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 embayments of Fossil Lake. The shoreline plant
assemblage had a rather subtropical appearance.
[Illustration: 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.
[Illustration: 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.