mountains and canyons

The Devil's Tower erupts like a monstrous tree stump above the Belle Fourche river in the plains country northwest of Sundance. Wyoming, in the United States. This flat-topped freak mountain is 265m (869ft) high, and its summit is 85m (279ft) across. Geologists estimate that its history began about 50 million years ago. when a mass of molten rock thrust up through sedimentary rocks laid down on the bed of an ancient inland sea millions of years before.

The molten rock cracked as it cooled, forming a mass of innumerable columns. Over succeeding millennia, the softer sedimentary rocks were eroded away by the weather and the Belle Fourche river. leaving the spectacular mountain of today.

Scientists have yet to unlock the secrets of the Grand Canyon through which the Colorado river runs in the western US state of Arizona. The canyon is more than 350km (217 miles) long. some 6 to 20km (4 to 13 miles) wide, and up to 1615m (5300ft) deep. More than 2 million million cubic metres (2.6 million million cubic yards) of rock were swept away in its making, but nobody has yet worked out where this vast quantity of debris went.

Until recently most scientists believed that the canyon formed during the world's last major period of mountain building. They thought that the river established its course. and then maintained it by eroding its bed at the same rate as the land rose during subsequent earth movements. However, these movements ended more than 50 million years ago, and no river deposits dating back more than 50 million years have been found downstream from the canyon.

In fact. there is evidence that. while the river above the canyon was established in its present course 30 to 40 million years ago, the river course below the canyon was formed less than 9 million years ago. Before then, the river must have followed another route to the sea: a route that has still to be found. Curiously, the river now only just reaches the sea. So much water is taken out of it by a series of canals and ten major dams for irrigation and drinking water that by the time it reaches the Gulf of California it is a mere trickle, no deeper than a child's paddling pool and only about 3m (10ft) wide. At times it dries out completely before reaching the sea.

Thousands of conifer trees were turned to stone to form a petrified forest in the US state of Arizona. Some 175 million years ago, the desert plateau on which the forest now stands was part of a swampy flood plain. Huge trees. up to about 60m (195ft) tall. flourished in the warm. moist climate. When they died they fell. and were covered by thick layers of mud and sand. which eventually hardened into shales and sandstones.

Ground water containing silica - the mineraI of which sand is made-seeped into the trunks. gradually replacing each cell with an exact copy in silica. As a result each solid stone trunk still has the precise shape and grain of the wood which patterned it. Millions of years later. earth movements lifted the rocks with the fossilised trunks. Then erosion did its work, gradually stripping away the layers, leaving the more resistant trunks high and dry. And, with the passage of time, the dead forest will 'grow', as erosion eats into yet lower rock layers.

Dome-shaped Stone Mountain, 25km (16 miles) northeast of Atlanta, in the southern US state of Georgia, is the largest mass of exposed granite in the world. It is some 2.5km (1.5 miles) across, and rises more than 250m (800ft) above the surrounding plain. It is also the South's answer to Mt Rushmore National Memorial in South Dakota. which carries the faces of four presidents-George Washington, Thomas Jefferson, Abraham Lincoln and Theodore Roosevelt. Stone Mountain features the mounted figures, each about 27m (90ft) high, of three Confederate heroes of the Civil War: Jefferson Davis (president of the Confederacy), General Robert E. Lee (commander in chief), and General Thomas 'Stonewall' Jackson.

Ayers Rock, close to the geographical centre of Australia, is a sacred place to the country's Aborigines. The native Pitjandjara people believe that each of its features represents an important person or event in their history, and some of the rock's caves contain Aborigine paintings and engravings representing human figures and epic journeys made by distant ancestors. One pothole, they believe, is the spot where a spear fell during the battle between rival clans of Aborigines. And a cave entrance is the mouth of a woman weeping for her lost son.

The rock, 348m (1143ft) tall and 9km (5.5 miles) round, sits alone on a desert plain, a remnant of a vast sandstone formation which once covered the entire region. It changes colour dramatically at sunset, finally glowing deep red - a spectacle that draws more than 50,000 visitors a year. Ayers Rock is not the world's largest monolith. It is surpassed by Mt Augustus in Western Australia, which is about twice its size.

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