Valley

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

Contents

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 small valley surrounded by mountains is known as a hollow. A deep, narrow valley is known as a coomb (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 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.

Some broad V examples are:

The original natural habitat of the human species was the large river valleys of the world, such as the Nile, Tigris-Euphrates, Yellow, Ganges, Amazon, Mississippi, etc. In pre-history, the rivers were used as a source of fresh water & food (fish and game animals), as well as a place to wash, and a sewer. The rivers carved the valleys. The valleys blocked the winds and shaded the inhabitants, creating cooler temperatures in the daytime, and warmer temperatures at night. The first civilizations grew from these river valley communities.

Glacial valleys

Tal-y-llyn U shaped valley at Dolgoch
Yosemite Valley from an airplane

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 include:

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 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

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]

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 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. 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

The Zin valley in the Negev desert of Israel.
The Lötschental Valley in the Swiss Alps
U-shaped valley carved by a glacier; Little Cottonwood Creek Valley, Wasatch Mountains, Utah.

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.

See also

A view of Orosí, Costa Rica.

References

  1. ↑ "Glossary of Glacier Terminology". U.S. Geological Survey. May 28, 2004. http://pubs.usgs.gov/of/2004/1216/h/h.html. Retrieved 2007-05-24. 


External links

Extraterrestrial valleys

River morphology
Large-scale features
Drainage basin • Drainage network • Strahler number (stream order) • River valley • River delta
Alluvial rivers
Meander • Point bar • Cut bank • Riffle • Stream pool • Braided river • Bar (river morphology) • Anabranch • River bifurcation • River channel migration • Oxbow lake • Floodplain • Riparian corridor • Avulsion (river) • Mouth bar • Thalweg • Channel pattern
Bedrock river
Canyon • Knickpoint • Plunge pool • Bedrock erosion
Bedforms
Lower plane bed • Current ripple • Dune • Upper plane bed • Antidune
Regional processes
Aggradation • Degradation (geology) • Base level • Erosion and tectonics
Mechanics
Playfair's Law • Hack's law • Sediment transport • Water erosion • Deposition (geology) • Exner equation
Category · Portal