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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.]]


# Deep cracks penetrate into a sandstone layer.
# 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.
# 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.
# 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.
# 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.
*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.]]


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.
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.
[[Natural Bridges National Monument]] in Utah is another area to view several natural bridges.


==Cave erosion==
==J erosion==
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.


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 [[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>
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>


==Arches as highways==
==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 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.


Revision as of 13:00, 19 October 2010

"Natural Bridges" redirects here. For the U.S. National Monument, see Natural Bridges National Monument.
The Delicate Arch in Arches National Park in Utah, United States.

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

File:Arch form.gif
A diagram showing the sequence of arch formation.
  1. Deep cracks penetrate into a sandstone layer. RJ!
  2. Erosion wears away exposed rock layers and enlarges the surface cracks, isolating narrow sandstone walls, or fins. J!
  3. Alternating frosts and thawing cause crumbling and flaking of the porous sandstone and eventually cut through some of the fins.J!
  4. The resulting holes become enlarged to arch proportions by rockfalls and weathering. Arches eventually collapse, leaving only buttresses that in time will erode.J!

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.

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.

Notable natural arches

References

  1. ^ Natural Arch and Bridge Society, FAQ.
  2. ^ American Geological Institute, Dictionary of Geological Terms, 1976, Doubleday Anchor
  3. ^ Trek Earth
  4. ^ Offbeat Tracks in Maharashtra - A Travel Guide - Book by Milind Gunaji ISBN 81-7154-669-2
Wikimedia Commons has media related to Natural arches.
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