Geology
The arch has formed on a concordant coastline where bands of rock run parallel to the shoreline. The rock strata are almost vertical, and the bands of rock are quite narrow. Originally a band of resistant Portland limestone ran along the shore, the same band appears one mile along the coast forming the narrow entrance to Lulworth Cove. Behind this is a 120-metre (390 ft) band of weaker, easily eroded rocks, and behind this is a stronger and much thicker band of chalk, which forms the Purbeck Hills.
The limestone and chalk are in closer proximity at Durdle Door than at Swanage, 10 miles (16 km) to the east, where the distance is over 2 miles (3 km). There are at least three reasons for this. Firstly, the beds are highly inclined (whilst more gently inclined at Swanage). Secondly, some of the beds have been cut out by faulting; and thirdly, the area around Durdle Door seems unusually shallow, so thinner bands of sediments were deposited. At Durdle Bay nearly all of the limestone has been removed by sea erosion, whilst the remainder forms a small headland. Erosion at the western end of the limestone band has resulted in the arch formation. UNESCO teams have been working on saving both the arch and adjacent beach.
The 120-metre (390 ft) 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 of 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.
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