LOWER MIOCENE AND UPPER OLIGOCENE ROCKS--Light-colored soft porous sand-
     stone and underlying white tuffaceous claystone and siltstone.  Arikaree Formation in 
     Denver Basin.  Tmo
ROCKS EQUIVALENT TO UPPER AND LOWER MIOCENE ROCKS AND WHITE RIVER 
     FORMATION--Light-colored sandstone, white tuffaceous blocky claystone, and
     siltstone.  Black Hills.  Tmo