Twin River Formation  search
BTChronostratigraphic Units
 .Phanerozoic
 ..Cenozoic
 ...Tertiary
 ....Twin River Formation
RTOligocene
 Miocene
 Washington
 upper Eocene
SNUpper Eocene to Miocene. Three mappable sequences recognized: lower member consisting of thin-bedded sandstone and siltstone; middle member of massive siltstone that grades westward into bedded siltstone and sandstone; and upper member composed chiefly of