arch

Coastal arch at Durdle Door, on the Dorset coast, England. Beds of resistant limestone form the Durdle promontory. Weaknesses in the limestone, such as faults and joints, have been exploited by forces of erosion and mass wasting (downslope movements of loose materials such as sand). The sea has eroded the limestone to form first a cave, and finally an arch, as the cave reaches through to the far side of the Durdle promontory.

In geomorphology (the study of landforms), any natural bridge-like land feature formed by erosion. Most sea arches are formed from the wave erosion of a headland where the backs of two caves have met and broken through. The roof of the arch eventually collapses to leave part of the headland isolated in the sea as a stack. A natural bridge is formed on land by wind or water erosion and spans a valley or ravine.

In some cases, as at Stair Hole, Dorset, England, an arch is formed when the sea has battered a hole through a cliff of hard rock (such as limestone) and has removed soft rock (such as clay) behind.