Valley

Valley

[val-ee]
Valley, The, town (2001 pop. 1,169), capital of the British dependency of Anguilla, in the West Indies. Located in the approximate center of the island, it is Anguilla's main town and its administrative center. Tourism is an economic mainstay. Among its attractions are the Anguilla National Museum, the Wallblake House (1787) and other examples of colonial architecture, and the Old Court House ruins.

In geology, a valley (also called a vale, dale, glen or strath and near or in Appalachia, a draw) is a depression with predominant extent in one direction. A very deep river valley may be called a canyon or gorge.

The terms U-shaped and V-shaped are descriptive terms of geography to characterize the form of valleys. Most valleys belong to one of these two main types or a mixture of them, at least with respect of the cross section of the slopes or hillsides.

River valleys

For a comprehensive list of world wide river valleys see: River valleys
A valley formed by flowing water, or river valley, is usually V-shaped. The exact shape will depend on the characteristics of the stream flowing through it. Rivers with steep gradients, as in mountain ranges, produce steep walls and a narrow bottom. Shallower slopes may produce broader and gentler valleys, but in the lowest stretch of a river, where it approaches its base level, it begins to deposit sediment and the valley bottom becomes a floodplain.

A V-shaped valley is formed by downcutting when the flowing stream erodes its channel at a higher rate than the sides are eroded. The resulting landform is a narrow canyon with fast water and little bank (floodplain) on the river sides.

Some broad V examples are:

Glacial valleys

A valley carved by glaciers, or glacial valley, is normally U-shaped. The valley becomes visible upon the recession of the glacier that forms it. When the ice recedes or thaws, the valley remains, often littered with small boulders that were transported within the ice. Floor gradient does not affect the valley's shape, it is the glacier's size that does. Continuously flowing glaciers - especially in the ice age - and large sized glaciers carve wide, deep incised valleys.

Examples of U-shaped valleys are found in every mountainous region that has experienced glaciation, usually during the Pleistocene ice ages. Most present U-shaped valleys started as V-shaped before glaciation. The glaciers carved it out wider and deeper, simultaneously changing the shape. This proceeds through the glacial erosion processes of glaciation and abrasion, which results in large rocky material (glacial till) being carried in the glacier. A material called boulder clay is deposited on the floor of the valley. As the ice melts and retreats, the valley is left with very steep sides and a wide, flat floor. A river or stream may remain in the valley. This replaces the original stream or river and is known as a misfit stream because it is smaller than one would expect given the size of its valley.

Other interesting glacially-carved valleys are the

Transition forms and valley shoulders

    Depending on the topography, the rock types and the climate, a lot of transition forms between V-, U- and plain valleys exist. Their bottoms can be broad or narrow, but characteristic is also the type of valley shoulder. The broader a mountain valley, the lower its shoulders are located in most cases. An important exception are canyons where the shoulder almost is near the top of the valley's slope. In the Alps - e.g. the Tyrolean Inn valley - the shoulders are quite low (100-200 meters above the bottom). Many villages are located here (esp. at the sunny side) because the climate is very mild: even in winter when the valley's floor is completely filled with fog, these villages are in sunshine.

In some stress-tectonic regions of the Rockies or the Alps (e.g. Salzburg) the side valleys are parallel to each other, and additionally they are hanging. The brooks flow into the river in form of deep gorges or waterfalls. Usually this fact is the result of a violent erosion of the former valley shoulders. A special genesis we find also at arêtes and glacial cirques, at every Scottish glen, or a northern fjord.

Hanging valleys

A hanging valley is a tributary valley with the floor at a higher relief than the main channel into which it flows. They are most commonly associated with U-shaped valleys when a tributary glacier flows into a glacier of larger volume. The main glacier erodes a deep U-shaped valley with nearly vertical sides while the tributary glacier, with a smaller volume of ice, makes a shallower U-shaped valley. Since the surfaces of the glaciers were originally at the same elevation, the shallower valley appears to be ‘hanging’ above the main valley. Often, waterfalls form at or near the outlet of the upper valley.

Valley floors

Usually the bottom of a main valley is broad - independent of the U or x shape. It typically ranges from about one to ten kilometers in width and is commonly filled with mountain sediments. The shape of the floor can be rather horizontal, similar to a flat cylinder, or terraced.

Side valleys are rather V than U-shaped; near the mouth clammies are possible if it is a hanging valley. The location of the villages depends on the across-valley profile, on climate and local traditions, and on the danger of avalanches or landslides. Predominant are places on terraces or alluvial fans if they exist.

Historic siting of villages within the mainstem valleys, however, have chiefly considered the potential of flooding.

Hollows

A hollow is a small valley or dry stream bed. This term is commonly used in New England, Arkansas, Missouri and Pennsylvania to describe such geographic features. The term is also used in Southern Appalachia, but pronounced "holler." Hollows may be formed by river valleys such as Mansfield Hollow or they may be relatively dry clefts with a notch-like characteristic in that they have a height of land and consequent water divide in their bases. A hollow such as this is Boston Hollow. Tourists in Europe can further visit a lot of karst, stalactite and ice hollows (e.g. in Slovenia and Austria).

Famous valleys

Rift valleys

Rift valleys, such as the Great Rift Valley, are formed by the expansion of the Earth's crust due to tectonic activity beneath the Earth's surface.

Extraterrestrial valleys

The other terrestrial planets and the moons of our Solar System can also have valley-like features. Lunar valleys can be formed from a linked chain of impact craters. Smaller valleys, known as rilles, may have originated from lava flows or from the contractions of cooling lava sheets.

Besides the lunar craters, the details of lunar mountain ranges have been well known for more than 300 years (e.g. J.H. Schröter's Selenotopographische Fragmente of 1791). A lot of linear phenomena like Rheita or Schröter valley and the famous Vallis Alpes (see also below) were observed with details less than 1 km (which corresponds to a coin seen from 5-10 km distance)—but the geological genesis was debated until the Apollo 11 mission in 1969. Astronomers have long been able to observe some highlands and mountains on Mars, and therefore guessed that there may be valleys, as well. In the 1970s this interpretation was proven correct by results from space probes. Valleys have also been found on Mercury and on the volcanic surfaces of Venus and Io.

The largest valley in our solar system is the Valles Marineris formation on Mars. The Valles (which were first detected in 1877 by Schiaparelli) are a huge canyon system spanning 4,500 x 600 km in area and having a depth up to 8 km. These enormous dimensions are 4-8 times greater than those of the American Grand Canyon. The Valles is currently understood to have been created by tectonic forces like the main grabens on Earth, rather than by running water. Later, though, it may have been expanded considerably by erosion, possibly including the action of surface water.

Icy moons of the gas planets Jupiter, Saturn and Neptune were also photographed by the two Voyagers as well as by other space probes. Some linear ruptures in the ice or apparent low areas between hills have been interpreted by astrogeologists as tectonic structures or valleys similar to grabens or active geologic rifts on Earth.

See also

References

External links

Extraterrestrial valleys

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