Natural arch

"Natural Bridges" redirects here; for the US National Monument, see Natural Bridges National Monument.

Delicate Arch, Utah, USA

A natural arch or natural bridge is a natural formation (or landform) where a rock arch forms, with a natural passageway through 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 (sub-aerial processes); the processes "find" weaknesses in rocks and work on them, making them bigger until they break through.

The choice of "bridge" vs "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^[2] defines a natural bridge as a "natural arch that spans a valley of erosion."

Coastline

On coasts this can form two different types of arches 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

Sequence of arch formation

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

• Many of these arches are found within Arches National Park and Rainbow Bridge National Monument in Utah, USA.

Water-eroded arches

A topographic map of Coyote Natural Bridge in Utah, USA 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 is a good place to view natural bridges.

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[1].

Arches as highways

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. 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

Natural Bridge National Historic Landmark in Virginia	Natural stone arch in Tirumala - Tirupati, India	London Arch, formerly "London Bridge" in Victoria, Australia	Pont d'Arc arch, S France.
Durdle Door, Dorset coast, United Kingdom	Rainbow Bridge was formed by a meandering watercourse. Utah (Navajo Nation), USA	Delicate Arch, Utah, USA	Arch in Timna valley Park, Negev Desert, Israel.
Rainbow Cave arch in the Galilee, Israel.	Natural Arch (sandstone) in Daniel Boone National Forest in Kentucky.	Pravčická brána in Bohemian Switzerland, Czech Republic.	Natural Bridge at Bryce Canyon National Park, Utah. Actually an arch.

See also

• 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
• El Arco de Cabo San Lucas, Mexico
• Bryce Canyon National Park, Utah, USA
• Castell de Castells, Spain
• Creelsboro Natural Bridge , Kentucky, USA
• Engetsu-tō, Shirahama, Wakayama, Japan
• Étretat, France
• Goat Rock Beach, California, USA
• Grosvenor Arch, Utah, USA
• Kolob Arch, Utah, USA
• Llanrhaeadr-ym-Mochnant, Wales, UK
• 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
• 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
• 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

Notes

1. ^ Natural Arch and Bridge Society, FAQ.
2. ^ American Geological Institute, Dictionary of Geological Terms, 1976, Doubleday Anchor

External links

Wikimedia Commons has media related to: Natural arches

• The Natural Arch and Bridge Society homepage