Durdle Door is a wall of limestone standing on its edge on the Dorset coast of southern England, with a doorway the sea punched clean through it about ten thousand years ago. The beds were laid flat on a Jurassic seabed and then tipped almost upright by the same squeeze that raised the Alps. In the arch's landward leg the grain runs up and down instead of side to side. What matters more is which way the wall faces. It runs along the coast rather than into it, so wherever the waves have broken through they meet soft clay and sand behind and spread sideways far faster than they can cut inland. That is why the bays on either side are almost circular. Lulworth Cove, a kilometre and a half east, is the textbook case; Man o' War Bay beside the arch is the same process caught earlier.
The steps down the eastern slope give you the whole scene: the arch, the shingle crescent and the white cliffs beyond. As the light drops the limestone warms to honey, then flattens into a black silhouette. The beach is steep shingle, and the seabed goes away from under you quickly into clear, pale-green water. When you swim out through the arch the rock hangs right over your head, and on the far side you meet the swell of the open Channel. For about a fortnight either side of the winter solstice the sun comes up far enough south-east to rise inside the doorway. It sits there a quarter of an hour, then clears the rock. None of this is public land. The Weld family bought the estate in 1641 and still own the arch, the beach and the cliffs, and in four centuries they have never closed the coast. Almost everyone turns back here, so the path west over Swyre Head empties within minutes. It drops to Bat's Head, where the sea is opening a small hole through a chalk headland — the next door, which will take tens of thousands of years, by which time this one will probably have fallen.