Elongate depression of the Earth's surface. Valleys are commonly drained by rivers and may be in a relatively flat plain or between ranges of hills or mountains. Valleys formed by rivers and slope denudation are typically V-shaped; those formerly occupied by glaciers are characteristically U-shaped. Valley evolution is controlled mainly by climate and rock type. Very narrow, deep valleys cut in resistant rock and having steep, almost vertical sides are called canyons. Smaller valleys of similar appearance are called gorges.