The Geologic History of Fossil Butte National Monument and Fossil Basin
NPS Occasional Paper No. 3
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Fossils of plants, invertebrates, and vertebrates are abundant in rocks exposed in the Fossil Basin. For example, Oyster Ridge received its name because of the well-preserved and abundant fossil oysters found there. Of course, Fossil Butte and Fossil Butte National Monument were so named because of the beautifully preserved fossil fish. A brief review of the main groups of fossils to be found in the basin follows.


Many of the sediments in the Fossil Basin contain an abundance of fossil plants. These consist of pollen, spores, wood, and leaf impressions. Those of the Green River Formation are best preserved and have received the most study.

Brown (1929, 1934) and Lesquereux (1883) have presented the best information on the flora of the Green River Formation. Much of the present knowledge is based on their work.

Brown (1929, 1934) concluded that the Green River flora is a mixed forest type. It contains some plants common to warm, wet lowlands and others adapted to cooler, drier uplands. Pollen studies support Brown's original concept that several plant communities, including mountain communities, existed around Fossil Lake.

The flora was situated in an inland, mountain basin. Fossil Lake was in the middle of the basin. Swamps and flood plains bordered the lake, while the flanking ridges and mountains provided altitudinal variations in the plant assemblage.

Spruce, fir, and pine are indicated by fossil pollen. The presence of these conifers suggest that at least some of the nearby mountains rose to heights of 6000—8000 ft.

Leaf imprints indicate that deciduous trees occurred on the lower slopes. Oak, elm, maple, and beech were common forms.

Closer to the lake where it was wetter and warmer there were willows, laurel, and fig trees. Palms grew in the sandier soils (Fig. 11). Cypress grew in quiet embay merits of Fossil Lake. The shoreline plant assemblage had a rather subtropical appearance.

fossil leaf impression
Fig. 11. Impression of a palm frond from the fish beds of Fossil Basin. Width of original, 160 mm. Collection of University of Wyoming.

Ground cover was provided by a mold of dead leaves, holly, liverworts, mosses, and ferns (Fig. 12). Prairie-type grasses are not present and large savanna areas probably were not developed.

fossil fern leaf
Fig. 12. A fern leaf from the Green River Shales of the Green River Basin. Width of original, 4 cm. Collection of University of Wyoming.

In the lake itself small, one-celled algae floated about in the waves and currents. Bacteria grew on the lake bottom and in the water and helped to contribute to the organic ooze that built up on the bottom. Fungi, wonderfully preserved in oil shales, are further indicators that Fossil Lake was a fresh-water lake and deep enough to have depths below the zone of sunlight penetration (Bradley 1964a). Reeds, rushes, and similar lake-shore plants grew in shallow, near-shore waters.

The large amount of organic material in the rocks is evidence of the prolific microscopic plant growth in the lake.

Occasional dry periods in the Fossil Basin are indicated by fossils of a plant called Ephedra. This plant had leaves with a thick, waxy cuticle to prevent desiccation during drought. The absence of saline minerals in the Green River Formation in Fossil Basin suggests that these arid periods were never of the intensity that occurred in the Green River Basin to the east.

The flora of Fossil Basin, especially that from the Green River Formation, is usually compared to that of the present-day Gulf Coast. Seaward from the Appalachians, the composition of the flora is very similar to that of the Green River. Spruce, pine, and fir grow in the Appalachians at elevations of 6000 or more feet, supporting the idea that similar elevations existed near Fossil Lake and that a general Gulf Coast climate prevailed in Fossil Basin.

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Last Updated: 01-Mar-2005