arch

The west door of Notre Dame Cathedral, Paris, France is a fine example of Early Gothic architecture (begun 1160). The period saw the introduction of Islamic influence in Christian church architecture, such as the ogival (pointed) arch.

A Gothic arch and a Roman arch.

In masonry, a curved structure that supports the weight of material over an open space, as in a bridge or doorway. The first arches consisted of several wedge-shaped stones supported by their mutual pressure. The term is also applied to any curved structure that is an arch in form only, such as the Arc de Triomphe, Paris, 1806–36.

The Romans are credited with engineering the earliest round keystone arches, used for aqueducts. Other forms of arch include the pointed arch, the corbelled arch of the Maya Indians, the medieval lancet and ogee arches, and the Islamic horseshoe arch.

History The principle of the arch was known to the ancient Egyptians, Mesopotamians, and Greeks but arches were seldom used until Roman times, when the semicircular arch was universally adopted. This form continued in fashion during the phases of architecture known as Romanesque in Western Europe, and Byzantine in Eastern Europe. Pointed arches, first used in India about the 6th century BC, were employed in Mesopotamia and were adopted by early Muslim architects; they were introduced into England and France, probably by returning crusaders, at the end of the 11th century. In Muslim countries, mainly south of the Mediterranean, the horseshoe arch, sometimes round and sometimes pointed, was popular. In Renaissance architecture, throughout Western Europe, round arches replaced the pointed type.

Terms and parts The principal terms connected with any form of arch are the span, rise, and springing line; and the various parts: abutment, archivolt (moulding around the face or underside of an arch), crown, extrados, impost, intrados, keystone, spandrel, springer, and voussoir.

Types The principal types of arch, according to their shape, are semicircular or ‘round’, segmental, stilted, round horseshoe, equilateral (pointed, with the radii equal to the span), lancet (pointed, with radii larger than the span), four-centred (pointed, with four arcs), pointed horseshoe, three-centred, cusped, and elliptical.

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.