Abstract
Polyak et al. (Reports, 7 March 2008, p. 1377) reported that development of the western Grand Canyon began about 17 million years ago. However, their conclusion is based on an inappropriate conflation of Plio-Quaternary incision rates and longer-term rates derived from sites outside the Grand Canyon. Water-table declines at these sites were more likely related to local base-level changes and Miocene regional extensional tectonics.
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