Durdle Door is a natural limestone arch on the Jurassic Coast near Lulworth in Dorset, England. It is privately owned by the Welds, a family who owns 12,000 acres (50 km2) in Dorset in the name of the Lulworth Estate. It is open to the public. The name Durdle is derived from the Old English ‘thirl’ meaning bore or drill.
The form of the coastline around Durdle Door is controlled by its geology—both by the contrasting hardnesses of the rocks, and by the local patterns of faults and folds. 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 that 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. These steeply dipping rocks are part of the geological structure known as the Lulworth crumple, itself part of a broader monocline (a kinked type of geological fold) produced by the building of the Alps during the mid-Cenozoic.
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). Around this part of the coast nearly all of the limestone has been removed by sea erosion, whilst the remainder forms the small headland which includes the arch. Erosion at the western end of the limestone band has resulted in the arch formation. UNESCO teams monitor the condition of 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, a narrow and compressed band of Cretaceous Wealden clays and sands, and then narrow bands of greensand and sandstone.
In Man O’ War Bay, the small bay immediately east of Durdle Door, the band of Portland and Purbeck limestone has not been entirely eroded away, and is visible above the waves as Man O’War Rocks. Similarly, offshore to the west, the eroded limestone outcrop forms a line of small rocky islets called (from east to west) The Bull, The Blind Cow, The Cow, and The Calf.
As the coastline in this area is generally an eroding landscape, the cliffs are subject to occasional rockfalls and landslides; a particularly large slide occurred just to the east of Durdle Door in April 2013, resulting in destruction of part of the South West Coast Path.