Canyons exist below the rim of the land, below the horizon. These ragged scars on the face of the planet descend hundreds to thousands of feet below their surrounding landscape, giving it depth. Their widths may stretch for miles or mere feet. Sunlight may fill them or may never reach their darkened bottom regions. Winding through many is water, possibly the most powerful force on the planet.
Sudden, tremendous events in Earth’s history did not produce these landforms. Instead, it was mainly the slow, orderly process of erosion, the wearing away of the planet’s surface through the action of wind and water. While wind has played a part in their formation, its effect has been subtle. The true creator of a canyon is water, primarily in the form of a river. Over millions of years, water has scoured and cut away layer upon layer of rock, lowering a canyon’s floor and widening its walls. Although perhaps much more slowly, canyons created millions of years ago continue to be shaped in the present day. The erosive power of water is unrelenting.
The shape of the land
A canyon may be defined as a narrow, deep, rocky, and steep-walled valley carved by a swift-moving river. Its depth may be considerably greater than its width. Some sources use the words gorge, ravine, and chasm interchangeably with canyon. Others say they are all variations of steep-sided valleys normally with a stream or river flowing through them. A few make the distinction that canyons are usually found in arid (dry) regions characterized by plateaus, which are relatively level, large expanses of land that rise some 1,500 feet (457 meters) or more above their surroundings and have at least one steep side.
Canyons are incredibly diverse in their forms. The walls of some canyons are V-shaped and ragged; the walls of others are steeper and almost
smooth. Some canyons have been carved through sandstone and limestone and other types of sedimentary rock (rock formed by the accumulation and compression of sediment, which may consist of rock fragments, remains of microscopic organisms, and minerals). Others have been carved through multiple layers of igneous (pronounced IG-nee-us) rock, which is formed by the cooling and hardening of magma, melted rock material from within Earth, and metamorphic (pronounced meh-tah-MORE-fik) rock, whose texture or composition has been changed by extreme heat and pressure. Some canyons are dry; others are filled with rushing rivers. Some cover vast spaces; others are so narrow a person can barely squeeze through the walls.