Although not seen within the boundaries of the Fossil Butte National
Monument, the Evanston Formation is exposed just south of Highway 30N,
1.5 miles southeast of the southeast corner of the monument. The
Evanston was not involved in the complex folding and faulting but it is
somewhat disturbed and rests under the Wasatch Formation with angular
uncomformity. The Evanston Formation bridges the time boundary between
the Cretaceous and Tertiary periods. In the lower part of the unit are
found many fossil leaves, pollen, and spores and a jaw of the horned
dinosaur _Triceratops_ that prove its Cretaceous age, and in the upper
part are found fossil mammals of Paleocene age.
The Evanston Formation has been studied in detail by Oriel and Tracey
(1970). These authors divide the formation into three members. The
lowest, which they called the Lower Member, is predominantly “gray to
very dark gray mudstone, siltstone, claystone and gray carbonaceous
sandstone.” The Lower Member reaches a thickness of 500 ft in some
places. Above and in part interfingering with the Lower Member is a 1000
ft thick unit that was named the Hams Fork Conglomerate Member. This
unit consists of beds of boulder conglomerate interstratified with thick
beds of coarse, partly conglomeratic brown sandstone and gray mudstone.
Where the Lower Member of the Evanston is missing, the Hams Fork
Conglomerate forms the base of the formation.
The Upper Member of the Evanston Formation is termed by Oriel and Tracey
(1970) the Main Body. It is more than a thousand feet thick and the
lower part intertongues with the Hams Fork Conglomerate. The Main Body
is “light to dark gray carbonaceous sandy to clayey siltstone
interbedded with gray, tan, yellow and brown sandstone and conglomerate
and carbonaceous to lignitic claystone.” It is this Main Body that can
be seen along the highway just southeast of the monument.
The types of sediments and fossils found in the Evanston indicate that
the formation was deposited by streams on flood plains and in marshes
and ponds. A subtropical climate is indicated and the area was heavily
wooded.