Photo: Khumbu Valley

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.

Glacier-Made Valleys

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.

Share

The Innovators Project

  • Abdel Kader Haidara - An Islamic scholar sorts through trunks of ancient manuscripts.

    Brave Sage of Timbuktu

    Abdel Kader Haidara had made it his life's work to document Mali's illustrious past. When the jihadists came, he led the rescue operation to save 350,000 manuscripts.

See more innovators »

Genographic

Phenomena

See more posts »

Latest Video

See more videos »

Shop Our Space Collection

  • 300x225_030414_spaceBooks3.jpg

    Be the First to Own Cosmos: A Spacetime Odyssey

    The updated companion book to Carl Sagan's Cosmos, featuring a new forward by Neil deGrasse Tyson is now available. Proceeds support our mission programs, which protect species, habitats, and cultures.

Shop now »