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in East Iceland , a rather flat valley]] A valley is a Landform , which can range from a few Square Mile s ( Square Kilometer s) to hundreds or even thousands of square kilometers in area. It is typically a low-lying area of Land , surrounded by higher areas such as Mountain s or Hill s. GENESIS OF VALLEYS Valleys are formed by numerous geographical processes. Glacial Valley s, 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 Valley s, 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 River s), which Erode s the landscape. . The shoulders of U or V valleys are often located ~''halfway up'' the slopes]] VALLEY FLOORS Usually the bottom of a Main Valley is broad - independent of the U or V shape. It ranges from about 1 to 5 km and is filled with mountain Sediment s. The shape of the floor can be rather Horizontal , similar to a flat Cylinder , or Terrace d. Side Valley s are rather V than U-shaped; near the mouth Clammies are possible if it is a Hanging Valley . The location of the Village s depends on the across-valley profile, on Climate and local traditions, and on the danger of Avalanches s or Landslide s. Predominant are places on terraces or Alluvial Fan s if they exist. The villages of the primary valleys, however, have to consider mainly the danger of possible Flooding s. HOLLOWS A hollow is loose name for a Valley in the Earth . It is commonly used in New England and Missouri to describe such geographic features. 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. A hollow such as this is Boston Hollow . Tourists in Europe can further visit a lot of Karst , Stalactite and Ice Hollows (e.g. in Slovenia and Austria ). FAMOUS VALLEYS
EXTRATERRESTRIAL VALLEYS Moons and the other Terrestrial Planet s of our Solar System can also have Valley-like features. Lunar Valleys could be formed due to a linked chain of impact Crater s. Smaller valleys, known as Rille s, can originated from Lava flows or because of contractions in cooling lava Sheet s. Besides the lunar Crater s, the details of mountain ranges ( Lunar Alps , Carpathes etc.) have been well known for more than 300 years (e.g. J.H. Schröter 's ''Selenotopographische Fragmente'' of 1791 ). 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. Astronomer s have long been able to observe some Highland s and the " Nix Olympica" on Mars, and therefore guessed that there may be extraterrestrial valleys, too. In the 1970s this interpretation was proven correct by a series of Space Probe s. 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. The ''Valles'' (which were first detected in 1877 by Schiaparelli ) are a huge Canyon system, 4,500 x 600 km in area and up to 8 km in depth. These enormous dimensions are 4-8 times greater than those of the American "Grand Canyon". The ''Valles'' is currently understood to have been created by Tectonic forces like the main Graben s on Earth, rather than by running water. In later stages, however, it has been ''expanded'' considerably by erosion, possibly including the action of surface water. Several volcanic or Icy Moon s of the gas planets Jupiter , Saturn and Neptune were also photographed by the two Voyager s, as well as other space probes. 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. SEE ALSO
WEBLINKS OF EXTRATERRESTRIAL VALLEYS
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