Formed by rivers making deep cuts within the Earth's terrain.

Bound by cliffs and cut by erosion, canyons are deep, narrow valleys in the Earth's crust that evoke superlatives and a sense of wonder. Layers of rock outline stories of regional geology like the table of contents to a scientific text.

The landforms commonly break parched terrain where rivers are the major force that sculpts the land. They are also found on ocean floors where the currents dig underwater grooves.

"Grand" is the word used to describe one of the most famous canyons of all. Cut by the Colorado River over the last few million years, the Grand Canyon is 277 miles (446 kilometers) long, more than 5,000 feet (1,500 meters) deep, but only 18 miles (29 kilometers) across at its widest yawn.

Layers of rock in the Grand Canyon tell much about the Colorado Plateau's formative years: a mountain range built with two-billion-year-old rock and then eroded away; sediments deposited from an ancient sea; more mountains; more erosion; another sea; a burst of volcanic activity; and the birth of a river that has since carved the chasm by washing the layers away.

Each layer erodes differently. Some crumble into slopes, others sheer cliffs. They stack together like a leaning staircase that leads to the river's edge. A mixture of minerals gives each layer a distinctive hue of yellow, green, or red.

And its immense size has been seen by very few. More people have orbited the Earth than have hked the Grand Canyon from end to end. Follow the trek of Pete McBride and Kevin Fedarko using this interactive map.

Types of Canyons

Other canyons start where a spring sprouts from the base of a cliff. Such cliffs are composed of permeable, or porous, rock. Instead of flowing off the cliff, water seeps down into the rock until it hits an impermeable layer beneath and is forced to leak sideways. Where the water emerges, the cliff wall is weakened and eventually collapses. A box canyon forms as sections of wall collapse further and further back into the land. The heads of these canyons are marked by cliffs on at least three sides.

Slot canyons are narrow corridors sliced into eroding plateaus by periodic bursts of rushing water. Some measure less than a few feet across but drop several hundred feet to the floor.

Submarine canyons are similar to those on land in shape and form, but are cut by currents on the ocean floor. Many are the mere extension of a river canyon as it dumps into the ocean and flows across the continental shelf. Others are gouged from turbid currents that occasionally plunge to the ocean floor.