Stair Holewest of Lulworth Cove in Dorset, southern England | Geology of the world and the Environment

Stair Holewest of Lulworth Cove in Dorset, southern England

Stair Hole is the big hole in the spine of hard Portland limestone that faces out to sea. It's been formed by sea erosion of a weakness in the rock, which is slowly, slowly producing a circular sea cove behind it that will one day, in the distant future look like Lulworth Cove, just over the other side of the headland. In fact it will probably merge with Lulworth Cove as a result of continued coastal erosion.

The rocks here are sedimentary, and, as you might expect, they once lay in nice, neat, horizontal layers made up of hard limestone alternating with soft mudstone.

The Crumple was produced when the tectonic plates carrying Europe and Africa crashed into one another about 65 million years ago. The force of the impact folded up parts of the land to form the Alps and the Pyrenees after a fashion that was not totally dissimilar to the way in which a car hood - or bonnet, as we like to call them in England - will bend in a head-on collision.

Shock waves spread out across the plates, like ripples in a pond, pushing up smaller hills like the chalk ridges in Purbeck. As the layers of rock at the Crumple were pushed against each other under the impact of those shock waves they bent so that some of the originally horizontal layers are now almost vertical, but the different types of rock performed differently under the huge pressures to which they were subjected. The hard limestone bent and cracked, but remained largely intact. Gravity caused the soft mudstone to slide between the harder layers of limestone to fill any spaces produced where it had bent. In this way the amazing folds that are the Lulworth Crumple were formed.

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#sedimentary #alternating #layers #horizontal_layers
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