Longitudinal Valley

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.

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