This work was carried out while the author was a Ph.D. student at Stanford University. Many thanks to Mike McWilliams for useful comments on the first drafts of the paper, and to Kate Huntington and Kip Hodges for statistical discussions. This paper greatly improved over the course of three rounds of review by Douglas Burbank, Mark Brandon, Daniel Stockli, Kelin Whipple and two anonymous reviewers. Samples A, B and C were dated by Paul O'Sullivan of A2Z Inc. Sample D was dated by Geotrack International Pty.