An Eocene (45 Ma) flora from Thunder Mountain caldera shows that montane conifer forest species from upper slopes descended to interfinger with mixed conifer-deciduous hardwood forest on the caldera floor then near 1700 m. Most species are allied to those in the western United States, but a few genera are in China. Precipitation was near 100 cm yearly, with most in summer.

This flora of 70 species is dominated by deciduous trees, many with descendants in China. Precipitation was 890 mm, mean annual temperature 12.5 C, the annual range 10 C, and freezing rare. Elevation was about 1000 meters. Comparison with the Horse Prairie flora, 30 miles east and across the present continental divide, indicates that the divide was then low and discontinuous, with warmer climate to the east."