bright to dull red, pink, purple, yellow, and gray color arranged in various patterns. This unit is best observed at a distance, especially after a rain. The bright hues of the Wasatch contrast markedly with the whites and tans of the overlying Green River Formation.

Individual bands of color range from 1 to 10 ft thick (Oriel and Tracey 1970:78). The colors are brightest in the upper part of the member and drabber in the coarser-grained lower part.

The predominant rock types in the Main Body are banded, variegated mudstone with interlayered sandstone, conglomerate, marlstone, siltstone, and claystone.

The upper part of the Main Body is mainly mudstone composed of fine silt and very fine, bedded sand with a clay binder. Conglomerates occur as channel fills and contain calcium carbonate as cement as do a number of sandstone and siltstone layers in the upper Main Body.

Conglomerates and sandstone are more common in the lower part of the Main Body. Some are part of Veatch’s old Almy Formation. They are best developed along the edge of Fossil Basin. The Main Body overlaps the Evanston Formation in some places and may rest directly on Mesozoic or Paleozoic rocks.

SANDSTONE TONGUE.

This is a tongue of cross-bedded sandstone. It is brown in color and is composed mostly of quartz with some black chert grains. This unit is limited in distribution to the south of Fossil Butte National Monument. The Sandstone Tongue thins and pinches out to the north and is not present within the monument. The area of pinchout represents the shoreline at one stage of Fossil Lake. The Sandstone Tongue wedges into the Fossil Butte Member of the Green River Formation. The sediment sources were probably the Uinta Mountains at the south edge of Fossil Basin. Tectonic events caused an uplift and erosion of Mesozoic and Paleozoic rocks in the Uintas and debris was deposited in Fossil Lake as an encroaching delta of sand. When deposition of the sand halted, the lake expanded and covered the sand, encasing it within the shales of the Green River Formation.

The Sandstone Tongue can be traced into the Main Body of the Wasatch, hence its assignment to the Wasatch Formation.

MUDSTONE TONGUE.

This tongue of the Wasatch Formation can be seen in the northwestern part of Fossil Butte National Monument and over most of the northern part of Fossil Basin. In the area of its distribution, the Mudstone Tongue separates the underlying Fossil Butte Member of the Green River Formation from the overlying Angelo Member of the Green River Formation. The tongue thins and disappears to the south, that is, basinward. As with the Sandstone Tongue, the edges of the Mudstone Tongue where it pinches out represent an ancient shoreline of Fossil Lake. To the north and west the Mudstone Tongue merges with the Tunp Member of the Wasatch Formation.

The Mudstone Tongue is a composite of dark-red mudstone which becomes lighter in color basinward, changing to light red, pink, or greenish-gray claystone. The unit is a mixture of silt and clay derived from the north and west, and was deposited as a large delta in Fossil Lake.

An interesting aspect of the Mudstone Tongue is the presence of algal logs. These are cylinders of limestone that apparently formed as an encrustation of calcium carbonate around logs and branches that fell into the edge of the lake. The calcium carbonate resulted from the action of algae which grew around the log. Successive growths of algae resulted in successive layers of calcium carbonate being deposited.

BULLPEN MEMBER.

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.

TUNP MEMBER.

This peripheral unit of the Wasatch Formation was first described by Oriel and Tracey in 1970. It is not exposed in the monument, but forms a belt of outcrops around the edge of Fossil Basin and in channels cut at right angles to the basin edge.

The Tunp Member is seen to intertongue with nearly all of the Wasatch and Green River formations. The member grades laterally basinward from coarse, angular conglomerates to fine mudstone. Two limestone tongues of the Green River Formation are interbedded with the Tunp, indicating that at least twice there was major expansion of the lake.

Lithologically, the Tunp Member is a diamictite. This is a sedimentary rock with a wide range of particle sizes. The Tunp is best described as a red, conglomeratic, sandy mudstone with angular, poorly rounded to smooth, well-rounded clasts with a size range from pebble to boulder. There is no bedding or orientation of clasts.

The Tunp Member probably originated from mudflows and gravity sliding (Tracey et al. 1961). Environmental studies indicate that the area had a warm, humid climate. This would cause deep weathering of the surrounding slopes. This weathered material would then be a prime source to be acted upon by rain, gravity, and possibly earthquakes. These agents of deposition would then cause the material to flow and be deposited with little chance for sorting and rounding of the rock particles. The result was a belt of coarse, unsorted detritus on the basin edges now called the Tunp Member.

AGE OF THE WASATCH FORMATION.

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.