Natural arch

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Delicate Arch in Arches National Park, Utah, United States Delicate Arch LaSalle.jpg
Delicate Arch in Arches National Park, Utah, United States

A natural arch, natural bridge, or (less commonly) rock arch is a natural rock formation 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).

Contents

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, by a significant margin, is the Xianren Bridge in China, with a span of 122 ± 5 meters (400 ± 15 ft). [3]

Coastline

The Azure Window, Malta, which collapsed in 2017 Malta Gozo, Azure Window (10264176345).jpg
The Azure Window, Malta, which collapsed in 2017

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 Arch previously 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

Metate Arch, Devils Garden (GSENM), a very thin arch near the end of its life 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.

Water-eroded arches

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. 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 225 feet (69 m). 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 234 feet (71 m), 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 400 feet (120 m) by the Natural Arch and Bridge Society in October 2010, with a precision of ±15 feet (4.6 m). [5] [6]

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. [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

Natural Bridge, Virginia 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.[ citation needed ]

In Europe, the Romanian village of Ponoarele has a road segment called God's Bridge that is 30 m (98 ft) long and 13 m (43 ft) wide, passing over a stone arch 22 m (72 ft) high and 9 m (30 ft) 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.[ citation needed ]

Notable natural arches

Africa

Natural arches in the La Cathedrale formation of Tadrart Rouge range, Algeria TadrartRouge3.jpg
Natural arches in the La Cathedrale formation of Tadrart Rouge range, Algeria

Antarctica

Asia

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

Europe

The arches at Marinha Beach, Caramujeira, Lagoa, Algarve, Portugal Praia da Marinha (2012-09-27), by Klugschnacker in Wikipedia (31).JPG
The arches at Marinha Beach, Caramujeira, Lagoa, Algarve, Portugal
Es Pontas is a natural arch on the coast of Mallorca, Spain Es pontas 2.jpg
Es Pontàs is a natural arch on the coast of Mallorca, Spain
Durdle Door, Dorset, the United Kingdom Durdle Door Dorset Sunset.jpg
Durdle Door, Dorset, the United Kingdom

North America

Canada

Perce Rock, Quebec, Canada QC Gaspesie RocherPerce tango7174.jpg
Percé Rock, Quebec, Canada

Caribbean

Mexico

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

United States

Holei Sea Arch, Hawaii Volcanoes National Park, Hawaii, United States Hawaii Volcanoes National Park 02.jpg
Holei Sea Arch, Hawaii Volcanoes National Park, Hawaii, United States
Landscape Arch, Utah, United States - one of the longest natural arches in the world Landscape Arch Utah (50MP).jpg
Landscape Arch, Utah, United States - one of the longest natural arches in the world
Rainbow Bridge, Utah, a natural bridge formed by a meandering watercourse Utah Rainbow Arch.jpg
Rainbow Bridge, Utah, a natural bridge formed by a meandering watercourse

Oceania

"The Hole in the Rock" on Piercy Island, Cape Brett, New Zealand Hole In The Rock In Bay Of Islands.jpg
"The Hole in the Rock” on Piercy Island, Cape Brett, New Zealand
The arch at Tunnel Beach, Dunedin, New Zealand Tunnel Beach Arch, New Zealand.JPG
The arch at Tunnel Beach, Dunedin, New Zealand

Australia

New Zealand

South America

See also

Related Research Articles

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Landscape Arch is the longest of the many natural rock arches located in Arches National Park, Utah, United States.

Sinkhole Depression or hole in the ground caused by collapse of the surface into an existing void space

A sinkhole, also known as a cenote, sink, sink-hole, swallet, swallow hole, or doline, is a depression or hole in the ground caused by some form of collapse of the surface layer. Most are caused by karst processes – the chemical dissolution of carbonate rocks or suffosion processes. Sinkholes vary in size from 1 to 600 m both in diameter and depth, and vary in form from soil-lined bowls to bedrock-edged chasms. Sinkholes may form gradually or suddenly, and are found worldwide.

Stack (geology) geological landform consisting of a steep and often vertical column or columns of rock

A stack or sea stack is a geological landform consisting of a steep and often vertical column or columns of rock in the sea near a coast, formed by wave erosion. Stacks are formed over time by wind and water, processes of coastal geomorphology. They are formed when part of a headland is eroded by hydraulic action, which is the force of the sea or water crashing against the rock. The force of the water weakens cracks in the headland, causing them to later collapse, forming free-standing stacks and even a small island. Without the constant presence of water, stacks also form when a natural arch collapses under gravity, due to sub-aerial processes like wind erosion. Erosion causes the arch to collapse, leaving the pillar of hard rock standing away from the coast—the stack. Eventually, erosion will cause the stack to collapse, leaving a stump. Stacks can provide important nesting locations for seabirds, and many are popular for rock climbing.

Natural Bridges National Monument national monument in San Juan County, Utah, United States

Natural Bridges National Monument is a U.S. National Monument located about 50 miles (80 km) northwest of the Four Corners boundary of southeast Utah, in the western United States, at the junction of White Canyon and Armstrong Canyon, part of the Colorado River drainage. It features the thirteenth largest natural bridge in the world, carved from the white Permian sandstone of the Cedar Mesa Formation that gives White Canyon its name.

Butte Isolated hill with steep, often vertical sides and a small, relatively flat top

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Rainbow Bridge National Monument national monument in San Juan County, Utah, United States

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Colorado Plateau plateau in the southwestern United States

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Double Arch (Utah) Natural arch in Utah, United States

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Geology of the Capitol Reef area

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Natural Bridge State Resort Park in the U.S. state of Kentucky

Natural Bridge State Resort Park is a Kentucky state park located in Powell and Wolfe Counties along the Middle Fork of the Red River, adjacent to the Red River Gorge Geologic Area and surrounded by the Daniel Boone National Forest. Its namesake natural bridge is the centerpiece of the park. The natural sandstone arch spans 78 ft (24 m) and is 65 ft (20 m) high. The natural process of weathering formed the arch over millions of years. The park is approximately 2,300 acres (9 km2) of which approximately 1,200 acres (5 km2) is dedicated by the Office of Kentucky Nature Preserves as a nature preserve. In 1981 this land was dedicated into the nature preserves system to protect the ecological communities and rare species habitat. The first federally endangered Virginia big eared bats, Corynorhinus townsendii virginianus, recorded in Kentucky were found at Natural Bridge State Resort Park in the 1950s.

Azure Window Former natural limestone arch in the Maltese island of Gozo

The Azure Window, also known as the Dwejra Window, was a 28-metre-tall (92 ft) natural arch on the island of Gozo, located just off the shores of Malta. The limestone feature, which was in Dwejra Bay close to the Inland Sea and Fungus Rock, was one of the island's major tourist attractions until it collapsed in stormy weather on 8 March 2017. The small rock over the water is no longer accessible by walking. The arch, together with other natural features in the area, has appeared in a number of international films and media productions.

Devils Garden (Arches National Park) rock formation in Arches National Park in Grand County, Utah, United States

Devils Garden is an area of Arches National Park, located near Moab, Utah, United States, that features a series of rock fins and arches formed by erosion. The Devils Garden Trail, including more primitive sections and spurs, meanders through the area for 7.2 mi (11.6 km). The trailhead leads directly to Landscape Arch after a 0.8 mi (1.3 km) outbound hike, while Tunnel Arch and Pine Tree Arch can be seen on spur trails on the way to Landscape Arch. Several other arches, including Partition, Navajo, Double O, and Private Arch, as well as the Dark Angel monolith and Fin Canyon, are accessed via the primitive loop trail and its spurs.

Arches National Park National park in Utah, United States

Arches National Park is a national park in eastern Utah, United States. The park is adjacent to the Colorado River, 4 miles (6 km) north of Moab, Utah. More than 2,000 natural sandstone arches are located in the park, including the well-known Delicate Arch, as well as a variety of unique geological resources and formations. The park contains the highest density of natural arches in the world.

Xianren Bridge is a natural arch created by flowing water that has the world's longest recorded span. Carved of limestone karst, the formation bridges Buliu River in the northern Guangxi Zhuang Autonomous Region, China. Because of its remote location, accessible only by a three-hour rafting trip, it was not discovered until recently and remains relatively obscure. An expedition in October 2010 by the Natural Arch and Bridge Society first measured the bridge's span and found it to be 121.9 ± 4.6 m (400 ± 15 ft) in length.

Devils Garden (Grand Staircase–Escalante National Monument) natural arches and hoodoo formations in the Grand Staircase-Escalante National Monument in Garfield County, Utah, United States

The Devils Garden of the Grand Staircase–Escalante National Monument (GSENM) in south central Utah, United States, is a protected area featuring hoodoos, natural arches and other sandstone formations. The United States Geological Survey (USGS) designated the name Devils Garden on December 31, 1979. The area is also known as the Devils Garden Outstanding Natural Area within the National Landscape Conservation System.

Scarp retreat

Scarp retreat is a geological process through which the location of an escarpment changes over time. Typically the cliff is undermined, rocks fall and form a talus slope, the talus is chemically or mechanically weathered and then removed through water or wind erosion, and the process of undermining resumes. Scarps may retreat for tens of kilometers in this way over relatively short geological time spans, even in arid locations.

A fin is a geologic formation that is a narrow, residual wall of hard sedimentary rock that remains standing after surrounding rock has been eroded away along parallel joints or fractures. Fins are formed when a narrow butte or plateau develops many vertical, parallel cracks. There are two main modes of following erosion. The first is when water flows along joints and fractures and opens them wider and wider, eventually causing erosion. The second is where the rock type (stratum) is harder and more erosion resistant than neighboring rocks, causing the weaker rock to fall away.

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