STRATIGRAPHY
The complete geologic history of the Fossil Basin involves many stratigraphic units. Most of these are not exposed within the monument itself but are well known from outcrops outside the boundaries of the monument and from deep wells ([Fig. 4]).
At the base of the stratigraphic section are rocks of Precambrian age. These have not been penetrated by wells within the Fossil Basin but should occur as much as 25,000 ft beneath the surface. They probably consist of metamorphic and intrusive rocks. Beginning about 600 million years ago, at the beginning of Cambrian time, sedimentary deposits accumulated. These are both marine and continental in origin and range in age from early Cambrian (600 million years old) to early Tertiary (50 million years old). Some 34 geologic formations are recognized in this thick section, a list of which follows:
| Eocene | Fowkes Formation |
| Wasatch-Green River Formations | |
| Paleocene | Evanston Formation (upper part) |
| Upper Cretaceous | Evanston Formation (lower part) |
| Adaville Formation | |
| Lazeart Sandstone | |
| Hilliard Shale | |
| Frontier Formation | |
| Lower Cretaceous | Aspen Shale |
| Bear River Formation | |
| Gannett Group | |
| Jurassic | Stump Sandstone |
| Preuss Sandstone | |
| Twin Creek Limestone | |
| Nugget Sandstone (upper part) | |
| Triassic | Nugget Sandstone (lower part) |
| Thaynes Limestone | |
| Woodside Formation | |
| Dinwoody Formation | |
| Permian | Phosphoria Formation |
| Pennsylvanian | Wells Formation |
| Mississippian | Brazer Limestone |
| Madison Limestone (upper part) | |
| Devonian | Madison Limestone (lower part) |
| Darby Formation | |
| Silurian | Not Present |
| Ordovician | Leigh Dolomite |
| Bighorn Dolomite | |
| Cambrian | Gallatin Limestone |
| Gros Ventre Formation | |
| Precambrian | Complex of metamorphic and intrusive rocks |
Within the monument itself the oldest rocks exposed are those of the Thaynes Limestone near Prow Point. As will be seen in the discussion of the structure of the Fossil Basin, all of the rocks beneath the Evanston Formation are folded and faulted and within the monument are buried by deposits of Tertiary age. For this reason detailed discussion of stratigraphic units will be confined to those rocks that can be seen within the boundaries of the monument or closely adjacent to it ([Fig. 5]). The distribution of the various formations within the monument is illustrated in Figs. [7] and [8].
Fig. 4. Geologic time scale.
Thaynes Limestone
At the very northeastern corner of Fossil Butte National Monument can be seen an outcrop of the Thaynes Limestone of Triassic age, a marine unit deposited some 200 million years ago. This formation is noted for its rich and varied assemblage of marine fossils, mostly forms related to oysters and clams. The Thaynes Limestone consists predominantly of sandy, gray limestone and green-gray limey siltstones. The formation weathers to a dark brown in the lower part and a grayish-yellow in the upper. The fact that the Thaynes Limestone is exposed in the monument at all is because of the faulting that took place just prior to the deposition of the Evanston Formation of latest Cretaceous and earliest Tertiary age (refer to section on structure) and the erosion of the Tertiary rocks that once covered it.