Scientists had believed it to be about 5 million to 6 million years old, but dating it has proved difficult because erosion on the sides of the canyon has destroyed much evidence of its antiquity.

A team from the University of New Mexico used new dating techniques to estimate the age of "mammillaries," thick white and yellowish-orange calcite deposits in caves along the walls of the canyon, where they are protected from erosion. The mammillaries mark the height of river water when they were formed. The dating technique that the team used charts the rate at which uranium spontaneously decays to lead.

Studying sites along the length of the canyon - which is 277 miles long and 18 miles across at its widest - geologist Victor Polyak and his colleagues concluded that the western Grand Canyon began eroding about 17 million years ago at a rate of a couple of inches every thousand years.

The evidence suggests that the eastern end was produced by a different river that began its erosive process much later from the Colorado River, the only river generally identified with the canyon.

When the two rivers joined about 5 to 6 million years ago to form the current Colorado, the erosion accelerated to a rate of about 8 inches to almost a foot every thousand years.

Not everyone agrees with the new estimate. Geologist Joel Pederson of Utah State University, for example, contends that there are no traces of sediment that would have been created by erosion before 6 million years ago.