A canyon is a deep valley with steep, rocky sides, usually carved over millions of years by a river cutting through rock and earth. The Grand Canyon in Arizona is perhaps the most famous example: it stretches 277 miles long, reaches depths of over a mile, and reveals layers of rock that tell the story of Earth's history going back nearly two billion years.

Canyons often form through a slow, powerful process. A river flows through the landscape, and over vast stretches of time, the water erodes the rock beneath it, cutting deeper and deeper. The sides stay steep because they erode more slowly than the riverbed. Some canyons, like steep gorges in the mountains, are narrow and dramatic. Others, like the Grand Canyon, are immense, with the river far below appearing as just a thin ribbon.

You'll find canyons throughout the American West, where rivers have carved through desert rock, creating some of the most spectacular natural formations on Earth.