Durdle Door - Lulworth, Dorset, UK - Natural Arches on Waymarking.com

'Durdle Door is one of the most photographed landmarks along the Jurassic Coast. This rock arch in the sea was formed as a result of the softer rocks being eroded away behind the hard limestones, allowing the sea to punch through them. Eventually the arch will collapse to leave a sea stack such as those that can be seen at Ladram Bay in East Devon.

Below the cliffs lies a sweeping beach that was once three separate coves. This popular beach has no facilities although there are public toilets at Durdle Door Holiday Park. In summer, a mobile kiosk on the path leading to Durdle Door provides ice creams and refreshments.

Access and car parking is via Durdle Door Holiday Park, where a short walk and steep steps lead to the beach.'

Ian West has some detailed information:visit link

'The natural arch of Durdle Door has been cut by the sea through the almost vertical Purbeck Caps and the top of the Portland Freestone. From the landward side, as seen here, the Portland Stone is hardly visible and the surfaces visible here are mainly those of the Purbeck stromatolitic limestones (the Soft Cap), like those of the Fossil Forest. Holes left by late Jurassic trees are visible. The soft Wealden strata, thin here partly because of strike faulting, have been eroded away on this side except for a narrow connecting peninsula protected to some extent by the wall of stone. Originally there was Kimmeridge Clay seaward of Durdle Door but these relatively soft strata have been easily removed by the sea.

North of the Wealden there is Gault and Upper Greensand. The main cliffs to the north are of Chalk, particularly of the Micraster Zones.

Outlying rocks mark the outcrop of the Portland Stone and basal Purbeck limestone.

The natural arch of Durdle Door is one of the physical features of the area which has retained a name given to it probably more than a 1000 years ago (Arkell, 1947), showing that the coast does not change very rapidly. The name " Durdle " is derived from an Old English word " thirl " , meaning to pierce (as in " nostril "). A similar arched rock in south Devon is known as the Thurlestone. At Durlston Bay, Swanage, there was probably an arched rock of which a stack at Durlston Head (of Purbeck Broken Beds with celestite) is most likely to be the relic. Damon (1884) also mentioned another name: " The most singular feature of this is the natural arch, known as the " Barn-door ", formed in the Purbeck Limestone and sufficiently high for a good-sized sailing boat to pass through it. "

The shape of the arch is largely controlled by two sets of joints at oblique angles. You can see these in some of the photographs above. Of particular interest are the structures here. The Purbeck Monocline plunges down to the east and here in the west we are near the very angular foresyncline. In this vicinity there is overturning which is not seen elsewhere and much strike faulting. Although there has been argument about the direction in which the incompetent strata moved here it could, in fact, only have gone up. There is no room beneath and there is an unconformity between the Jurassic and middle Cretaceous strata.'