Caves, Arches & Stacks

Waves will widen weakness or cracks in the rock face by the processes of hydraulic action, wave pounding, abrasion and solution. Gradually these cracks get larger, developing into small caves.

Further erosion widens the cave and where the fault lines runs through the headland, two caves will eventually erode into the back of each other forming an arch, passing right through the headland. Alternatively waves from the seaward side continue to erode the back of the cave until the headland is broken through.
A combination of wave attack at the base of the arch, and weathering of the roof of the arch (by frost, wind and rain), weakens the structure until eventually the roof of the arch collapses inwards leaving a stack; a column of rock which stands separate from the rest of the headland.
		The stack will continue to erode, eventually collapsing to form a stump which will be covered by water at high tide.

The unusual animation below shows the formation of caves, arches stacks and stumps using modeling clay!

The south west coast is littered with caves where the sea has worn away weaker rocks.

A well known coastal arch can be found just outside the boundary of the South West group, at Lulworth in Dorset. Durdle door was formed as a result of the softer rocks being eroded away behind the hard limestones, allowing the sea to punch through them. The name Durdle is derived from an Old English word 'thirl' meaning bore or drill. Eventually the arch will collapse to leave a sea stack.

Is this the past and future of Durdle Door?

Some of the best examples of sea stacks in the British Isles can be found in Ladram Bay in East Devon. The stacks are the remains of caves and arches that have collapsed, leaving behind the stumps. The rock is Otter Sandstone. The cliff in the background is High Peak, and the central layer of stone is Sidmouth Mudstone. The top of this layer is where the trees begin.

Photo © Copyright Robin Lucas and licensed for reuse under this

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