Durdle Door (sometimes spelled Durdle Dor) is a natural limestone arch on the Jurassic Coast near Lulworth in Dorset, England.

Geology

The arch has formed on a concordant coastline where bands of rock run parallel to the shoreline. Here the rock strata are nearly vertical, and the bands of rock are quite narrow.

Originally a band of resistant Portland limestone ran along the shore, the same band which can be seen two miles down the coast forming the narrow entrance to Lulworth Cove. Behind this is a 400-foot (120 m) band of various weaker rocks which are easily eroded, and behind this is a stronger and much thicker band of chalk, which forms the Purbeck Hills. The limestone and chalk are much closer together here than at Swanage, 10 miles (16 km) to the east, where the distance between them is over 2 miles (3 km). There are at least three reasons for this. First, the beds are highly inclined here, and more gently angled at Swanage. Secondly, some of the beds have been cut out by faulting at Durdle Door; and thirdly, the area around Durdle Door appears to have been unusually shallow, so a much thinner sequence of sediments were deposited here.

At Durdle Bay all except a short stretch of the limestone has been completely eroded away by the sea and the remainder forms a small headland where it has protected the clay behind. At the western end this band of limestone has been eroded through, creating the natural arch.

The 400-foot (120 m) isthmus which joins the limestone to the chalk is made of a 50-metre (160 ft) band of Portland limestone, which is less resistant than the Purbeck beds, a narrow and compressed band of Cretaceous clays—Wealden Clay, sands and chert beds—and then narrow bands of Greensand and sandstone. In Man o' War bay, the small bay immediately east of Durdle Door the Portland stone has not been entirely eroded away, and at low tide the band of Portland stone is partially revealed.

[Source: wikipedia]

Worth: 6 points

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