A valley is a low-lying area of land that is usually found at the foot of mountains or hills. The most common way valleys are formed is by the erosion of land from running water. River valleys are formed by the action of the river. As a river flows downhill, it cuts through the land like a knife. Over thousands of years the river erodes the land to form a valley, usually in the shape of a V. In contrast, valleys formed by glaciers are often U-shaped, because they are formed by rocks carried in the glacier that erode the soil.