Natural arch

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File:Delicate Arch LaSalle.jpg
Delicate Arch in Arches National Park, Utah, United States
File:The Great Arch Saudi Arabia.jpg
The Great Arch, Tabuk Province, Saudi Arabia

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.[1] By contrast, the Dictionary of Geological Terms defines a natural bridge as a "natural arch that spans a valley of erosion."[2]

The largest natural arch on Earth, by a significant margin, is the Xianren Bridge in southern China, with a span of Template:Convert.[3]

Coastline

File:Malta Gozo, Azure Window (10264176345).jpg
The Azure Window, Malta, before it collapsed in 2017
File:Darwinarch.jpg
Darwin's Arch, Galápagos Islands, Ecuador, before it collapsed in 2021

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

File:Metate Arch, Devils Garden, Grand Staircase-Escalante National Monument, Utah, USA.jpg
Metate Arch, Devils Garden (GSENM), a very thin arch near the end of its life

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.[4]

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. Template:Wide image

Water-eroded arches

File:Coyote Natural Bridge map.jpg
A topographic map of Coyote Natural Bridge in Utah shows how the meandering Coyote Gulch carved a shorter route through the rock under the arch. The old riverbed is now higher than the present water level.

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 Template:Convert. 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 Template:Convert, 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 Template:Convert by the Natural Arch and Bridge Society in October 2010, with a precision of ±Template:Convert.[5][6]

Cave erosion

File:London Bridge before collapse.jpg
London Bridge, Victoria, Australia, before its partial collapse in 1990

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.[7]

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

File:Natural Bridge State Park (31044316221).jpg
Natural Bridge, Virginia

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.[8] 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.Script error: No such module "Unsubst".

In Europe, the Romanian village of Ponoarele has a road segment called God's Bridge that is Template:Cvt long and Template:Cvt wide, passing over a stone arch Template:Cvt high and Template:Cvt thick.[9]

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.Script error: No such module "Unsubst".

Notable natural arches

Africa

File:TadrartRouge3.jpg
Natural arches in the La Cathedrale formation of Tadrart Rouge range, Algeria
File:Hole In The Wall.jpg
Hole-in-the-Wall

Antarctica

Asia

File:Natural stone arch in tirumala.JPG
Natural Arch, Tirumala, India
File:Arches in Timna Park in summer 2011 (7).JPG
Arch in Timna Valley Park, Negev Desert, Israel

Europe

File:Praia da Marinha (2012-09-27), by Klugschnacker in Wikipedia (31).JPG
The arches at Marinha Beach, Caramujeira, Lagoa, Algarve, Portugal
File:Es pontas 2.jpg
Es Pontàs is a natural arch on the coast of Mallorca, Spain
File:Durdle Door Dorset Sunset.jpg
Durdle Door, Dorset, the United Kingdom
File:Мала прераст у нањону Вратне.jpg
Little Prerast in eastern Serbia

North America

Canada

File:QC Gaspesie RocherPerce tango7174.jpg
Percé Rock, Quebec, Canada

Caribbean

Mexico

File:CaboSanLucasLandsEnd.JPG
The Arch of Cabo San Lucas, Mexico

United States

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File:Hawaii Volcanoes National Park 02.jpg
Holei Sea Arch, Hawaii Volcanoes National Park, Hawaii, United States
File:Landscape Arch Utah.jpg
Landscape Arch, Utah, United States – one of the longest natural arches in the world[16]
File:Utah Rainbow Arch.jpg
Rainbow Bridge, Utah, a natural bridge formed by a meandering watercourse

Oceania

File:Hole in the Rock at Motukokako Island (Piercy Island).jpg
"The Hole in the Rock” on Piercy Island, Cape Brett, New Zealand
File:Rock arch south of Tunnel Beach.jpg
The arch at Tunnel Beach, Dunedin, New Zealand

Australia

New Zealand

South America

File:20030924PPedraFurada9.jpg
Stone arch at Pedra Furada, Piauí, Brazil
File:Morro da Igreja - Pedra Furada - Zoom.jpg
Pedra Furada, Santa Catarina, Brazil

See also

References

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External links

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Template:Coastal geography

  1. ↑ Natural Arch and Bridge Society Template:Webarchive, FAQ.
  2. ↑ American Geological Institute, Dictionary of Geological Terms, 1976, Doubleday Anchor
  3. ↑ Big 14 Tour - Fairy Bridge Template:Webarchive, The Natural Arch and Bridge Society
  4. ↑ Script error: No such module "citation/CS1".
  5. ↑ Jett, Stephen C.China Diary Template:Webarchive, The Natural Arch and Bridge Society
  6. ↑ Script error: No such module "citation/CS1".
  7. ↑ Script error: No such module "citation/CS1".
  8. ↑ Script error: No such module "citation/CS1".
  9. ↑ Script error: No such module "citation/CS1".
  10. ↑ Script error: No such module "citation/CS1".
  11. ↑ Offbeat Tracks in Maharashtra - A Travel Guide - Book by Milind Gunaji Template:ISBN
  12. ↑ Script error: No such module "citation/CS1".
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  15. ↑ Script error: No such module "citation/CS1".
  16. ↑ Script error: No such module "citation/CS1".
  17. ↑ "Koger Arch, Kentucky" Template:Webarchive. naturalarches.org. The Natural Arch and Bridge Society. Retrieved November 6, 2017.
  18. ↑ Script error: No such module "citation/CS1".
  19. ↑ "Natural Arch of Kentucky" Template:Webarchive. naturalarches.org. The Natural Arch and Bridge Society. Retrieved November 6, 2017.
  20. ↑ Script error: No such module "citation/CS1".
  21. ↑ "Sea stack at Tettegouche toppled by powerful winter storm" 2019-12-02
  22. ↑ "Big South Fork National River and Recreation Area: Twin Arches". nps.gov. April 14, 2015. Retrieved March 14, 2018.
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