In the northern part of Fossil Basin, the Mudstone Tongue of the Wasatch
Formation separates the Angelo Member of the Green River Formation from
the Fossil Butte Member. Further south, the two members rest directly on
one another. The southern extent of the Angelo Member has not been
determined as yet, but Oriel and Tracey (1970:32) believe it extends up
to, and intertongues with, Wasatch conglomerates near the Uinta
Mountains. Toward other edges of Fossil Basin, the Angelo Member thins
and pinches out into the Wasatch Formation. The Bullpen Member of the
Wasatch rests on top of the Angelo Member.
The Angelo Member consists of white to blue-white weathering limestone,
marlstone, and mudstone. Some sandstone lenses, claystone, oil shale,
and siliceous limestone are present as well. In general, Oriel and
Tracey (1970) have found that buff limestones prevail to the north and
white, siliceous limestone, to the south. Like the Fossil Butte Member,
the Angelo Member shows a facies change from deep-water to shore.
This member forms the very uppermost, rounded slopes of Fossil Butte in
contrast to the more vertical cliffs formed by the Fossil Butte Member.