The age of the Green River Formation is dated on its intertonguing
relationships with the Wasatch Formation because the latter has datable
mammal fossils. The well-preserved fossils of fish, leaves, and insects
from the Green River are, unfortunately, of little value in dating the
Green River Formation.
Although no Lostcabinian mammals are known from Fossil Basin, the Green
River Formation is believed to be of that age (Gazin 1959; Schaeffer and
Mangus 1965).