The Needles' pointed shape is a result of their unusual geology. The strata have been so heavily folded during the Alpine Orogeny that the chalk is near vertical. This chalk outcrop runs through the centre of the Island from Culver Cliff in the east to the Needles in the west, and then continues under the sea to the Isle of Purbeck, forming Ballard Cliff (near Swanage), Lulworth Cove and Durdle Door. At Old Harry Rocks (east of Studland and north of Swanage) these strata lines moving from horizontal to near vertical can be seen from the sea.