As I drove into work today, I heard a story on NPR discussing revisions to dating of the Grand Canyon. Apparently, some kind of helium dating technique when applied to soil samples from the canyon produces dates of about 70 million years, whereas the canyon has previously been thought to be only a few million years old.
My first reaction is that they are dating the age of the sediments that make up the walls of the canyon. I don't think there is a problem with the canyon walls being much older than the river system that cut through them.
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