| 134th | Top landforms |
in East Iceland, a rather flat valley]] showing a characteristic U-shape, the bottom's rocky 'rubble' accretion and the broad shoulders]]
in Montana, United States.]]
In geology, a valley (also called a vale, dale, glen or strath and near or in Appalachia, a draw) is a depression with predominant extent in one direction. A very deep river valley may be called a canyon or gorge.
The terms U-shaped and V-shaped are descriptive terms of geography to characterize the form of valleys. Most valleys belong to one of these two main types or a mixture of them, at least with respect of the cross section of the slopes or hillsides.
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A valley formed by flowing water, or river valley, is usually V-shaped. The exact shape will depend on the characteristics of the stream flowing through it. Rivers with steep gradients, as in mountain ranges, produce steep walls and a narrow bottom. Shallower slopes may produce broader and gentler valleys, but in the lowest stretch of a river, where it approaches its base level, it begins to deposit sediment and the valley bottom becomes a floodplain.
Some broad V examples are:
The original natural habitat of the human species was the large river valleys of the world, such as the Nile, Tigris-Euphrates, Yellow, Ganges, Amazon, Mississippi, etc. In pre-history, the rivers were used as a source of fresh water & food (fish and game animals), as well as a place to wash, and a sewer. The rivers carved the valleys. The valleys blocked the winds and shaded the inhabitants, creating cooler temperatures in the daytime, and warmer temperatures at night. The first civilizations grew from these river valley communities.
A valley carved by glaciers, or glacial valley, are normally U-shaped. The valley becomes visible upon the recession of the glacier that forms it. When the ice recedes or thaws, the valley remains, often littered with small boulders that were transported within the ice. Floor gradient does not affect the valley's shape, it is the glacier's size that does. Continuously flowing glaciers - especially in the ice age - and large sized glaciers carve wide, deep incised valleys.
Examples of U-shaped valleys are found in every mountainous region that has experienced glaciation, usually during the Pleistocene ice ages. Most present U-shaped valleys started as V-shaped before glaciation. The glaciers carved it out wider and deeper, simultaneously changing the shape. This proceeds through the glacial erosion processes of glaciation and abrasion, which results in large rocky material (glacial till) being carried in the glacier. A material called boulder clay is deposited on the floor of the valley. As the ice melts and retreats, the valley is left with very steep sides and a wide, flat floor. A river or stream may remain in the valley. This replaces the original stream or river and is known as a misfit stream because it is smaller than one would expect given the size of its valley.
Other interesting glacially-carved valleys are the
| , Utah, with very striking shoulders]] |
Depending on the topography, the rock types and the climate, a lot of transition forms between V-, U- and plain valleys exist. Their bottoms can be broad or narrow, but characteristic is also the type of valley shoulder. The broader a mountain valley, the lower its shoulders are located in most cases. An important exception are 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 (100-200 meters above the bottom). Many villages are located here (esp. at the sunny side) because the climate is very mild: even in winter when the valley's floor is completely filled with fog, these villages are in sunshine. |
In some stress-tectonic regions of the Rockies or the Alps (e.g. Salzburg) the side valleys are parallel to each other, and additionally they are hanging. The brooks flow into the river in form of deep gorges or waterfalls. Usually this fact is the result of a violent erosion of the former valley shoulders. A special genesis we find also at arêtes and glacial cirques, at every Scottish glen, or a northern fjord.
flowing from a hanging valley.]]
A hanging valley is a tributary valley with the floor at a higher relief than the main channel into which it flows. They are most commonly associated with U-shaped valleys when 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. [1].
Usually the bottom of a main valley is broad - independent of the U or V shape. It typically ranges from about one to ten kilometers in width and is commonly 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 avalanches or landslides. Predominant are places on terraces or alluvial fans if they exist.
Historic siting of villages within the mainstem valleys, however, have chiefly considered the potential of flooding.
A hollow is a small valley or dry stream bed. This term is commonly used in New England, Arkansas, Missouri and Pennsylvania 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).
Valley in the Swiss Alps]]
, Utah.]]
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.
The other terrestrial planets and the moons of our Solar System can also have valley-like features. Lunar valleys can be formed from a linked chain of impact craters. Smaller valleys, known as rilles, may have originated from lava flows or from the contractions of cooling lava sheets.
Besides the lunar craters, the details of lunar mountain ranges 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 debated until the Apollo 11 mission in 1969. Astronomers have long been able to observe some highlands and mountains on Mars, and therefore guessed that there may be valleys, as well. In the 1970s this interpretation was proven correct by results from space probes. 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 spanning 4,500 x 600 km in area and having a depth up to 8 km. 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. Later, though, it may have been expanded considerably by erosion, possibly including the action of surface water.
Icy moons of the gas planets Jupiter, Saturn and Neptune were also photographed by the two Voyagers as well as by 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 grabens or active geologic rifts on Earth.
, Costa Rica.]]
| Look up Valley in Wiktionary, the free dictionary. |
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French vallée, Old French valee, from Latin vales
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Plural |
valley (plural valleys)
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| This page or section does not have any sources. You can help Wikipedia by finding sources, and adding them. Tagged since August 2009 |
A valley is a type of landformation. A valley is a long "depression" (or low part) in the land, between two higher parts which might be hills or mountains. Valleys often start as a downward fold between two upward folds in the surface of the Earth. A valley is made deeper by a stream of water or a river as it flows from the high land to the lower land, and into a lake or sea. Some valleys are made by glaciers which are slow-moving rivers of ice. When a valley is made deeper or wider by water or ice, this is called erosion. Wind can also make valleys larger by erosion.
A valley has a "head" where it begins in the mountains or hills, "sides" where it rises up on either side, a "floor" which is where the valley is most flat. Some valleys have an "entrance" where the valley opening can be seen between two hills or mountains or cliffs. A place where a valley is very narrow and has high walls is sometimes called a "gorge". (This word "gorge" is sometimes used to mean the "throat" on a human body).
Many of the people of the world live in valleys because there is often a river or stream in a valley for fresh water, and there is often good soil in a valley to grow crops.
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Mountains and hills are made when the layers of rock and soil (called "strata") get folded. There are always valleys in hills and mountains, in between the highest parts which are the "peaks". If the mountains are very high, then people who want to travel to the other side of the mountains must go by the valleys. A valley that is used for people to travel through the mountains is called a "pass".
Valleys that are high in the mountains are usually made deeper by a stream or small river running fast down the mountainside, from a place where there is lots of rainwater or melting snow. The mountain stream winds around the biggest rocks and washes the soil away as it flows. It cuts a passage for itself through the softest soil and smallest stones. A small stream can cut a very deep valley. Valleys that are high in the mountains are usually V-shaped. There are many valleys like this in hills and mountains all over the world.
Iaovalley
Iao Valley in Hawaii shows a perfect V-shape. |
Blick zum Muretto-Pass
The Muretto Pass is a V-shaped valley between the mountains of the Swiss Alps. |
Vallata dell'
The Amendolea River winds between the mountains of Reggio-Calabria in Italy |
West
Mountains and valleys in the Czech Republic |
In country that has hills, but is not very steep, a river or stream runs more slowly. It makes a wider valley that often has some large bends as the river flows around the hills, aways following the lowest way. Water running down from the hillsides often carries soil that spreads out across the valley, making flat land that is good for growing food crops and raising cattle and other animals. Many farms are in valleys that are in hilly land. Many towns are built on the sloping sides of valleys. Famous valleys of this type are the Loire Valley and the Lower Rhine Valley in Europe and the Thames Valley in England.
Chinon and the Loire from the
The Loire Valley in France is famous for its old towns and castles. |
080110 wolf
Grapes to make white wine are grown in the valley of the Moselle River. Germany |
Shona farms
Farming in a valley in Zimbabwe |
Some valleys are almost flat, like a large saucer. Valleys of this type often have a very large river with many "tributaries" (streams that are like branches) running through them. The tributaries carry water from the hills or mountains that may be far from the main river. After heavy rain, lots of water rushes into the main river so that it rises and floods over the flat floor of the valley. When the flood waters spread, they drop lots of soil which has washed down from the hills. The soil that drops on the valley floor makes the valley flatter. Valleys of this type are very useful for growing food crops. The widest valleys in the world are like this. Famous valleys of this type are the Mississippi-Missouri Basin in North America, the Amazon Basin in South America, the Lower Danube in Europe, the Ganges River Valley in India, the Nile River Flood Plain in Africa and the Darling River Basin in Australia.
Vallee fertile du Nil a
Growing crops in the Nile Valley |
Fields of
Grain growing in the Euphrates Valley in Iraq |
Argeş County Romania
The Danube Valley in Romania |
Ganges near
The Ganges River Valley at Varanasi. |
A glacier is like a frozen river. Many countries do not have any glaciers. A glacier starts in very high mountains where there is snow and ice all the year. The snow and ice starts to move down a valley that has been made by a fast-flowing stream. As the ice starts to slide down the mountainside, it does not flow around the rocks; it pushes the rocks out of the way. As a glacier moves, it picks up more ice and gets bigger and bigger. A big glacier cuts through the soil and softer rock of the valley and piles up the rocks on either side, or pushes them in front of it. When a big glacier melts, it leaves a valley of a deep U-shape. Many valleys like this were made in the Ice Ages. In mountainous countries like Switzerland many people live in the valleys that were made by glaciers. Some of the deepest valleys in the world were made by glaciers. The fjords of Norway and "sounds" of New Zealand are where glaciers went into the sea.
Glacier du Tour 100
A glacier is a river of ice. |
Franz Josef
The Franz Josef glacier in New Zealand |
Randa
The Mattertal, a glacial valley in Switzerland, showing a recent landslide. |
A plateau is high land that is flat on top, or gently rolling, not pointed like mountains or rounded like hills. In hilly or mountainous country, the bands of a soil and rock are folded, but in a plateau the strata are in flat layers. The water that makes streams on the top of a plateau cuts down in wide valleys with sides that are steep cliffs and a bottom that is quite flat. Valleys like this are often deep and very narrow. Some valleys like this are very deep and wide. They are called "canyons". Famous valleys of this type are the Grand Canyon in the United States and the Megalong Valley in Australia.
Grand Canyon
The Grand Canyon, Arizona, USA. |
IMG 7431-Sharyn
Sharyn River Canyon, Kazakhstan. |
Foz de Arbayú
Foz de Arbayún in Spain |
Sometimes a valley has been formed in the hills near the coast of a country. Movement in the earth's surface may cause the land to sink lower and become flooded by the sea. The shape of the valley can still be seen from the tops of the hills that stick out from the water. Some of the hills may become islands, and others become the shore of a bay. Sunken valleys often make good harbours. The east coast of Australia has many sunken valleys of which the most famous is Sydney Harbour.
Sydney
Sydney Harbour is a sunken valley. |
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