An elongated trough in the Earth's surface, most commonly formed by the erosional action of rivers over a long period of time. It may also be carved out by a glacier, in which case it is U-shaped rather than (as in a river valley) V-shaped. Extensional movements of the Earth's crust may produce large rift valleys by faulting.
Genesis of valleys
Valleys are formed by numerous geographical processes. Glacial valleys, which are usually U- rather than V-shaped, were formed tens of thousands of years ago (most likely during the last Ice Age) by the massive erosive power of glaciers. Several glacial valleys can be found in the English Lake District and many can be found in Alpine countries. 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. Valleys are, however, most commonly formed by fluvial activity (the action of running water, such as rivers), which erodes the landscape.
Valley floors
Usually the bottom of a main valley is broad - independent of the U or V shape.
The villages of the primary valleys, however, have to consider mainly the danger of possible flooding.
Hollows
A hollow is loose name for a valley in the earth. 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.
Famous valleys
California Central Valley (United States) Copper Canyon Danube Valley (Eastern Europe, Wachau, Iron Gate) Death Valley (United States) Grand Canyon (United States) 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) Upper Rhine Valley (an old graben system) (France) Rhone Valley from the Matterhorn to Grenoble and Lyon (France) Shenandoah Valley (United States) Sonoma Valley, California, USA Valley of the Kings (Egypt) San Fernando Valley (United States) Santa Clara Valley, perhaps better known as "Silicon Valley" (United States)Extraterrestrial valleys
Moon and the other terrestrial planets of our Solar System can also have valley-like features. Lunar valleys could be formed due to a linked chain of impact craters. Smaller valleys, known as rilles, can originated from lava flows or because of contractions in cooling lava sheets. A lot of linear phenomena like Rheita or Schröter valley and the famous Vallis Alpes (see also below) were observed with details less than 1 km (which corresponds to a coin seen from 5-10 km distance)—but the geological genesis was in discussion up to the Apollo 11 mission of summer 1969. Valleys have also been found on Mercury and on the volcanic surfaces of Venus and Io.
The largest valley in our solar system is the Valles Marineris formation on Mars. Some linear ruptures in the ice or apparent low areas between hills have been interpreted by astrogeologists as tectonic structures or valleys similar to graben or active geologic lines on Earth.
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