Most existing valleys have been formed by geological conditions that happened thousands of years ago. Rivers and streams are responsible for forming most of the modern valleys. The flowing waters cut valleys in "V" or "U" shapes, carving steep-walled sides and narrow floors. Other valleys have been made from large glaciers. The massive blocks of snow and ice slowly move downhill, leaving in their wake a hollowed-out valley.
The world's deepest valley is the Kali Gandaki River in Nepal. It lies between two 8,000-meter Himalayan peaks known as Dhaulagiri and Annapurna. The Himalayas are one of Earth's most active tectonic uplifts, and this valley illustrates the rapid down-cutting that occurs in areas with a rapid uplift.
The most well-known valley is the Grand Canyon of the Colorado River in Arizona. The Grand Canyon is 1 mile deep and 180 meters (590 feet) to 30 kilometers (19 miles) wide. It was formed when the Colorado River incised into a tall upward of sedimentary rocks.
As valley river waters wind closer to the sea, they follow the natural curves in the land by stripping sediment from outside bends and depositing it in the inside bends. Down-cutting, rock and dirt dredged from the middle of the channel, leads to deep, slender chasms such as the Black Canyon in Colorado's Gunnison National Park.