Canyon, a deep valley with steep sides. A narrow canyon with almost perpendicular sides is known as a gorge. Canyons are also called chasmsor gulches. They were formed over a period of thousands of years by river erosions. Canyons are found not only on land, but also on ocean bottoms near continental shores and near the mouths of some large rivers. One of the most awe-inspiring canyons is the Grand Canyon of the Colorado River.