The exact age of the units within the Wasatch Formation can be
determined only if fossils are present. None has been found in the basal
conglomerates but it is believed to be earliest Eocene. The Lower Member
is also not dated with fossils but is believed to be very early Eocene.
A number of fossil mammals are known from the Main Body of the Wasatch
Formation. The lower part is early, early Eocene as demonstrated by the
presence of a very primitive ungulate, _Haplomylus speirianus_. In the
upper part of the Main Body another primitive ungulate, _Hyopsodus
browni_, is found which indicates a mid to early Eocene age. The
Mudstone and Sandstone tongues are not dated by means of fossils but the
stratigraphic relationships indicate an early Eocene age. Fossil
mollusks in the Bullpen Member are not completely diagnostic but suggest
either a late early Eocene (Lostcabinian) (Fig. 6) or mid-Eocene age.
The Tunp Member interfingering as it does with the entire Wasatch
section represents all of early Eocene age.