Some facts and figures

Since the latter part of the nineteenth century, the topography of the Canyon has been further mapped and measured by teams of surveyors laboriously traversing the very difficult terrain by mule and on foot, carrying heavy and cumbersome theodolites and plane tables. More recently, aerial photographs gave extra detail on some of the most inaccessible terrain and now satellite imagery can perform in a second or two what it took Powell and his team 95 days to complete.

The gorge of the Grand Canyon lies entirely in northwestern Arizona and extends for nearly 448 km (278 miles) from Lake Powell in the east, created by the completion of the Glen Canyon Dam in 1963, to Lake Mead in the west, created by the completion of the Hoover Dam (originally called the Boulder Dam) in 1936. The gorge was excavated by the powerful erosive force of the Colorado River as it cut down through the rock strata of the Colorado Plateau.

The essential ingredient in the formation of the Canyon was the process of vertical uplift of the Colorado Plateau over a period of nearly six million years to form a vast uplifted tableland over a large portion of the states of Arizona, Colorado, New Mexico and Utah. The uplift rejuvenated and re-empowered the ancient river to dramatic effect. The altitude of the Canyon and its flanking cliffs ranges from 2793 m down to 518 m above sea level. The maximum depth of the Canyon at any one location is about 1829 m (6000 ft) from rim to floor, but is more typically around 1.5 km deep, and its width varies between 0.5 and 30 km.

The water that flows through the Canyon is derived from four rivers, the Green, Colorado, San Juan and Little Colorado, and together they drain a huge area of many hundreds of square kilometres. Continuing erosion by both the permanent and seasonal rivers produces many impressive waterfalls and rapids (over 100 are named) along the Canyon. Although the Canyon is neither the deepest nor longest gorge in the world, it nevertheless has achieved icon status in global terms.

Protection of the Grand Canyon has quite a long history, dating back to 1893 when it was declared a forest reserve, but protection was very limited as mining, lumbering and hunting were permitted. The Canyon's remaining wildlife was protected in 1906 when it upgraded to a game reserve. Redesignation as a national monument followed in 1908 and eventually led on February 26th, 1919 to an Act of Congress that declared the Grand Canyon as a National Park. Finally in the US context, in 1975 the National Park was enlarged to nearly half a million hectares (4930 sqkm or 1900 sq miles) with the incorporation of some adjacent national recreation areas, while 34,000 ha were removed into the Havasupai Indian Reservation. From the international perspective the Canyon was inscribed on the World Heritage List in 1979.

From the scientific investigation of the Canyon's strata we now know that it took the Colorado River a 'mere' 3-5 million years to cut its way down through what geologists call layer-cake strata of the Canyon's rocks. Layer upon layer of horizontal strata are laid out one upon the other. The youngest and most recent layers at the top of the pile form the plateau landscape surface. But these 'recent' surface rocks are now known to be some 270 million years old. Peering into the

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