Canyon

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A canyon (occasionally spelled cañon) or gorge is a deep ravine between cliffs often carved from the landscape by a river. Rivers have a natural tendency to reach a baseline elevation, which is the same elevation as the body of water it will eventually drain into. This forms a canyon.[1] Most canyons were formed by a process of long-time erosion from a plateau level. 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 wet areas because physical weathering has a greater effect in arid zones. The wind and water from the river combine to erode and cut away less resistant materials such as shales. The freezing and expansion of water also serves to help form canyons. Water seeps into cracks between the rocks and freezes, pushing the rocks apart and eventually causing large chunks to break off the canyon walls.[2] Canyon walls are often formed of resistant sandstones or granite. Submarine canyons form underwater, generally at the mouths of rivers. The word canyon is Spanish in origin (cañón). The word canyon is generally used in the United States, while the word gorge is more common in Europe and Oceania, though it is also used in some parts of the United States and Canada. The military derived word defile is occasionally used in the United Kingdom.

A famous example is the Grand Canyon in Arizona with an average depth of one mile and a volume of 4.17 trillion cubic meters.[3] In the southwestern United States, canyons are important archeologically because of the many cliff-dwellings built there, largely by the earlier inhabitants, Ancient Pueblo Peoples.

The Yarlung Zangbo Grand Canyon (or Tsangpo Canyon), along the Yarlung Tsangpo River in Tibet, China, is regarded by some as the deepest canyon in the world at 18,000 feet[4], and is slightly longer than Grand Canyon making it one of the world's largest.[5]

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

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 Dales in Yorkshire, England.

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