earliest Tertiary age (refer to section on structure) and the erosion of the Tertiary rocks that once covered it.

Evanston Formation

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.

Wasatch Formation

The term Wasatch was first used by Hayden (1869:91) as follows:

Immediately west of Fort Bridger commences one of the most remarkable and extensive groups of Tertiary beds seen in the West. They are wonderfully variegated, some shade of red predominating. This group, to which I have given the name of Wasatch group, is composed of variegated sands and clays. Very little calcareous matter is found in these beds.

In Echo and Weber Canyons are wonderful displays of conglomerates, fifteen hundred to two thousand feet in thickness. Although this group occupies a vast area, and attains a thickness of three to five thousand feet, yet I have never known any remains of animals to be found in it. I regard it, however, as of middle Tertiary age.

The Wasatch is well exposed in Fossil Basin. There the unit was regarded by Veatch (1907) as a group and divided by him into three formations: the Almy, Fowkes, and Knight. He wrote (1907:88):

In the Wasatch group as thus defined by Hayden the field work of the season 1905 showed three divisions: 1) a basal member composed of reddish-yellow sandy clays, in many places containing pronounced conglomerate beds, which has been named the Almy Formation; 2) a great thickness of light-colored rhyolitic ash beds containing intercalated lenses of white limestones with fresh-water shells and leaves—the Fowkes Formation; and 3) a group of reddish-yellow sandy clays with irregular sandstone beds (the Knight Formation) closely resembling 1) lithologically and separated from 1) & 2) by a pronounced period of folding and erosion.

Veatch, however, erred in his field work and did not realize that the Fowkes Formation had been downfaulted into the position in which he saw it (Tracey and Oriel 1959; Eardley 1959). The Fowkes is actually the youngest of the three formations of Veatch and is considerably later in age than the true Wasatch.

The Almy and Knight formations are not separable (Oriel and Tracey 1970:16), and can be seen to grade into each other at the basin edges.

Fig. 5. Intertonguing relationship of latest Cretaceous and Tertiary stratigraphic units of Fossil Basin.

Fig. 6. Subdivisions of Paleocene and Eocene time.

The Almy and Knight, as defined by Veatch, are probably different facies of Wasatch, the Almy being a more peripheral facies and the Knight a more basinal facies (Oriel and Tracey 1970). These authors proposed that the terms Almy and Knight be dropped and that the name Wasatch be applied to all these rocks to avoid confusion.

The latest subdivision of the Wasatch Formation in the Fossil Basin is that of Oriel and Tracey. The members of the Wasatch Formation they propose are described in order from oldest to youngest.

BASAL CONGLOMERATE MEMBER.

This member has only local development in Fossil Basin. Where found, it is a lenticular conglomerate with pebbles and cobbles of buff and tan sandstone from the Nugget Sandstone and limestone fragments from the Thaynes and Twin Creek formations.

The basal conglomerate is essentially a channel fill in ancient stream beds cut into Mesozoic rocks.

LOWER MEMBER.

This is an irregular sequence of flood-plain and stream-channel deposits. It is exposed along the southern part of the Tunp Range and extends into the far western section of Fossil Butte National Monument just below Prow Point.

Mudstone is the main rock type. It can be tan, brown, pink, red, or gray in color. Black, carbonaceous siltstones are also present. Gray sandstone that weathers yellow or brown and coarse-grained, cross-bedded conglomerate and sandstone are also prominent. Limestone occurs as thin lenses and is often brown, platy, and carbonaceous.

It is interesting to note that the Lower Member is intermediate not only in stratigraphic position but also in color and composition between the underlying Evanston Formation and overlying Main Body of the Wasatch Formation. The Lower Member thus appears to represent a gradual change in either climatic and/or sedimentary conditions in Fossil Basin (Oriel and Tracey 1970).

Fig. 7. Geologic map of Fossil Butte National Monument (after Rubey et al. 1968).

Fig. 8. NW-SE section across Fossil Butte National Monument, Sage and Kemmerer quadrangles (mapped by Rubey et al. 1968).

MAIN BODY.

This unit of the Wasatch Formation is that part of the formation which produces the spectacular red-colored badlands in Fossil Butte National Monument. Particularly typical exposures can be seen in the south-facing scarp of Fossil Butte where the Main Body makes up the lower portion of the butte.

The most remarkable feature of the Main Body is its color. On the lower slopes of Fossil Butte are bands of