Valley, an elongated depression between bluffs, hills, or mountains. Small valleys surrounded by hills are often called hollows, glens, dales, or dells. Small stream valleys are gullies or ravines. Valleys vary from deep, steep-sided canyons to wide, fertile plains between low, gentle slopes. Most valleys are formed by rivers, but some are scooped out by glaciers. Others are made by movements of the earth's crust.
River valleys are carved out by the streams that flow through them. In the early stages of a river valley, the pronounced deepening action of the stream cuts gorges, gulches, and canyons. These early forms, known as young valleys, are sharply V-shaped with steep, rugged slopes. The valley floor is little wider than the stream itself. Examples are the Grand Canyon of the Colorado River in Arizona and the gorge of the Niagara River.
Under normal circumstances, a valley passes eventually into a mature stage. The slopes of the valley are worn down by tributary streams and by the widening action of the river. The deepening action slows down as the rate of descent of the river is reduced, and the valley widening is pronounced. The river winds in great loops, called meanders, through a wide floodplain built of materials deposited by the river during floods. Mature valleys are found in the region of the Allegheny Plateau (western Pennsylvania and West Virginia) and the Cumberland Plateau (Kentucky and Tennessee).
A third stage, called old age, is reached when the valley floor is many times wider than the river and its meanders. Tributaries feed the river more heavy material than it can carry, and much of this material settles on the river bed, making the river bed shallower. The valley becomes a wide, level plain. The Mississippi Valley is an example.
River valleys reach maturity and old age faster in regions where the stream cuts through relatively soft rock, and where normal rainfall helps wear down the slopes. Even under these circumstances, the process may take millions of years.
Glacial valleys, or troughs, are formed as glaciers move down young, V-shaped stream valleys, cutting them out in a rounder shape. Structural valleys are made by the folding or faulting (breaking) of the earth's crust. When the crust folds, valleys are formed between parallel mountain ranges. Another type of structural valley, the rift valley, is formed by the dropping of a block of the earth's surface between parallel faults. The Rhine Valley between the Vosges Mountains and the Black Forest is an example of a rift valley. Hanging valleys are the valleys of tributary streams that join the trunk (main) valley at a higher level, forming waterfalls.
illustration titled Rivers and Related Features.
