The Grand Canyon of the Colorado River is the largest gorge in the world-a 290-mile-long gash across the face of the Colorado Plateau in northern Arizona. Rim to rim, it measures up to 18 miles across, with an average width of 10 miles; its average depth is one mile. Within this Delaware-size area of eroded rock rise mountains higher than any in the eastern United States and that dark walls of gorges millions of years old. Agent of this scene, the Colorado River drops 2,200 feet over nearly 200 rapids as it roars through the Canyon toward the Gulf of California.
Numbers, though, tell only part of the canyon's story and merely hint at the magic of its myriad hues, strata, spires, and gorges. The place is more than the sum of its parts-so much more that neither the eye nor the mind of the beholder can encompass more than a small part of it at one time. As John Wesley Powell, whose party in 1869 became the first to traverse the canyon by river, wrote, "You cannot see the Grand Canyon in one view, as if it were a changeless spectacle from which a curtain might be lifted. "
At the canyon's bottom, a mile below the rim, the Colorado River slices through Granite Gorge, exposing some of the oldest rocks visible anywhere on the earth. Nearly two billion years old, the Vishnu schist is the gleaming black remnant of a once towering mountain range. Some 500 million years after it formed, vast rifting and faulting laid it down the tilted, colorful sediments of the Grand Canyon Series atop schist. Ten distinct layers of sandstone, limestone, and shale bespeak the advance and retreat of ancient seas, the building up and wearing down of mountains, the meandering of rivers over 600 million years.
At either rim, visitors to date perch atop creamy Kaibab limestone cliffs studded with fossilized sponges, corals, snails, and shellfish that inhabited a warm inland sea 240 million years ago. Though rock layers once covered this ancient seabed, all geologic signs of the more recent Mesozoic and Cenozoic eras wore away eons ago.
Compared with the nearly two-billion-year process of deposition, erosion has set a brisk pace. The Grand Canyon itself is less than six million years old, created only since the Colorado River changed course and began flowing through the ancestral Colorado plateau. In just two million years, the river sliced to within 500 feet above its current depth. Wind, rain, snow, heat, and cold have helped the process along. So has the flow of hundreds of tributaries, many of which are dry washes, filled intermittently by snowmelt and summer thunderstorms. Over eons these streams have created " a composite of thousands, of tens of thousands, of gorges," as Powell marveled. "Every one of these...is a world of beauty in itself."
The Grand Canyon is not only a slice of North America's geologic history but also a cross section of ecozones. Between rim and river, travelers find the same variety of ecological regions they would encounter on a trip from Canada to Mexico - from the snowy evergreen woods of the boreal zone to the arid depths of the lower Sonoran, where summer temperatures soar above 100oF and tenacious shrubs like creosote and ocotillo predominate. In all, the Grand Canyon provides rich and diverse habitat for more than 400 vertebrate species and 1,500 plants.
The human presence here stretches back at least 4,000 years, beginning with hunter-gatherers who deposited delicate split-willow animal figures in limestone caves. Before disappearing about A.D. 1150, ancient Pueblo peoples who had lived in and around the canyon for a millenium or more left a rich legacy of pottery, baskets, pictographs, and granaries and other structures in thousands of sites. The Havasupai people in the region today trace their presence back hundreds of years.
During the 20th century, humans have had a far more profound impact on the Grand Canyon than in all of the past. By 1900, the canyon was already a well-known destination, thanks to written accounts by explorers and scientists and to the glowing canvases of Thomas Moran. And after Grand Canyon National Park was established in 1919, the number of visitors continued to increase exponentially: About five million people now visit the canyon each year by car, on foot, atop mules, on motorized rafts, and in helicopters. Some of the human impact, such as the view-marring haze drifting in from power plants and urban centers, has been indirect. In 1963, the Glen Canyon Dam ended the Colorado River's free flow at the Grand Canyon's entrance, and that single action has changed the canyon more than any other event in human times.
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