A natural arch or natural bridge 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. 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.
Weather-eroded arches 
- 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.
Water-eroded arches 
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.
Cave erosion 
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 additional 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.
Another 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 effectively used for traffic.
Notable natural arches 
- Aloba Arch, Chad
- Bogenfels, Namibia
- Goedehoop natural rock bridge, South Africa
- Tassili n'Ajjer, national park in Algeria with many arches
- Tukuyu natural bridge, Tanzania
- Engetsu-tō, Shirahama, Wakayama, Japan
- Hazarchishma Natural Bridge, Bamyan Province, Afghanistan
- Jabal Umm Fruth Bridge, Jordan
- Jebel Kharaz, Wadi Rum, Jordan
- Natural Arch, Tirumala hills, Tirumala, India
- Paradise Point, Pakistan
- Punarjani Guha, natural tunnel in Thrissur district of Kerala, India
- Rainbow Cave (actually an arch) in the Galilee, Israel
- Rock Bridge of Gulanchwadi, Narayangaon, Maharashtra, India
- Senkanmatsu-shima, Iwami, Tottori, Japan
- Shipton's Arch, Xinjiang, China
- Xianren Bridge, China
- Arco Naturale, Capri, Italy
- Azure Window, Gozo, Malta
- Castell de Castells, Spain
- Chaos de Montpellier-le-Vieux, France
- Durdle Door, Dorset, England
- Es Pontàs, Spain
- Étretat, France
- Green Bridge of Wales, Pembrokeshire, Wales
- Inland Sea, Gozo, Malta
- Kuhstall, Germany
- Malá Pravčická brána, Czech Republic
- Marvelous Bridges, Bulgaria
- Pistyll Rhaeadr, Wales
- Podul lui Dumnezeu, Ponoarele, Mehedinţi, Romania
- Pont d'Arc, France
- Pravčická brána, Czech Republic
North America 
- 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
- Bryce Canyon National Park, Utah, USA
- Creelsboro Natural Bridge, Kentucky, USA
- El Arco de Cabo San Lucas, Mexico
- Goat Rock Beach, California, USA
- Grosvenor Arch, Utah, USA
- Koger Natural Arch, Kentucky, USA 
- Kolob Arch, Zion National Park, Utah, USA
- Landscape Arch, Utah, USA
- Lost Creek Wilderness, Colorado, USA
- Natural Bridge, Aruba (collapsed 2005)
- Natural Bridge, Virginia, USA
- Natural Bridge Caverns, Texas, USA
- Natural Bridge Park, Alabama, USA
- Natural Bridge State Park, Kentucky, USA
- Natural Bridge State Park, Massachusetts, USA
- Natural Bridges National Monument, Utah, USA
- Natural Bridges State Beach, California, USA
- Percé Rock, Quebec, Canada
- Rainbow Bridge National Monument, Utah, USA
- Rattlesnake Canyon, Colorado, USA
- Sewanee Natural Bridge, Tennessee, USA
- Sipapu Bridge, Utah, USA
- Tonto Natural Bridge, Arizona, USA
- Wrather Arch, Arizona, USA
- London Arch, Australia (collapsed 1990)
- Oparara Basin Arches, New Zealand
- Piercy Island, New Zealand
- Springbrook National Park, Queensland, Australia
South America 
- Arch Islands, Falkland Islands
- Icononzo, Colombia
- La Portada, Chile
- Puente del Inca, Argentina
- Sete Cidades National Park, Brazil
See also 
- Natural Arch and Bridge Society, FAQ.
- American Geological Institute, Dictionary of Geological Terms, 1976, Doubleday Anchor
- Trek Earth
- Offbeat Tracks in Maharashtra - A Travel Guide - Book by Milind Gunaji ISBN 81-7154-669-2
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