The time of U migration in deposits in sandstone can be determined by correlating apparent age calculations, based on radiochemical analyses, with the geology of a particular deposit. Data were obtained from U ore samples representing deposits above the water table, deposits just above and below perched water tables, and deposits at least 250 ft. below the water table in the Hulett Creek area, Wyoming. The first U deposition occurred more than 250,000 years ago for the deposits now at or above the water table. Approximately 60,000 to 80,000 years ago these deposits were oxidized, leached, and locally enriched. Accumulation of U in the deposits below the water table probably did not start before 180,000 years ago and has continued to the present.