![]() 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. 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. 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 upper valley, the stream will most effectively erode its bed through corrasion to produce a steep-sided V-shaped valley. Typically the flow will increase downstream and the gradient will decrease. 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. Valley of Palakaria river springing from Vitosha Mountain, seen in the background, in Bulgaria The valley floor is typically formed by river sediments and may have fluvial terraces. The flat (or relatively flat) portion of a valley between its sides is referred to as the valley floor. All three processes can contribute to the development of a valley over geological time. Other valleys may arise principally through tectonic processes such as rifting. ![]() 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. 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. Most commonly, they arise from erosion over long periods by moving water and are known as river valleys. ![]() Valleys may arise through several different processes. Many different types of valleys are described by geographers, using terms that may be global in use or else applied only locally. Rift valleys arise principally from earth movements, rather than erosion. In areas of limestone bedrock, dry valleys may also result from drainage now taking place underground rather than at the surface. In desert areas, valleys may be entirely dry or carry a watercourse only rarely. These glaciers may remain present in valleys in high mountains or polar areas.Īt 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. Some valleys are formed through erosion by glacial ice. Most valleys are formed by erosion of the land surface by rivers or streams over a very long period. 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.
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