Valley

A valley is an elongated low area, often running between hills or mountains, that typically carries a river or stream from one end to the other. Most valleys are cut by the long term erosive action of rivers and streams, though some form through erosion by glacial ice, and a valley created during an ice age may lose its glacier and later be occupied by an ordinary river once the climate warms. In dry regions a valley may hold water only intermittently or not at all, and where the underlying rock is limestone a valley can run dry because drainage has moved underground; rift valleys, by contrast, form mainly through tectonic activity rather than erosion. River valleys have long attracted human settlement, and early civilizations grew up in valleys such as those of the Nile, the Tigris and Euphrates, the Indus, the Ganges, the Yangtze, the Yellow River, the Mississippi and the Amazon.