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."
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.
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).
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.
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 arch, a cave erosion arch made of limestone, is located in Carter Caves State Resort Park and it has a paved road on top. The second arch, a weather-eroded sandstone arch with a dirt road on top, is located 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 South America, 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, Peru.
The Arch Islands (Spanish: Islotes Franceses) are a group of small islands, off Port Albemarle on West Falkland in the Falkland Islands. They are uninhabited, and accessible only by boat. They are so called because the largest of the group has a natural arch in it, large enough to allow a fair sized boat through.
The islands include:
Big Arch Island
Albemarle RockArch Rock (Tasmania)
Arch Rock is a sandstone islet, with an area of 0.44 ha and containing a natural arch, in south-eastern Australia. It is part of the Partridge Island Group, lying close to the south-eastern coast of Tasmania, in the D'Entrecasteaux Channel between Bruny Island and the mainland.Arco Naturale
The Arco Naturale is a natural arch on the east coast of the island of Capri. Dating from the Paleolithic age, it is the remains of a collapsed grotto. The arch spans 12 m at a height of 18 m above ground and consists of limestone.Azure
Azure may refer to:
Azure (color), a hue of blue
Shades of azure, shades and variations
Azure (barley), a malting barley variety
Azure (design magazine), Toronto, Ontario
Azure (magazine), a periodical on Jewish thought and identity
Azure (heraldry), a blue tincture on flags or coats of arms
Azure (album), an album by jazz flugelhornist Art Farmer
"Azure" (song), by Duke Ellington
Azure, Alberta, a locality in Canada
Azure, Montana, a census-designated place in the United States
Azure Window, a former natural arch in Malta
Bentley Azure, a car
Microsoft Azure, a cloud computing platform
Mozilla Azure, a graphics abstraction APIAzure Window
The Azure Window (Maltese: it-Tieqa Żerqa), also known as the Dwejra Window (Maltese: it-Tieqa tad-Dwejra), was a 28-metre-tall (92 ft) natural arch on the island of Gozo in 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 arch, together with other natural features in the area, has appeared in a number of international films and media productions.
The rock formation, which consisted of a pillar rising from the sea joined to the cliff by a horizontal slab, was created by the collapse of a sea cave, probably during the 19th century. The final collapse followed a century of natural erosion, during which large sections of the limestone arch had broken off and fallen into the sea.Butt of Lewis
The Butt of Lewis (Scottish Gaelic: Rubha Robhanais) is the most northerly point of Lewis in the Outer Hebrides. The headland, which lies in the North Atlantic, is frequently battered by heavy swells and storms and is marked by the Butt of Lewis Lighthouse. The nearest populated area is the village of Eoropie, about 1 mile (1.6 km) to the south.The road to the lighthouse passes a sheltered cove called Port Stoth. Agricultural lazy beds are also visible along the coast. The Butt of Lewis features some of the oldest rocks in Europe, having been formed in the Precambrian period up to 3000 million years ago. Southwest from the lighthouse is a natural arch called the "Eye of the Butt" (Scottish Gaelic: Sùil an Rubha), best viewed from the Habost machair.Castell de Castells
Castell de Castells (Valencian: [kasˈteʎ de kasˈteʎs], Spanish: [kasˈtel de kasˈtels]) is a municipality high in the mountains of the Marina Alta on the Costa Blanca in southeastern Spain. The village is at the source of the Rio Jalón and surrounded by mountains. It is approximately 12 kilometres (7.5 mi) from the nearest village of Benigembla, and 20 kilometres (12 mi) from Jalón, the nearest large town. It is approximately one hour's drive to the beaches, the Fuentes de Algar, and Terra Mítica theme park. Also in the area are the caves of Vall d'Ebo and Benidoleig.
The highest peak of the Serrella mountain range, Malla de Llop, is over 1,300 metres (4,300 ft) high. The remains of an Arabic castle, Penya Castellet, and many trails are found in the area, popular for walking and mountain biking. The area has a rich history with three settlements around the village: Pla de Petracos, Bitlla and Pla d'Alt (The High Place). The ruins of these settlements are now being rebuilt as modern homes.
Five kilometres outside the village in the area known as Pla de Petracos are ancient cave paintings said to have been painted over 5000 years ago. Caves at the archaeological site have a viewing platform, and the paintings are explained on six interpretive panels.
Three kilometres outside the village is an enormous natural arch, Els Arcs.Cebolla Wilderness
Cebolla Wilderness is a 61,600-acre (24,900 ha) Wilderness area located within the El Malpais National Conservation Area in New Mexico. The area was added to the National Wilderness Preservation System on December 31, 1987 by Public Law 100-225. The area is a bordered by the Acoma Pueblo to the east and the El Malpais National Monument and New Mexico Highway 117 to the west. Elevations range from 7,000 feet (2,100 m) to 8,200 feet (2,500 m). The sandstone canyons and mesas of this rimrock area include features such as La Ventana natural arch.Delicate Arch
Delicate Arch is 52-foot-tall (16 m) freestanding natural arch located in Arches National Park, near Moab in Grand County, Utah, United States. The arch is the most widely recognized landmark in Arches National Park and is depicted on Utah license plates and a postage stamp commemorating Utah's centennial anniversary of admission to the Union in 1996. The Olympic torch relay for the 2002 Winter Olympics passed through the arch.Landscape Arch
Landscape Arch is the longest of the many natural rock arches located in Arches National Park, Utah, United States.Natural Arch, Tirumala hills
Natural Arch, Tirumala hills, a notified National Geo-heritage Monument, is a distinctive geological feature 1 km (0.6 mi) north of the Tirumala hills temple, near the Chakra Teertham in the state of Andhra Pradesh, India. The arch is also called Silathoranam (శిలాతోరణం) in the local language (Telugu language: sila means ‘rock’ and thoranam means a garland strung over a threshold, connecting two vertical columns or an ‘arch’ as in this case). The arch measures 8 m (26.2 ft) in width and 3 m (9.8 ft) in height, and is naturally formed in the quartzites of Cuddapah Supergroup of Middle to Upper Proterozoic (1600 to 570 Ma) due to natural erosive forces.Natural Bridge, Virginia
Natural Bridge is an unincorporated community in Rockbridge County, Virginia, United States. The community is the site of Natural Bridge, a natural arch which gives the town its name. Natural Bridge is located at the junction of U.S. Route 11 and State Route 130. Natural Bridge has a post office with ZIP code 24578, which opened on June 1, 1800.Paradise Point, Pakistan
Paradise Point, on the Arabian Sea, is a beach in Karachi, Sindh, Pakistan. Paradise Point is a sandstone rock promontory which once had a natural arch.
The beach has attractions for families and tourists, including beachside horse and camel rides, amusement parks, restaurants, and swimming in the Arabian Sea. Paradise Point Beach is accessible through Mauripur Road (formerly Hawkes Bay Road) or the Mubarak Goth Road from Karachi. Nathiagali Beach is located west of Paradise Point Beach.Other beaches close to the city include Sandspit Beach, Hawke's Bay, and Clifton Beach.Pulpit Rock, Portland
Pulpit Rock is a coastal feature at Portland Bill, the southern tip of the Isle of Portland, Dorset, England. Representing an open bible leaning on a pulpit, Pulpit Rock was formed in the 1870s after a natural arch was cut away by quarrymen at Bill Quarry. As a quarrying relic, the rock is similar to that of Nicodemus Knob, another quarrying landmark on the island.
Pulpit Rock has become a popular tourist attraction on the island and is often photographed. Despite the danger, for many decades it has been a popular place for tombstoning. Pulpit Rock is also a popular point for wrasse anglers. The British record Ballan wrasse was caught there in 1998 by Pete Hegg.The geological succession up from sea level is: Portland Cherty Series (up to the level of the neighbouring quarried platform), then Portland Freestone (the oolitic limestone quarried inland of Pulpit Rock), then a cap of thin-bedded limestones which are part of the basal Purbeck Formation.Rainbow Bridge National Monument
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.Sewing-Machine Needles
Sewing-Machine Needles (62°58′11.8″S 60°29′39.1″W) is a group of three prominent rock needles, the highest 45 m, lying close southeast of Rancho Point, Deception Island, in the South Shetland Islands. The name Sewing-Machine Rock was given by whalers for what was originally a conspicuous natural arch. Needles is now considered the more suitable descriptive term; an earthquake tremor in 1924 caused the arch to collapse.Sheep Island, Argyll
Sheep Island is a small uninhabited island situated off the southern tip of the Kintyre peninsula in Scotland.
Sheep Island, along with Sanda Island and Glunimore Island, form a small group of islands approximately 3 kilometres (1.9 mi) south of Kintyre at grid reference NR733055.
Running roughly north to south, the island is around 600 metres (660 yd) long, and is steep and precipitous along the west coast, rising to a maximum height of 41 metres (135 ft). There is a cave on the west coast and a natural arch at the northwest of the island. A considerable reef of drying rocks surrounds the island and makes landing on the island problematic. There are a number of strong tidal races in the area, Tum ba nach is one that extends north from Sheep Island to around halfway across the Sound of Sanda.
As the name suggests, sheep have traditionally been grazed here, however the island is more notable as the primary breeding ground for puffins in the Clyde. From here the birds are returning to nearby Ailsa Craig, where they were once wiped out. Guillemots and razorbills also nest on Sheep Island.Stack (geology)
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.
Isolated steep-sided, rocky oceanic islets, typically of volcanic origin, are also loosely called "stacks" or "volcanic stacks".Tarner Island
Tarner Island is a triangular shaped island in Loch Bracadale just off the coast off the Harlosh peninsula of Skye in Scotland. It is about 28 hectares (69 acres) in extent.
The coastline is largely cliff-lined and rocky and there is a natural arch to the north. Tarner Island is only about 1 kilometre (0.62 mi) from mainland Skye and there are several skerries including Sgeir Mhòr and Sgeir Bheag that lies just offshore to the north east between the island and Colbost Head. Wiay and the tidal islet of Oronsay lie about 2 kilometres (1.2 mi) to the south.