Valley

A valley is a long, narrow landform that descends between two elevated sides such as mountains, hills or ridges. Most valleys contain a watercourse—a river, stream or brook—that flows along the bottom and has carved the land over long periods.

Valleys are found on all continents and also occur on the ocean floor. Their dimensions vary widely: some are narrow ravines only a few meters across, while others are broad lowlands extending for hundreds of kilometres. Many people have historically settled in valleys because of the availability of water, fertile soil and easy travel routes.

Formation

The most common process is water erosion: a river carries rock and sediment, and over time carves an increasingly deep path through the land. As the slopes on either side wear down and rains wash material away, the valley widens. Glaciers also form valleys, creating U-shaped forms with flat floors, whereas river valleys typically have V-shaped profiles. Tectonic movement of the earth's crust produces long, straight valleys known as rift valleys.

Types

A V-valley is narrow and found where a river is young, with steeply cut sides. A U-valley was shaped by glaciers and has a broad floor. A deep, narrow valley with steep rock walls on either side is called a canyon. Rift valleys, such as those in East Africa, form where the earth's crust pulls apart and the land between sinks.

People and use

Valleys accumulate sediment brought by river water, making their soils suitable for farming. Many early civilizations began in river valleys. Valleys also provide easy passage through mountains and are used to build roads and railways. However, valleys pose a flooding hazard, especially when heavy rains concentrate in a single channel.