentary rock, volcanic and sedimentary rocks can sometimes be found very close together.

Giant's Causeway, Co Antrim

The famous Giant's Causeway has formed from the erosion of hexagonal basalt columns made by volcanic activity.

Farne Islands, Northumberland

On the Farne Islands and at Dunstanburgh Whin Sill, an extremely resistant volcanic rock has formed cliffs and low islands, now home to many species of breeding birds.

Caves, arches and stacks

Associated with cliffs are some interesting features that form over time when the sea starts to erode the different rock types.

When under pressure, sedimentary rocks are very prone to crack and form fault lines. The fault lines are weak points, mercilessly exploited by the sea which erodes the cliff to form caves, arches and stacks.

The Llŷn Peninsula, Gwynedd

The 'Whistling Sands' of Porthor on the Llŷn Peninsula, Gwynedd
© NTPL / Joe Cornish

As the sea attacks a fault line the rock is eroded and a cave may form that goes back many metres. Limestone forms many caves as can be found at the coast on the the Llŷn Peninsula.

Sometimes on a headland the lower cliffs have been eroded into caves. Where caves have formed on a cliff with constant erosion the back of the cave can be broken through to the cave on the other side of the headland. In this situation an arch of rock will form. As further material is taken away eventually the arch may collapse completely leaving a pinnacle of rock called a stack.

Old Harry Rocks, Dorset

At Purbeck, the chalk has formed the stack of Old Harry Rocks.

The Needles, the Isle of Wight

On the western end of the Isle of Wight are the famous Needles, which were once part of the chalk on Tennyson Down.

Wave cut platforms

The sea is at its most destructive in the zone where large waves can dash against the cliff, carrying boulders and pebbles in the swash (the ‘up’ the beach part of the breaking wave) which further helps to break down the cliff.

After a long time the cliffs can be eroded to just a low expanse of flat rock called a wave cut platform which is usually exposed at low tide.

Robin Hood's Bay, North Yorkshire

One of the best examples of this feature is to be found in Robin Hood's Bay where the sedimentary rock has eroded to its layers. The wave cut platform forms many rock pools at low tide.

Worms Head near Rhossili, Swansea

Another wave cut platform is found at Worms Head near Rhossili where a fast current races across the flat expanse as the tide comes in.

Wembury Point, Devon

At Wembury, the wave cut platform extends right out to sea to the Mewstone at the mouth of Plymouth Sound.

Old Harry Rocks at Studland on the Corfe Castle Estate, Dorset

The National Trust

Cliffs