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Home > Landscape & Geology > Dorset Coast and Geology > Stair Hole Lulworth - the Formation of a cove in progress
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Stair Hole is a small cove situated immediately west of Lulworth Cove. Along much of the coastline the steeply dipping Portland limestone and basal Purbeck limestones form an outer barrier against the sea. However in places such as Stair Hole the sea has breached the outer rocks forming small natur...
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Picture 1 of 15 - St...
Sparsely joined Portland limestone is the most resistant rock along the Purbeck Coast. However wave action exploits weak points forming caves, which eventually become arches as the sea breaches through the rock. A number of processes contribute to this, including: Hydraulic action – a process i...
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Picture 2 of 15 - St...
Over time as these processes continue more caves and arches appear.
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At the western end of Stair hole is a gap. This is a collapsed arch and eventually the other arches will collapse in the same way. The open gap here has caused increased erosion of the shales and limestones of the Purbeck Formation, landward of the beach. In storms the seas directly attack this.
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Picture 4 of 15 - St...
Having breached the Portland stone the waves have access to the Purbeck beds beyond. Here you can see where the sea has been most successful at cutting back and causing collapse of the Purbeck limestones. These are more easily eroded away and much of this rock has been removed at stair hole behind ...
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Picture 5 of 15 - St...
The process of erosion of the Purbeck beds can be seen in the main bay at Lulworth. The beds are greatly jointed having layers of weak clay between harder limestones.
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The wave action and battering from the sea erodes away the weaker clay (through hydraulic action, abrasion/corrasion and corrosion) – leaving the more resistant limestone blocks. Eventually these collapse and fall onto the beach leaving isolated pillars of stacks
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Picture 7 of 15 - St...
The boulders are further broken up by the waves and eroded away to leave smaller rocks and particles.
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Picture 8 of 15 - St...
Beyond, the Purbeck Beds at stair hole lays the Wealden Beds. The mottled clays and sands easily wash away. Slumping of the Wealden strata has occurred and there is the development of mudslides. As the Purbeck beds are further eroded support for the Wealden will be removed this will increase. Onc...
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Picture 9 of 16 - St...
This picture shows the area where the barrier of Purbeck rock has been lowered, opposite the open gap. It has been said that Stair Hole is a " Lulworth Cove in the making " but it is being formed in a different manner. This cove is being formed from caves being eroded through the Portlan...
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The Wealden beds are the most rapidly eroding rock. Rain water drains quickly down through the sandstone until it reaches bands of clay below. Once it reaches this point it forms streams and the sands above literally slip and slide along the clay and crumble into the sea, where they are quickly was...
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After periods of prolonged rain large mud flows and slides can occur in the Wealden Clay. These rocks are also very prone to Human erosion.
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Picture 12 of 16 - S...
Eventually the embryonic cove at Stair Hole will expand as the Wealden Clay is washed away and join with the main Lulworth cove. Leaving the remaining outcrops of Portland Stone as sea stacks.
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As the cove enlarges more of the Greensand and Chalk will be washed away. The Greensand, although better drained than the Wealden beds, is very soft and easily eroded by abrasion/corrasion.
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At the back of the cove the chalk cliffs are more resistant to erosion only collapsing when the pounding of the waves carves out a wave cut notch at the base of the cliff destabilising the material above.
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Once a cliff fall has occured the debris protect the cliff until these have been washed away. The chalk is prone the weathering - frost shattering – after periods of wet and then cold weather. The cliffs are also slowly worn away by corrosion as the limestone reacts with the seawater.
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