Natural arch

Natural arch
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. By contrast, the Dictionary of Geological Terms defines a natural bridge as a "natural arch that spans a valley of erosion."
The largest natural arch, by a significant margin, is the Xianren Bridge in China, with a span of .
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 Archpreviously known as London Bridgein 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 ±.
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
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
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, India
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
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
Chaos de Montpellier-le-Vieux, France
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
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
Wied il-Mielaħ Window, Gozo, Malta
North America
Canada
Percé Rock, Quebec
Caribbean
Natural Bridge, Aruba (collapsed 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
Ayres Natural Bridge Park, Wyoming
Bell Smith Springs, Illinois
Blackwater Natural Bridge, Wyoming
Bryce Canyon National Park, Utah
Corona Arch, Utah
Creelsboro Natural Bridge, Kentucky
Delicate Arch, Utah
Druid Arch, Utah
Eye of the Needle, Montana
Goat Rock Beach, California
Grosvenor Arch, Utah
Holei Sea Arch, Hawaii Volcanoes National Park, Hawaii
Honopū Arch, Hawaii
Hickman Bridge, Utah
Koger Arch, Kentucky
Kolob Arch, Zion National Park, Utah
Landscape Arch, Utah
La Ventana Arch, El Malpais National Monument, New Mexico
Mesa Arch, Utah
Natural Arch, Kentucky
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.
Three Arch Rocks National Wildlife Refuge, Oregon
Tonto Natural Bridge, Arizona
Twin Arches, Big South Fork National River and Recreation Area, Tennessee
Window Rock, Arizona
Wrather Arch, Arizona
Yahoo Arch, Kentucky
Oceania
Australia
London Arch, 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
Tunnel Beach arch, Dunedin, New Zealand
Spörings Arch, Tolaga Bay, New Zealand
Mercury Bay, New Zealand (collapsed)
South America
Arch Islands, Falkland Islands
Darwin's Arch, Galápagos Islands, Ecuador (collapsed in 2021)
Icononzo, Colombia
La Portada, Chile
Pedra Furada, Ceará, Brazil
Pedra Furada, Piauí, Brazil
Pedra Furada, Santa Catarina, Brazil
Puente del Inca, Argentina
Sete Cidades National Park, Brazil
San Rafael Falls, Ecuador (formed in 2020, collapsed in 2021)
La Catedral, Paracas National Reserve, Peru (Collapsed in 2007)
See also
List of longest natural arches
References
External links
The Natural Arch and Bridge Society
World map of many natural arches
Coastal and oceanic landforms
Coastal geography
Erosion landforms