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Specimen Ridge

Ridge Known for Its Petrified Trees

Ridge in Yellowstone NP, Rocky Mountains near Buffalo Ford, WY

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The fossil forests of Specimen Ridge contain 50 million year old trees buried by ancient volcanos. Before today’s magma hot spot moved under Yellowstone National Park, an earlier volcanic province existed. The vast province stretched from Montana’s Gallatin Range to Wyoming’s Absaroka Range with two northwest trending belts of volcanos that erupted periodically. At Specimen Ridge, Dr. Erling Dorf identified 27 layers of buried forests, testifying to the amount of regional volcanic activity. Mount Washburn inside the park, and Stinkingwater Peak east of the park, are remnant volcanos of that period.

Geologists initially thought that volcanic ash fallout smothered the trees, which included sequoia, fir, and many deciduous types, and then fossilized them. Then, scientists applied the knowledge gained from the eruption of Mount St. Helens and changed the process of fossilization to involve mudflows on the volcano slope instead of airborne ash covering. Today, we believe it is a combination of both airborne ash and mudflows that turned the layered forests into stone. (Dorf 1980, 14-23; Fritz 2011, 43-45; Knowlton 1928, 3-16; Viney)

In a simplified version, using just one volcano, and ignoring the effects of adjacent volcanos, the distance from the volcano vent determines the fate of one forest layer.

The vent obliterated the trees immediately nearby.

Down from the summit vent, a landslide of mud and hot debris knocked trees over and unearthed stumps, and quickly transported them in a mud-debris-tree mixture down the eroding volcano slope. Hot rock continued melting more mountain snow and ice into water and mud, channeling the now horizontal trees off the slope to their burial in the swollen and choked ancient streambeds below.

Farther from the vent in the wider valleys below, the mud-debris-tree mixture torrent slowed. The forest trees here endured the impact, remaining upright and buried in mud roots and all.

Adding to the mudflow burial process, airborne volcanic ash accumulated over the forest. The volcano deposited thick ash near its vent and downwind. Ash deposits thinned with distance. Upright trees far from the vent and out of the mudflow path faced ash entombment.

Years after the volcano, forest trees living and buried, continued to be buried. Rains, snowmelt, and earthquakes triggered mudslides across the landscape.

Deprived of oxygen beneath the ground, the trees then had their wood cells replaced with silica or calcite, turning them into stone.

To see Specimen Ridge’s Fossil Forest start at the Fossil Forest Trailhead.

Collecting any natural resources, including rocks and fossils, is illegal in Yellowstone National Park.

Confluence of Lamar River and Slough Creek with Specimen Ridge forming the skyline, Jacob W. Frank (NPS) two photographs above.

Illustrations and Maps

Generalized geologic map of the Eocene Absaroka volcanic field (modified from Smedes and Prostka, 1972 and “Geology of Wyoming”). Black masses represent locations of volcanic centers (see Chadwick, 1970). Single dot-dashed line is the boundary of Yellowstone National Park.

The 27 fossil forests buried in Specimen Ridge.

Geologic hazards at volcanoes.

Historical Photographs

Specimen Ridge, view north across the confluence of Slough Creek with Lamar River. Petrified tree trunk, Fossil Forest. Yellowstone National Park, Wyoming. ca. 1890. Joseph Paxson Iddings (USGS)

One of the larger trees appears to have been prostrated before it was fossilized (see Pl. CVIII [108]).. It is about 4 feet in diameter and is exposed for a length of 40 feet. There is nothing to indicate the portion of the trunk in its relation to roots and branches, but neither shows on the exposed part. There is no appreciable diminution in diameter, and consequently it must have been a very tall trunk. (Knowlton p. 759 in Hague 1889 Monograph 32)

[Upright] Petrified tree trunk [shown below]. Lamar Valley, looking east from the top of Specimen Ridge toward the base of Mount Norris. Yellowstone National Park, Wyoming. ca. 1890. Joseph Paxson Iddings (USGS)

On the Web

On Wikipedia

Specimen Ridge, el. 8,379 feet (2,554 m) is an approximately 8.5-mile (13.7 km) ridge along the south rim of the Lamar Valley in Yellowstone National Park. The ridge separates the Lamar Valley from Mirror Plateau. The ridge is oriented northwest to southeast from the Tower Junction area to Amethyst Mountain. The ridge is known for its abundance of amethyst, opal and petrified wood. It was referred to as Specimen Mountain by local miners and was probably named by prospectors well before 1870. The south side of the ridge is traversed by the 18.8-mile (30.3 km) Specimen Ridge Trail between Tower Junction and Soda Butte Creek. The trail passes through the Petrified Forest and over the summit of Amethyst Mountain el. 9,614 feet (2,930 m).

Sources

Dorf, Erling and NPS. Petrified Forests of Yellowstone. Washington D.C.: U.S. Government Printing Office, 1980. http://archive.org/details/petrifiedforests00nati.

Fisher, Mark P., Ken Steele, and Deb D. Steele. “Absaroka Volcanic Province.” Geology of Wyoming. Accessed November 11, 2023. https://www.geowyo.com/absaroka-volcanic-province.html.

Fritz, William J., and Robert C. Thomas. Roadside Geology of Yellowstone Country. Second Edition. Missoula MT: Mountain Press Pub., 2011.

Hague, Arnold and USGS. “Geological History of the Yellowstone National Park.” Yellowstone, 1912. http://npshistory.com/publications/yell/hague/index.htm.

Knowlton, F. H. “The Fossil Forests of the Yellowstone National Park.” Washington D.C.: USGS, 1928. http://npshistory.com/publications/yell/fossil-forests-1928.pdf.

NPS. “Fossils.” Yellowstone. Accessed November 18, 2023. https://www.nps.gov/yell/learn/nature/fossils.htm.

Santucci, V. L. “The Yellowstone Paleontological Survey.” NPS, 1998. Yellowstone Center for Resources. https://doi.org/10.13140/RG.2.1.4572.1369.

USGS, Michael Serio, and Madison Myers. “The Underappreciated Fossil Riches of Yellowstone,” June 6, 2022. https://www.usgs.gov/observatories/yvo/news/underappreciated-fossil-riches-yellowstone.

Viney, M. “The Fossil Trees of Specimen Ridge.” The Virtual Petrified Wood Museum. Accessed December 17, 2023. https://petrifiedwoodmuseum.org/PDF/SpecimenRidge.pdf.

Yuretich, R.F. (1984). Yellowstone fossil forests: New evidence for burial in place: Geology, v. 12, p. 159-162.