Valley
A valley is an elongated low area often running between hills or mountains and typically containing 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. Some valleys are formed through erosion by glacial ice. These glaciers may remain present in valleys in high mountains or polar areas.
At lower latitudes and altitudes, 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 result from drainage now taking place underground rather than at the surface. Rift valleys arise principally from earth movements, rather than erosion. Many different types of valleys are described by geographers, using terms that may be global in use or else applied only locally.
Formation of valleys
Valleys are depressions in the morphology that slope in one direction along their entire length. They tend to be longer than wide and come in a variety of dimensions. Valleys may arise through several different processes. Most commonly, they are created from erosion over long periods by moving water and are known as river valleys. The processes that alter the river valley surface include corrasion, abrasion, pot-holing, corrosion, and weathering.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 altitudes, 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. All three processes can contribute to the development of a valley over geological time. The flat portion of a valley between its sides is referred to as the valley floor. The valley floor is typically formed by river sediments and may be flanked by fluvial terraces.
River valleys
The development of a river valley is affected by the character of the bedrock or alluvium over which the river or stream flows, the elevational difference between its top and bottom, and 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 erode 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 valleys.
File:Tal-y-llyn-valley-Dolgoch-01.jpg|thumb|upright=1.35|U-shaped valley on the Afon Fathew near Dolgoch, Wales
Some sections of a 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 localized uplift of the land surface or 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.
Glacial valleys
There are various forms of valleys 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. This parabola shape is caused by glacial erosion removing the contact surfaces with greatest resistance to flow, and the resulting section minimises friction.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 that have previously been shaped by water rather than ice. Abrasion by rock material embedded within the moving glacial ice causes the widening and deepening of the valley to produce the characteristic U or trough shape with 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 can result from differing erosion rates due to both the nature of the bedrock 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.
File:Pirin ezera Pano Chairski ezera.jpg|upright=1.35|A panoramic view of two merging U-shaped valleys in Pirin mountain, Bulgaria|thumb
Other interesting glacially carved valleys include:
- Yosemite Valley
- 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.
Tunnel
Meltwater
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 valleys, 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 valleys can form. The floor or bottom of these valleys can be broad or narrow, but all valleys have a shoulder. The broader a mountain valley, the lower its shoulders are located in most cases. An important exception is 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. Many villages are located here because the climate is very mild: even in winter when the valley's floor is filled with fog, these villages are in sunshine.In some stress-tectonic regions of the Rocky Mountains or the Alps, the side valleys are parallel to each other, and are hanging. Smaller streams flow into rivers as deep canyons or waterfalls.
Hanging tributary
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 underwater. The branches of Sognefjord are much shallower than the main fjord. The mouth of Fjærlandsfjord is about deep while the main fjord nearby is deep. The mouth of Ikjefjord is only deep while the main fjord is around 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.