Valleys

negative, linearly extended landforms having a uniform, although frequently uneven drop along the thalweg; they are formed by the erosive action of running water. Ravines and gulleys formed by temporary (periodic) streams are the embryonic forms of river valleys. Valleys usually form entire systems; one valley opens into another and it, in turn, into a third, and so on until their merging streams run through a common channel into a basin. In valleys the two chief components that are distinguished are its sides (or slopes) and its floor. The floor is often undeveloped in young valleys, and the sides come up to the river, thereby forming its banks. Valley sides consisting of original rocks may be high or low, steep or gently sloping, and in profile they may be convex, straight, concave, or step-like (terraced). In their steepness and profile the two valley sides may be the same or different (asymmetrical valleys). In asymmetrical valleys of the northern hemisphere the right side is generally steeper, and in the southern hemisphere the left side is. In more developed valleys, a flat floor covered with river deposits (alluvium) results from the horizontal meandering of the river. In this case the side of the valley is no longer the bank of the river. Depending on such factors as the stage of development, the geological structure of the terrain, and the combination of exogenous factors, the transverse profile of the valley may have the shape of a canyon, gorge, trough, or trapezoid, or it may be V-shaped or U-shaped.

Valleys are called tectonic if their initial stage was determined by the presence of negative geological structures in the relief (grabens, synclinal troughs). Examples are the Alai Valley in Middle Asia and the California Valley in North America. These valleys should not be confused with erosion valleys in areas of dislocated (folded or inclined) bedding of sedimentary layers, where erosion has acted on a previously existing graben, synclinal fold, and so on. According to their relationship to the strike of structures and mountain ranges, a distinction is made between longitudinal valleys, which coincide with the direction of the strike of the beds, and transverse valleys, which are perpendicular to the axes of folds; diagonal valleys are encountered less often. In turn, longitudinal valleys may be synclinal, monoclinal, or anticlinal, depending on whether they were formed along the axis of a syncline, on the limb of a fold, or along an anticlinal vault. Monoclinal valleys are frequently asymmetrical in their transverse profile; the steep side lies in the direction of the dip of the beds. Longitudinal valleys are broader and show little morphological change over considerable distances. Transverse valleys, on the contrary, frequently change their character over short distances, alternating between narrow gorges and basin-like expanded areas (“lenticular valleys”).

Valleys that cut through a mountain chain across the strike are called water gaps. They may be formed by the development of valleys under different, previously existing geomorphological conditions on a surface which was located above the present-day mountain ranges that they now intersect and in rocks of different composition, with a different tectonic structure (epigenetic valleys). Water-gap antecedent valleys are formed as a result of the cutting through of a young mountain range which has begun to develop across the valley of an existing river; they are also formed in the process of the beheading of the upper reaches of a different river flowing along the opposite slope of the range.

Mountainous and level valleys differ greatly in morphology. The former are characterized by a considerable depth, a relatively small width, and an uneven drop in the longitudinal profile. The latter are generally broad, with walls that are not very deep or steep and with small gradients.

In its upper reaches a valley usually ends in a watershed cone or glacial cirque. Open valley heads not cut off by walls are encountered less often. The valley mouth is often accompanied by a delta or debris cone or forms an inlet basin such as a bay or estuary, into which the river falls.

The valleys of large rivers present a unique landscape that differs sharply from the spaces between rivers. The floodplains of level rivers provide good meadows for growing hay, and in areas protected against erosion they may be plowed; the terraces above them may also be used for planting many kinds of crops. These plains often serve as locations for population centers, especially in mountainous regions. The valley sides are frequently covered with trees.

The geomorphological analysis of valleys helps to explain the recent geological history of the development of a region, to disclose placer deposits of minerals, and to draw correct conclusions when designing the hydroengineering structures of hydroelectric power plants and irrigation canals.