I.

Introduction

Grand Canyon, deep, steep-walled canyon in northwestern Arizona, excavated by the Colorado River. The Grand Canyon is 446 km (277 mi) long, up to 29 km (18 mi) wide, and more than 1,500 m (5,000 ft) deep. It is the deepest and longest canyon in North America. The canyon contains towering buttes, mesas, and valleys within its main gorge. A spectacular section of the canyon, together with plateau areas on the north and south rims of the gorge, is preserved as the Grand Canyon National Park, which was established in 1919 and receives millions of visitors each year. UNESCO made the Grand Canyon a World Heritage Site in 1979.

The Grand Canyon cuts steeply through an arid plateau region that lies between about 1,500 and 2,700 m (about 5,000 and 9,000 ft) above sea level. This region, although lacking year-round streams in recent years, is sharply eroded, showing such characteristic forms as buttes; it is interspersed with old lava flows, hills composed of volcanic debris, and intrusions of igneous rock. The plateau area has a general downward slope to the southwest and in its upper reaches is sparsely covered with such evergreens as juniper and piñon. Parts of the northern rim of the canyon are forested. The south rim has a more desertlike climate. Vegetation in the depths of the valley consists principally of such desert plants as agave and Spanish bayonet. Wildlife around the canyon includes unique species of squirrels, as well as pumas, deer, bighorn sheep, and other animals found in the Southwest region. In general the entire canyon area has little soil. The climate of the plateau region above the canyon is severe, with extremes of both heat and cold. The canyon floor also becomes extremely hot in summer, but seldom experiences frost in the wintertime.