Photograph by Larry Fellows, Arizona Geological Survey
Down in the valley, the land is depressed—scoured and washed out by the conspiring forces of gravity, water, and ice. The scars left behind are known by their shapes and where they lie. Some hang; others are hollow. They all take the form of a "U" or "V."
Rivers and streams make most primary valley cuts, carving steep-walled sides and a narrow floor that from afar looks like the letter "V." The gradient of the river—how quickly it drops—helps define the steepness of the sides and the width of the floor. Mountain valleys, for example, tend to have near-vertical walls and a narrow channel, but out on the plains the slopes are shallow and the channel wide.
As the waters wind toward the sea, they accentuate natural curves in the land by stripping sediment from the outsides of bends and dumping it on the insides. The bulk of the rock and dirt is dredged from the bottom of the channel, a process called down cutting that can ultimately lead to deep, slender chasms like Black Canyon in Colorado's Gunnison National Park.
Some river and stream valleys, especially those in the mountains or located near the North and South Poles, are transformed by glaciers.
The massive blocks of snow and ice slowly creep downhill along the path of least resistance—valleys already cut by rivers and streams. As the glaciers ooze, they pick up rocks and grind away at the valley floor and sides, pressing the "V" into a "U." When the glacier melts, a U-shaped valley marks the spot where the snow and ice once flowed.
Side valleys are formed by tributaries to streams and rivers and feed the main stem. Where the main channel is carved deeper than the tributary, as commonly occurs during glaciations, the side valleys are left hanging. Waterfalls often cascade from the outlet of the upper valley into the drainage below.
Hollows, like those in Appalachia, are small valleys nestled between mountains or hills. Giant valleys, called rifts, are found where two pieces of Earth's crust are separated or split apart. One such example is the Great Rift Valley, a rift system stretching from the Middle East to southern Africa.
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