The migration of U may be studied by the distribution of the radioactive daughter products, which serve as natural tracers in the migration of U. The distribution of the daughter products is determined by radiochemical analyses of samples from ore deposits in sandstone, and the apparent minimum and maximum dates of U introduction or redistribution may be calculated from the Pa 231 /Th 230 ratio. The primary assumption required is that the Pa and Th do not migrate in measurable quantities from the place where they were produced by the decay of the parent U isotopes. The upper limit of age determination is about 250,000 years, based on the half-lives of Pa 231 and Th 230 . The difference in the half-lives of these isotopes is reflected in their differential rates of growth and decay corresponding to migrations of the parent U during the time range considered. The growth and decay patterns, analyzed mathematically, are used to determine the apparent date of U migration. Calculations based on analyses of samples from the Hulett Creek area, Wyoming, illustrate the results for typical sandstone ore deposits that are above and at the water table.