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A canyon, or Gorge, is a valley walled by cliffs. Most canyons originate by a process of long-time erosion from a plateau level, with a stream gradually carving out its valley. The cliffs form because harder rock strata that are resistant to erosion and weathering remain exposed on the valley walls. Canyons are much more common in arid areas than in wetter areas because weathering has a lesser effect in arid zones. Canyons walls are often formed of resistant sandstones or granite.

The word canyon is Spanish in origin; in some parts of the United States, canyons are sometimes called gorges or notches.

In the southwestern United States, canyons are important archeologically because of the many cliff-dwellings built there, largely by the Anasazi people.

Sometimes large rivers run through canyons as the result of gradual geologic uplift. These are called entrenched rivers, because they are unable to easily alter course. The Colorado River and the Snake River in the northwestern United States are two examples of this.

Canyons often form in areas of limestone rock. Limestone is to a certain extent soluble, so cave systems form in the rock, when these collapse a canyon is left, for example in the Mendip Hills in Somerset and Yorkshire Moors in Yorkshire, England.

The world's two largest canyons are the:

Other well-known canyon systems include: The Blue Mountains west of Sydney in New South Wales, Australia contain many canyons carved into the sandstone rock.

See also: list of landforms, geomorphology, geology, earth science.