DEFINITION OF GRAND CANYON

The Great Gorge of the Colorado River,

known as the Grand Canyon, is a one-mile (1.5-km) deep,

4–18-mile (6.5–29-km) wide, 217-mile (350-km) long gorge

in the Colorado Plateau of northwestern Arizona. The

canyon was created by more than 8 million years of erosion

as the Colorado Plateau has been uplifted by tectonic forces,

and it exposed more than 1.7 billion years’ worth of history

of the region in its stratified rock layers and deeper metamorphic

and igneous rocks. The canyon was set aside as a

national monument in 1908, and 673,575 acres of the

canyon were designated a national park in 1919. The

198,280-acre primitive Grand Canyon National Monument

presently adjoins the national park on the west.

The canyon was first explored by boat by a party led by

John W. Powell in 1869, after being described by Spanish

explorer Garcia Lopez de Cardenas in 1540. Before that, the

canyon was inhabited by hundreds of Indian pueblos, many

of which are still visible on the lower canyon walls and rim.

From top to base, the canyon exposes a stratigraphic

sequence ranging from the Kaibab Limestone, Toroweap Formation,

Coconino Sandstone, Hermit Shale, Supai Formation,

Redwall Limestone, Muav Limestone, Bright Angel

Shale, Tapeats Sandstone, and Vishnu Schist. Most of the

rocks are sedimentary except for the Precambrian schists and

intrusions of Zoroaster Granite. Two major unconformities

are present, one above the Vishnu Schist and other rocks of

the Precambrian sequence, and one, known as the Great

Unconformity, beneath the Tapeats Sandstone.

The Kaibab Limestone is the highest unit in the canyon

stratigraphy, and it is composed of limestone with minor

shale and sandstone in its upper parts. Fossils include

crinoids, corals, brachiopods, and other shellfish. This formation

is known as the giant bathtub ring around the edge of

the canyon, because of its resemblance to rings found around

many older ceramic tubs. The white color of the formation is

caused by the fossilized white sponges, which have been

replaced by silica, forming chert. The Kaibab Limestone is

therefore strong and erosionally resistant and forms steep

cliffs around the edge of the canyon.

The Torweap Formation is a dark yellow to gray limestone

underlying the Kaibab, and is in turn underlain by the

Coconino Sandstone. This unit consists of light-colored fossil

sand dunes with large-scale cross-bedding. Fossil footprints

and burrows of invertebrates are common. The Hermit Shale

underlies the Coconino Sandstone and consists of soft, rustcolored

shale that erodes easily, forming a slope instead of a

cliff. This formation contains abundant fossilized plants and

footprints of amphibians and reptiles.

The Supai Formation is a red shale formed in a deltaic

environment, preserving terrestrial plant fossils and amphibian

tracks, with more marine conditions toward the west.

This is underlain by the marine Redwall Limestone, a hard

cliff-forming brown limestone that is stained red by hematite

that has washed down from the overlying red shale. Fossils

include marine brachiopods, snails, fish, corals, and trilobites.

The sparsely fossiliferous Muav Limestone underlies

the Redwall Limestone.

The Bright Angel Shale is composed of a colorful variety

of mudstone and shale with fossils of brachiopods, trilobites,

and other marine organisms. It is underlain by the Tapeats

Sandstone, a cliff-forming rippled brown sandstone, with

occasional fossils of trilobites.

Underlying the Great Unconformity at the base of the

Tapeats sandstone is the Precambrian sequence, consisting of

the Vishnu Schist and intrusive Zoroaster Granite on the

southwest side of the canyon. On the northeast side of the

canyon, the Vishnu Schist is overlain by another Precambrian

sequence, including a variety of sandstones, shales, mudstones,

stromatolitic limestone, basaltic lava, and quartzite.

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