Uranium has been mined in this area of southeastern Wyoming from poorly consolidated sandstone units in the Eocene Wind River Formation, under conditions made difficult by loose ground and abundant water. Mineralization and ore lie mostly along the periphery of tongues of altered sand, recognized by greenish to rusty color. The major tongue is as much as 70 feet thick, hundreds of feet across, and several miles long. The best ore occurs on geochemical fronts, or rolls, at terminal edges of the altered zones and on their upper and lower surfaces. This concept has been successfully applied to exploration drilling, mining projection, and reserve estimates.