Valley
This article is about the physical-geographic term. For places named "Valley" see Valley (disambiguation). |
Mt.Gilbert & Right Fork Canyon, Nevada. The valley shoulders in the right foreground are broken by two by-valleys |
A
valley is a
landform, which can range from a few
square miles (
square kilometers) to hundreds or even thousands of square miles in area. It is typically a low-lying area of
land, surrounded by higher areas such as
mountains or
hills.
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.
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Schematic cross section of a typical valley in the Eastern Alps. The shoulders of U or V valleys are often located roughly halfway up the slopes |
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
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
avalanchess or
landslides. Predominant are places on terraces or
Alluvial fans if they exist.
The villages of the primary valleys, however, have to consider mainly the danger of possible
floodings.
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).
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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)
*
Valley of the Kings (Egypt)
*
San Fernando Valley (United States)
*
Santa Clara Valley, perhaps better known as "
Silicon Valley" (United States)
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.
Besides the lunar
craters, 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.
Astronomers have long been able to observe some
highlands 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 probes. Valleys have also been found on
Mercury and on the volcanic surfaces of
Venus and
Io.
 |
Valles Marineris on Mars. On Earth these huge Canyon system would cross half Eurasia or the whole USA. |
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
grabens 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
moons of the gas planets
Jupiter,
Saturn and
Neptune were also photographed by the two
Voyagers, 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.
*
Canyon,
Glen,
Vale,
Gorge,
Channel,
Gully*
Geography,
Geomorphology,
Geodynamics,
Glaciology*
List of landforms,
List of mountain ranges*
Geological features of the solar system,
List of Lunar valleys*
Martian mountains,
Lineaments on Europa,
Geologic features on Titan, (
valles,
scarps and
ruptures).
Extraterrestrial valleys
*
ESA image:
Vallis Alpes, bisecting the Lunar Alps
*
Valles Marineris and Ophir Chasma,
bilingual website (
English and
German)
