Monday, December 3, 2007
This article is about the physical-geographic term. For places named "Valley" see Valley (disambiguation).
In geology, a valley 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.
A valley carved by glaciers, or glacial valley, is normally U-shaped. If we can see the valley, it means the glacier that formed it is no longer there. 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 - espec. 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 are the
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. Glacial valleys
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.
Transition forms and valley shoulders
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.
Usually the bottom of a main valley is broad - independent of the U or V shape. It typically ranges from about one to ten kilometres 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.
California Central Valley (United States)
Danube Valley (Eastern Europe, Wachau, Iron Gate)
Death Valley (United States)
Glen Coe (Scotland)
Grand Canyon (United States)
Great Glen (Scotland)
Great Rift Valley (from Jordan to the Red Sea and Lake Victoria)
Indus Valley (Pakistan)
Loire Valley with its famous castles (France)
Napa Valley (United States)
Okanagan Valley (Canada)
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, USA
Valley of the Kings (Egypt)
Valley of the Sun (Phoenix, Az, US)
San Fernando Valley (United States)
Santa Clara Valley, perhaps better known as "Silicon Valley" (United States)
South Wales Valleys (Wales)
Valley of Mexico (Mexico), also known as "El Valle de México" see Mexico city Famous valleys
- ▼ December (5)