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Jai Ganesh

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Registered: 2005-06-28

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Valley

Valley

Gist

A valley is a low area of land between hills or mountains, often with a river or stream flowing through it. It's essentially a depression in the landscape, shaped by natural processes like erosion from water or ice.

The term "the Valley" can refer to different locations depending on context. In the Los Angeles area, it typically refers to the San Fernando Valley. However, it could also refer to Silicon Valley, particularly in the context of northern California. Additionally, "the Valley" can refer to other places like The Valleys in South Wales, UK, or even a specific village in Ireland.

Summary

A valley is an elongated low area often running between hills or mountains and typically containing a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams over a very long period. Some valleys are formed through erosion by glacial ice. These glaciers may remain present in valleys in high mountains or polar areas.

At lower latitudes and altitudes, these glacially formed valleys may have been created or enlarged during ice ages but now are ice-free and occupied by streams or rivers. In desert areas, valleys may be entirely dry or carry a watercourse only rarely. In areas of limestone bedrock, dry valleys may also result from drainage now taking place underground rather than at the surface. Rift valleys arise principally from earth movements, rather than erosion. Many different types of valleys are described by geographers, using terms that may be global in use or else applied only locally.

Formation of valleys

Valleys may arise through several different processes. Most commonly, they arise from erosion over long periods by moving water and are known as river valleys. Typically small valleys containing streams feed into larger valleys which in turn feed into larger valleys again, eventually reaching the ocean or perhaps an internal drainage basin. In polar areas and at high altitudes, valleys may be eroded by glaciers; these typically have a U-shaped profile in cross-section, in contrast to river valleys, which tend to have a V-shaped profile. Other valleys may arise principally through tectonic processes such as rifting. All three processes can contribute to the development of a valley over geological time. The flat (or relatively flat) portion of a valley between its sides is referred to as the valley floor. The valley floor is typically formed by river sediments and may have fluvial terraces.

Details

Valleys are depressed areas of land–scoured and washed out by the conspiring forces of gravity, water, and ice. Some hang; others are hollow. They all take the form of a "U" or "V."

Rivers and streams make most primary valley cuts, carving steep-walled sides and a narrow floor that from afar looks like the letter "V." The gradient of the river—how quickly it drops—helps define the steepness of the sides and the width of the floor. Mountain valleys, for example, tend to have near-vertical walls and a narrow channel, but out on the plains, the slopes are shallow and the channel is wide.

As waters wind toward the sea, they add to natural twists in the land by stripping sediment from the outsides of bends and dumping it on the insides. The bulk of the rock and dirt is dredged from the bottom of the channel, a process called down cutting that can ultimately lead to deep, slender chasms like Black Canyon in Colorado's Gunnison National Park.

Types of Valleys

Some river and stream valleys, especially those in the mountains or located near the North and South Poles, are transformed by glaciers.

The massive blocks of snow and ice slowly creep downhill where they will meet the least resistance: valleys already cut by rivers and streams. As the glaciers ooze, they pick up rocks and grind away at the valley floor and sides, pressing the "V" into a "U." When the glacier melts, a U-shaped valley marks the spot where the snow and ice once flowed.

Side valleys are formed by tributaries to streams and rivers and feed the main stem. Where the main channel is carved deeper than the tributary, as commonly occurs during glaciations, the side valleys are left hanging. Waterfalls often cascade from the outlet of the upper valley into the drainage below.

Hollows, like those in Appalachia, are small valleys nestled between mountains or hills.

Giant valleys, called rifts, are found where two pieces of Earth's crust are separated or split apart. One such example is the Great Rift Valley, a rift system stretching from the Middle East to southern Africa.

Additional Information

A valley is an 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 Kāli Gandaki River in Nepal. Lying between two 8,000-metre (26,000-foot) Himalayan peaks, Dhaulāgiri and Annapūrna, 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 (1 mile) 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.

Geomorphic characteristics

The relief of valleys and canyons is produced by the incising action of rivers. Hillslope processes are indeed critical in the development of valley sides (see below), but it is rivers that lower the level of erosion through degradation. Rivers ultimately adjust to a baselevel, defined as the lowest point at which potential energy can be transformed to the kinetic energy of river flow. In most cases, the ultimate baselevel for rivers is sea level. Some rivers drain to enclosed basins below sea level, as, for example, the Jordan River, which flows to the Dead Sea in Israel and Jordan. Moreover, rivers may adjust to local baselevels, including zones of resistance to incision, lakes, and dams (both natural and artificial).

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