In 1872 congress set aside land to create our first national park (96% in WY, 3% in MT, and 1% in ID). Yellowstone is a Hot Spot, a volcano produced by a plume of magma that “punches” through the Earths’ crust. The current Yellowstone Caldera was created by a massive eruption 640,000 years ago. Major eruptions occur at 600,000 to 900,000 year intervals. Yellowstone National Park is well known for its wildlife and geothermal features. Today, the forests of Yellowstone help to define a Sub Alpine ecosystem. Lodgepole pine makes up 80% of the forested areas. Lodgepole pine trees need fire to open up their cones. Engelmann spruce, douglas-fir, subalpine fir, whitebark pine, and aspen are also common. Petrified wood deposits found in Yellowstone reveal a past environment and ecosystem very different from the one we enjoy today.

In Yellowstone remnants of 27 fossil forests are embedded within 366m of sediment (Kenrick & Davis, 2004, p. 61). Clusters of fossilized trees, exposed at Specimen Ridge, represent a succession of over a dozen of these forest remnants (Yuretich, 1984, p. 159). Many trees are preserved in their original growth position (Fritz, 1980, p. 313, Retallack, 1981, p. 52, Yuretich, 1984, p. 161). The hike to Specimen Ridge begins at a pullout on the south side of the road 5.3 miles east of Tower Junction in the Lamar Valley. The 1.5-mile hike to the trees has a gain of 1,200 feet. The round trip is 3 miles and takes 2 to 3 hours.

The fossil plant deposits were formed during the Eocene by periodic volcanic activity some 48 million years ago as indicated by radiometric dating and biostratigraphic correlation (Fritz, 1984, p. 638). During the Eocene two northwest trending subparallel volcanic chains formed the Western and Eastern Absaroka Belts. The Absaroka eruptive centers were 25 to 60 km apart with a narrow intermontane basin in-between them. The Larmar River Formation petrified wood deposits are found between these two volcanic chains (Fritz, 1980, p. 312).

During volcanic eruptions mud flows and braided streams originating on surrounding stratovolcanic peaks of the Absaroka Volcanic Supergroup transported plant parts from higher to lower elevations as well as buried plants in place. Transported trees were stripped of bark, branches, and roots (Fritz, 1980, p. 312). Fossil trees preserved in situ have bark and intact root systems penetrating the substrate (Fritz, 1981, p. 54).

Stumps at specimen ridge are rooted in fine-grained tuffaceous sandstone that represents immature soils. Conglomerates that overlie these root-zone sandstones formed from volcanic sediments that flowed around and buried the trees where they grew (Yuretich, 1984, p. 161). The burial of these plants by intermittent volcanic sediment flows occurred in localized areas (Fritz, 1980, p. 312, 1984, p. 638). During quiescent times, new soil layers formed and a new forest would grow. Growth rings suggest that some of the forests grew for 500 years.

The tree composition of these ancient forests was typical of warm temperate to subtropical floras. This contrasts with today’s subalpine ecosystem. More than 80 kinds of trees, shrubs, and herbs are known. Redwoods, maples, oaks, chestnuts, magnolias, walnuts, persimmons, dogwoods, laurels, and bays are some of the more common trees. The flora also contained some exotic trees whose relatives are now found in East Asia. Fossil woods reported from Specimen Ridge include: pine, fir, redwood, cypress, oak, beech, sycamore, willow, and laurels (Beyer, 1954, p. 567). You can take a virtual hike up Specimen Ridge by clicking on the picture to the right.

Yuretich, R.F. (1984). Reply to Comment on “Yellowstone fossil forests: New evidence for burial in place”: Geology, v. 12, p. 638-639).