What is a valley in geology?

In geology, valleys are elongated depressions on Earth's surface, typically formed by rivers or glacial activity . Most valleys are created by river erosion, resulting in various shapes depending on the landscape and geological structures. Glacial valleys often have a characteristic U-shape, formed as glaciers deepen and widen the valley floor. River valleys, on the other hand, can range from narrow, deep canyons in upland areas to wider floodplains and terraces downstream. Tectonic activity can also create valleys, known as rift valleys , which are fault-bounded troughs that existed before the river.

How are valleys formed?

Valleys are elongate depressions in the Earth's surface, most commonly drained by rivers and situated in a relatively flat plain or between hills or mountains. While tectonic activity can create rift valleys, most valleys result from river erosion, where rivers carve the valleys in which they flow. The shape of a valley depends on base level, rock type, and rock structures. During the initial development of a valley, relief increases as rivers expend energy vertically. Over time, rivers smooth the valley floor, and more energy is spent laterally, broadening it. Other types of valleys include: * Rift valleys Valleys formed through tectonic activity. * Glacial valleys Stream valleys that have been glaciated into a U-shape.

What is the difference between a V-shaped valley and a U-shaped valley?

The primary distinction between V-shaped and U-shaped valleys lies in their formation and resulting morphology. V-Shaped Valleys: These valleys form through erosion caused by rivers, which carve narrow, deep channels, especially in areas with rapid uplift and intense fluvial action. They are characterized by steep, knife-edge ridges and valley slopes that converge in a V-shape. U-Shaped Valleys: These valleys are the product of glaciation . Glaciers, having greater viscosity and cross-section than rivers, erode valley walls and floors, creating a parabolic or U-shaped cross profile with wide, flat bottoms and steep, even vertical sidewalls.

How do rivers shape valleys over time?

Rivers shape valleys over time through continuous erosion and deposition processes. The river's energy is primarily used for vertical entrenchment, deepening the valley. Over time, the river smooths the valley floor, eliminating waterfalls and reaching its base level. As the river approaches its base level, its energy shifts to lateral erosion, widening the valley floor. The valley's shape depends on base level, rock type, and geological structures. In areas with folds or faults, these structures control the orientation and form of valleys. The longitudinal profile of a valley, steep in the headwaters and gentle in the lower reaches, reflects the river's adjustment to its base level.

What role do glaciers play in forming valleys?

Glaciers play a significant role in shaping valleys through erosion and deposition. As a glacier moves, it picks up rock debris from the valley's walls and floor. Glacial erosion occurs through abrasion and quarrying. Abrasion happens as the ice grinds over the ground, while quarrying or plucking involves the glacier plucking rocks from its bed. Glaciers transform V-shaped stream valleys into U-shaped valleys because the U-shape offers the least resistance to the moving glacier. This process results in relatively wide, flat valley bottoms and steep, even vertical sidewalls. Smaller tributary glaciers erode their troughs less rapidly than the main glacier, leading to hanging valleys.

What are some famous valleys around the world, and how were they formed?

Valleys are elongated depressions on Earth's surface, commonly drained by rivers, situated in flat plains or between hills or mountains. Formation Valleys are created through various geological processes: * River erosion Most valleys are carved by rivers. The steeper a stream's gradient, the more power it has to erode the streambed. * Tectonic activity Rift valleys form when a segment of the Earth's crust subsides between dip-slip or normal faults. * Glacial activity Glaciers reshape V-shaped stream valleys into U-shaped valleys by eroding the base and sidewalls. Examples * Kāli Gandaki River Valley The world's deepest subaerial valley is located in Nepal, between the Himalayan peaks Dhaulagiri and Annapurna. * Grand Canyon A well-known canyon carved by the Colorado River in northern Arizona. * East African Rift System An extensive system of continental rift valleys.

valley

Images

cratered plains and cratered highlands in Lunae Planum

False-colour oblique view of central Valles Marineris. The eastern edge of Candor Chasma (left) and Ophir Chasma (centre) show evidence of erosional collapse. This picture combines data from high-resolution black-and-white and low-resolution colour images taken by the Viking spacecraft.

At a Glance

valley summary

valley

geology

Written by

Victor R. Baker

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History

Top Questions

What is a valley in geology?
In geology, valleys are elongated depressions on Earth's surface, typically formed by rivers or glacial activity. Most valleys are created by river erosion, resulting in various shapes depending on the landscape and geological structures.

Glacial valleys often have a characteristic U-shape, formed as glaciers deepen and widen the valley floor. River valleys, on the other hand, can range from narrow, deep canyons in upland areas to wider floodplains and terraces downstream. Tectonic activity can also create valleys, known as rift valleys, which are fault-bounded troughs that existed before the river.
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Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How are valleys formed?
Valleys are elongate depressions in the Earth's surface, most commonly drained by rivers and situated in a relatively flat plain or between hills or mountains. While tectonic activity can create rift valleys, most valleys result from river erosion, where rivers carve the valleys in which they flow.

The shape of a valley depends on base level, rock type, and rock structures. During the initial development of a valley, relief increases as rivers expend energy vertically. Over time, rivers smooth the valley floor, and more energy is spent laterally, broadening it.

Other types of valleys include:
* Rift valleys Valleys formed through tectonic activity.
* Glacial valleys Stream valleys that have been glaciated into a U-shape.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What is the difference between a V-shaped valley and a U-shaped valley?
The primary distinction between V-shaped and U-shaped valleys lies in their formation and resulting morphology.
- V-Shaped Valleys: These valleys form through erosion caused by rivers, which carve narrow, deep channels, especially in areas with rapid uplift and intense fluvial action. They are characterized by steep, knife-edge ridges and valley slopes that converge in a V-shape.
- U-Shaped Valleys: These valleys are the product of glaciation. Glaciers, having greater viscosity and cross-section than rivers, erode valley walls and floors, creating a parabolic or U-shaped cross profile with wide, flat bottoms and steep, even vertical sidewalls.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do rivers shape valleys over time?
Rivers shape valleys over time through continuous erosion and deposition processes. The river's energy is primarily used for vertical entrenchment, deepening the valley. Over time, the river smooths the valley floor, eliminating waterfalls and reaching its base level.

As the river approaches its base level, its energy shifts to lateral erosion, widening the valley floor. The valley's shape depends on base level, rock type, and geological structures. In areas with folds or faults, these structures control the orientation and form of valleys. The longitudinal profile of a valley, steep in the headwaters and gentle in the lower reaches, reflects the river's adjustment to its base level.
2 Britannica Sources
1. valley: Valley longitudinal profiles
2. river: Valley evolution
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What role do glaciers play in forming valleys?
Glaciers play a significant role in shaping valleys through erosion and deposition. As a glacier moves, it picks up rock debris from the valley's walls and floor. Glacial erosion occurs through abrasion and quarrying. Abrasion happens as the ice grinds over the ground, while quarrying or plucking involves the glacier plucking rocks from its bed.

Glaciers transform V-shaped stream valleys into U-shaped valleys because the U-shape offers the least resistance to the moving glacier. This process results in relatively wide, flat valley bottoms and steep, even vertical sidewalls. Smaller tributary glaciers erode their troughs less rapidly than the main glacier, leading to hanging valleys.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are some famous valleys around the world, and how were they formed?
Valleys are elongated depressions on Earth's surface, commonly drained by rivers, situated in flat plains or between hills or mountains.

Formation Valleys are created through various geological processes:
* River erosion Most valleys are carved by rivers. The steeper a stream's gradient, the more power it has to erode the streambed.
* Tectonic activity Rift valleys form when a segment of the Earth's crust subsides between dip-slip or normal faults.
* Glacial activity Glaciers reshape V-shaped stream valleys into U-shaped valleys by eroding the base and sidewalls.

Examples
* Kāli Gandaki River Valley The world's deepest subaerial valley is located in Nepal, between the Himalayan peaks Dhaulagiri and Annapurna.
* Grand Canyon A well-known canyon carved by the Colorado River in northern Arizona.
* East African Rift System An extensive system of continental rift valleys.
6 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

valley, elongate depression of the Earth’s surface. Valleys are most commonly drained by rivers and may occur in a relatively flat plain or between ranges of hills or mountains. Those valleys produced by tectonic action are called rift valleys. Very narrow, deep valleys of similar appearance are called gorges. Both of these latter types are commonly cut in flat-lying strata but may occur in other geological situations.

Wherever sufficient rainfall occurs, opportunity exists for the land surface to evolve to the familiar patterns of hills and valleys. There are, of course, hyperarid environments where fluvial activity is minimal. There also are geomorphological settings where the permeability of rocks or sediments induce so much infiltration that water is unable to concentrate on the land surface. Moreover, some landscapes may be so young that insufficient time has elapsed for modification by fluvial action. The role of fluvial action on landscape, including long-term evolutionary processes, is considered here in detail. For additional information on fluvial and hillslope processes relating to valley formation, see river.

Probably the world’s deepest subaerial valley is that of the Kāli Gandaki River in Nepal. Lying between two 8,000-metre (26,000-foot) Himalayan peaks, Dhaulāgiri and Annapūrna, the valley has a total relief of six kilometres (four miles). Because the Himalayas are one of the Earth’s most active areas of tectonic uplift, this valley well illustrates the principle that the most rapid downcutting occurs in areas of the most rapid uplift. The reason for this seeming paradox lies in the energetics of the processes of degradation that characterize valley formation. As will be discussed below, the steeper the gradient or slope of a stream, the greater its expenditure of power on the streambed. Thus, as uplift creates higher relief and steeper slopes, rivers achieve greater power for erosion. As a consequence, the most rapid processes of relief reduction can occur in areas of most rapid relief production.

Perhaps the most famous example of a canyon is the Grand Canyon of the Colorado River in northern Arizona. The Grand Canyon is about 1.6 km (1 mile) deep and 180 metres (590 feet) to 30 km (19 miles) wide and occurs along a 443-km- (275-mile-) long reach where the Colorado River incised into a broad upwarp of sedimentary rocks.

Geomorphic characteristics

The relief of valleys and canyons is produced by the incising action of rivers. Hillslope processes are indeed critical in the development of valley sides (see below), but it is rivers that lower the level of erosion through degradation. Rivers ultimately adjust to a baselevel, defined as the lowest point at which potential energy can be transformed to the kinetic energy of river flow. In most cases, the ultimate baselevel for rivers is sea level. Some rivers drain to enclosed basins below sea level, as, for example, the Jordan River, which flows to the Dead Sea in Israel and Jordan. Moreover, rivers may adjust to local baselevels, including zones of resistance to incision, lakes, and dams (both natural and artificial).

Valley longitudinal profiles

The longitudinal profile of a valley is the gradient throughout its length. Valleys formed by river action typically have a concave upward profile, steep in the headwaters and gentle in the lower reaches. The lower end of such a profile is adjusted to an effective lower limit of erosion defined by the baselevel.

In an ideal case of river adjustment to uniformly resistant materials, the longitudinal profile of a stream assumes a characteristic form that minimizes variations in transporting power. Power in a river derives from the rate of transfer of potential energy, dE/dt, which depends on the rate of fall in elevation of water, dy/dt, according to Equation. where E is energy, t is time, m is mass, g is the acceleration of gravity, and y is elevation. The rate of fall in elevation, in turn, can be expressed as follows:where S is the slope (fall in elevation, dy, with downstream horizontal distance, dx) and V is the flow velocity (change in horizontal distance, dx, with time, dt).

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Combining equations (1) and (2) and using the fluid density ρ (mass per unit volume of water), one obtains Equation. where W is channel width, D is channel depth, L is a unit length of stream, and the other parameters are as defined above. Because flow discharge Q is defined asthe power per unit length of flow, Ω, can be expressed as

It should be noted that in order to minimize variation in power, a river increasing its discharge in a downstream direction must decrease its slope. Thus, slope must be constantly decreasing downstream, explaining the concave upward character of the longitudinal profile.

The idealized concave upward longitudinal profile defined purely by energy considerations, noted above, only occurs where channel bed resistances and adequate adjustment time permit. Resistant zones of bedrock require greater power for a stream to incise at a given discharge Q than do less resistant zones. Therefore, by equation (5) the stream gradient S must be locally steeper at resistant zones. Similarly, a rapid base-level change, such as a fall of sea level, may not allow adequate time for the entire longitudinal profile to adjust. One indication of such effects on a longitudinal profile is a nick point, or abrupt change in slope of the profile.

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Geomorphic characteristics

Valley longitudinal profiles