Periods was probably much longer than the Oligocene, Miocene and Pliocene combined.  It witnesses the appearance, development, migration and extinction of the earlier orders of the higher mammals.



The principal localities where Eocene mammals hair been found, hair been searched and researched.  There are still however, large areas unexplored or where the search has been hasty or incomplete.



The Fort Union beds cover a vast extent of Tertiary in Montana and Wyoming and are now known to be equivalent in time with the earlier Eocene