A valley is a generally elongated hollow form in the landscape that is created by the interplay of erosion and denudation . The linear erosion occurs through a river ( river valley ), the extensive denudation through gravitational mass movements . The deepest line is called the depth line , valley floor or valley floor , depending on the shape . This has a monotonous (same-direction) gradient . In the case of a deviating shape or other formation processes (e.g. in the case of glacial erosion ), one speaks of a valley or valley-like shape.
The word goes back to the Indo-European root dhel- "Bend, Höhlung, Völbung " and is related to the word dent .
Lines of water (rivers) that run off and can erode material (erosion) lead to the lowering of the river bed. It is irrelevant whether the deep or side erosion predominates. With the lowering of the river bed, denudation attacks the valley slopes (or walls). This also removes the boundaries of the valley and places them deeper, but never deeper than the river. Rivers play a key role in the creation of talent, as they form the deepest line for their catchment area, up to which the entire erosion can work. They thus represent a regional erosion base , making them the regulator of land formation. Due to the different types of erosion and denudation as well as the different erosion speeds, different valley shapes arise.
Open hollow forms that were not created by fluvial erosion should not be referred to as valleys . These include above all tectonic trenches, on the bottom of which a river can flow, but the slopes of which were not created by erosion. Also, elongated hollow forms between the dunes back (z. B. Draa dunes ), beach ridges , moraines u. Ä. Are not referred to as valleys, but described as valleys, valley ridges, valley furrows, furrows, depressions or hollows. Likewise, forms that are created by temporary erosion processes with rapid change (such as is the case with badlands , gullies or runs ) are not referred to as valleys.
The formation of the valley is modified by the geological storage conditions of the rocks and any existing tectonic processes ( e.g. mountain formation ). The climatic conditions, which significantly influence the amount of runoff and weathering intensity, also have a significant influence on the formation of the valley.
Basic valley shapes
A ravine is created when a river is very deeply eroded. Side erosion and denudation play no or only a subordinate role. That is why gorge valleys have walls as valley boundaries, the river completely fills the valley floor.
A notch valley is created when there is an approximate balance between deep erosion and slope erosion. Notched valleys are V-shaped in cross-section. The valley floor is completely or almost completely filled by the river flowing in it.
If a river carries out both deep and side erosion, a clearly formed valley floor is created. The river no longer completely fills the valley floor. Depending on the occurrence or absence of denudation, a distinction is made between bottom valleys with slopes as a boundary or box valleys bounded by walls.
There are transition types between the gorge, Kerbtal and the sole / box valleys, which are called Kerbsohlental or Sohlenkerbtal.
Trough valleys form in areas of heavy denudation . In contrast, both deep and side erosion recede. The strong slope removal leads to flat valley slopes. The transition to the actual valley floor is indistinct. Basin valleys are mainly formed in the periglacial areas and in the humid tropics . There is strong denudation in both climatic regions.
A high valley is a valley in the upper section of a mountain range that has not been formed by severe erosion. Such valleys are usually clearly separated from other valleys and, for example, only accessible via one elevation step.
An asymmetrical valley is a valley with a noticeably uneven (asymmetrical) slope.
Special valley shapes
The valley shapes listed here arise under certain storage conditions of the rocks, during tectonic activities or under special climatic conditions.
A narrow valley or a narrow valley is the umbrella term for a valley section with a very narrow valley floor. The cross-section is V-shaped or steeply U-shaped. By nature, the water has practically no freedom of movement in the valley floor. In the traffic-technical sense one speaks of valley pass , in place names there is also the expression Klause .
A ravine is a narrow, steep valley cut, usually in a mountain range. The valley boundary consists of walls or steep slopes. Gorges are created by cutting even smaller watercourses into stable, massive rock. It preferentially forms in deep rock such as granite . The deep erosion strongly outweighs the denudation.
A canyon is created when a river cuts deep into a plateau with layers of rock lying horizontally. The rocks have a different resistance to erosion. The slopes of a canyon are therefore stepped like stairs and alternately steep and significantly flatter. It acts both deep erosion and denudation.
Breakthrough valleys cut through a mountain range. Antecedent breakthrough valleys are created by the cutting of a river into the mountains created by tectonic uplift from a plain. A river can cut its meanders acquired in the lowlands deep into the resulting mountains (forced meanders). Epigenetic breakthrough valleys also arise without tectonics, when a river encounters a hardwood during deep erosion and then erosively attacks it.
Wadis and dry valleys
A wadi is a dry valley in desert areas that only temporarily carries water after heavy rainfall. But there are also dry valleys in humid climates today when the conditions that led to the formation of the valley are no longer given. An example of this are the numerous dry valleys in northern Germany or the dry valleys in karst areas such as the Swabian and Franconian Alb. These emerged when, in the Ice Age, rainwater could not seep away due to the permafrost that existed at the time and was forced to run off above ground.
Hollow forms are referred to as valley-like shapes that are more or less similar to valleys, but have different conditions of formation.
Trough valleys and fjords
Glacial valleys (actually broad bottom valleys), which carried away the meltwater from the Ice Age inland ice advances parallel to the ice edge, have only limited the properties of a valley today due to later overprinting. Above all, the gradient in the same direction usually no longer exists today.
Glacial gullies and fjords
Glacial channels were created under the glacier by meltwater erosion. Its current shape is largely based on the shape of the dead ice blocks that once filled the channel. In most cases, therefore, they do not have a gradient in the same direction. Förden are drowned glacial channels.
A ria is a sea bay that penetrates deep into the mainland and was created by flooding a former river valley.
General valley development
Especially in the upper reaches of the rivers, the water, which flows rapidly due to the great gradient, cuts deeply erosively into the rock and leads to the formation of a ravine or a gorge. Depending on the nature of the rock, a valley or a gorge follows downstream . These are mainly to be found in the high mountains. The deep erosion is so strong that the weathering of the slopes and their erosion cannot follow. The rock walls of the gorges become almost vertical and only as wide as their river bed. The maximum depth of a gorge is determined by the critical height of the rock walls. If this is exceeded, the instability leads to rock falls and landslides. The slopes are shifted back and the vertical valley slopes flatten. This results in intense lateral erosion of the slopes.
If the valley is predominantly V-shaped, it is called a notched valley. The relocation is characterized by heavy material removal on the slopes, which withstands the deep erosion of the river. The flatter the slopes of the valley, the more discontinuous the denudation . If rockslides regularly occur in a deep gorge at the beginning, lateral erosion only occurs later when the corresponding meteorological events occur. These denudations can then be observed by sliding or sliding as well as in the form of mudslides and other currents.
Notched bottom valleys are characterized by less deep erosion and low, but existing, side erosion. The transition from the Kerbtal to the Sohlental is fluid. Due to the erosion on the slopes, the water is forced to move the eroded material. If the gradient in the valley is lower, the water begins to meander . On the impact slope , it then often hits the slope of the valley and attacks it intensively. Deposits of material take place at the same time on the sliding slope , which widens the bottom of the valley.
The notched mountain range is completely eroded over time and leveled in the process; the difference in altitude between the valley floor and mountain peaks decreases again. In the increasingly flatter terrain, the Kerbsohlental becomes a wider sole valley. The reason for the reduction in the height difference between the valley floor and the mountains does not have to be due to erosion. It can also be due to tectonics .
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