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 speleothem data leading to their inference that the western Grand Canyon incised much earlier than previously thought. This contradicts several lines of published geological knowledge in the region, hinges upon unjustified hydrogeological assumptions, and is based on two anomalous data points for which we offer alternative explanations.
authors.affiliation: Department of Geology, Utah State University, Logan, UT 84322-4505, USA. joel.pederson@usu.edu
authors.forename: Joel
authors.initials: J
authors.lastname: Pederson
authors.forename: Richard
authors.initials: R
authors.lastname: Young
authors.forename: Ivo
authors.initials: I
authors.lastname: Lucchitta
authors.forename: L Sue
authors.initials: LS
authors.lastname: Beard
authors.forename: George
authors.initials: G
authors.lastname: Billingsley