The coast as a sculpture
Many of the most striking shapes on the Cornish coast were not built but carved. Waves, weather and time work on the rock, and where it is weakest they cut deepest. The sequence from cove to cave to arch to stack is one of the clearest examples of how a coastline changes, although the process takes a very long time and rarely follows a neat order.
How a cove forms
A cove is a small, sheltered bay, often with a narrow entrance and a wider interior. Typically a band of weaker rock lies behind a tougher outer barrier. Once waves break through a gap in the barrier, they can spread out inside and wear away the softer material faster than the headlands on either side. Over time this leaves a rounded hollow with a beach at its back, flanked by headlands of harder rock.
Streams reaching the shore also play a part. A small valley that ends at the sea often gives a ready-made notch for the waves to enlarge, which is why so many coves sit at the foot of a steep, narrow valley.
How waves carve a cave
Sea caves usually start with a weakness: a crack, a joint, a fault or a band of softer rock. Waves strike the base of a cliff again and again. Water and air are forced into the crack under pressure with each wave, and this repeated force slowly widens it. Sand and pebbles thrown against the rock add an abrasive effect, grinding the surface like sandpaper. Salt and wetting and drying cycles also help break the rock down.
Over many years, the crack becomes a notch, then a hollow, then a cave. Where the roof of a cave has a vertical weakness reaching the surface, waves may force air and spray upward and form a blowhole. Blowholes can be loud and spectacular in heavy seas, but the ground around them can be unstable.
Caves are more common where rock is jointed and fractured, which is why they cluster in some stretches of coast and are rare in others. The type of rock in a particular place, whether granite, slate or something else, strongly affects what forms.
From cave to arch
If a cave forms on both sides of a narrow headland, the two can eventually meet. The result is an arch: a bridge of rock with the sea running through. Arches are temporary features in geological terms. The roof is gradually undercut and loaded with its own weight, and cracks in it widen through weathering as well as wave action.
From arch to stack
When the roof of an arch finally falls, the outer part of the headland is left standing alone as a sea stack, a column or pillar of rock separated from the cliff. Stacks are often the last survivors of a former headland. Waves keep working on them, and each eventually reduces to a stump, then a low rock visible only at low tide, then nothing.
This is also why the coast has many offshore rocks and islets. They are fragments of land that once joined the mainland. Their story links to the wider pattern of cliffs retreating, which is covered in the article on cliff erosion.
What this means for the shape of the coast
Because hard rock lasts longer than soft rock, a stretch of coast with mixed geology develops a ragged outline of headlands, coves and islets. This unevenness is part of what makes the Cornish coast so varied, and part of why parts of the county have been regarded as a hazardous coastline by sailors, a subject treated separately.
Other features worth knowing
- Wave-cut platforms are flat shelves of rock at the foot of a cliff, exposed at low tide. They form as the cliff retreats and are often where rock pools collect.
- Geos are narrow, steep-sided inlets, sometimes formed when a cave roof collapses, and sometimes by waves exploiting a line of weakness.
- Raised beaches and other signs of former sea levels sometimes appear above the present shoreline, showing that the relationship between land and sea has changed over long periods.
Staying safe around caves and coves
Caves and rocky coves look inviting but carry real risks, and it is wise to treat them with respect.
- Tides. Many caves and coves can be cut off as the tide rises. Check tide times before going, and leave plenty of time to return.
- Swell. Waves can surge into caves unexpectedly, even on calm-looking days. A larger set of waves may arrive after a long lull.
- Falling rock. Cave roofs, arches and the cliffs above can shed stones or larger pieces without warning.
- Slippery rock. Wet seaweed and algae make footing treacherous.
- Poor communication. Mobile signal can be weak or absent in coves and caves.
Stay out of caves unless you are certain of the conditions, and never climb on arches or stacks. Do not swim into a cave. If you want a closer look, view features from the beach or the coast path in good light and at low tide, and heed any local warning signs. Organized, guided coasteering and kayaking trips exist, but this guide cannot assess any particular provider, so check qualifications and conditions for yourself.
Reading the shapes
Next time you look at an isolated rock offshore or a hole in a headland, you are seeing a snapshot in a long process. Sea caves, arches and stacks are all stages in the slow retreat of the coast, and they will look different to those who visit in future.