This uppermost member of the Wasatch Formation is found mostly to the
west and south of the monument. A few, small, isolated caps on the top
of the Green River Formation east of Prow Point occur within the
monument. These low hills, mere bumps, are remnants of a once more
extensive distribution of the Bullpen Member.
Veatch (1907:99) originally considered what is now called the Bullpen
Member as being the Bridger Formation, this was based mostly on the
Bullpen’s position above the Green River Formation rather than on any
lithologic resemblance to the Bridger. The Bullpen is much redder in
color than the Bridger.
In early work Tracey and Oriel (1959:729) called these beds an upper
tongue of the Wasatch and have since named them the Bullpen Member.
The Bullpen Member is lithologically very like the Main Body of the
Wasatch and can be traced into the peripheral units of the Wasatch
Formation.
The rocks included within the Bullpen Member are layered sequences of
red, pink, gray, and green claystone and mudstone. Bentonite is present
in some claystone beds and causes sloughing because of its property of
expanding when wet. Some sandstones are present in the northern areas of
the member’s distribution. Limestones are also present. They are thin
and slabby and have a brown, white, or gray color. Some contain varying
amounts of clay. A conglomerate is present in the upper part of the
Bullpen toward the basin periphery and merges with the Tunp Member of
the Wasatch Formation.
The Bullpen Member is conformable with the underlying Angelo Member of
the Green River Formation. The contact is a transitional one reflecting
a gradual change from the lake environment of the Green River Formation
through a swamp environment to that of a flood plain. The light-colored,
fine-grained shales, marlstones, and limestones of the Green River
Formation grade upward into drabber, coarser grained clay and mudstones
of the Bullpen Member as a reflection of this change of environments.