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Amethyst Mountain
Amethyst Mountain, el. 〔 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.〔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.〕
In 1872, it was named Amethyst Mountain by the United States Geological Survey for amethysts found on its summit.〔Gannett, Henry (1905) ''The Origin of Certain Place Names in the United States.'' Bulletin no. 258, United States Geological Survey, Reston, Virginia.〕 This mountain is known for its abundance of amethyst, opal and exposures of well known and visited petrified forests.〔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 unpublished MS thesis, Walla Walla College, 114 pp.〕〔National Park Service (1980) ''Petrified forests of Yellowstone.''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.〕 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.〔Schneider, B (2003) ''Hiking Yellowstone National Park.'' Falcon Press, Guilford, Connecticut. 101 pp. ISBN 0-7627-2539-7〕
==Geology==
Amethyst Mountain consists of a geological formation known as the Lamar River Formation. Within the Amethyst Mountain area, it is over 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 to 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 to . 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.〔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.〕
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.〔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.〕
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