A natural arch or natural bridge is a natural geological formation where a rock arch forms, with an opening underneath. Most natural arches form as a narrow ridge, 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. By contrast, the Dictionary of Geological Terms defines a natural bridge as a "natural arch that spans a valley of erosion." 
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.
- Deep cracks penetrate into a sandstone layer.
- Erosion wears away exposed rock layers and enlarges the surface cracks, isolating narrow sandstone walls, or fins.
- Alternating frosts and thawing cause crumbling and flaking of the porous sandstone and eventually cut through some of the fins.
- The resulting holes become enlarged to arch proportions by rockfalls and weathering. Arches eventually collapse, leaving only buttresses that in time will erode.
- Many of these arches are found within Arches National Park and Rainbow Bridge National Monument in Utah.
Some natural bridges may look like arches, but they form in the path of streams that wear away and penetrate the rock. Pothole arches form by chemical weathering as water collects in natural depressions and eventually cuts through to the layer below.
Natural Bridges National Monument in Utah is another area to view several natural bridges.
Like all rock formations, natural bridges are subject to continued erosion, and will eventually collapse and disappear. One example of this was the double-arched Victorian coastal rock formation, London Bridge, which lost an arch after storms increased erosion.
Arches as highways
In a few places in the world, natural arches are truly natural bridges because there are roads running across them. Two such arches are found in Kentucky. One, a cave erosion arch made of limestone is located in Carter Caves State Park and it has a paved road on top. Another, a weather-eroded sandstone arch with a dirt road on top is located on the edge of Natural Bridge State Resort Park in Kentucky. It is called White's Branch Arch (also known as the Narrows) and the road going over it is usually referred to as the Narrows Road.
The third one is found in Ponoarele village, in Romania. It is 60 m long, 13 m wide, features a stone arch 4 m thick, 20 m high, with a 9 m span. It is called God's Bridge (Podul lui Dumnezeu) and it is the only one in the world effectively used for traffic.
Pont d'Arc arch in Southern France.
Natural Bridge at Bryce Canyon National Park, Utah, United States.
Notable natural arches
- Anacapa Island, Channel Islands National Park, California, USA
- Arch Creek Historic and Archeological Site, Florida, USA
- Arches National Park, Utah, USA
- Ayres Natural Bridge State Park, Wyoming, USA
- Azure Window, Gozo, Malta
- Bryce Canyon National Park, Utah, USA
- Castell de Castells, Spain
- Creelsboro Natural Bridge, Kentucky, USA
- El Arco de Cabo San Lucas, Mexico
- Engetsu-tō, Shirahama, Wakayama, Japan
- Étretat, France
- God's bridge, Ponoarele, Mehedinţi, Romania
- Goat Rock Beach, California, USA
- Grosvenor Arch, Utah, USA
- Kolob Arch, Utah, USA
- Llanrhaeadr-ym-Mochnant, Wales, UK
- Natural Bridge, Alabama, USA
- Natural Bridge Caverns, Texas, USA
- Natural Bridge State Park, Kentucky, USA
- Natural Bridge State Park, Massachusetts, USA
- Natural Bridge, Virginia, USA
- Natural Bridges National Monument, Utah, USA
- Natural Bridges State Beach, California, USA
- Natural Arch, Tirumala hills - Tirumala, India
- Hazarchishma Natural Bridge, Bamiyan, Afghanistan
- Parque Nacional de Sete Cidades, Brazil
- Percé Rock, Quebec, Canada
- Pont d'Arc, France
- Pravčická brána, Czech Republic
- Punarjani Guha - natural tunnel, India
- Rainbow Cave (actually an arch) in the Galilee
- Jebel Kharaz (Jordan)
- Rattlesnake Canyon, Colorado, USA
- Rock Bridge of Gulanchwadi, Narayangaon Maharashtra, India
- Senkanmatsu-shima, Iwami, Tottori, Japan
- Sewanee Natural Bridge, Tennessee, USA
- Shipton's Arch, Xinjiang, China
- Springbrook National Park, Queensland, Australia
- Tassili n'Ajjer - National Park in Algeria with many arches
- Tonto Natural Bridge, Arizona, USA
- Tukuyu natural bridge, Tanzania
- ^ Natural Arch and Bridge Society, FAQ.
- ^ American Geological Institute, Dictionary of Geological Terms, 1976, Doubleday Anchor
- ^ http://www.nature.nps.gov/geology/parks/arch/
- ^ Trek Earth
- ^ Offbeat Tracks in Maharashtra - A Travel Guide - Book by Milind Gunaji ISBN 81-7154-669-2
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Look at other dictionaries:
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