A coastline that is always moving
The tall cliffs along the Cornish coast look permanent, and in human terms they often are. Yet they are slowly being worn back, and sometimes parts of them fall suddenly. Erosion is a natural process that has shaped the coast for thousands of years, and it continues today. Understanding it explains why warning signs appear on so many cliff paths.
What makes a cliff
A cliff forms where land meets the sea at a steep angle. Waves attack the base, and the rock above is left unsupported. How quickly this happens depends on several factors: the type of rock, how it is layered and cracked, how exposed the coast is to waves, and the weather.
Cornwall has a varied geology. Hard rocks such as granite generally resist erosion well and often form bold headlands. Layered and folded sedimentary rocks, including slates and shales, are more varied: some layers are tough, others break easily, and the direction in which the layers tilt can help or hinder stability. Coastlines with softer rocks or loose material, such as clay and old glacial or wind-blown deposits, can erode much faster.
This is why some cliffs seem barely to change in a lifetime, while others have clear signs of recent loss.
How cliffs retreat
Erosion is not a single process. Several work together.
Wave attack at the base. Waves hit the foot of the cliff with great force. They hurl sand and pebbles at it and force water and air into cracks. Over time this undercuts the cliff, forming a notch. Caves can form where the rock is weaker, as described in the article on coves and caves.
Weathering from above. Rain, frost, wind and salt spray break the rock down. Water soaks into cracks and, in cold spells, can freeze and expand, although frost is less frequent in Cornwall’s mild climate than inland. Plant roots can also pry into joints.
Groundwater and rainfall. Water moving through the cliff adds weight and reduces the strength of soft material. After heavy or prolonged rain, cliffs are more likely to fail. Many falls occur in or after wet weather.
Gravity. Once the base is undercut and the rock is weakened, gravity does the rest. A section may fall away as a rockfall, slide as a landslip, or slump downward in a rotating movement, especially in softer material.
Storms. Big waves at high tide, particularly during storms, deliver the most energy. A single winter can cause more retreat than many calm years combined, although the effect is uneven and depends on the cliff.
Types of cliff failure
- Rockfalls, where blocks or slabs detach from a steep face.
- Landslips and slumps, where a mass of earth or loose rock moves downslope, often leaving a stepped or hummocky surface.
- Mudflows, in wet weather, where saturated soil flows over a cliff.
- Collapse of undercut sections, including cave roofs, arches and overhangs.
Most are unpredictable. The cliff top may look sound while the rock below the surface is cracked or hollowed out.
How fast do cliffs retreat
There is no single answer. Rates vary hugely from place to place, from almost nothing on resistant headlands to a more noticeable loss in softer sections. Anyone claiming a precise figure for the whole coast should be treated with caution. Local coastal managers monitor certain stretches, and any published information about a specific site is the best guide.
The material removed does not vanish. It is broken up, carried along the shore and may become part of beaches or dunes. In that sense cliff erosion feeds some of the sand and pebbles described in the guide to Cornish beaches.
What erosion has left behind
Retreating cliffs leave visible traces: wave-cut platforms at the cliff foot, offshore rocks and stacks, and in some places old paths or field boundaries cut off by the edge. The coast path itself is sometimes rerouted inland when a section of cliff is lost, and clifftop features such as Cornish hedges or old buildings can end up close to the edge as the land retreats.
Why edges should be avoided
For visitors, the key message is simple: cliffs are not as stable as they look.
- Edges can be undercut. The ground at the top may overhang a hollow below. Grass can hide a crack or an unstable lip.
- Failure can come without warning. A section may collapse under a person’s weight, or fall by itself.
- Weather increases the risk. Wet ground, wind and gusts, and poor visibility all make a fall more likely.
- The base is dangerous too. Rockfalls land on the beach below. Sitting or sheltering under a cliff is risky, especially after rain, and cliff-foot paths can be cut off by the tide.
- Slippery ground and steep slopes. Dew, rain and loose gravel make footing uncertain.
Sensible habits include:
- Stay on the path and well back from the edge.
- Obey fencing, signs and diversions, which often mark known problems.
- Keep children and dogs close, especially near drops.
- Avoid walking too near the edge in poor light or fog.
- Do not climb on cliff faces or go below overhangs, and do not dig into cliffs or collect fossils from unstable faces.
The guide to walking the coast path responsibly and the wider overview of coastal safety build on these points.
Cliffs and wildlife
Cliffs also support life. Ledges are used by coastal seabirds for nesting, and cliff-top vegetation, including coastal heathland, depends on the thin soil and salt winds. Erosion creates fresh bare ground that some plants and insects use. Keeping to paths protects these habitats as well as people.
A moving edge
Erosion is a natural process, slow in most places and abrupt in some. Appreciating the cliffs from a respectful distance lets visitors enjoy them while respecting the fact that they are changing.