U-shaped valley
A glaciated valley or U- valley is a great valley shape strong in relief, reshaped by glacial mountains. It is characterized by a typical in the lower part a U- shaped transverse profile.
Form of treasure
The lower U-shaped profiled valley was carved out by glaciers a Trogtals flows and of them carried fragments of rock and sometimes completely filled. Depending on the distance to the ridge, this was done so far in the current ice age, more or less often. In high mountain regions such as the Alps this valley, with its sometimes extremely steep slopes and wide valley floor sunk into a usually significantly less steep valley V- shaped cross section. This occurs at the side of the valley at the top of the U- shaped part, a characteristic kink profile. It is called the trough edge or trough edge and above the often existing flattening trough shoulder. Since the surface, unlike the slope schrofigen games about can be smoothed, the trough shoulder is also called sanding board. And the upper edge may be very pronounced, and is then referred to as a cut groove.
For the longitudinal profile is typical that the slope of the valley floor is uneven and the Talzug can be divided by bars even, between which are filled by lakes wells may extend or plain, formed of unconsolidated sediments valley floors. In the high mountains shaped glacial valleys often begin with trough-shaped high valleys that may be framed by Karwänden facing uphill. The steep-sided glaciated valley often only starts below it with a clear gradient step; this can be very striking, if there several valleys meet ( Konfluenzstufe ). Side valleys of hollows can be either high-lying former Firnmulden, or also show trough shape and lead mostly as hanging valleys.
Formation
Most U-shaped valleys are only about formations previously applied fluvial valleys out its surface formation results from the interaction of the different driving forces of erosion in the course of a river of ice and the respective resistance of the rock subsoil. As a glacier transported the rock material not as a mountain stream mainly transported to the sole and deposited again and again, but over the entire body of ice distributed mostly to the end carries with it an ice stream also tends not to form a balanced longitudinal profile. Levels in longitudinal profile, for example triggered by crossing more resistant rocks, which enhance the vertical erosion at the base levels, which leads to these places often overdeepened basin.
The erosive power of a glacier is particularly strong where the pressure conditions in the increasingly consolidated firn finally changing thawing and refreezing ( refreezing ) lead to the Eisbasis and thus particularly effective erosion processes such as Detraktion. However, the effluent at the glacier base pressurized meltwater developed a strong erosive power. From this area often is suddenly an increased vertical erosion of the glacial stream, which testifies after thawing of the glacier, a sometimes abrupt valley of Trogtals. The fact that an all-round side erosion is effective, show the sometimes slightly overhanging trough walls.
A second focus of the erosive events is the edge of the glacier. At the glacier beginning very steep back walls of the cirques may be free of ice and then steepened at the upper limit of the Firnes through increased frost weathering on the so-called black and white border on. At the trough shoulders the smoothing goes partly to the sanding earlier, higher glacier extents back - the cut throat is then formed there by the increased freeze-thaw cycles on the border of glacier and rock - and partly to erosion forms of meltwater channels that can accompany a river of ice - the cut throat is then more likely a result of lateral erosion of such Flankengerinne. For medium mountain adhere relief forms a trough shoulder occurs less in evidence, especially since they often merges into surrounding undulating, over-molded part by former Plateauvergletscherung plateaus.
After the melting of the valley glacier keep only the U-shaped valleys in particularly steady rocks in their form; usually put in their abutment deprived of glacial ice slopes and valley walls landslides of varying size and speed. They range from gradual subsidence and landslides, which are increasingly constrict the broad valley floor to mountain falls, which can completely spill it. The lateral moraines are deposited on the valley slopes and unstable form scree slopes that blur the original trough shape of the valley on. The levels of the valley are initially overcome by the streams in waterfalls, but then often clammy -like cut (example: Aare ). Underlying basin can become dry in this way, not only by the radiation emanating from the mouths of tributaries fill with gravel.
Similar shapes
Occasionally, high-altitude valley terraces as the Inn valley terraces or Kames be confused with trough shoulders. Nor are the upper edges of fluvial erosion valleys that have been cut later in the valley bottoms stepped trough valleys, trough shoulders dar. If such V-shaped valleys later partially re- filled with sediments, they are also to speak not as a U-shaped valleys, but rather as a box valleys.
Importance on cultural landscapes
Typically pronounced U-shaped valleys, despite their width and track have poverty gap of only limited settlement and transport favor. The valley floor is filled with river gravel and often marshy. The meandering rivers occur widely on the shore. The marshes are now mostly drained and straightened rivers. Above all, the lowest valleys are frost by cold air lakes. Not too steep to higher altitudes with suitability for human settlement and agriculture represent mostly the alluvial fan and debris einmündender streams dar. In some low-lying hollows as the Lauterbrunnen Valley, the trough shoulders have proven to be the more suitable settlement areas.
A greater importance for traffic received deep alpine trough valleys associated with the railway and highway construction. Through them spaced low-lying basis points arise quite close for tunnels through the main ridges of the mountains. Previously set the levels of the valley and the cut of narrow gorges bolt in the course of many U-shaped valleys more serious traffic barriers is lowered by as many Gletschertransfluenzen mountain passes. Well-known examples are the Via Mala or Schoellenen.
However, large importance for the Alpine settlements have less steep and typical U-shaped valleys, which are largely filled by Talschottern. Together with the inner alpine valleys longitudinal they represent a network besiedelbarer valley areas with favorable climatic conditions
Distribution of trough valleys
U-shaped valleys are most typical of the mountains with extant or past glaciation. The degree of retention of their typical form of treasure, however, is dependent on the age of the last glacial overprint and since then the intensity of erosion ( fluvial dissection, frost debris formation).
Known examples of Trögtäler are, next to the fjord coastline of North and South America, Norway and New Zealand, the Yosemite Valley in California, the Ordesa valley in the Pyrenees or the Lauterbrunnen valley in the Alps.
Examples of other U-shaped valleys in the Alps: