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Science 19 September 2008:
Vol. 321. no. 5896, p. 1634
DOI: 10.1126/science.1158019
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Technical Comments

Comment on "Age and Evolution of the Grand Canyon Revealed by U-Pb Dating of Water Table–Type Speleothems"

Joel Pederson,1* Richard Young,2 Ivo Lucchitta,3 L. Sue Beard,3 George Billingsley3

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.

1 Department of Geology, Utah State University, Logan, UT 84322–4505, USA.
2 Department of Geological Sciences, State University of New York–Geneseo, Geneseo, NY 14454, USA.
3 United States Geological Survey, Flagstaff, AZ 86001, USA.

* To whom correspondence should be addressed. E-mail: joel.pederson{at}usu.edu

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Science. ISSN 0036-8075 (print), 1095-9203 (online)