WASHINGTON (Reuters) - The Grand Canyon, carved out over the eons by rushing river water, began to form 17 million years ago, making it nearly three times older than previously thought, scientists said on Thursday. The general consensus among geologists had been that the famed natural landmark in Arizona was about 6 million years old. But now University of New Mexico scientists say it is far older based on their findings using a technique called uranium-lead isotope to date mineral deposits in caves from nine sites in the canyon's walls. The cave formations provided a record of a dropping water...