immense canyon cut by the Colorado River into the high-plateau region of northwestern Arizona, U.S., noted for its fantastic shapes and coloration.

The Grand Canyon lies in the southwestern portion of the Colorado Plateau, which is a large area of the southwestern United States consisting essentially of horizontal, layered rocks and lava flows. The broad, intricately sculptured chasm of the canyon contains between its outer walls a multitude of imposing peaks, buttes, gorges, and ravines. It ranges in width from about 0.1 to 18 miles (0.2 to 29 km) and extends in a winding course from the mouth of the Paria River, near Lees Ferry and the northern boundary of Arizona, to Grand Wash Cliffs, near the Nevada line, a distance of about 277 miles (446 km). Its greatest depths lie more than a mile (some 6,000 feet 【1,800 metres】) below its rim. The canyon includes many tributary side canyons and surrounding plateaus. The deepest and most impressively beautiful section, 56 miles (90 km) long, is within the central part of Grand Canyon National Park, which encompasses the river's length from Lake Powell (formed by Glen Canyon Dam in 1963) to Lake Mead (Mead, Lake) (formed by Hoover Dam in 1936). In its general colour, the canyon is red, but each stratum or group of strata has a distinctive hue—buff and gray, delicate green and pink, and, in its depths, brown, slate-gray, and violet. At approximately 8,200 feet (2,500 metres) above sea level, the North Rim is some 1,200 feet (3,658 metres) higher than the South Rim.

Although its awesome grandeur and beauty are the major attractions of the Grand Canyon, perhaps its most vital and valuable aspect lies in the time scale of Earth history that is revealed in the exposed rocks of the canyon walls. No other place on Earth compares with the Grand Canyon for its extensive and profound record of geologic events. The canyon's record, however, is far from continuous and complete. There are immense time gaps; many millions of years are unaccounted for owing to gaps in the strata that resulted either from vast quantities of materials being removed by erosion or because there was little or no deposition of materials. Thus, rock formations of considerably different ages are separated only by a thin, distinct surface that reveals the vast unconformity in time.

Briefly summarized, the geologic history of the canyon strata is as follows. The crystallized, twisted, and contorted unstratified rocks of the inner gorge at the bottom of the canyon are Archean granite and schist more than 2.5 billion years old. Overlying these very ancient rocks is a layer of Proterozoic limestones, sandstones, and shales that are more than 540 million years old. On top of these are Paleozoic rock strata composed of more limestones, freshwater shales, and cemented sandstones that form much of the canyon's walls and represent a depositional period stretching over 300 million years. Overlying these rocks in the ordinary geologic record should be a thick sequence of Mesozoic rocks (248 to 65 million years old). Rocks dating from the Mesozoic Era in the Grand Canyon, however, have been entirely eroded away. These rocks are found, nonetheless, in nearby southern Utah, where they form precipitous butte remnants and vermilion, white, and pink cliff terraces. Of relatively recent origin are overlying sheets of black lava and volcanic cones that occur a few miles southeast of the canyon and in the western Grand Canyon proper, some estimated to have been active within the past 1,000 years. (See also Grand Canyon Series.)

Plant and animal fossils are not abundant in the Grand Canyon's sedimentary rocks because of the age of the rocks and are confined mostly to primitive algae and mollusks, corals, trilobites, and other invertebrates.

Jeremy Schmidt, The Grand Canyon National Park: A Natural History Guide (1993); Ann Zwinger, Downcanyon: A Naturalist Explores the Colorado River Through the Grand Canyon (1995); and Stephen R. Whitney, A Field Guide to the Grand Canyon, 2nd ed. (1996), cover the natural history of the park and present its flora and fauna. Barbara J. Morehouse, A Place Called Grand Canyon (1996), discusses the physical and cultural history of the area from early Native American presence to modern times.John Wesley Powell's 1869 journey through the Grand Canyon is the subject of Edward Dolnick, Down the Great Unknown (2001). Powell's second journey in 1871 and 1872, recorded by the assistant topographer of the expedition, is detailed in Frederick Dellenbaugh, A Canyon Voyage (1908; reissued 1991). An interesting companion to these is Eilean Adams, Hell or High Water: James White's Disputed Passage Through Grand Canyon, 1867 (2001), written by White's granddaughter.Donald L. Baars, Rex C. Buchanan, and John R. Charlton, The Canyon Revisited: A Rephotography of the Grand Canyon, 1923/1991 (1994), combines photos taken by the Birdseye expedition of 1923 with photographs taken in 1991. Other notable photographic records are C.C. Lockwood, Beneath the Rim (1996); and Craig Leland Childs, Grand Canyon: Time Below the Rim (1999), with photographs by Gary Ladd.