Among the wonders of Yellowstone National Park are the spectacular fossil forests of Amethyst Mountain and Specimen Ridge in the northeastern section of the park and the Gallatin Fossil Forests in the northwestern section. In 1898, John Muir, who was instrumental in establishing the US National Park System, wrote: “Yonder is Amethyst Mountain … beneath the living trees the edges of petrified forests are exposed to view ... standing on ledges tier above tier where they grew, solemnly silent in rigid crystalline beauty after swaying in the wind thousands of centuries ago, opening marvelous views back into the years and climates and life of the past time.” Muir’s visit to Amethyst Mountain was no doubt prompted by the early descriptions and diagrams showing multiple layers of fossil forests there (Figure 1A) (Holmes 1878, 1879). Specimen Ridge and the Gallatin Fossil Forests also have successive tiers of fossil forests. Erling Dorf’s 1964 Scientific American article “The Petrified Forests of Yellowstone Park” includes an illustration of Specimen Ridge with more than 20 layers of fossil forests within a vertical section of some 2,000 feet of volcanics. It is unclear how many different volcanic eruptions were involved in creating these massive accumulations of petrified stumps and logs and the time span represented, as well as how long-lived were the individual forests. The classic paper on the geology of the region is by Smedes and Prostka (1972), who concluded that the Lamar River Formation in which Amethyst Mountain and Specimen Ridge occur and the Sepulcher Formation in which the Gallatin Fossil Forests occur are comparable in age, approximately 50 million years old.