Natural arch

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 larger 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]



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

A diagram showing the 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.[3]

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 layer below.

Natural Bridges National Monument in Utah is another area to view several 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.[4]

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 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.

The third one is found in Ponoarele village, in Romania. It is 60 m long, 13 m wide, features a stone arch 4 m thick, 20 m high, with a 9 m span. It is called God's Bridge (Podul lui Dumnezeu) and it is the only one in the world effectively used for traffic.


Notable natural arches

See also


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

External links

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • natural arch —    1. A residual portion of the roof of a subsurface karst cavity which has not collapsed. Such a natural arch may occur as a surface topographic feature, or as a part of a cave system [20].    2. A rock arch or very short natural tunnel;… …   Lexicon of Cave and Karst Terminology

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  • Natural Bridge — or Natural Bridge can refer to several things: Natural arch, a land formation sometimes referred to as a natural bridge Natural Bridge, Alabama Natural Bridge, New York Natural Bridge, Queensland, in the Gold Coast hinterland, Australia Natural… …   Wikipedia

  • Arch 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… …   Wikipedia

  • Arch Rock — may refer to:* Arch Rock (Mackinac Island), Michigan, USA * Arch Rock (Tasmania), Australia * Three Arch Rocks National Wildlife Refuge, Oregon, USAee also* Natural arch * Arches National Park …   Wikipedia

  • Natural Bridge State Park (Wisconsin) — Coordinates: 43°20′39″N 89°55′47″W / 43.34417°N 89.92972°W / 43.34417; 89.92972 …   Wikipedia

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  • Natural Bridge — a natural limestone bridge in western Virginia. 215 ft. (66 m) high; 90 ft. (27 m) span. * * * or natural arch Naturally created arch formation resembling a bridge. Most are erosion features that occur in sandstone or limestone. Some are formed… …   Universalium

  • arch — arch1 /ahrch/, n. 1. Archit. a. a curved masonry construction for spanning an opening, consisting of a number of wedgelike stones, bricks, or the like, set with the narrower side toward the opening in such a way that forces on the arch are… …   Universalium

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