title: Comment on "Age and evolution of the Grand Canyon revealed by U-Pb dating of water table-type speleothems".
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.
authors.affiliation: Arizona Geological Survey, 416 West Congress Street, Suite 100, Tucson, AZ 85701, USA. phil.pearthree@azgs.az.gov
authors.forename: Philip A
authors.initials: PA
authors.lastname: Pearthree
authors.forename: Jon E
authors.initials: JE
authors.lastname: Spencer
authors.forename: James E
authors.initials: JE
authors.lastname: Faulds
authors.forename: P Kyle
authors.initials: PK
authors.lastname: House