Durdle Door is a natural sea arch created by wave erosion in the limestone cliffs along the Dorset coast of southwest England, near West Lulworth. The arch has formed on a concordant coastline where bands of rock possessing contrasting hardness run parallel to the shore. The rocks here are of Jurassic Age, originally consisting of a band of resistant Portland limestone that has been drastically tilted upwards. With a rock strata bedding that is nearly vertical, weaker, easily eroded limestone beds are selectively coupled with stronger, much thicker bands of chalk. This geologic feature has allowed mechanical weathering through wave attack to specifically remove the weaker rock layers, creating this spectacular arch.

The arch is actually on private property as part of the Lulworth Estate, but is currently open to the public. Around this part of the coast nearly all of the limestone has been removed by sea erosion, with the remainder forming the small headland that includes the arch. UNESCO teams continually monitor the condition of both the arch and adjacent beach. The name Durdle is derived from the Old English 'thirl' meaning bore or drill. (Photograph taken 30 September 2015 by Gary Griggs, University of California, Santa Cruz [UCSC], Santa Cruz, California, U.S.A.)