Natural arch: Difference between revisions
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[[Image:arch form.gif|thumb|300px|A diagram showing the sequence of arch formation.]] |
[[Image:arch form.gif|thumb|300px|A diagram showing the sequence of arch formation.]] |
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# Deep cracks penetrate into a sandstone layer. |
# Deep cracks penetrate into a sandstone layer. RJ! |
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# Erosion wears away exposed rock layers and enlarges the surface cracks, isolating narrow sandstone walls, or fins. |
# Erosion wears away exposed rock layers and enlarges the surface cracks, isolating narrow sandstone walls, or fins. J! |
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# Alternating frosts and thawing cause crumbling and flaking of the porous sandstone and eventually cut through some of the fins. |
# Alternating frosts and thawing cause crumbling and flaking of the porous sandstone and eventually cut through some of the fins.J! |
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# The resulting holes become enlarged to arch proportions by rockfalls and weathering. Arches eventually collapse, leaving only buttresses that in time will erode. |
# The resulting holes become enlarged to arch proportions by rockfalls and weathering. Arches eventually collapse, leaving only buttresses that in time will erode.J! |
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*Many of these arches are found within [[Arches National Park]] and [[Rainbow Bridge National Monument]] in Utah. |
*Many of these arches are found within [[Arches National Park]] and [[Rainbow Bridge National Monument]] in Utah. |
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[[Image:Coyote Natural Bridge map.jpg|thumb|A [[topographic map]] of Coyote Natural Bridge in Utah shows how the [[meander]]ing [[Coyote Gulch]] carved a shorter route through the rock under the arch. The old riverbed is now higher than the present water level.]] |
[[Image:Coyote Natural Bridge map.jpg|thumb|A [[topographic map]] of Coyote Natural Bridge in Utah shows how the [[meander]]ing [[Coyote Gulch]] carved a shorter route through the rock under the arch. The old riverbed is now higher than the present water level.]] |
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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 |
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 J below. |
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[[Natural Bridges National Monument]] in Utah is another area to view several natural bridges. |
[[Natural Bridges National Monument]] in Utah is another area to view several natural bridges. |
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== |
==J erosion== |
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Natural bridges can form from natural [[limestone]] [[cave]]s, where paired [[sinkhole]]s collapse and a ridge of stone is left standing in between, with the cave passageway connecting from sinkhole to sinkhole. |
Natural bridges can form from natural [[limestone]] [[cave]]s, where paired [[sinkhole]]s collapse and a ridge of stone is left standing in between, with the cave passageway connecting from sinkhole to sinkhole. |
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Like all |
Like all J formations, natural bridges are subject to continued erosion, and will eventually collapse and disappear. One example of this was the double-arched [[Victoria (Australia)|Victoria]]n coastal rock formation, [[London Arch|London Bridge]], which lost an arch after storms increased erosion.<ref>[http://www.trekearth.com/gallery/Oceania/Australia/photo206866.htm Trek Earth]</ref> |
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==Arches as |
==Arches as J-ways== |
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In a few places in the world, natural arches are truly natural bridges because there are roads running across them. Two such arches are found in Kentucky. One, a cave erosion arch made of limestone is located in [[Carter Caves State Resort Park|Carter Caves State Park]] and it has a paved road on top. Another, a weather-eroded sandstone arch with a dirt road on top is located on the edge of [[Natural Bridge State Park (Kentucky)|Natural Bridge State Resort Park]] in [[Kentucky]]. It 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 a few places in the world, natural arches are truly natural bridges because there are roads running across them. Two such arches are found in Kentucky. One, a cave erosion arch made of limestone is located in [[Carter Caves State Resort Park|Carter Caves State Park]] and it has a paved road on top. Another, a weather-eroded sandstone arch with a dirt road on top is located on the edge of [[Natural Bridge State Park (Kentucky)|Natural Bridge State Resort Park]] in [[Kentucky]]. It 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. |
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Revision as of 13:00, 19 October 2010
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![](http://upload.wikimedia.org/wikipedia/commons/thumb/1/13/Delicate_Arch_LaSalle.jpg/220px-Delicate_Arch_LaSalle.jpg)
A natural arch or natural bridge is a natural geological formation where a rock arch forms, with an opening underneath. Most natural arches form as a narrow ridge, walled by cliffs, become narrower from erosion, with a softer rock stratum under the cliff-forming stratum gradually eroding out until the rock shelters thus formed meet underneath the ridge, thus forming the arch. Natural arches commonly form where cliffs are subject to erosion from the sea, rivers or weathering (subaerial processes); the processes "find" weaknesses in rocks and work on them, making them bigger until they break through.
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]
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, e.g., London Bridge in (Victoria, Australia). When these 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 breaks through the strong rock and then erodes the weak rock very quickly. Good examples of this are at Durdle Door and Stair Hole near Lulworth Cove on the Dorset Jurassic Coast in south England, although these are on an area of concordant coastline. When Stair Hole eventually collapses, it will form a cove.
Weather-eroded arches
- Deep cracks penetrate into a sandstone layer. RJ!
- Erosion wears away exposed rock layers and enlarges the surface cracks, isolating narrow sandstone walls, or fins. J!
- Alternating frosts and thawing cause crumbling and flaking of the porous sandstone and eventually cut through some of the fins.J!
- The resulting holes become enlarged to arch proportions by rockfalls and weathering. Arches eventually collapse, leaving only buttresses that in time will erode.J!
- Many of these arches are found within Arches National Park and Rainbow Bridge National Monument in Utah.
Water-eroded arches
![](http://upload.wikimedia.org/wikipedia/commons/thumb/7/7a/Coyote_Natural_Bridge_map.jpg/220px-Coyote_Natural_Bridge_map.jpg)
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 J below.
Natural Bridges National Monument in Utah is another area to view several natural bridges.
J 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 J 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.[3]
Arches as J-ways
In a few places in the world, natural arches are truly natural bridges because there are roads running across them. Two such arches are found in Kentucky. One, a cave erosion arch made of limestone is located in Carter Caves State Park and it has a paved road on top. Another, a weather-eroded sandstone arch with a dirt road on top is located on the edge of Natural Bridge State Resort Park in Kentucky. It 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.
Gallery
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Natural Bridge National Historic Landmark in Virginia, United States.
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Pont d'Arc arch in Southern France.
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Durdle Door on the Dorset coast, United Kingdom.
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The Arch of Cabo San Lucas in Mexico.
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Pravčická brána in Bohemian Switzerland, Czech Republic.
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Natural Bridge at Bryce Canyon National Park, Utah, United States.
Notable natural arches
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Anacapa Island, Channel Islands National Park, California, USA
Arch Creek Historic and Archeological Site, Florida, USA
Arches National Park, Utah, USA
Ayres Natural Bridge State Park, Wyoming, USA
Azure Window, Gozo, Malta
Bryce Canyon National Park, Utah, USA
Castell de Castells, Spain
Creelsboro Natural Bridge, Kentucky, USA
El Arco de Cabo San Lucas, Mexico
Engetsu-tō, Shirahama, Wakayama, Japan
Étretat, France
God's bridge, Ponoarele, Mehedinti, Romania
Goat Rock Beach, California, USA
Grosvenor Arch, Utah, USA
Kolob Arch, Utah, USA
Llanrhaeadr-ym-Mochnant, Wales, UK
Natural Bridge, Alabama, USA
Natural Bridge Caverns, Texas, USA
Natural Bridge State Park, Kentucky, USA
Natural Bridge State Park, Massachusetts, USA
Natural Bridge, Virginia, USA
Natural Bridges National Monument, Utah, USA
Natural Bridges State Beach, California, USA
Natural Arch, Tirumala hills - Tirumala, India
Parque Nacional de Sete Cidades, Brazil
Percé Rock, Quebec, Canada
Pont d'Arc, France
Pravčická brána, Czech Republic
Punarjani Guha - natural tunnel, India
Rainbow Cave (actually an arch) in the Galilee
Rattlesnake Canyon, Colorado, USA
Rock Bridge of Gulanchwadi, Narayangaon Maharashtra, India[4]
Senkanmatsu-shima, Iwami, Tottori, Japan
Sewanee Natural Bridge, Tennessee, USA
Shipton's Arch, Xinjiang, China
Springbrook National Park, Queensland, Australia
Tassili n'Ajjer - National Park in Algeria with many arches
Tonto Natural Bridge, Arizona, USA
Tukuyu natural bridge, Tanzania
References
- ^ Natural Arch and Bridge Society, FAQ.
- ^ American Geological Institute, Dictionary of Geological Terms, 1976, Doubleday Anchor
- ^ Trek Earth
- ^ Offbeat Tracks in Maharashtra - A Travel Guide - Book by Milind Gunaji ISBN 81-7154-669-2
External links
![](http://upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/30px-Commons-logo.svg.png)