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.
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.
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.