Significations et usages de Valley

  U-shaped valley in Glacier National Park in Montana, United States.

  Fljótsdalur in East Iceland, a rather flat valley (in Scotland, this type of valley is called a "strath")

  A glaciated valley in the Mount Hood Wilderness showing a characteristic U-shape, the bottom's rocky 'rubble' accretion and the broad shoulders

In geology, a valley or dale 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.

  Name

A valley in its broadest geographic sense is also known as a dale. A valley through which a river runs may also be referred to as a vale. A small, secluded, and often wooded valley is known as a dell or in Scotland as a glen. A wide, flat valley through which a river runs is known in Scotland as a strath. A mountain cove is a small valley, closed at one or both ends, in the central or southern Appalachian Mountains which sometimes results from the erosion of a geologic window. A small valley surrounded by mountains or ridges is sometimes known as a hollow. A deep, narrow valley is known as a coon (also spelled combe or coombe). Similar geological structures, such as canyons, ravines, gorges, gullies, and kloofs, are not usually referred to as valleys.

  River valleys

For a comprehensive list of world wide river valleys see: River valleys (category)

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 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.

Some broad V examples are:

• North America: Black Canyon of the Gunnison National Park, and others in Grand Canyon NP
• Europe:
  • Austria: narrow passages of upper Inn valley (Inntal), affluents of Enns a.s.o
  • Switzerland: Napf region, Zurich Oberland, Engadin
  • Germany: affluents to the middle reaches of Rhine and Mosel

Some of the first human complex societies originated in river valleys, such as that of the Nile, Tigris-Euphrates, Indus, Ganges, Yangtze, Huang He, Mississippi, and arguably Amazon. In prehistory, the rivers were used as a source of freshwater and food (fish and game), as well as a place to wash and a sewer. The proximity of water moderated temperature extremes and provided a source for irrigation, stimulating the development of agriculture. Most of the first civilizations developed from these river valley communities.

  Vale

In geography, a vale is a wide river valley, usually with a particularly wide flood plain or flat valley bottom. Vales commonly occur between the escarpment slopes of pairs of chalk downs, where the chalk dome has been eroded, exposing less resistant underlying rock, usually clay.

  Glacial valleys

  U shaped valley on the Afon Fathew near Dolgoch

  Yosemite Valley from an airplane

There are various forms of valley associated with glaciation that may be referred to as glacial valleys.

  U-shaped valley

A valley carved by glaciers 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 include:

• 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 in Montana, USA.

  Tunnel valley

A tunnel valley is a large, long, U-shaped valley originally cut under the glacial ice near the margin of continental ice sheets such as that now covering Antarctica and formerly covering portions of all continents during past glacial ages.[1]

A tunnel valley can be up to 100 km (62 mi) long, 4 km (2.5 mi) wide, and 400 m (1,300 ft) deep (its depth may vary along its length).

Tunnel valleys were formed by subglacial erosion by water and served as subglacial drainage pathways carrying large volumes of melt water. Their cross-sections exhibit steep-sided flanks similar to fjord walls, and their flat bottoms are typical of subglacial glacial erosion.

  Meltwater valley

In northern Central Europe, the Scandinavian ice sheet during the various ice ages advanced slightly uphill against the lie of the land. As a result its meltwaters flowed parallel to the ice margin to reach the North Sea basin, forming huge, flat valleys known as Urstromtäler. Unlike the other forms of glacial valley, these were formed by glacial meltwaters.

  Transition forms and valley shoulders

  Look from Paria View to a valley in Bryce Canyon, Utah, with very striking shoulders

Depending on the topography, the rock types and the climate, a lot of transitional 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 canyons 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

  Bridal Veil Falls in Yosemite National Park flowing from a hanging valley.

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.^[1] Hanging valleys are also the product of varying rates of erosion of the main valley and the tributary valleys. The varying rates of erosion are associated with the valleys rock composition of the adjacent rocks in the different valley locations. The tributary valleys are eroded and deepend by glaciers at a slower rate than that of the main valley floor. Thus the difference in the two valleys depth increases over time. The tributary valleys that were composed of more resistant rock then hangs over the main valley.^[2]

  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, similar to a flat cylinder, or terraced.

Side valleys are rather V than U-shaped; near the mouth waterfalls 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

  Wheat in the Hula Valley, Israel

A hollow is a small valley or dry stream bed. This term is commonly used in New England, Appalachia, Arkansas and Missouri to describe such geographic features, in Appalachia it may be pronounced as "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.

  Famous valleys

  The Valley of Flowers in Uttarakhand, India.

  The Harau Valley in West Sumatra, Indonesia

  Hell's Gate, British Columbia

  A view of Orosí, Costa Rica

• Aburra Valley (Colombia)
• Barossa Valley (Australia)
• Cauca Valley (Colombia)
• Danube Valley (Eastern Europe)
• Death Valley (California, United States)
• Fraser Canyon (British Columbia, Canada)
• Fraser Valley (British Columbia, Canada)
• Glen Coe (Scotland, United Kingdom)
• Grand Canyon (Arizona, United States)
• Great Glen (Scotland, United Kingdom)
• Hell's Gate (British Columbia, Canada)
• Hunter Valley (Australia)
• Hutt Valley (New Zealand)
• Indus Valley (Pakistan)
• Iron Gate (Romania/Serbia)
• Jordan Valley
• Las Vegas Valley (Nevada, United States)
• Little Cottonwood Creek Valley (Utah, United States)
• Loire Valley with its famous castles (France)
• Nant Ffrancon (Wales, United Kingdom)
• Napa Valley (California, United States)
• Nile Valley (Egypt/Sudan/Ethiopia/Uganda)
• Okanagan Valley (British Columbia, Canada)
• Ottawa Valley (Ontario/Quebec, Canada)
• Palo Duro Canyon (Texas, United States)
• Panjshir Valley (Afghanistan)
• Upper Rhine Valley, an old graben system. (France)
• Rhone Valley from the Matterhorn to Grenoble and Lyon (France)
• Rio Grande Valley (Texas, United States)
• Saint Lawrence Valley, the most densely populated region of Canada. (Ontario/Quebec/New York) (Canada/United States)
• San Fernando Valley (California, United States)
• Shenandoah Valley (Virginia/West Virginia, United States)
• Silicon Valley, (California, United States)
• Sonoma Valley, (California, United States)
• South Wales Valleys (Wales, United Kingdom)
• Valley of flowers (India)
• Valley of the Kings (Egypt)
• Valley of Mexico (Mexico), also known as "El Valle de México", see Mexico City
• Valley of the Sun (Arizona, United States)
• Willamette Valley, (Oregon, United States)
• Jordan Rift Valley (Israel)

  Rift valleys

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

  See also

Geography portal

• Canyon, Grass valley, Stream channel, Gully
• Cliff, Glacial landforms, Side valley
• Geography, Geomorphology, Geodynamics, Glaciology
• Landform
  • List of landforms
  • List of mountain ranges
• Geological features of the solar system
  • List of Lunar valleys
  • List of lineae on Europa
  • List of valles on Mars
  • List of largest rifts and valleys in the Solar System
  • List of geological features on Titan, (escarpments and ruptures).

  References

  External links

Look up Valley in Wiktionary, the free dictionary.

Wikimedia Commons has media related to: Valleys

• NPS.gov, University of Wisconsin
• Glacial moraine types (LEO dictionary)
• UWSP.edu, Glossary of Alpine Glacial Landforms
• BGU.ac.ilPDF, SAR interferometry (analysis of valley forms in Fig.2 and 6)
• Braggiotourismus.ch, Shoulder of the Swiss Calanca valley near Braggio
• Zum.de, Typical valley sections (valleys and terrace valleys)
• Kented.org.uk, V-shaped valley
• Valleys.com, Valleys of the World

  Extraterrestrial valleys

• ESA image: Vallis Alpes, bisecting the Lunar Alps
• Valles Marineris and Ophir Chasma, bilingual website (English and German)