A natural arch, natural bridge, or (less commonly) rock arch is a natural landform where an arch has formed with an opening underneath. Natural arches commonly form where inland cliffs, coastal cliffs, fins or stacks are subject to erosion from the sea, rivers or weathering (subaerial processes).
Most natural arches are formed from narrow fins and sea stacks composed of sandstone or limestone with steep, often vertical, cliff faces. The formations become narrower due to erosion over geologic time scales. The softer rock stratum erodes away creating rock shelters, or alcoves, on opposite sides of the formation beneath the relatively harder stratum, or caprock, above it. The alcoves erode further into the formation eventually meeting underneath the harder caprock layer, thus creating an arch. The erosional processes exploit weaknesses in the softer rock layers making cracks larger and removing material more quickly than the caprock; however, the caprock itself continues to erode after an arch has formed, which will ultimately lead to collapse.
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 on Earth, by a significant margin, is the Xianren Bridge in southern 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. Two examples of this type of arch are London Bridge in Victoria, Australia, and Neill Island in the Andaman Islands, India. When these arches 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 along concordant coastlines breaks through the strong rock and then erodes the weak rock very quickly. Good examples of this type of arch are the Durdle Door and Stair Hole near Lulworth Cove on Dorset's Jurassic Coast in south England. When Stair Hole eventually collapses it will form a cove.
Weather-eroded arches
Weather-eroded arches begin their formation as deep cracks which penetrate into a sandstone layer. Erosion occurring within the cracks wears away exposed rock layers and enlarges the surface cracks isolating narrow sandstone walls which are called 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. The arches eventually collapse leaving only buttresses that in time will erode.
Many weather-eroded arches are found in Arches National Park, Canyonlands National Park, and Grand Staircase–Escalante National Monument (GSENM), all located in southern Utah, United States.
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 protects the area surrounding three large natural bridges, all of which were formed by streams running through canyons, the largest of which is named Sipapu Bridge with a span of . The Rainbow Bridge National Monument's namesake was also formed by flowing water which created the largest known natural bridge in the Western Hemisphere with a span of , based on a laser measurement made in 2007. Xianren Bridge, also known as Fairy Bridge, in Guangxi, China is currently the world's largest known natural bridge with a span recorded at by the Natural Arch and Bridge Society in October 2010, with a precision of ±.Jett, Stephen C.China Diary , The Natural Arch and Bridge Society
Cave erosion
Natural bridges can form from natural limestone caves, where paired sinkholes collapse and a ridge of stone is left standing in between, with the cave passageway connecting from sinkhole to sinkhole.
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.
Moon Hill in Yangshuo, Guizhou Province, China, is an example of an arch formed by the remnant of a karst limestone cave.
Arches as highway or railway bridges
In a few places in the world, natural arches are utilized by humans as transportation bridges with highways or railroads running across them.
In Virginia, US Route 11 traverses Natural Bridge. Two additional natural arch roadways are found in Kentucky. The first, a cave erosion arch made of limestone, is in Carter Caves State Resort Park and has a paved road on top. The second, a weather-eroded sandstone arch with a dirt road on top, is on the edge of Natural Bridge State Park in Kentucky. The latter arch 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.
In Europe, the Romanian village of Ponoarele has a road segment called God's Bridge that is long and wide, passing over a stone arch high and thick.
The railroad from Lima, Peru crosses the Rio Yauli on a natural bridge near kilometer 214.2 as it approaches the city of La Oroya.
Notable natural arches Africa
Aloba Arch, Chad Acacus Tedrart Arch, Libya Boatswain Bird Island, Ascension Island Bogenfels, Namibia Goedehoop natural rock bridge, South Africa Hole-in-the-Wall, Eastern Cape, South Africa Tassili n'Ajjer and Tadrart Rouge, two mountain ranges with many arches, Algeria Tukuyu natural bridge, Tanzania Wolfberg Arch, Cederberg, Western Cape, South Africa
Antarctica The Kerguelen Arch, Christmas Harbour, the Kerguelen Islands (collapsed ca. 1910) Scott Island in the Antarctic has a natural arch
Asia
Keshet Cave, Galilee, Israel Burdah Bridge, Wadi Rum, Jordan Baatara gorge waterfall in Tannourine, Lebanon has three natural bridges. Engetsu Island, Shirahama, Wakayama, Japan Hawrah Natural Bridge, Neill Island, Andaman Islands, India Hazarchishma Natural Bridge, Bamyan Province, Afghanistan Jabal Umm Fruth Bridge, Jordan Jebel Kharaz, Wadi Rum, Jordan "Jisr al-Hajar" or the "Stone Bridge" in Mzaar Kfardebian, Lebanon Elephant Trunk Hill, Guilin, Guangxi province, China Moon Hill, Guangxi province, China Natural Arch, Tirumala hills, Tirumala, India Pasilagon Point Natural Arch, Banton, Romblon, Philippines The Pigeons' Rock known as Raouché in Beirut, Lebanon Punarjani Guha, natural tunnel in Thrissur district of Kerala, India Rock Bridge of Gulanchwadi, Narayangaon, Maharashtra, IndiaOffbeat Tracks in Maharashtra - A Travel Guide - Book by Milind Gunaji Shipton's Arch, Xinjiang, China Steller's Arch on Bering Island, Russia Tianmen Mountain, Zhangjiajie, China Xianren Bridge, China The Great White Hole of Krabi, Krabi, Thailand Mountain Angel Eye, Cao Bằng, Vietnam The Great Arch, Hizma, Tabuk Province, Kingdom of Saudi Arabia
Europe
Albandeira Arch, Lagoa, Algarve, Portugal Arco Naturale, Capri, Italy As Catedrais beach, Ribadeo, Spain Azure Window, Gozo, Malta (collapsed in 2017) Blue Window or the Seal cave of the Corinthian Gulf, in the front of the Alcyonides islands, Greece Bow Fiddle Rock, Portknockie, Scotland Capu Tafunatu, France Chaos de Montpellier-le-Vieux, France Devil's Gate (Crimea) Dore Holm, Shetland Islands Drangarnir, Faroe Islands Durdle Door, Dorset, England Dyrhólaey, Iceland Els Arcs, Castell de Castells, Spain Es Pontàs, Spain Étretat, France Geropotamos bridge, Crete, Greece Għar Qawqla, Gozo, Malta (collapsed in the 20th century) Green Bridge of Wales, Pembrokeshire, Wales Hajdučka vrata, Čvrsnica mountain, Bosnia and Herzegovina Stone Passage on Dobropoljka (Bistrica) river, Bosnia and Herzegovina Stone Passage on Paljanska Miljacka, Bosnia and Herzegovina Inland Sea, Gozo, Malta Kuhstall, Germany Lalaria beach arch, Skiathos Island, Greece Malá Pravčická brána, Czech Republic Marinha Beach, Caramujeira, Lagoa, Algarve, Portugal Marsden Rock, South Shields, England (collapsed in 1996) Marvelous Bridges, Bulgaria Monte Forato, Tuscany, Italy Ófærufoss, Iceland (collapsed in winter 1992/1993) Pistyll Rhaeadr, Wales Ponoarele, Romania (Podul lui Dumnezeu, or God's Bridge) Pont d'Arc, France Pravčická brána, Czech Republic Puentedey, Spain (Puentedei, or God's Bridge) Trypitòs Arch of Paxi Island, Greece Samar Natural Arch, Serbia Šuplja Stena Natural Arch, Serbia Tour Percée, France Wied il-Mielaħ Window, Gozo, Malta Vratna Gates, Serbia
North America Canada
Percé Rock, Quebec
Caribbean Natural Bridge, Aruba (collapsed in 2005)
Mexico
El Arco de Cabo San Lucas, Baja California Sur
United States
Anacapa Island, Channel Islands National Park, California Angel Arch, Utah Arch Creek, Florida Arch Rock, Michigan Arturos Bridge, California Ayres Natural Bridge Park, Wyoming Balcony Bridge, California Bell Smith Springs, Illinois Blackwater Natural Bridge, Wyoming Bryce Canyon National Park, Utah Corona Arch, Utah Creelsboro Natural Bridge, Kentucky Delicate Arch, Utah Devil's Doorway (Wisconsin) Druid Arch, Utah Eye of the Needle, Montana Goat Rock Beach, California Great Arch Rock, Farallon Islands, California Great West Arch, Farallon Islands, California Grosvenor Arch, Utah Hōlei Sea Arch, Hawaii Volcanoes National Park, Hawaii (collapsed in 2026) Honopū Arch, Hawaii Hickman Bridge, Utah Koger Arch, Kentucky"Koger Arch, Kentucky" . naturalarches.org. The Natural Arch and Bridge Society. Retrieved November 6, 2017. Kolob Arch, Zion National Park, Utah Landscape Arch, Utah La Ventana Natural Arch, New Mexico Mesa Arch, Utah Mobius Arch, California Natural Arch, Kentucky"Natural Arch of Kentucky" . naturalarches.org. The Natural Arch and Bridge Society. Retrieved November 6, 2017. Natural Bridge, Virginia Natural Bridge Caverns, Texas Natural Bridge of Arkansas Natural Bridge Park, Alabama Natural Bridge State Park, Kentucky Natural Bridge State Park, Massachusetts Natural Bridge State Park, Wisconsin Natural Bridges National Monument, Utah Natural Bridges State Beach, California Rainbow Bridge National Monument, Utah Rattlesnake Canyon, Colorado Rialto Beach, Olympic National Park, Washington Rock Bridge Memorial State Park, Missouri Rockbridge State Nature Preserve, Ohio Sewanee Natural Bridge, Tennessee Sipapu Bridge, Utah An arch at Tettegouche State Park, Minnesota, collapsed in 2010."Sea stack at Tettegouche toppled by powerful winter storm" 2019-12-02 Three Arch Rocks National Wildlife Refuge, Oregon Tonto Natural Bridge, Arizona Twin Arches, Big South Fork National River and Recreation Area, Tennessee"Big South Fork National River and Recreation Area: Twin Arches". nps.gov. April 14, 2015. Retrieved March 14, 2018. Window Rock, Arizona Wrather Arch, Arizona Yahoo Arch, Kentucky
Oceania
Australia London Bridge, Port Campbell National Park, Victoria, Australia (one of the two spans collapsed in 1990) Natural Bridge, Springbrook National Park, Queensland, Australia Nature's Window, Kalbarri National Park. Western Australia, Australia
New Zealand Mangapohue Natural Bridge, New Zealand Oparara Basin Arches, New Zealand "The Hole in the Rock", Piercy Island, Cape Brett, New Zealand