Most of us walk past the clearest evidence of how the British coast is changing without noticing it. A wet shelf of rock at low tide, a pillar of chalk standing offshore, a gap in a headland where a cave used to be — all of these are readable. Learn the shapes and you can work out what the sea has already taken, and roughly where it will go next.
This is a field guide to two of the most distinctive results of coastal erosion: wave-cut platforms and sea stacks. No geology training needed. Boots, a tide table and a bit of patience will do.
Four things do most of the work at the foot of a cliff. Hydraulic action is the sheer force of water compressing air into cracks, which prises rock apart. Abrasion is sand and pebbles acting as grinding tools against the rock face. Attrition is those same pebbles wearing each other down into smaller, rounder shapes. Solution is chemistry: chalk and limestone slowly dissolve in slightly acidic seawater.
Waves rarely work alone. Rain, frost and groundwater weaken the cliff from above, and mass movement — slumps, falls, mudflows — delivers material to the shore where the sea can cart it away. Cliffs that look purely wave-carved are usually the product of weather and waves taking turns.
Because rock resistance varies hugely, harder bands survive as headlands while softer ones are hollowed into bays. That is why the British coastline is so ragged: it is a jumble of different rocks meeting the same sea in different moods.
A wave-cut platform is not built by the sea; it is what is left behind. As a cliff retreats, the abrasion at its base planes off the rock into a gentle shelf that extends seawards. The cliff and the platform are two halves of the same story.
Go at low tide, ideally on a spring tide when the widest area of rock is exposed.
Width matters. A wide platform means the cliff has retreated a long way, and it also protects that cliff, because waves break and lose energy as they cross it. A tall cliff with almost no platform beneath it is a different situation entirely — the sea is hitting solid rock directly, and retreat is likely to be quicker. Reading the width of the shelf is often more useful than any figure you will find online.
Good examples to study include the chalk platforms around Flamborough Head in East Yorkshire, the limestone shelves of the Gower and Pembrokeshire, and the broad rocky foreshores along the Antrim coast.
A sea stack is not a separate landform. It is the fourth act of a sequence that begins on a headland. Once you know the order, you can spot the early stages before the dramatic ones appear.
You can often see several stages side by side on a single headland. Old Harry Rocks near Studland in Dorset, The Needles on the Isle of Wight, and the Old Man of Hoy in Orkney — one of Britain's tallest, at well over 100 metres — are all chalk or sandstone stacks at different points in their lives. Durdle Door in Dorset and the Green Bridge of Wales in Pembrokeshire show the arch stage; Bow Fiddle Rock in Moray is a quartzite arch doing the same job on a much smaller scale.
Two related features are worth knowing. A geo is a narrow inlet cut inland along a fault or joint. A blowhole is a vertical shaft connecting a cave roof to the cliff top, through which spray bursts during heavy seas.
Chalk and limestone are soluble and well jointed, which is why they produce clean cliffs, neat arches and stacks. Granite, slate and gneiss are far more resistant, and erosion there follows joint patterns at a pace that can seem almost static in a human lifetime. Glacial till, such as the soft cliffs of Holderness in East Yorkshire, has no strong structure at all: it slumps and flows rather than falling in blocks, and those cliffs are among the most rapidly eroding stretches in Europe.
Scotland adds a twist. Parts of the land are still rebounding after the weight of the last ice sheet was removed, so old shorelines and platforms now sit stranded above the present sea level. If a wave-cut platform appears well above the water, you may be looking at a raised beach rather than a modern one.
The practical lesson: never compare erosion rates between two bays without checking what they are made of. Two cliffs a mile apart can behave completely differently.
Photo: Alexandra_Koch / Pixabay
