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A longitudinal valley is an elongated valley found between two almost parallel mountain chains in geologically young fold mountains such as the Alps, Carpathians, Andes or the highlands of Central Asia. They are often occupied and shaped by a subsequent stream. The term is frequently used if a mountain range also has prominent transverse valleys, where rivers cut through the mountain chains in so-called water gaps.
Many longitudinal valleys follow the strike of the rock strata or significant geological fault lines. These are formed in conjunction with the tectonic movements during the mountain building, which in turn are due to plate tectonic processes. The faults be structures that reach deep into the lower part of the earth's crust, that are already in place before the actual mountain building phase and are later reactivated as, for example, is the case in the Periadriatic Seam in the Alps. For the formation of longitudinal valleys, however, nappe overthrusts also play a major, if not the most important, role. The nappes that are present in many young fold mountain ranges is responsible to a large extent for the morphological division of a mountain belt into more or less parallel chains. In such cases, longitudinal valleys generally run along the so-called leading edge of the napped (the overthrust front), that are oriented at right angles to the direction of movement of the tectonic nappes, which in turn corresponds to the direction of movement of the colliding continental blocks. From this configuration results a course that runs with the strike of the geological units, which is an important criterion for the definition of a longitudinal valley. By contrast a transverse valley cuts across the strike.
Particularly long valley systems that are occupied by several rivers, sometimes running in opposite directions, are known in German as Längstalfurchen ("longitudinal troughs") as opposed to the usual Längstäler, although no such distinction is made in English. The Eastern Alps and other Alpine ranges have many such troughs that are almost straight for a distance of several hundred miles and which were accentuated by glacial processes during the Pleistocene. Several examples are:
Even the line of the Upper Rhone and Anterior Rhine valleys – albeit separated by the Furka and Oberalp Passes - can be seen as a longitudinal trough.