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A valley is an elongated low area often running between hills or mountains, which will typically contain 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 of time. Some valleys are formed through erosion by glacial ice. These glaciers may remain present in valleys in high mountain 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 taking place underground rather than at the surface. Rift valleys arise principally from earth movements, rather than erosion. Many different types of valley 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 of time 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 altitude, 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. It is possible for all three processes to 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 and is typically formed by river sediments and may be terraced.
River valleys 
The development of a river valley is affected by the character of the bedrock over which the river or stream flows, the elevational difference between its top and bottom, and indeed the climate. Typically the flow will increase downstream and the gradient will decrease. In the upper valley, the stream will most effectively erode its bed through corrasion to produce a steep-sided V-shaped valley. The presence of more resistant rock bands, of geological faults, fractures, and folds may determine the course of the stream and result in a twisting course with interlocking spurs. In the middle valley, as numerous streams have coalesced, the valley is typically wider, the flow slower and both erosion and deposition may take place. More lateral erosion takes place in the middle section of a river's course, as strong currents on the outside of its curve eats at the bank. Conversely, deposition may take place on the inside of curves where the current is much slacker, the process leading to the river assuming a meandering character. In the lower valley, gradients are lowest, meanders may be much broader and a broader floodplain may result. Deposition dominates over erosion. A typical river basin or drainage basin will incorporate each of these different types of valley.
Some sections of stream or river valleys may have vertically incised their course to such an extent that the valley they occupy is best described as a gorge, ravine or canyon. Rapid down-cutting may result from localised uplift of the land surface or a rejuvenation of the watercourse as a result for example of a reduction in the base level to which the river is eroded, e.g. lowered global sea level during an ice age. Such rejuvenation may also result in the production of river terraces.
There are various forms of valley associated with glaciation. True glacial valleys are those that have been cut by a glacier which may or may not still occupy the valley at the present day. Such valleys may also be known as glacial troughs. They typically have a U-shaped cross-section and are characteristic landforms of mountain areas where glaciation has occurred or continues to take place.
The uppermost part of a glacial valley frequently consists of one or more 'armchair-shaped' hollows, or 'cirques', excavated by the rotational movement downslope of a cirque glacier. During glacial periods, for example the Pleistocene ice ages, it is in these locations that glaciers initially form and then, as the ice age proceeds extend downhill through valleys which have previously been shaped by water rather than ice. Abrasion by the movement of ice and particularly by rock material embedded within it causes the widening and deepening of the valley to produce the characteristic U or trough shape relatively steep, even vertical sides and a relatively flat bottom. Interlocking spurs associated with the development of river valleys are preferentially eroded to produce truncated spurs, typical of glaciated mountain landscapes. The upper end of the trough below the ice-contributing cirques may be a trough-end. Valley steps (or 'rock steps) can result from differing erosion rates due to both the nature of the bedrock (hardness and jointing for example) and the power of the moving ice. In places, a rock basin may be excavated which may later be filled with water to form a ribbon lake or else by sediments. Such features are found in coastal areas as fjords. The shape of the valley which results from all of these influences may only become visible upon the recession of the glacier that forms it. A river or stream may remain in the valley; if it is smaller than one would expect given the size of its valley, it can be considered an example of a misfit stream.
Other interesting glacially carved valleys include:
- Yosemite Valley (United States)
- Side valleys of the Austrian river Salzach for their parallel directions and hanging mouths.
- That of the St. Mary River in Glacier National Park in Montana, United States.
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. Such valleys 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 water erosion. They once 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.
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 shoulders
Depending on the topography, the rock types and the climate, a variety of transitional forms between V-, U- and plain[clarification needed] valleys can form. Their bottoms can be broad or narrow, but characteristic is also the type of valley shoulder[clarification needed]. 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 are hanging. Smaller streams flow into rivers as deep canyons or waterfalls.
A hanging valley is a tributary valley that is higher than the main valley. They are most commonly associated with U-shaped valleys, where 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.
Hanging valleys also occur in fjord systems under water. The branches of Sognefjord are much shallower than the main fjord. The mouth of Fjærlandsfjord is about 400 meters deep while the main fjord nearby is 1200 meters deep. The mouth of Ikjefjord is only 50 meters deep while the main fjord is around 1300 meters at the same point.
Glaciated terrain is not the only site of hanging streams and valleys. Hanging valleys are also simply the product of varying rates of erosion of the main valley and the tributary valleys. The varying rates of erosion are associated with the composition of the adjacent rocks in the different valley locations. The tributary valleys are eroded and deepened by glaciers or erosion at a slower rate than that of the main valley floor; thus the difference in the two valleys' depth increases over time. The tributary valley, composed of more resistant rock, then hangs over the main valley.
Trough-shaped valleys also form in regions of heavy topographic denudation. By contrast with glacial U-shaped valleys, there is less downward and sideways erosion. The severe downslope denudation results in gently sloping valley sides; their transition to the actual valley bottom is unclear. Trough-shaped valleys occur mainly in periglacial regions and in tropical regions of variable wetness. Both climates are dominated by heavy denudation.
Box valleys have wide, relatively level floors and steep sides. They are common in periglacial areas and occur in mid-latitudes, but also occur in tropical and arid regions.
Terms for valleys
There are many terms used for different sorts of valleys. They include:
- Dell: A small, secluded and often wooded valley.
- Cove: 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.
- Hollow: A term used in some regions for a small valley surrounded by mountains or ridges or alternatively (in Ireland, New England, Appalachia, and the Ozarks of Arkansas and Missouri) a small valley or dry stream bed.
- A steephead valley is a deep, narrow, flat bottomed valley with an abrupt ending.
- Erosional valley: A valley formed by erosion.
- Structural valley: A valley formed by geologic events such as drop faults or the rise of highlands.
- Dry valley: A valley not created by sustained surface water flow.
- Longitudinal valley: An elongated valley found between two nearly-parallel mountain chains.
- A strike valley is typically developed parallel to a cuesta from more readily eroded strata
British regional terms for valleys
The terms corrie, glen and strath are all anglicisations of Gaelic terms and are commonly encountered in place-names in Scotland and other areas where Gaelic was once widespread. Strath signifies a wide valley between hills, the floor of which is either level or slopes gently. A glen is a river valley which is steeper and narrower than a strath. A corrie is a basin-shaped hollow in a mountain. Each of these terms also occurs in parts of the world formerly colonised by Britain. Corrie is used more widely by geographers as a synonym for (glacial) cirque, as is the word cwm borrowed from Welsh.
The word dale occurs widely in place-names in the north of England and, to a lesser extent, in southern Scotland. As a generic name for a type of valley, the term typically refers to a wide valley, though there are many much smaller stream valleys within the Yorkshire Dales which are named "(specific name) Dale". Clough is a word in common use in northern England for a narrow valley with steep sides. Gill is used to describe a ravine containing a mountain stream in Cumbria and the Pennines. The term combe (also encountered as coombe) is widespread in southern England and describes a short valley set into a hillside. Other terms for small valleys such as hope, dean, slade, slack and bottom are commonly encountered in place-names in various parts of England but are no longer in general use as synonyms for valley.
The term vale is used in England and Wales to describe a wide river valley, usually with a particularly wide flood plain or flat valley bottom. In Southern England, vales commonly occur between the outcrops of different relatively erosion-resistant rock formations, where less resistant rock, often claystone has been eroded. An example is the Vale of White Horse in Oxfordshire.
Some of the first human complex societies originated in river valleys, such as that of the Nile, Tigris-Euphrates, Indus, Ganges, Yangtze, Yellow River, Mississippi, and arguably the Amazon. In prehistory, the rivers were used as a source of fresh water 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. Siting of settlements within valleys is influenced by many factors, including the need to avoid flooding and the location of river crossing points.
- List of valleys in India
- List of valleys in Pakistan
- Beqaa Valley (Lebanon)
- Emin Valley (Kazakhstan)
- Ihlara, Turkey
- Dang Valley (Western Nepal)
- Jordan Rift Valley (Jordan - Israel)
- Jordan Valley
- Kathmandu (Nepal)
- Mahaweli (Sri Lanka)
- Panjshir Valley (Afghanistan)
- Valleys of China
- Dalen, Telemark (Telemark, Norway)
- Bergensdalen (Vestland, Norway)
- Danube Valley (Eastern Europe)
- Glen Coe (Scotland, United Kingdom)
- Great Glen (Scotland, United Kingdom)
- Gudbrandsdalen (Oppland, Norway)
- Hallingdalen (Buskerud, Norway)
- Heddal (Telemark, Norway)
- Iron Gate (Romania/Serbia)
- Lauterbrunnen Valley (Bern, Switzerland)
- Loire Valley with its famous castles (France)
- Midt-Telemark (Telemark, Norway)
- Nant Ffrancon (Wales, United Kingdom)
- Numedalen (Buskerud, Norway)
- Østerdalen (Hedmark, Norway)
- Po Valley, (Italy)
- Rhone Valley from the Matterhorn to Grenoble and Lyon (France)
- Romsdalen (Møre Og Romsdal, Norway)
- Setesdal (Agder, Norway)
- South Wales Valleys (Wales, United Kingdom)
- Upper Rhine Valley or Upper Rhine Plain, an old graben system. (France and Germany)
- Vestfjorddalen (Norway)
- Caguas (Puerto Rico)
- Central Valley (California)
- Coachella Valley (California)
- Cumberland Valley (Maryland/Pennsylvania)
- Death Valley (California)
- Fraser Canyon (British Columbia)
- Fraser Valley (British Columbia)
- Grand Canyon (Arizona, United States)
- Hell's Gate (British Columbia)
- Hudson Valley (New York)
- Imperial Valley (California)
- Las Vegas Valley (Nevada)
- Missouri River Valley (Missouri)
- Monument Valley (Arizona, Utah)
- Napa Valley (California)
- Okanagan Valley (British Columbia)
- Ottawa Valley (Ontario/Quebec)
- Palo Duro Canyon (Texas)
- Valley of the Sun (Arizona)
- Rio Grande Valley (Texas)
- Saint Lawrence Valley (Ontario/Quebec/New York)
- Salt Lake Valley (Utah)
- San Fernando Valley (California)
- Shenandoah Valley (Virginia/West Virginia)
- Sonoma Valley (California)
- Toluca Valley (Mexico)
- Valley of the Gods (Utah)
- Valley of Mexico (Mexico)
- Willamette Valley (Oregon)
- Yosemite Valley (California)
Numerous elongate depressions have been identified on the surface of Mars, Venus, the Moon, and other planets and their satellites and are known as valles (singular: 'vallis'). Deeper valleys with steeper sides (akin to canyons) on certain of these bodies are known as chasmata (singular: 'chasma'). Long narrow depressions are referred to as fossae (singular: 'fossa'). These are the Latin terms for 'valley, 'gorge' and 'ditch' respectively. The German term 'rille' or Latin term 'rima' (signifying 'cleft') is used for certain other elongate depressions on the Moon.
- List of valleys on Mars,
- List of chasmata on Mars,
- List of valleys on the Moon,
- List of largest rifts, canyons and valleys in the Solar System
- Canyon – Deep ravine between cliffs
- Grass valley – Meadow within a forested and relatively small drainage basin
- Stream channel – Type of landform in which part of a body of water is confined to a relatively narrow but long region
- Gully – Landform created by running water and/or mass movement eroding sharply into soil
- Monkhouse, F.J. (1971). Principles of Physical Geography (Seventh ed.). London: University of London Press Ltd. pp. 152–157. ISBN 0340090227.
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- Monkhouse, F.J. (1971). Principles of Physical Geography (Seventh ed.). London: University of London Press Ltd. pp. 161–164. ISBN 0340090227.
- "Vale of Eden". Britannica. Retrieved 20 December 2020.
- Monkhouse, F.J. (1971). Principles of Physical Geography (Seventh ed.). London: University of London Press Ltd. pp. 230–234. ISBN 0340090227.
- Jørgensen, Flemming; Peter B.E. Sandersen (June 2006). "Buried and open tunnel valleys in Denmark—erosion beneath multiple ice sheets". Quaternary Science Reviews. 25 (11–12): 1339–1363. Bibcode:2006QSRv...25.1339J. doi:10.1016/j.quascirev.2005.11.006.
- "Glossary of Glacier Terminology". U.S. Geological Survey. May 28, 2004. Retrieved 2007-05-24.
- Nesje, A., & Whillans, I. M. (1994). Erosion of Sognefjord, Norway. Geomorphology, 9(1), 33-45.
- "Illustrated Glossary of Alpine Glacial Landforms - Hanging Valley". .uwsp.edu. Retrieved 2011-10-03.
- Goudie, Andrew, ed. (2004). Encyclopedia of Geomorphology. Psychology Press. p. 98. ISBN 9780415327381.
- "Early History, Santa Clara County". National Park Service. National Park Service. Retrieved January 16, 2015.
Santa Clara Valley is a structural valley, created by mountain building, as opposed to an erosional valley, or one which has undergone the wearing away of the earth's surface by natural agents.
- Ross, David (2001). Scottish Place-names (First ed.). Edinburgh: Birlinn. p. 203. ISBN 1841581739.
- Ross, David (2001). Scottish Place-names (First ed.). Edinburgh: Birlinn. p. 99. ISBN 1841581739.
- Ross, David (2001). Scottish Place-names (First ed.). Edinburgh: Birlinn. p. 53. ISBN 1841581739.
- Whitten, D.G.A.; Brooks, J.R.V. (1972). Dictionary of Geology (First ed.). London: Penguin. p. 83. ISBN 0140510494.
- "Bing maps". Microsoft Bing. Retrieved 20 December 2020.
- Gelling, Margaret (1984). Place-names in the Landscape: the Geographical roots of Britain's place-names (First ed.). London: J M Dent. p. 88. ISBN 0460860860.
- Gelling, Margaret (1984). Place-names in the Landscape: the Geographical roots of Britain's place-names (First ed.). London: J M Dent. p. 99. ISBN 0460860860.
- "Combe". Merriam–Webster's Dictionary. Merriam–Webster. 2011.
- "Gazetteer of Planetary Nomenclature, feature types". International Astronomical Union. Retrieved 20 December 2020.
- "Gazetteer of Planetary Nomenclature, Welcome". International Astronomical Union. Retrieved 20 December 2020.
- NPS.gov, University of Wisconsin
- UWSP.edu, Glossary of Alpine Glacial Landforms
- "BGU.ac.il" (PDF)., SAR interferometry (analysis of valley forms in Fig.2 and 6)
- Valleys.com, Valleys of the World
- Valley definitions, YourDictionary