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natural arch
A natural arch, natural bridge or, less commonly, a rock arch is a natural rock formation where a rock arch forms, with an opening underneath. Most natural arches form as a narrow bridge, walled by cliffs, become narrower from erosion, with a softer rock stratum under the cliff-forming stratum gradually eroding out until the rock shelters thus formed meet underneath the ridge, thus forming the arch. Natural arches commonly form where cliffs are subject to erosion from the sea, rivers or weathering (subaerial processes); the processes "find" weaknesses in rocks and work on them, making them larger until they break through.
The choice between ''bridge'' and ''arch'' is somewhat arbitrary. The Natural Arch and Bridge Society identifies a bridge as a subtype of arch that is primarily water-formed.〔(Natural Arch and Bridge Society ), FAQ.〕 By contrast, the ''Dictionary of Geological Terms'' defines a natural bridge as a "natural arch that spans a valley of erosion."〔American Geological Institute, ''Dictionary of Geological Terms'', 1976, Doubleday Anchor〕
The largest natural arch, by a significant margin, is the Xianren Bridge in China, with a span of .〔(Big 14 Tour - Fairy Bridge ), The Natural Arch and Bridge Society〕
==Coastline==
On coasts two different types of arches can form depending on the geology. On discordant coastlines rock types run at 90° to the coast. Wave refraction concentrates the wave energy on the headland, and an arch forms when caves break through the headland, e.g., London Bridge in (Victoria, Australia). When these eventually collapse, they form stacks and stumps. On concordant coastlines rock types run parallel to the coastline, with weak rock (such as shale) protected by stronger rock (such as limestone) the wave action breaks through the strong rock and then erodes the weak rock very quickly. Good examples of this are at Durdle Door and Stair Hole near Lulworth Cove on the Dorset Jurassic Coast in south England, although these are on an area of concordant coastline. When Stair Hole eventually collapses, it will form a cove.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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