V-shaped valley
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The term V-shaped is used in Geography to characterize the form of steep eroded valleys. The cross section of the valley resembles the letter V, and some canyons look similar.
Similar to the V-shaped valley is the U-shaped valley. An U valley is generally formed by glacial erosion. Most of the mountainous valleys, glens or dales belong to these two types - or a mixture of them.
The terms U-shaped and V-shaped are descriptive terms of Geography to characterize the form of valleys. Most of the mountainous valleys, glens or dales belong to these two main types or a mixture of them, at least with respect of the cross section of the slopes or hillsides.
Other (more special) types are e.g. the clammy (steep cliffs), the hollow-formed hillsides and river plain valleys, mostly with a broad river bed.
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[edit] Two mountainous valley types
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A V-shaped valley is a narrow valley with steep walls (slopes) and a narrow valley bottom with a stream channel. The valley shape resembles a V in cross section.
It is created by fast flowing brooks or rivers with steep gradients. Flowing downhill quickly, it removes more sediment from the stream channel bottom than from the sides. This "downcutting" can be compared to a knife cutting through a cake. The resulting landform is a narrow canyon with fast water and little bank (floodplain) on the river sides. A lot of examples are found in many mountain ranges; some broad V examples are:
- USA: Black Canyon of the Gunnison National Park, and others in Grand Canyon NP
- Alpine Europe:
A U-shaped valley has a flat or slightly rounded floor and steepened slopes, both formed by glaciers flowing downhill (not by the river presently existing here). U-shaped valleys were carved by glaciers and are called U-shaped for in cross section they resemble the letter U. Floor gradient does not affect the valley's shape, it is the glacier's size that does. Continuously flowing glaciers - espec. in the ice age - and large sized glaciers carve wide, deep incised valleys.
Examples of U-valleys are found are found in every mountainous region with glacial periods - like the Alps or Rocky mountains but also in Scotland, Scandinavia or Canada. Much interesting are the
- side valleys of the Austrian river Salzach for their parallel directions and hanging mouthes,
- or some Scottish Glens full with bushes and flowers.
- A classic glacial trough was also formed by St. Mary River in Glacier National Park in Montana, USA.
[edit] Genesis (V becomes U) and river plain valleys
V-shaped valleys are the rather original landforms crated by alpine rivers, due to a drastic erosion of the valley slopes (mountain sides) which is called denudation. However,
U-shaped valleys (in German "Trogtal" = trough valley) are the destiny of V profiles if a glacier fills it. It makes the valley wider and mostly deeper, too. The maximum erosion takes place at two height niveaus: at the valley floor (see also ground moraine) and near his edges (see border- or lateral moraine). In many cases this edge is approximately halfway up - which forms the later valley shoulders. Below them the slope inclination has a maximum (see 2nd figure).
Because at any time glaciers came into most alpine valleys of the northern hemisphere, the U-type is more frequent. Just at very narrow or "young" valleys the V-type is predominant. However,
River plain valleys are typical for the lower reaches of rivers. They have broad flat valley floors with sides of varying angles (usually more shallow near to the Ocean). The valley bottom is formed by a meandering river. Plain valleys are created by slow moving rivers flowing at moderate to low angle slopes. A river at this gradient tends to erode more along the sides of the channel than along the bottom. Thus, the river slowly sweeps across the valley floor, forming a broad river plain.
[edit] Transition forms and valley shoulders
Depending on the topography, the rock types and the climate, a lot of transition 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.
[edit] External links
- Univ.of Wisconsin
- Glacial moraine types (LEO dictionary)
- Glossary of Alpine Glacial Landforms
- SAR interferometry (analysis of valley forms in Fig.2 and 6)
- Shoulder of the Swiss Calanca valley near Braggio
- Typical valley sections (=> U valleys and terrace valleys)
- V-shaped valley
[edit] See also
- clammy, cliff, gorge
- valley shoulder, plain valley
