Grand Canyon

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.

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.

III

GEOLOGY OF THE GRAND CANYON

Although the canyon itself may be of comparatively recent origin, the rocks exposed in its walls are not. Most of the rock strata (layers) were originally deposited as marine sediment, indicating that for long periods of time the canyon area was the floor of a shallow sea.

In a typical section of the canyon, toward its eastern end, nine separate rock layers can be seen, piled vertically like a stack of pancakes. The topmost layer is a limestone, the Kaibab limestone. Below this layer is a thick deposit of sandstone, called the Coconino sandstone, and below that a layer of soft, shaly rock known as the Hermit shale. Still lower is a series of shales and sandstones interbedded with each other, collectively termed the Supai formation. The fossils found in the Supai and the rocks above it suggest that these rocks were all deposited in the Permian Period at the end of the Paleozoic Era. However, the Supai may be slightly older still. Next comes a deposit of light gray-blue limestone, the Redwall limestone, which in many places has been colored red by seepage from the Supai beds above. The Redwall is 152 m (500 ft) thick and is easily identified because of the prominent sheer cliffs that it forms in the canyon walls. This layer has been identified as belonging to the Mississippian Period. A thin layer of sandstone, the Temple Butte, beneath the Redwall, gives evidence of having originated in the Devonian Period. The next three rock layers, consisting of the brown Muav limestone, the green Bright Angel shale, and the Tapeats sandstone, all belong to the Cambrian Period, at the dawn of the Paleozoic Era. Beneath these layers, at the bottom of the canyon, are the most ancient rocks of all, Precambrian schists and gneisses, from half a billion to a billion years old.

IV

HISTORY OF THE GRAND CANYON

The first Europeans to see the canyon were members of a group headed by the Spanish explorer Francisco Vásquez de Coronado, which set out from New Spain (now Mexico) in February 1540. The sighting was made later that year. Because of the inaccessibility of the canyon, it was not until more than three centuries later that it was fully explored. Beginning about 1850, a series of expeditions commanded by officers of the United States Army surveyed the canyon and the surrounding area. The first passage of the canyon was accomplished in 1869 by the American geologist John Wesley Powell and ten companions, who made the difficult journey through the length of the gorge in four rowboats.

The construction of the Glen Canyon Dam in Arizona in 1963 dramatically reduced the natural flow of sand and nutrients down the Colorado River and into the Grand Canyon, and created Lake Powell. The color of the Colorado River also changed from reddish to greenish below the dam, and the river’s temperature became much colder from water deep in Lake Powell. In recent decades, the federal government has occasionally released large amounts of water from the dam to create artificial floods similar to the natural spring floods that once occurred along the river. Such controlled floods carry new sediments to help restore sandbars and beaches in the canyon, as well as habitat for fish and plants.