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).
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 122 ± 5 meters (400 ± 15 ft).
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 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 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.
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).
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.
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. [ citation needed ]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 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.
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 ]
Landscape Arch is the longest of the many natural rock arches located in Arches National Park, Utah, United States.
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.
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 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.
In geomorphology, a butte is an isolated hill with steep, often vertical sides and a small, relatively flat top; buttes are smaller landforms than mesas, plateaus, and tablelands. The word butte comes from a French word meaning knoll ; its use is prevalent in the Western United States, including the southwest where mesa is used for the larger landform. Because of their distinctive shapes, buttes are frequently landmarks in plains and mountainous areas. To differentiate, geographers use the rule of thumb that a mesa has a top that is wider than its height, while a butte has a top that is narrower than its height.
Rainbow Bridge National Monument is administered by Glen Canyon National Recreation Area, southern Utah, United States. Rainbow Bridge is often described as the world's highest natural bridge. The span of Rainbow Bridge was reported in 1974 by the Bureau of Reclamation to be 275 feet (84 m), but a measurement of span according to definition by the Natural Arch and Bridge Society in 2007 resulted in a value of 234 feet (71 m). At the top it is 42 feet (13 m) thick and 33 feet (10 m) wide. The bridge, which is of cultural importance to a number of area Native American tribes, has been designated a Traditional Cultural Property by the National Park Service.
A mesa is an isolated, flat-topped elevation, ridge or hill, which is bounded from all sides by steep escarpments and stands distinctly above a surrounding plain. Mesas characteristically consist of flat-lying soft sedimentary rocks capped by a more resistant layer or layers of harder rock, e.g. shales overlain by sandstones. The resistant layer acts as a caprock that forms the flat summit of a mesa. The caprock can consist of either sedimentary rocks such as sandstone and limestone; dissected lava flows; or a deeply eroded duricrust. Unlike plateau, whose usage does not imply horizontal layers of bedrock, e.g. Tibetan Plateau, the term mesa applies exclusively to the landforms built of flat-lying strata. Instead, flat-topped plateaus are specifically known as tablelands.
The exposed geology of the Bryce Canyon area in Utah shows a record of deposition that covers the last part of the Cretaceous Period and the first half of the Cenozoic era in that part of North America. The ancient depositional environment of the region around what is now Bryce Canyon National Park varied from the warm shallow sea in which the Dakota Sandstone and the Tropic Shale were deposited to the cool streams and lakes that contributed sediment to the colorful Claron Formation that dominates the park's amphitheaters.
The Colorado Plateau, also known as the Colorado Plateau Province, is a physiographic and desert region of the Intermontane Plateaus, roughly centered on the Four Corners region of the southwestern United States. This province covers an area of 336,700 km2 (130,000 mi2) within western Colorado, northwestern New Mexico, southern and eastern Utah, northern Arizona, and a tiny fraction in the extreme southeast of Nevada. About 90% of the area is drained by the Colorado River and its main tributaries: the Green, San Juan, and Little Colorado. Most of the remainder of the plateau is drained by the Rio Grande and its tributaries.
Double Arch is a close-set pair of natural arches in Arches National Park in southern Grand County, Utah, United States, that is one of the more known features of the park.
The exposed geology of the Capitol Reef area presents a record of mostly Mesozoic-aged sedimentation in an area of North America in and around Capitol Reef National Park, on the Colorado Plateau in southeastern Utah.
The Big South Fork National River and Recreation Area, commonly known as Big South Fork, preserves the Big South Fork of the Cumberland River and its tributaries in northeastern Tennessee and southeastern Kentucky. In addition, the former mining community of Blue Heron is preserved and interpreted via signage.
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.
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 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 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.
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 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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