THE LAND

Forming of the Grand Canyon
Geologists have a difficult time pinpointing the age of the Grand Canyon itself, though it is far younger than even the most recent rock layers—those on the rim, which are about 250 million years old. These lay near sea level 70 million years ago, when the earth's crust began a slow uplift. Some time after that the ancestral Colorado River settled on its present course and began to carve the Canyon. The big question is when! Too much of the geologic record is missing to provide a conclusive answer yet. Studies of river deposits have led to conflicting theories of the birth of the Grand Canyon: any time between 5 and 70 million years ago. Even the direction of the ancestral Colorado is in dispute—some geologists think that the Little Colorado and upper Colorado once drained northward; a look at a map of Marble Canyon supports this view because the tributaries suggest a tilt of the land to the north. Or perhaps the Colorado River followed its present course but in the opposite direction, later to be reversed by tilting of the Colorado Plateau. Other theories claim that the Colorado and Little Colorado flowed southward either into a giant lake or into the Rio Grande to the Gulf of Mexico. A second, younger river could have carved the lower Grand Canyon and eventually captured the upper river near the present junction of the Colorado and Little Colorado Rivers. Or perhaps the lake had filled to overflowing from other sources, then broken through and carved the Little Colorado and Colorado Rivers in a catastrophic flood about 5 million years ago. Still other geologists stick with John Wesley Powell's theory that the Colorado has always followed its present course after the uplift began about 70 million years ago. The problem with this idea is that no river deposits older than 5 million years have been found near the lower end of the Grand Canyon.

No matter how the Grand Canyon did form, gradual uplift continued, giving the waters a steeper gradient and thus greater power. Today, the South Rim reaches elevations of 7,000-7,500 feet, while the North Rim towers about 1,500 feet higher. The Colorado River drops through the Canyon at an average gradient of 7.8 feet per mile, 25 times that of the lower Mississippi.

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