II.

Formation of the Grand Canyon

The Grand Canyon has been sculpted mainly by water erosion. Working from the sides, streams and small rivers flowed into the powerful Colorado River, which created the main channel and now flows through the canyon’s lowest portions more than 1,500 m (more than 5,000 ft) below the canyon’s rim. Scientific research continues into the origin and early history of the canyon. A widely accepted theory holds that the river began eroding the canyon about 6 million years ago, gradually cutting through the numerous rock layers of the Colorado Plateau. Other researchers have proposed alternative scenarios. Some geologists have interpreted evidence from calcite deposits in caves in the region as indicating the canyon formation process began much earlier, perhaps as far back as 17 million years ago.

The geologic uplifting that formed the Colorado Plateau and the Rocky Mountains also tilted the land. This tilting amplified gravitational forces, adding to the river’s erosion power as it rushed along its course from high in the mountains of present-day Wyoming to the Gulf of California (an arm of the Pacific Ocean).

Other factors have also played a part in the formation of the canyon. The Kaibab Plateau, which forms the northern canyon rim, is about 365 m (about 1,200 ft) higher than the Coconino Plateau, which forms the southern rim. Water from the northern side has flowed into the canyon, forming tributary valleys, while the streams of the southern plateau flow away in a southerly direction without carving valleys in the canyon walls. The underlying rock beds also have a southwestern slant, with the result that groundwater from the north finds its way into the canyon, but water from the south does not. In the entire canyon region, the rocks have been broken by jointing and faulting, and fractures in the rocks resulting from these processes have contributed to the relatively rapid erosion of the gorge.