Kerbtal
Notched valleys (also called V valleys due to their frequent, simplified, symmetrical valley slopes ) are valleys that are mainly located in low and high mountain ranges . Often they are so narrow at their sole that they can be considered a narrow valley . The genesis same notched sole valleys other hand, are the sole valleys assigned. Notched valleys are formed when there is a predominantly predominant deep erosion and simultaneous denudation on the valley slopes.
geomorphology
precursor
The strong flowing water , especially in the upper reaches of the rivers due to the great gradient , partly in the form of waterfalls , cuts erosively into the rock and leads to the formation of a ravine . Depending on the nature of the rock, this results in a notch valley or a gorge (gorge). These are mainly to be found in the high mountains. The deep erosion is so strong that weathering of the slopes and their erosion cannot follow. The rock walls of the ravines 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.
Emergence
If the critical height of the rock walls 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 now predominantly V-shaped, one speaks of a notched valley. A distinction is also made between ideal-typical symmetrical and asymmetrical notch valleys, which is mainly determined by the homogeneous slope of the valley slopes. The relocation is characterized by heavy material removal on the slopes, which withstands the deep erosion of the river. The flatter the valley slopes, the more discontinuous the denudation appears . If rockslides regularly occur in a deep ravine at the beginning, lateral erosion only occurs later when there are corresponding meteorological events. 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. Due to the impenetrable layers in the geological structure of the rock, these can sometimes mark sections of a notch valley. A special form of V-shaped valleys are the canyons . The transition from the Kerbtal to the Sohlental is fluid. Due to the erosion on the slopes, the water is forced to be able to move the removed 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. How strong the effect against the slope is in such places can be observed during floods. Deposits of material take place at the same time on the sliding slope , which widens the bottom of the valley.
In terms of their shape, notch valleys can resemble the breakthrough valleys of large rivers, but in contrast to these, they do not penetrate the attacked mountains. They are therefore steeper along their entire length.
successor
Notched valleys are not the last valley shape that can form when mountains are eroded. In high mountains with glaciers , notched valleys lead the glaciers that expand during ice ages. This leads to intensive lateral glacial erosion, while deep erosion ceases almost completely. The ice continues to grind down the valley walls ( deterion ) and partially blows out rock material due to the enormous pressure. This mainly happens in the narrowest part of the valley, i.e. just above the bottom, as this is where the weight pressure of the glacier is highest. At the edge of the glacier, further material is carried away by detraction and has an erosive effect itself. The resulting valley shapes are trough valleys or fjords (U valleys).
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 mountains, the already wide Kerbsohlental becomes a 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 plate tectonics or clod tectonics . In the case of clods in particular, it is possible that they sink and so the water itself continues to sediment the Kerbtal , i.e. fill it up. If the floe subsides enormously, the flowing water in the flat valley develops into a steppe lake . Later the Kerbtal is completely leveled in a sedimentary basin .
Soil science
Valley floor
The soils in a Kerbtal are marked by deposits and erosion of the water flowing through them. If soils of large hollow valleys can still be assigned to floodplain soils almost everywhere in the sole area , it is more difficult with notched valleys. The soil in the Kerbtal is built up very differently through sedimentation . In shallower passages as well as on the sliding slope there are deposits that can be very humus and loess depending on the nature of the mountain. In the Kerbsohlental, in which the floor is wider and therefore there are many more normally dry areas for sedimentation even during floods, there are already pronounced and extensive floodplain soils.
Especially in the Kerbsohlental there are also old arms of the flowing water. The soils there are shaped by the gravel and gravel layers of the old stream bed and can develop into floodplain leyas over time . On the other hand, these courses of an old brook bed are shaped by flooding with currents and erosion even during floods.
Slopes
The type of soil on the slope is generally strongly characterized by rocks, sometimes over a large area interrupted by rocks. Interestingly, in the course and also in the cross-section of a valley, a wide variety of soils can occur on the slopes within a few hundred meters. The type of soil formed on the slope depends on various factors, including above all the orientation in the wind system (entry of loess and dust) and the slope of the slope (strength of erosion and density of vegetation). The raw soils of the Syroseme are mainly found where large areas of the slope have been torn open by erosion . They consist of thick gravel and loose rock layers with a thin humus horizon.
In the temperate climates, brown soils formed on the upper slopes along the valley . Depending on the bedrock of the mountains, these are mostly limestone soils. Compared to brown earth on level ground, these are still very flat.
Climate and Weather
The local climate in notched valleys differs from that in the surrounding mountains. The reason for this is of course the lower position, but also the different radiation from the sun. This gives rise to various phenomena.
Basics
Depending on the direction of a valley, the duration of sunshine in the valley floor is very different. Valleys facing north or south tend to be disadvantaged, as a shadow is cast through the valley slopes in the morning and evening hours. Valleys that run in an east-west direction are therefore lit for longer if their northern slope is not too steep. It therefore also plays a role on what geographical latitude the valley lies.
A climatic determination of the valley is made - in addition to the prevailing macroclimate, of course - also by its location in the mountains. The decisive factor for the microclimate is whether the valley is on the windward or lee side .
Wind systems
In the valley there are two wind systems that are related and related in their formation. On the valley walls there is slope wind circulation and in the valley direction itself there is valley wind . Together these form the mountain-valley wind system . The cause of wind is always a pressure difference between air masses in two areas. In the case of notched valleys, this pressure difference is caused by the different levels of radiation from the sun.
Since this narrow wind effect requires intensive solar radiation, it can be observed mainly in summer with high pressure weather conditions. However, partial processes of the phenomenon can also occur in winter due to the almost exclusive irradiation of the mountains. However, the effect is also overlaid by the winds of the general weather conditions.
The slope wind circulation occurs when the upper valley slopes are intensively warmed by sunshine in the morning. The warm air rises there and draws air from the valley. Since there is still cool air a few meters above the slope, the slope wind does not rise vertically upwards, but remains roughly on the slope, i.e. noticeable as wind. The cold air above falls off and moves back into the valley to compensate. The slow but steady updraft is used, for example, when paragliding .
In the afternoon, the upflow on the slope across the valley is almost completely stopped, as the bottom of the valley is now also warmed up and is completely reversed in the evening and at night, when cold air falls down on the valley slope. At the bottom of the valley, however, heat is emitted all night long, which is why the air rises vertically there.
The valley wind blowing in the direction of the valley follows the vertical directions of the slope wind with a time delay. In the mornings, it blows down the valley, bringing the cool air from the high altitudes into the foreland of the valley. Its direction is not determined by the temperature differences between the valley floor and the upper slope, but by the pressure difference between the foreland and the altitude at the upper end of the Kerbtal valley. The main reason for the valley wind, however, is the inequalities of the slope wind circulation. Around noon, the falling air masses are no longer sufficient to compensate for the rising air masses: A wind comes out of the foreland of the valley and blows up the valley. In the middle of the night, in turn, larger air masses fall into the valley than rise in the middle, creating the downward wind in the direction of the valley, which continues until sunrise.
The lowest temperatures in the valley are therefore reached shortly before sunrise (in the valley). The wind system also influences the local formation of precipitation.
Precipitation
In the way that precipitation occurs, notch valleys are mainly characterized by specific fog phenomena . The fog is significantly influenced by the wind system of the valleys. The fog known as the valley fog is caused by the low temperatures in the valley. Since the temperature is lowest in the last hours of the night due to the air mass balancing, fog does not appear until then and persists into the early hours of the day. Fog in itself is not precipitation, but is mainly deposited as dew on the vegetation of the slopes.
The local amount of precipitation is influenced not only by the orientation of the valley but also by the valley wind. If this pushes back the moist air masses flowing into the valley, the amount of precipitation increases. The updraft carries rain areas out of the valley and thus lowers the amount of precipitation.
biogeography
Geobotany
Due to the geological, hydrological, but also meteorological spatiality of the cross-sectional shape of the Kerbtal, a very different plant community also forms in the vegetation . However, precisely because of the limited space available, the ideal-typical societies can also penetrate one another and thus cannot be clearly delimited from one another everywhere.
The valley floor of a Kerbtal is naturally covered, especially in the temperate latitudes, by alluvial forests, which are divided into the only a few meters wide softwood floodplain close to the water and the hardwood floodplain . However, due to the greater space available at water level, an alluvial forest can in fact only expand in kerboal valleys. Depending on the slope of the valley, the softwood floodplain can also be completely absent there, as it is destroyed by regular floods with ice displacement and bed load in the spring. In such places, shallow water embankments are only populated by flat shrubs and perennials such as forest shrubs.
The character of the valley flanks of a Kerbtal differs from point to point and so it is not uncommon for typical ravine forests , which have adapted to the extreme incline and rock content, to shape the shape of the settlement, especially in the lower part of the slope . In Central Europe, ravine forests belong to the mixed beech forests and also tolerate the soil type of the hardwood alluvial forest in the valley floor, so that both forest types interpenetrate at the transition, especially towards the water. The ravine-like character of a valley flank is often formed by the impact slope of the water, which extends right up to the slope. There is no alluvial forest in these places, the ravine forest extends up to and often beyond the water.
Further mixed beech forests form a natural biotope on the higher or flatter slopes. The predominant soil types are essential for the formation of specific forest communities of the beech (in Central Europe). Due to the stony subsoil of possible brown earth soils, dry-slope limestone beech forests occur in the notch valleys. Oak dry forests form on suitable southern slopes. In a dry climate, the slopes of the Kerbtal offer particularly unfavorable conditions compared to the sole, as there is hardly any groundwater there. Exposed south-facing slopes are often free of vegetation or populated by small hard- leaved plants, especially if heat and light are reflected through the stony subsoil.
The most common tree species in the notched valleys of the northern hemisphere belong to the beech and birch family . In general, it can also be stated that tree species in the valley floors tend to be shallow roots , while on the flanks they tend to be deep roots . In the ravine forests, there are often only those species that can give themselves sufficient support via tap roots .
The flowing waters have a high flow velocity, which means that the river beds are covered with coarse gravel and stones. With these severe conditions, hardly any aquatic plants grow in the waters of the valley valleys. Above all, protected places in stone crevices, but also tree trunks or roots, give plants a hold in the flowing waters and thus the possibility of sedimentation of fine material.
In some places in natural notch sole valleys, the water can be dammed or slowed down to create pond or swamp-like interruptions. At these points, sedimentation in the water and on the banks creates particularly good conditions for the vegetation. Due to the deep valley location and the surrounding higher forest vegetation, there are no tall grasses in such places, as are often found in lowlands in the lowlands.
Phytogeography
The interaction between plants and the spatial shape of the Kerbtal mainly affects the geomorphological process of material displacement. Plants absorb water on the slopes and thus provide natural support. This leads directly to the fact that the soil on the slope is protected from erosion . By flattening the peaks of the watercourse in the valley, the vegetation on the slope also indirectly protects the bottom of the valley floor from erosion and supports sedimentation.
On the other hand, loose surface material on the slopes, especially leaves and branches, is very easily removed and deposited in the valley floor. Organic material is therefore shifted from the slope into the valley floor, particularly through water. This explains the often thin humus layer on the slope despite the vegetation.
Since the valley flanks are denuded in the Kerbtal despite vegetation, the flowing water carries fluvial sediment . In addition to geomorphological factors, the vegetation close to the water also contributes to sedimentation by slowing down the water as a flooded obstacle.
Geozoology
The Kerbtal also offers the animal world a wide variety of habitats that are used in combination by many species and between which there is such a strong relationship. The south-facing slopes with a widespread stony surface offer shelter, warmth and light , especially for cold-blooded animals . The canyon forest is therefore a habitat with better living conditions, especially for reptiles and amphibians, than a flat and dense forest. Many species also need the high temperature differences on the slope, which prevail on the one hand between day and night and on the other hand between exposed and shady areas.
The north-facing slope offers much cooler living conditions and is nowhere near as dry. These conditions are rather ideal for nocturnal homoiothermic animals. In the case of stony ground, this slope is also divided into stone crevices, caves and overhangs and thus offers hiding places.
The cooler and light-poor valley floor is not far from the lighter and warmer valley slopes, but compared to the valley flank it is the widespread moister habitat. For insects and amphibians, the moist room plays an important role in the reproduction with larvae. Since they mostly need fish-poor or fish-free waters for their larvae, the high dynamics of the flowing water play a role in the formation of small-scale water arms. The character of concrete notch valleys as a habitat therefore also depends on how rich in water the flowing water is.
The flowing water offers a great abundance of fish in all sufficiently warm biomes . The mountain stream habitat is usually directly connected to other bodies of water such as large rivers and ultimately the sea . As a very oxygen-rich body of water, it is even used by some marine fish to reproduce. The reason for the abundance of oxygen is the steep valley shape of the Kerbtal and the associated movement and turbulence of the water.
Anthropogeography
Notch valleys offer advantages for people as a place to live and do business , but also as a course for infrastructure, such as protection from wind and sun. Apart from exceptions in the subtropics, they also ensure a fresh water supply. They cause problems for people through floods , which essentially cause the formation of the valleys, but endanger people in the form of flash floods .
colonization
In contrast to the gorge or steep breakthrough valleys, the Kerbtal is habitable for humans thanks to adapted forms of settlement . Due to their narrowness, settlement in the form of villages and towns is possible almost exclusively in the direction of the valley. In such valleys, forms of the row village were formed . In addition to historical-political classifications, human settlements in valley valleys are also very clearly limited by natural occurrences (narrow passages and steep passages). When creating settlements, it should be noted that there is always a risk of flooding in a Kerb valley. Especially in the curves of the water, which form very strong impact and sliding slopes in the Kerbtal, they can cause strong shifts in the waterfront and thus endanger the settlements.
Villages in notched valleys developed into cities, especially in places where short side valleys flowed into a larger notch valley and allowed a lateral ascent over the ridge or at least to high plains. Paths, roads and modern transport connections are led by notch valleys. Important places developed where crossing points developed in valleys. This is the case, for example, at larger branches of a valley, but it is also possible in places where side valleys allow a comfortable crossing of the valley.
Small and medium-sized towns could well develop in notch valleys, but a kerbtal in the narrow sense does not offer enough space for large cities , but is taken up by the expansion of the cities (which then lie in neighboring basin-like valleys). Examples of such large cities in Germany, which were able to penetrate into valley valleys through incorporation and growth, are Freiburg im Breisgau with an altitude difference of about 1000 meters in the urban area as well as Dresden , Wiesbaden and Würzburg in some parts .
A special form of urban settlement of a Kerbtal is Wuppertal , which is located in the Kerbsohlental of the Wupper and was able to develop and expand in this. In places the valley floor is extended like a basin, but in parts the populated valley is only about 300 meters wide. The side valleys are partly used for traffic development and are usually urbanized.
In the Alps , where large-scale settlement is only possible in the valleys, large cities developed in the trough valleys, from which curb valleys were developed in urban areas. Innsbruck is an example of settlement out of a trough valley.
Management
Notched valleys can be managed in different ways:
- Steep notched valleys usually only allow forestry on their flanks . The removal of the felled wood is difficult. The hillside forest is not accessible everywhere by forest roads, so that back horses were used in the past . Since the development (especially via logging routes ) for large technical equipment such as the forwarder has not improved, the use of backhorses (apart from organic farms) is hardly economical. That is why more and more forests on the valley slopes are no longer used for logging. Forestry in the Kerbtal today means in particular conservation.
- Agriculturally , they often offer fertile soils due to the water. Basically, notched valleys in the valley floor can be used for agriculture in any way that the climate in the valley allows. However, a Kerbtal is limited in its duration of sunshine and thus vegetation time ( see flora and fauna ) by the steep slope. In addition to the pure width of the valley floor, the water and its old river beds also limit the usable areas of the valley. A high proportion of the valley floor can therefore also be characterized by gravel and crushed stone layers that are difficult to manage.
- Narrow valleys in Europe are currently used on the valley floor primarily for pasture and meadow farming. Especially when keeping cattle on pastures, they offer the advantage of a water supply compared to plateaus. They only allow arable farming in small plots in wider areas .
- The slope, which is favored by the sun's rays, can often be used in the moderate latitudes for fruit growing and sometimes also for viticulture . In the higher ridges, fruit is often grown in the form of orchards , which allow more efficient use of the soil.
- Another use of the valley is water management . At the present time, this is almost exclusively limited to the operation of reservoirs , the dams of which close curb valleys at narrow points and thus damming the valley's flowing waters. The purpose of the reservoirs is to be seen in energy generation and flood protection . However, the fast flowing waters of the Kerbtal have been used to generate energy for many centuries. The original technology is the water wheel with which mills were operated. Since the Kerbtal is steeper, it has a higher density of potential and kinetic energy of the water. This also enables the installation of water mills with an advantageous "overshot water wheel". The slope of the water of the valley (or a branched course) is artificially reduced in order to let the kinetic energy act selectively on the water wheel.
- Watermills were most widespread in Central Europe during early industrialization. During that time, in addition to grain, basic materials for glass and paint production as well as spices and coffee were ground. Water was used to make paper and wooden boards. Sawmills operated by hydropower in the valleys had the main advantage that they were very close to the woodworking area, the forests on the valley slope (see above). Due to its proximity to the mining area, hydropower was also of particular value for grinding ore in the valley valleys.
- In mining , notched valleys often provide access to mines that are driven vertically into the mountain through a tunnel . In such places, in addition to the corresponding steep slopes, they also have to provide enough space to store the cut material. A special form of these tunnels is the "water solution gallery", which drains mines. The Kerbtal is used for drainage from the point where the tunnel emerges. When creating large areas with a corresponding depth, such a tunnel must sometimes have a considerable length up to a sufficiently deep Kerbtal. The Rothschönberger Stolln in the Ore Mountains is more than 50 kilometers long, crosses under the insufficiently deep valley of the Freiberg Mulde and therefore drains into the Kerbtal of the Triebisch .
Transport route
Notched valleys are the starting point for passes that lead across mountains. Above all, the protective roles of the valley contribute to the Kerbtal as a useful transport route. Both humans and animals being transported are better protected from wind and sun in the valley. Especially when goods were still very often transported with animals, the distributed and secure water supply in the valley played a major role. For example, notched valleys in subtropical regions were often the only alternative for transport routes, provided that their body of water carried water continuously.
The role of the Kerbtal as a carrier of infrastructure increased again with the construction of the railway in the 19th century . The low gradeability of the first railways was the reason for creating routes in the valleys. At the upper end of such a passage, the ascent to the higher elevations of the mountains took place via a ramp built on the slope. In high mountains, tunnels and passes lead from one valley to another. Depending on the height of the tunnel system, one speaks of a base tunnel if it crosses the mountain at its "base" or of the apex tunnel if it is first ascended via ramps. If the Kerbtal is too steep or the area on the slope is not sufficient for a ramp, circular tunnels were used to gain height in a spiral ascent “next to” the valley. A well-known example is on the Wutach Valley Railway in the Black Forest .
The greatest problem of the traffic routes in valley valleys is strong flooding. Especially railway tracks, which consisted of wooden sleepers for a very long time, were flooded and washed away. The substructures of roads and railway systems are also prone to erosion and even total destruction. Long-distance railway lines are threatened to this day.
With the construction of the high-speed lines , the role of the Kerbtal for the railroad has changed. The Kerbtal has become more of an obstacle due to more powerful trains but also cheaper tunnels and bridges. Newly built railway lines are similar in their course to the construction of motorways . In the German low mountain range, motorways have been designed almost from the beginning in such a way that they cross the valley valleys in order to run in a more straight line. Since motorways can still have steeper gradients, the course with direct transitions from bridges to tunnels in the low mountain range is rather rare. This construction method is used over long distances on many new lines.
The Saxon-Franconian Magistrale , which was completed in 1869 and runs from east to west on the flat panel of the Ore Mountains, is archetypal for the course of modern railway lines . The route crosses many of the Ore Mountains notch valleys via large structures, including the Göltzschtalbrücke or the Hetzdorf Viaduct (closed in 1992). The course of this railway line was more due to the parallel course to the Erzgebirge ridge than to the achievement of high speeds.
Geomorphological transformation
The use of the valley valleys by humans has not left its mark in numerous places. To improve usability, humans changed the shape of valleys, sometimes considerably. The most obvious changes are caused by reservoirs that, on the one hand, contribute to artificial sedimentation, and on the other, erode the valley slopes much more homogeneously than flowing water.
But man himself also targeted the slopes of the valley. In the case of mountains with particularly valuable or valuable rock such as sandstone , basalt , granite or marble , it was most likely to reach the rock layers on valley flanks through quarries . At the tunnel exits, people tried to deposit mined rock in the immediate vicinity - i.e. in the valley. He thus contributed artificially to sedimentation in some places. As soon as a mining area had reached a certain size, material was started to be transported up in the mine, as the storage space in the valley is very limited.
The valley slopes were also attacked when building wider traffic routes. Railway lines in particular have a limited curve radius and thus forced the valley floor to be widened. In order to widen the valley, valley slopes were blown up, and in some places original notch valleys were transformed into gorges or notch sole valleys. The resulting rocky outcrops have to be secured against erosion and falling rocks after they have been blasted open. Alternatively, tunnels are also built that run parallel to the valley and are not used to leave the valley.
Ultimately, humans also have an impact on the form-forming processes in the valley through their water control. Artificially secured banks, especially if they are defined by solid walls, prevent even denudation in the valley. Normally, a valley slope breaks away evenly at those points where a curve of water extends directly to the slope. As a result of human safety measures, there is only significant erosion on the slope at such locations during floods. In addition to protecting residential buildings, the main purpose of security is to protect traffic routes in the valley.
Biogeographical transformation
During the agricultural development of the Kerbtal, human activities mainly changed the biotopes of the valley floor in notched valley valleys. The extensive alluvial forests ( see section Geobotany ) were cleared there in favor of pasture and fodder meadows. In many places, people also cleared local and small-scale swamp areas in order to be able to manage them better. Depending on the depth of the water and the associated depth of the groundwater, litter meadows or wet meadows form in the valley . Both biotopes are dependent on management because otherwise they would quickly be overgrown by tall herbaceous vegetation and later taken up again by forests.
On the slopes, forests were displaced in favor of meadow orchards and, above all, on the southern slope in favor of vineyards.
The pasture landscapes interrupted by hedges are now considered to be a cultural landscape worthy of protection , as they are very species-rich, especially when they are supplemented by permanently flooded parts of meadows and natural hedges and embankments.
See also
literature
- Frank Ahnert: Introduction to Geomorphology . UTB, 2003, ISBN 3-8252-8103-5 .
- Matthias Kuhle: Glacial Geomorphology . Knowledge Buchges., Darmstadt 1991, ISBN 3-534-06892-0 .
- Hartmut Leser: Geomorphology . Westermann, Braunschweig 1998, ISBN 3-14-160294-8 .
- Herbert Wilhelmy: Geomorphology in key words II. Exogenous morphodynamics . Borntraeger, Berlin 2002, ISBN 3-443-03113-7 .
