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Coming upon the Grand Canyon long ago, an old prospector is supposed to have said in amazement, “Something awful happened here.”

The something appears to have started happening 17 million years ago, geologists concluded in a study in the journal Science on Friday. If correct, this is at least 11 million years earlier than previous estimates.

By dating mineral deposits inside caves up and down the canyon walls, the geologists said they determined the water levels over time as erosion carved out the mile-deep canyon as it is known today. They concluded that the canyon started from the west, then another formed from the east, and the two broke through and met as a single majestic rent in the earth some six million years ago.

Previous theories had posited six million years as the earliest age for the beginning of the entire Grand Canyon of the Colorado River.

The new research was conducted by geologists at the University of New Mexico, led by Victor Polyak. The researchers, supported by the National Science Foundation, used an improved uranium-lead dating technique that yields ages of minerals back tens to hundreds of million years.

Tests at nine sites established the ages of thick white and yellowish-orange calcite deposits lining walls of cave interiors as a measure of lowering water levels as the canyon deepened. The results indicated that the erosion rate was much slower in the western canyon than in the eastern section, the geologists reported.

“We were surprised by our results,” Dr. Polyak said in a telephone interview. “There have been a lot of theories over the last 100 years, and we didn’t expect the canyon history to go back so far.”

In an accompanying commentary, two British geologists who were not involved in the research wrote that some scientists had suspected the greater time scale for the canyon’s formation, but that the new study “demonstrates it firmly for the first time.”

The geologists, Tim C. Atkinson of University College, London, and Michael Leeder of the University of East Anglia in Norwich, England, said the “new age for such a major valley confirms that on geological time scales the relief of the continents is continually remade by the interplay” of erosion and crustal uplift.

Carol Hill, a member of the research team from the University of New Mexico, said small streams had eroded the western and eastern canyons. The single Colorado River, which John Wesley Powell navigated in his heroic 1869 expedition through the canyon depths, emerged only after the breakthrough between the separate chasms. The merger apparently occurred where the river today, coming from the north, bends to the west, in the area known as the Kaibab Arch.