Valley

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This article is about the physical-geographic term. For places named "Valley" see Valley (disambiguation).

Image:Fljótsdalur.jpg Image:MountGilbertNV.jpg A valley is a landform, which can range from a few square miles (square kilometers) to hundreds or even thousands of square kilometers in area. It is typically a low-lying area of land, surrounded by higher areas such as mountains or hills.

1 Genesis of Valleys
2 Valley floors
3 Hollows
4 Famous valleys
5 Extraterrestrial valleys
6 See also
7 Weblinks of extraterrestrial valleys

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Genesis of Valleys

Valleys are formed by numerous geographical processes. Glacial valleys, which are usually U- rather than V-shaped, were formed tens of thousands of years ago (most likely during the last Ice Age) by the massive erosive power of glaciers. Several glacial valleys can be found in the English Lake District and many can be found in Alpine countries. 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. Valleys are, however, most commonly formed by fluvial activity (the action of running water, such as rivers), which erodes the landscape.

Image:ValleyProfile3.png

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Valley floors

Usually the bottom of a main valley is broad - independent of the U or V shape. It ranges from about 1 to 5 km and is 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 avalanchess or landslides. Predominant are places on terraces or Alluvial fans if they exist.
The villages of the primary valleys, however, have to consider mainly the danger of possible floodings.

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Hollows

A hollow is loose name for a valley in the earth. It is commonly used in New England and Missouri to describe such geographic features. 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).

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Famous valleys

California Central Valley (United States)
Copper Canyon
Danube Valley (Eastern Europe, Wachau, Iron Gate)
Death Valley (United States)
Grand Canyon (United States)
Great Rift Valley (from Jordan to the Red Sea and Lake Victoria)
Loire Valley with its famous castles (France)
Napa Valley (United States)
Upper Rhine Valley (an old graben system) (France)
Rhone Valley from the Matterhorn to Grenoble and Lyon (France)
Shenandoah Valley (United States)
Valley of the Kings (Egypt)
San Fernando Valley (United States)
Santa Clara Valley, perhaps better known as "Silicon Valley" (United States)

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Extraterrestrial valleys

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

Besides the lunar craters, the details of mountain ranges (Lunar Alps, Carpathes etc.) 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 in discussion up to the Apollo 11 mission of summer 1969.

Astronomers have long been able to observe some highlands and the "Nix Olympica" on Mars, and therefore guessed that there may be extraterrestrial valleys, too. In the 1970s this interpretation was proven correct by a series of space probes. Valleys have also been found on Mercury and on the volcanic surfaces of Venus and Io.

Image:ValMarinerisPIA00422 modest.jpg 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, 4,500 x 600 km in area and up to 8 km in depth. 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. In later stages, however, it has been expanded considerably by erosion, possibly including the action of surface water.

Several volcanic or icy moons of the gas planets Jupiter, Saturn and Neptune were also photographed by the two Voyagers, as well as 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 graben or active geologic lines on Earth.

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