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A study in March dated the initial carving of the Grand Canyon at 16 to 17 million years ago, but a more recent study suggests the carving from water and wind may have begun when dinosaurs still roamed the American Southwest.

“The Colorado River, with some help from the wind, ultimately carries the detritus away pebble by pebble, sand grain by sand grain,” said one of the authors of the new study, Brian Wernicke of Caltech. “From this point of analysis, the unanswered questions about how and why the canyon formed start to pile up.”

The new thinking, detailed in the May issue of the Geological Society of America Bulletin, indicates that the canyon, or at least a prototype of it, actually formed about 55 million years ago ?w and possibly sooner ?w in younger rock layers that lay above the ones the canyon is cut into today. Erosion wore down these layers while water continued to carve out the canyon, essentially moving the whole landscape down through the rock sequence until it got to its present-day position.

Wernicke and his team used grains of the mineral apatite in the canyon’s ancient sandstone walls to find clues as to when the different rock layers were uncovered by erosion.

Apatite contains the radioactive elements uranium and thorium. As these elements decay, they spit out helium atoms. By comparing the relative abundances of the elements scientists can date the apatite grains.

These crystals form deep in the Earth, where temperatures are much hotter than at the surface. With each mile of depth, temperatures increase by about 72 degrees Fahrenheit (40 degrees Celsius). At temperatures above 158 F (70 C), the helium escapes from the mineral, but once the grains cool below that temperature, the helium becomes trapped. So the date of the apatite grains tells scientists the last time the rock layer was buried deep underground.

The dates the team got from samples taken from the bottom of the Upper Granite Gorge and the top of the surrounding plateau show that the two different rock sequences were both about 131 F (55 C) from 55 to 28 million years ago, then cooled to near-surface temperatures about 15 million years ago, Wernicke said.

If the rock sequences were at the same temperature for all those millions of years, they must also have been at the same depth, Wernicke and his team surmised. This suggests to the researchers that a canyon must have existed at least 55 million years ago in the younger rock layers that once lay atop the southwestern plateaus. It might have emerged sooner, they say.

The finding challenges the notion that the upper layers of the rock sequence were eroded away before the canyon formed. Instead, the team’s findings suggest that a canyon had formed in the upper layers and then the top rock layers were eroded away, with the river-cutting of the canyon keeping pace and continually lowering the position of the canyon.