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Science 19 September 2008:
Vol. 321. no. 5896, p. 1634
DOI: 10.1126/science.1159762
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Technical Comments

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

Victor Polyak,* Carol Hill, Yemane Asmerom

Pederson et al. and Pearthree et al. offer critical comments on our study of the age and evolution of the Grand Canyon. Both sets of authors question our use of incision rates from two sample sites located outside the canyon and present alternative interpretations of our data. As we explain, even without the sites in question, our data support a "precursor" western Grand Canyon older than 6 million years.

Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA.

* To whom correspondence should be addressed. E-mail: polyak{at}unm.edu

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