GLACIAL VALLEY

also called glacial trough stream valley that has been glaciated, usually to a typical U-shaped cross section. U-shaped valleys occur in many parts of the world and are characteristic features of mountain glaciation. These glacial troughs may be several thousand feet deep and tens of miles long. When allowed to spread out, a glacier erodes the landscape uniformly, but when confined within valley walls it tends to deepen and widen the valley floor. The commonly V-shaped stream valley is converted to a U-shaped valley because the U-shape provides the least frictional resistance to the moving glacier. Because a glacier cannot turn as a stream can, the valley becomes straighter and smoother. The valley walls may be almost vertical and may be striated by passing boulders; the floor may be covered with till or moraines. Because thickness of the ice is the dominant factor in the deepening process, smaller tributary glaciers erode their troughs less rapidly than the main glacier does. When the glaciers melt, the tributary troughs are left as hanging valleys high on the walls of the main glacial valley. Postglacial streams may form waterfalls from the mouths of the hanging valleys; an example is Yosemite Falls, California.