VALLEY

elongate depression of the Earth's surface. Valleys are commonly drained by rivers and may be in a relatively flat plain or between ranges of hills or mountains. Valleys formed by the incision of rivers and by slope denudation are typically V-shaped; those formerly occupied by glaciers are characteristically U-shaped. Valley evolution is mainly controlled by climate and rock type; most valleys are in balance with the stream regime flowing through them. Formerly, all valleys were thought to be great chasms in the Earth opened up by cataclysmic tectonic events. Depressions formed in this way are not true valleys, although they are often called such; examples are Death Valley and the Great Valley of California. Very narrow, deep valleys cut in resistant rock and having steep, almost vertical sides are called canyons. They may reach depths of several thousand feet. Smaller valleys of similar appearance are called gorges. Both types are commonly cut in flat-lying strata but may occur in other geological situations. elongate depression of the Earth's surface. Valleys are most commonly drained by rivers and may occur in a relatively flat plain or between ranges of hills or mountains. Those valleys produced by tectonic action are called rift valleys. Very narrow, deep valleys of similar appearance are called gorges. Both of these latter types are commonly cut in flat-lying strata but may occur in other geological situations. Wherever sufficient rainfall occurs, opportunity exists for the land surface to evolve to the familiar patterns of hills and valleys. There are, of course, hyperarid environments where fluvial activity is minimal. There also are geomorphological settings where the permeability of rocks or sediments induce so much infiltration that water is unable to concentrate on the land surface. Moreover, some landscapes may be so young that insufficient time has elapsed for modification by fluvial action. The role of fluvial action on landscape, including long-term evolutionary processes, is considered here in detail. For additional information on fluvial and hillslope processes relating to valley formation, see river. Probably the world's deepest subaerial valley is that of the Kali Gandak River in Nepal. Lying between two 8,000-metre (26,000-foot) Himalayan peaks, Dhaulagiri and Annapurna, the valley has a total relief of six kilometres (four miles). Because the Himalayas are one of the Earth's most active areas of tectonic uplift, this valley well illustrates the principle that the most rapid downcutting occurs in areas of the most rapid uplift. The reason for this seeming paradox lies in the energetics of the processes of degradation that characterize valley formation. As will be discussed below, the steeper the gradient or slope of a stream, the greater its expenditure of power on the streambed. Thus, as uplift creates higher relief and steeper slopes, rivers achieve greater power for erosion. As a consequence, the most rapid processes of relief reduction can occur in areas of most rapid relief production. Perhaps the most famous example of a canyon is the Grand Canyon of the Colorado River in northern Arizona. The Grand Canyon is about 1.6 km (100 miles) deep and 180 metres (590 feet) to 30 km (19 miles) wide and occurs along a 443-km- (275-mile-) long reach where the Colorado River incised into a broad upwarp of sedimentary rocks. Victor R. Baker Additional reading Descriptions of valleys can be found in geomorphology textbooks, including M.J. Selby, Earth's Changing Surface: An Introduction to Geomorphology (1985); and Arthur L. Bloom, Geomorphology: A Systematic Analysis of Late Cenozoic Landforms (1978). The relevant fluvial phenomena are treated in David Knighton, Fluvial Forms and Processes (1984); and the relevant hillslope phenomena in M.J. Selby, Hillslope Materials and Processes (1982). More specialized topics include the following: Victor R. Baker, The Channels of Mars (1982); Theodore Oberlander, The Zagros Streams: A New Interpretation of Transverse Drainage (1965); and Julius Bdel, Climatic Geomorphology (1982; originally published in German, 1977). Victor R. Baker