|Elevation||9,609 ft (2,929 m)|
|Location||Yellowstone National Park, Park County, Wyoming, U.S.|
|Topo map||Amethyst Mountain|
Amethyst Mountain, el. 9,609 feet (2,929 m) is the highest peak and central part of a northwest – southeast trending ridge that lies between the Lamar River to the northeast and Deep Creek to the southwest within Park County, Wyoming. From northwest to southeast, this ridge consists of Specimen Ridge, Amethyst Mountain, and the Mirror Plateau in Yellowstone National Park. The nearest town is Silver Gate, Montana, which is 19.2 miles away.
In 1872, it was named Amethyst Mountain by the United States Geological Survey for amethysts found on its summit. This mountain is known for its abundance of amethyst, opal and exposures of well known and visited petrified forests. The summit of Amethyst Mountain is traversed by Specimen Ridge Trail, which also traverses the south side of Specimen Ridge between Tower Junction and Soda Butte Creek and through the Yellowstone Petrified Forest.
Amethyst Mountain consists of a geological formation known as the Lamar River Formation. Within the Amethyst Mountain area, it is over 440 feet (130 m) thick and consists predominantly of conglomerate and lesser proportions of tuffaceous sandstone and siltstone. Volcanic breccia is absent and only a very few thin airfall volcanic ash beds have been identified in the exposures at Amethyst Mountain. The conglomerates consist of a variety of mudflow deposits (lahars) and braided and meandering stream deposits. The lahar (mudflow) deposits consist of conglomerates that occur in massive and structureless beds. These deposits typically consist of matrix-supported, subangular, poorly sorted gravel that ranges in size from 1 cm (0.39 in) to 2 meters (6.6 ft) in diameter. Fluvial conglomerates are normally well-bedded and cross-bedded. They typically consist of grain-supported, subrounded, and moderately well-sorted gravel that typically ranges in size from 1 cm (0.39 in) to 30 cm (12 in). Most of the petrified wood and buried tree trunks occur within the conglomerates. Almost all of the fossil leaves, needles, pollen, and cones are found in the tuffaceous sandstones and siltstones. These tuffaceous sandstones and siltstones accumulated either along the banks of either braided or meandering rivers or within their abandoned channels. In the Amethyst Mountain area, the sediments comprising the Lamar River Formation consist of volcanic material eroded from and deposited downslope of surrounding stratovolcanoes. Locally, the sediments of the Lamar River Formation accumulated in a basin lying between two belts of Eocene volcanoes and at the base of the northern edge of Eocene 'Washburn Volcano'. The Lamar River Formation is part of the Washburn Group.
The Lamar River Formation is part of thick sequence, the Absaroka Volcanic Supergroup, of volcanic rocks erupted from and sediments eroded from two belts of Eocene stratovolcanoes and deposited within an intermountain basin. The Lamar River Formation unconformably lies upon Paleozoic sedimentary rocks and Precambrian crystalline rocks. Dark colored pyroxene andesite lava flows, volcaniclastic rocks, and basalts of the Sunlight Thorofare Creek Group overlie the Lamar River Formation. The Lamar River Formation has been assigned a Middle Eocene age based on radiometric dates and by floral comparisons.
Yellowstone Petrified Forest
A noteworthy feature of Amethyst Mountain are the multiple horizons of well-preserved petrified forests and concentrations of silicified wood found within the Lamar River Formation that is exposed on it slopes. These concentrations of petrified wood are known as the Yellowstone Petrified Forest. Extensive exposures on the northeast slopes of Amethyst Mountain comprise a classic study area, known as the Fossil Forest, that has been studied in detail by a number of researchers for more than 130 years. Some of the beds contain prehistoric forests buried in place (in situ) by volcanic lahars. Other beds are concentrations of fossilized upright stumps, flat-lying logs, and logs lying at various angles that have been transported from the higher slopes of adjacent volcanoes and buried by braided and meandering streams and volcanic lahars. Notably, virtually identical lahar and stream deposits that contain buried upright standing trunks of forests and beds of transported logs and upright stumps were created by the 1980 eruption of Mount St. Helens, and other Quaternary and Holocene eruptions of other Cascade Range volcanoes. The Late Pleistocene lahar and stream deposits of Mount St. Helens contain buried prehistoric logs and in place (in situ) upright tree trunks that are in the initial stages of being naturally petrified by silica.
Detailed studies of the fossil wood and the Lamar River Formation have greatly revised interpretations of their origin. In the past, these petrified wood accumulations have been classically interpreted to be prehistoric forests buried in place by air-fall volcanic ash and volcanic breccia. As noted above, it is now known that the petrified wood found within the Lamar River Formation is a mixture of material that were either buried in place or transported downslope by lahars and streams. In addition, more recent research has found that the lateral extent of the beds containing either transported wood and stumps or an in place fossil forest is, contrary to assumptions made by earlier studies, quite limited. Instead of being of regional extent as previously thought, these beds are quite limited in extent. The localized distribution of both petrified wood-bearing and fossil-forest beds reflects restricted areas that were buried by individual lahars.
In regard to these fossil forests and other fossils, collecting of fossils in Yellowstone National Park is illegal. In addition, visitors should stay on marked and maintained trails.
- "Amethyst Mountain". Geographic Names Information System, U.S. Geological Survey. Retrieved 2013-09-24.
- US Geological Survey (1986) Amethyst Mountain Quadrangle Wyoming-Park Co. 7.5-minute series topographic. scale 1:24,000, US Geological Survey, Denver, Colorado.
- US Geological Survey (1989) Opal Creek Quadrangle Wyoming-Park Co. 7.5-minute series topographic. scale 1:24,000, US Geological Survey, Denver, Colorado.
- Gannett, Henry (1905) The Origin of Certain Place Names in the United States. Bulletin no. 258, United States Geological Survey, Reston, Virginia.
- Holmes, WH (1878) Report on the geology of the Yellowstone National Park. US Geological Survey Territories of Wyoming and Idaho (1883 edition). Twelfth annual report. Part 2, 57 pp.
- Knowlton, FH (1914) The Fossil Forests of Yellowstone National Park. National Park Service, Department of the Interior, Office of the Secretary, Washington DC. 31 pp. Last accessed September 26, 2013.
- Fritz, JW (1977) Paleoecology of Petrified Woods from the Amethyst Mountain "Fossil Forest", Yellowstone National Park, Wyoming. unpublished MS thesis, Walla Walla College, 114 pp.
- National Park Service (1980) Petrified forests of Yellowstone. Handbook no. 108. National Park Service, US Department of the Interior, Washington, DC.
- Schneider, B (2003) Hiking Yellowstone National Park. Falcon Press, Guilford, Connecticut. 101 pp. ISBN 0-7627-2539-7
- Fritz, WJ (1980a) Depositional Environment of the Eocene Lamar River Formation in Yellowstone National Park. Doctoral Dissertation, The University of Montana, Missoula, Montana.
- Fritz, WJ (1980b) Stratigraphic framework of the Lamar River formation in Yellowstone National Park. Northwest Geology. vol. 9, pp. 1-18.,
- Fritz, WJ (1981) Reinterpretation of the depositional environment of the Yellowstone fossil forests: Reply. Geology. 9(2):53-54.
- Fritz, WJ (1982) Geology of the Lamar River Formation, northeast Yellowstone National Park. In SG Reid and DJ Foote, eds., pp. 73-101, Geology of Yellowstone Park Area. Wyoming Geological Association Guidebook, 33rd Annual Field Conference, 1982, Wyoming Geological Association, Casper, Wyoming.
- Feeley, TC, MA Cosca, MA and CR Lindsay (2002) Petrogenesis and implications of calc-alkaline cryptic hybrid magmas from Washburn Volcano, Absaroka Volcanic Province, USA. Journal of Petrology. 43(4):663-703.
- Smedes, HW, and HJ Prostka, 1972, Stratigraphic framework of the Absaroka Volcanic Supergroup in the Yellowstone National Park region. Professional Paper no. 729-C, U. S. Geological Survey, Reston, Virginia. 33 pp.
- Holmes, WH (1879) Fossil forests of the volcanic Tertiary formation of Yellowstone National Park. Geological and Geographical Survey of the Territories. Bulletin of the Survey vol. 5, no. 1, pp. 125-132. United States Geological and Geographical Survey, Department of the Interior, Washington DC
- Dorf, E (1960) Tertiary fossil forests of Yellowstone National Park, Wyoming. in DE Campau, HW Anisgard, and RL Egbet, pp. 253-260, Guidebook, 11th Annual Field Conference, Billings Geological Society, Casper, Wyoming.
- Dorf, E (1964) The 'petrified forests' of Yellowstone Park. Scientific American. 210(4):106-114.
- Dorf, E (1980) Petrified Forests of Yellowstone. National Park Service, U.S. Department of Interior, Washington, D.C., 31 pp.
- Karowe, AL. and TH Jefferson (1987) Burial of trees by eruptions of Mount St. Helens, Washington; implications for the interpretation of fossil forests. Geological Magazine. 124(3):191-204.
- Fritz, WJ, 1980c. Reinterpretation of the depositional environment of the Yellowstone "fossil forests". Geology. 8:(7):309-313.
- Fritz, WJ, and S Harrison (1985) Transported trees from the 1982 Mount St. Helens sediment flows--Their use as paleocurrent indicators. Sedimentary Geology 42(1-2):49-64.
- Cameron, KA, and PT Pringle (1991) Prehistoric buried forests of Mount Hood. Oregon geology. 53(2):34-43.
- Anonymous (nd) Petrified Forest, Yellowstone Photo Collection, Yellowstone National Park, National Park Service, Washington DC.
- Viney, M (2008) Specimen Ridge, The Virtual Petrified Wood Museum.