Guardian: The Colorado river and its tributaries took a hundred million years or two to carve the Glen Canyon out of the pink and scarlet sandstone which marks out the American southwest. Its myriad gorges, sheer cliffs and towering spires remain ed largely hidden. Prehistoric peoples farmed part of the canyon and Navajo Indian communities built camps close to the river, but few modern Americans ventured there besides explorers until they disappeared under a man-made wonder, the vast Lake Powell, with the...