Valleys are among the many common landcreates on the surconfront of the planet. They are sculpted by flowing water or flowing ice with the process of erosion, which is the steady wearing away of Earth surdeals with with the activity of wind and also water. Valleys take on a large selection of forms—from steep-sided canyons, such as the Grand also Canyon on the Coloraperform River, to broad plains, such as the reduced Mississippi River valley. The form of a valley depends on many kind of components, including what is eroding it, the slope of the land also surchallenge, the nature of the soil or rock wright here the valley is being created, and time.

The form of the land also

A valley is a fairly big hollow or depression bounded by hills or mountains on Earth's surchallenge that is occurred by stream erosion or glacial task and also that is drained externally. A basin is an additional hollow or depression on Earth's surface, yet it has actually no outlet to drain the water. (For even more information, see the Basin chapter.) Geologists describe any kind of body of running water moving downslope in a identified channel as a stream (a river is a huge stream). A glacier is a large body of ice that created on land by the compactivity and also recrystallization of scurrently, survives year to year, and also mirrors some authorize of movement downhill as a result of gravity. The 3 primary forms of valleys are the V-shaped valley, the flat-floored valley, and the U-shaped valley.

A V-shaped valley is a narrow valley that has a profile saying the form of the letter "V," defined by steeply sloping sides. It results from a stream eroding downward, a process referred to as downcutting. V-shaped valleys form in hills or other highland areas where streams are in their beginning or "youthful" phase and are flowing promptly down steep slopes. The bottom of a valley is called its floor. In highland also areas




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Blue Ridge Mountain Valley, North Carolina. Valleys are sculpted by flowing water or flowing ice with the process of erosion, which is the steady wearing amethod of Planet surdeals with through the action of wind and also water . PHOTOGRAPH REPRODUCED BY PERMISSION OF THE
CORBIS CORPORATION
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near a stream's source, or start, the valley sides slope dvery own almost straight to the stream's financial institutions, or sides of its channel. The valley floor in this region is narrow or also missing.


Valley: Words to Know

Alpine glacier: A relatively tiny glacier that forms in high elevations near the tops of mountains. Base level: The level listed below which a stream cannot erode. Erosion: The steady wearing away of Earth surfaces via the activity of wind and water. Fjord: A deep glacial trough subcombined via seawater. Floodplain: An location of almost level land also bordering a stream that is normally subject to regular flooding. Glacial trough: A U-shaped valley sculpted out of a V-shaped stream valley by the motion of a valley glacier. Glacier: A huge body of ice that developed on land also by the compaction and recrystallization of scurrently, survives year to year, and also mirrors some sign of movement downhill as a result of gravity. Hanging valley: A shpermit glacial tturbulent that leads into the side of a larger, main glacial trough. Levee (natural): A low ridge or mound alengthy a stream bank, created by deposits left when floodwater slows down on leaving the channel. Mass wasting: The spontaneous movement of product down a slope in response to gravity. Meander: A bfinish or loop in a stream's course. Rift valley: The deep main crevice in a mid-sea ridge; likewise, a valley or trough developed between two normal faults. Stream: Any body of running water that moves downslope under the affect of gravity in a characterized channel on Earth's surchallenge. U-shaped valley: A valley developed by glacial erosion that has a profile suggesting the form of the letter "U," defined by steep sides that may curve inward at their base and also a vast, practically level floor. Valley glacier: An alpine glacier flowing downward via a preexisting stream valley. V-shaped valley: A narrow valley developed by the downcutting action of a stream that has a profile suggesting the form of the letter "V," defined by steeply sloping sides.

Downstream, as the gradient or slope of the stream's channel lessens and also becomes more gentle, the floor of the valley widens. A stream in this phase is no longer taken into consideration youthful but "mature." A stream flowing at a moderate to low gradient has a tendency to erode more alengthy the banks of its channel than along its bed or bottom. Hence, the stream gradually sweeps throughout the valley floor in a series of S-shaped bends called meanders (pronounced me-AN-ders). Over time, as the stream continues to meander, it erodes amethod product on the valley floor, ever before widening it. The form of the valley transforms increasingly from a sharp V to a broader V to one that has actually a flat floor. This latter form of valley, through a floor that is horizontal and often wide, is recognized as a flat-floored valley. This form of valley is the the majority of widespread.

Streams are systems for moving sediment—rock debris such as clay, silt, sand, gravel, or also larger material—across Earth's surface to the world's oceans and seas. Naturally and also consistently, the flow of water in a stream may end up being too high to be accommodated in the stream's channel. The water then flows over the stream's banks and also floods the surrounding land. As it does so, the water instantly slows dvery own and also drops any type of sediment it was transporting. Coarser sediment is deposited near the channel. In time, as the process is repetitive over and over, the sediment forms mounds called herbal dams alengthy the stream's financial institutions. Finer sediment (sands, silt, and clay) lugged by the water is spreview farther amethod from the channel prior to it is lastly deposited. The level or gently sloping surface developed by the recurring deposition of sediment along a stream is dubbed the stream's floodordinary. A floodordinary may cover all or just a portion of the valley floor. (For better information, check out the Stream and also river and also Floodplain chapters.)

A U-shaped valley is a valley that has actually a profile saying the form of the letter "U," characterized by steep sides that may curve inward at their bases and also a broad, nearly level floor. Specifically, a U-shaped valley is one carved by glacial erosion. Thick masses of ice that move progressively over Earth's surchallenge, glaciers are discovered in regions close to the North and also South Poles, the excessive northernthe majority of and southernmost points on the world, or in mountains at high elevations. Those that form over large areas of continents cshed to the poles are called continental glaciers or ice sheets. Two continental glaciers are uncovered on Earth: one covers 85 percent of Greenland in the Northern Hemisphere and the other covers even more than 95 percent of Antarctica in the Southern Hemispright here. Relatively little glaciers that create in high elevations close to the tops of mountains are referred to as alpine or mountain glaciers. (For better information, watch the Glacial landcreates and functions chapter.)

Because of their size and weight, continental glaciers damage topography, leveling the surface features of a landscape. The effect of alpine glaciers is not as too much. They flow dvery own pre-existing V-shaped valleys produced by streams to form U-shaped valleys. Because of this, they are additionally recognized as valley glaciers. Whereas a stream just occupies the floor (or a primary percentage of it) in a V-shaped valley, a valley glacier occupies the entire valley, eroding both the sides and the floor as it moves, deepening and also widening the valley. U-shaped valleys are also well-known as glacial troughs.

Draining a room, streams tend to flow throughout the landscape in a treeprefer pattern: the smallest streams (the outer branches) flow right into bigger streams (the inner branches), which circulation into the major stream (the trunk). Smaller streams that circulation into bigger ones are well-known as tributaries. When valley glaciers fill a space, they may occupy the valleys developed


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by a major stream and also its nearemainder tributaries. Tributary valleys virtually always join the main stream valley at the very same altitude or height, yet glacial action deepens the primary valley even more than its side valleys. The shalreduced glacial troughs produced by these smaller sized valley glaciers (recognized as tributary glaciers) are called hanging valleys. If temperatures in the location boost, bring about the glaciers to melt and retreat, meltwater from the glaciers might develop streams that run through these hanging valleys. In spectacular instances, streams in hanging valleys frequently autumn to the floor of the major valley as a waterfall.

In particular instances, a valley glacier might flow all the method to a shore, carving out a deep, narrow U-shaped valley. If the glacier melts, the valley might end up being flooded via seawater, forming an inlet recognized as a fjord (pronounced fee-ORD).

The term rift valley is used to describe a space wbelow a section of Earth's crust (thin, solid outermost layer of the planet) has actually dropped dvery own in between two normal faults (cracks or fractures in Earth's crust along which rock on one side has actually relocated relative to rock on the other). Essentially, whenever the planet's crust is subjected to compression or stress caused by heat forces beneath the crust, faults develop, and some blocks of rock drop alengthy these fractures loved one to the ground on either side. A rift valley is not a true valley. The dropped-down block or crust that formed a depression on the surconfront of the ground is appropriately dubbed a graben (pronounced GRAH-bin). Rift valleys, which are discovered approximately the people on continents and the sea floors, are typically sites of volcanic and also earthquake activity. (For additionally indevelopment, watch the Fault chapter.)

Forces and changes: Construction and devastation

The dominant agent of erosion on the surconfront of the earth is flowing water. Although the ice in glaciers is solid, it likewise flows (though even more progressively than water in a stream channel). Both streams and glaciers move downhill under the affect of gravity, the primary driving force behind virtually all agents of erosion. As they move downward, they carry sediment they have picked up or that has actually fallen into or onto them.

Stream erosion

The shaping of stream valleys is due to a combination of erosion by flowing water and also mass wasting, the spontaneous motion of Earth material dvery own a slope in response to gravity. This does not incorporate material transported downward by streams, winds, or glaciers. Thunstable mass wasting, product from greater elevations is moved to lower elevations wright here streams, glaciers, and wind pick it up and move it to also lower elevations. Mass wasting occurs consistently on all slopes. While some mass-wasting processes act very gradually, others take place extremely unexpectedly. The general term landslide is provided to explain all relatively quick forms of mass wasting. (For further information, check out the Landslide and also various other gravity movements chapter.)


The Literary Landscape

"The famed Yosemite Valley lies at the heart of it, and it includes the head waters of the Tuolumne and Merced rivers, two of the many songful streams in the world; innumerable lakes and waterfalls and silky lawns; the noblest woodlands, the loftiest granite domes, the deepest ice-sculptured cañon, the brightest crystalline pavements, the snowy hills soaring right into the sky twelve and thirteenager thousand feet, arrayed in open up ranks and also spiry pinnacled groups partially separated by remarkable cañons and also amphitheatres; gardens on their sunny brows, avalanches thundering dvery own their lengthy, white slopes, cataracts roaring gray and foaming in the crooked rugged gorges, and also glaciers in their shadowy recesses functioning in silence, slowly completing their sculpture; new-born lakes at their feet, blue and also green, totally free or encumbered via drifting icebergs favor miniature Arctic Oceans, shining, sparkling, calm as stars."

—John Muir, Our National Parks , 1901.


Once a stream creates, it looks for out its base level, which is the level listed below which the stream cannot erode. Base level is the elevation of the big body of water, such as a lake or ocean, right into which a stream drains. The basic or ultimate base level for the majority of bigger streams is sea level. For


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The glacier-sculpted crescent-shaped Hunza Valley in Pakistan . PHOTOGRAPH REPRODUCED BY PERMISSION OF THE

a tributary, base level is the allude wbelow it empties right into a larger stream. Aided by gravity, a stream tries to reach its base level as quickly as feasible. The bigger the difference in height in between the stream and also its base level, the greater the erosive or cutting force of the stream. Temporary base levels may form along a stream's course, such as those created by a lake or resistant rock.

The resistance of the rock over which a stream flows determines the form of its initial stream valley. In a highland also area, a stream's erosional force is directed along its bed, not its banks, as it seeks to reduced its channel to base level. If the rock over which the stream flows in this area is difficult and resistant to erosion, the stream will certainly reduced a narrowhead, steep-walled canyon. The valley developed exists as a virtually vertical notch via no floor. If the rock is much less resistant to erosion, the stream will still downcut, but the sides of the valley will certainly take on the characteristic V shape as mass wasting—rockfalls and other types of landslides—widens the upper portion of the valley.

Rock hardness is a critical factor in erosion, yet a stream's ability to reduced counts on exactly how a lot sediment it carries and also how quick it is flowing. The sediment brought by a stream might have actually been eroded from the bed and banks of the stream, or it may have fallen into the stream after a mass-wasting event relocated it down a slope bordering the stream's channel. Water relocating at a low velocity or speed can relocate just little, fine pposts such as sand also, silt, and clay. Water moving at a high velocity deserve to relocate a bigger amount of both little particles and also huge, coarse pshort articles such as boulders. Water that flows erratically, providing rise to swirls and eddies, is sassist to be unstable, and rough water have the right to relocate the coarsest and also best amount of particles.

A stream erodes via hydraulic action and abrasion. Hydraulic action is the pressure exerted by the water itself. This force functions alengthy the banks of streams, attacking and also undermining layers of soil and also rock. Abrasion is the grinding and scraping of the stream's financial institutions and also bed by the sediment carried by the stream.

When a stream flows out of a highland location, reaching an elevation that is closer to its base level, it starts to sluggish down. In this state, the stream is still eroding downward, yet its primary pressure of erosion is directly laterally or horizontally versus its financial institutions. It starts meandering, winding back and also forth, carving out a valley floor between valley wall surfaces or bluffs. Periodically, the stream will flood all or a component of its valley, depositing sediment on its arising floodplain.

At the reduced reaches of a stream, wright here it philosophies base level, the valley it flows through will be open, wide, and flat-bottomed. Valley wall surfaces, if they even exist, will be much amethod from the stream channel.

Glacial erosion

Moving glaciers not just transport material as they move, they additionally sculpt and carve ameans the land beneath them. A glacier's weight, linked with its steady activity, deserve to considerably reform the landscape.

A valley glacier moves to reduced elevations under the pressure of gravity through a mix of interior circulation and also sliding at its base. The ice in a glacier is so dense and also under such press that it begins to behave actually prefer a thick tar, flowing outside and downward. Glacial movement with this inner flow is incredibly sluggish. On average, it steps only an inch or 2 (a couple of centimeters) a day. In a valley glacier, ice in the top main component moves much faster than ice at the sides, wright here it is in contact via the valley walls. The significant weight of a valley glacier additionally creates massive push at its base, and this push lowers the temperature at which ice melts. A layer of water develops in between the glacial ice and also the ground. The water reduces friction by lubricating the ground and permitting the glacier to slide on its bed.

A valley glacier is capable of eroding and also transferring astronomical quantities of sediment. As the glacier moves, its ice flows into and refreezes in fractures in the rock walls and floor of the valley. The glacier then plucks that rocky material away via it, some of which may be boulders the size of residences. This


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The 150-mile-lengthy Shenandoah Valley in north Virginia . PHOTOGRAPH REPRODUCED BY PERMISSION OF THE

material then becomes installed in the ice at the base and also alengthy the sides of the glacier. As the glacier proceeds to move, the installed material abrades or scrapes the rock bordering the glacier prefer a gigantic file.

Following an existing V-shaped valley, a glacier erodes the valley, deepening and also widening it. It might boost the width of the valley by as much as tens of miles (kilometers). The glacier also flattens the valley floor because ice often tends to cut dvery own over a more comprehensive location than flowing water. When the glacier lastly retreats, a thick layer of glacial sediment called ground moraine stays, filling in any irregularities in the valley floor.

Spotlight on renowned develops

Napa Valley, California

California's Napa Valley is world-renowned for having actually a vast selection of soils and also climate conditions that help develop grapes that are made right into high quality wines. The narrow valley is 30 miles (48 kilometers) lengthy and 1 to 6 miles (1.6 to 9.6 kilometers) wide. It occupies an area of around 300,000 acres (120,000 hectares).

At its northern finish lies the region's biggest hill, Mount St. Helena. Its southern finish opens up to San Pablo Bay, an arm of the San Francisco Bay system. The valley is bordered by two hill ranges, the Vaca Mountains on the east and the Macayamas Mountains on the west. Flowing with the middle of this valley is the 50-mile-long (80-kilometer-long) Napa River.

The valley began to develop around 4 million years ago once warmth forces beneath Earth's crust in this area compelled land also on either side of the contemporary valley upward right into hills. Certain locations between the hills were widened and lowered into troughs as an outcome. Napa Valley is such a space. After the valley's formation, eruptions from a number of volcanoes bordering the valley blanketed the land in volcanic cinders and ash that accumulated to layers hundreds of feet thick.

Shenandoah Valley, Virginia

The Shenandoah Valley in north Virginia is component of the Great Valley of the Appalachians, which stretches from Pennsylvania to Alabama. The Shenandoah Valley, which is about 150 miles (241 kilometers) lengthy, lies in between the Blue Ridge Mountains on the eastern and the Allegheny Mountains on the west.

Both hill ranges are part of the Appalachian Mountain device, which developed over 300 million years ago. The effective erosive pressures of water, wind, and also frost have actually greatly eroded the Appalachians considering that then. Water runoff has actually sculpted the mountains' distinctive alternating pattern of ridges and also valleys.

The Shenandoah River, which runs via the valley, is a tributary of the Potomac River. It was the initially of the tributaries in the location to reach the soft limerock layer that is currently the base of the Shenandoah Valley. Intercepting stream after stream west of the Blue Ridge Mountains, the Shenandoah River thrived. As it did, it sculpted out the Shenandoah Valley, disfixing the limerock and transferring the sediments north to the Potomac.

Yosemite Valley, The golden state

Lying in the Sierra Nevada Mountains in main The golden state is a 0.5-mile (0.8-kilometer) deep depression. Measuring 7 miles (11 kilometers) lengthy and 1 mile (1.6 kilometers) wide, Yosemite Valley lies at the heart of Yosemite National Park, which encompasses 761,170 acres (304,468 hectares).

The Merced River runs through the valley. Geologists estimate that a couple of million years back, it started to carve a valley wbelow the contemporary


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Yosemite Valley, seven miles long and also one mile throughout, was created throughout six key geologic stperiods that arisen over numerous years . PHOTOGRAPH REPRODUCED BY PERMISSION OF THE


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Yosemite Valley lies. The Sierra Nevada Mountains were uplifted and tilted westward, leading to the river to slowly, then swiftly, erode a deep, steep-walled canyon. About 2 million years back, North America's climate began to cool and also glaciers began to create. Throughout one of the significant glacial periods, around 700,000 years back, ice thickness in the location was up to 6,000 feet (1,829 meters). Massive, flowing glacial ice caused incredible amounts of rock to be carved and also transported to the location, producing huge rock formations that still exist.

The circulation and also retreat of glaciers via Yosemite Valley developed many kind of times until about 10,000 years ago, once the last significant glacial duration finished in The United States and Canada. What continued to be was a U-shaped valley that was actually deeper than the present-day Yosemite Valley. Rock debris from the glacier had actually dammed the valley creating ancient Lake Yosemite, which covered the valley floor. Streams from hanging valleys above fell in towering waterfalls. The tallest of these is Yosemite Falls, which presently drops 2,425 feet (739 meters) to the valley floor. At some point, sediment filled in the lake till it developed the existing valley floor.


For More Information

Books

Huber, N. King. The Geologic Story of Yosemite National Park . Washington, D.C.: UNITED STATE Geological Survey, 1987.

Net Sites

"A Glacier Carves a U-Shaped Valley." U.S. Geological Survey and also the National Park Service . http://wrgis.wr.usgs.gov/docs/parks/glacier/uvalley.html (accessed on August 14, 2003).

"Valley and also Stream Erosion." Bryant Watershed Project . http://www.watersheds.org/earth/valley.htm (accessed on August 14, 2003).

Valley Glaciers . http://www.zephryus.demon.co.uk/geography/resources/glaciers/valley.html (accessed on August 14, 2003).