Durdle Door is a natural limestone arch on the Jurassic Coast near Lulworth in Dorset, England. While open to the public, it is privately owned by the Welds, a family who own 12,000 acres (50 sq. km) in Dorset in the name of the Lulworth Estate. The name Durdle is derived from the Old English ‘thirl’ meaning bore or drill.

According to jurssiccoast.org, the natural arch of Durdle Door formed due to the effect of the erosive power of the sea on the vertical layers of different types of rock. At some point in the past the sea would have begun to breach the hard Portland Limestone and form a string of caves along the coast.

The much softer rocks behind would have quickly been eroded away creating caves and natural arches. Eventually the arches collapsed leaving stacks, which would in turn be broken and washed away by the power of the waves. Durdle Door is part of only a small strip of hard Portland limestone that is left. The remnants of old arches can still be seen in the form of ‘stumps’ of limestone only just visible in the waves. One day that is all that will remain of Durdle Door as well. [Source: jurassiccoast.org]