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Valley

In geology, a '''valley''' (also called a '''vale''', '''dale''', '''glen''' or '''strath''' and near or in Appalachia, a '''draw''') is a Depression (geology)|depression with predominant extent in one direction. A very deep river valley may be called a canyon or '''gorge'''. The terms U-shaped valley|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 (geometry)|cross section of the slopes or hillsides.

River valleys

- ''For a comprehensive list of world wide river valleys see: :Category:River valleys|River valleys (category)'' A valley formed by River|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:
- USA: Black Canyon of the Gunnison National Park, and others in Grand Canyon National Park|Grand Canyon NP
- Alpine Europe:
    - Austria: narrow passages of upper Inn valley (Inntal), affluents of Enns River|Enns a.s.o
    - Switzerland: Napf region, Zurich Oberland, Engadin
    - Germany: affluents to the middle reaches of Rhine and Moselle River|Mosel

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 Age|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|glaciation and Abrasion (geology)|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
- Yosemite Valley (USA)
- Side valleys of the Austrian river Salzach for their parallel directions and hanging mouths.
- Some Scottish glens full with bushes and flowers.
- That of the St. Mary River in Glacier National Park (United States)|Glacier National Park in Montana, USA.

Transition forms and valley shoulders

In some stress-tectonic regions of the Rocky mountains|Rockies or the Alps (e.g. Salzburg) the side valleys are parallel to each other, and additionally they are hanging valleys|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 Cirque (landform)|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 V 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 plane|horizontal, similar to a flat Cylinder (geometry)|cylinder, or stream terrace|terraced. Side valleys are rather V than U-shaped; near the mouth clammy|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 (hydrology)|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

- California Central Valley (United States)
- Copper Canyon
- Danube Valley (Eastern Europe, Wachau, Iron Gate (Danube)|Iron Gate)
- Death Valley (California)
- Glen Coe (Scotland)
- Grand Canyon (Arizona)
- Great Glen (Scotland)
- Great Rift Valley (from Jordan to the Red Sea and Lake Victoria)
- Indus Valley (Pakistan)
- Little Cottonwood Creek Valley, Utah
- Loire Valley with its famous castles (France)
- Nant Ffrancon Valley (Wales)
- Napa County, California|Napa Valley (California)
- Nile Valley (Egypt)
- Okanagan Valley (Canada)
- Owens Valley (California)
- Panjshir Valley
- Upper Rhine Valley (an old graben system) (France)
- Rhone Valley from the Matterhorn to Grenoble and Lyon (France)
- Rio Grande Valley (United States)
- Shenandoah Valley (United States)
- Sonoma Valley, California, United States|USA
- Valley of flowers (India)
- Valley of the Kings (Egypt)
- Valley of the Sun (Phoenix, Az, USA)
- San Fernando Valley (California)
- Santa Clara Valley, perhaps better known as "Silicon Valley" (California)
- South Wales Valleys (Wales)
- Valley of Mexico (Mexico), also known as "El Valle de México" see Mexico city

Rift valleys

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

Extraterrestrial valleys

The other terrestrial planets and the natural satellites|moons of our Solar System can also have valley-like features. List of valleys on the Moon|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 Impact crater|craters, the details of lunar mountain ranges have been well known for more than 300 years (e.g. Johann Hieronymus Schröter|J.H. Schröter's ''Selenotopographische Fragmente'' of 1791). A lot of linear phenomena like Vallis Rheita|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 Highland (geography)|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 (planet)|Mercury and on the volcanic surfaces of Venus and Io (moon)|Io. The largest valley in our solar system is the Valles Marineris formation on Mars. The ''Valles'' (which were first detected in 1877 by Martian canals|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 plate tectonics|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. Ice|Icy natural satellite|moons of the gas planets Jupiter, Saturn and Neptune were also photographed by the two Voyager program|Voyagers as well as by other space probes. Some linear ruptures in the ice or apparent low areas between hills have been interpreted by astrogeology|astrogeologists as tectonic structures or valleys similar to grabens or active geologic Rift (geology)|rifts on Earth.