Amethyst Mountain, el. is the highest peak and central part of a northwest – southeast trending ridge that lies between the

Lamar River The Lamar River is a tributary of the Yellowstone River, approximately long, in northwestern Wyoming in the United States. The river is located entirely within Yellowstone National Park. History Prior to the 1884–85 Geological Survey of the ...

to the northeast and Deep Creek to the southwest within

Park County, Wyoming Park County is a county in the U.S. state of Wyoming. As of the 2020 United States Census, the population was 29,624. The county seat is Cody. Park County is a major tourism destination. The county has over 53 percent of Yellowstone National P ...

. From northwest to southeast, this ridge consists of Specimen Ridge, Amethyst Mountain, and the Mirror Plateau in

Yellowstone National Park Yellowstone National Park is an American national park located in the western United States, largely in the northwest corner of Wyoming and extending into Montana and Idaho. It was established by the 42nd U.S. Congress with the Yellowst ...

. 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 Opal is a hydrated amorphous form of silica (SiO2·''n''H2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline form ...

and exposures of well known and visited

petrified forests Petrified wood, also known as petrified tree (from Ancient Greek meaning 'rock' or 'stone'; literally 'wood turned into stone'), is the name given to a special type of '' fossilized wood'', the fossilized remains of terrestrial vegetation. ' ...

.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 Soda Butte Creek is an approximately long major tributary of the Lamar River in Yellowstone National Park. It is named for a now-extinct geyser (Soda Butte) near its mouth. Soda Butte and the creek were named by A. Bart Henderson, a Cooke City m ...

and through the Yellowstone Petrified Forest.Schneider, B (2003) ''Hiking Yellowstone National Park.'' Falcon Press, Guilford, Connecticut. 101 pp.

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

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

aceous

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

and siltstone.

Volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

is absent and only a very few thin airfall

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer ...

beds have been identified in the exposures at Amethyst Mountain. The conglomerates consist of a variety of

mudflow A mudflow or mud flow is a form of mass wasting involving fast-moving flow of debris that has become liquified by the addition of water. Such flows can move at speeds ranging from 3 meters/minute to 5 meters/second. Mudflows contain a significa ...

deposits (

lahars A lahar (, from jv, ꦮ꧀ꦭꦲꦂ) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley. Lahars are extremel ...

) 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 Petrified wood, also known as petrified tree (from Ancient Greek meaning 'rock' or 'stone'; literally 'wood turned into stone'), is the name given to a special type of '' fossilized wood'', the fossilized remains of terrestrial vegetation. ' ...

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 The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...

volcanoes A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...

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 An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

lies upon

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

s and Precambrian crystalline rocks. Dark colored pyroxene

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predo ...

lava flows,

volcaniclastic Volcaniclastics are geologic materials composed of broken fragments ( clasts) of volcanic rock. These encompass all clastic volcanic materials, regardless of what process fragmented the rock, how it was subsequently transported, what environment it ...

rocks, and

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

s 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.

Yellowstone Petrified Forest

A noteworthy feature of Amethyst Mountain are the multiple horizons of well-preserved

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.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 DCDorf, 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. Some of the beds contain prehistoric forests buried in place (''

in situ ''In situ'' (; often not italicized in English) is a Latin phrase that translates literally to "on site" or "in position." It can mean "locally", "on site", "on the premises", or "in place" to describe where an event takes place and is used in ...

'') 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.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. 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 On March 27, 1980, a series of volcanic explosions and pyroclastic flows began at Mount St. Helens in Skamania County, Washington, United States. A series of phreatic blasts occurred from the summit and escalated until a major explosive er ...

, and other Quaternary and

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togeth ...

eruptions Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often ...

of other Cascade Range volcanoes.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. The Late Pleistocene lahar and stream deposits of Mount St. Helens contain buried prehistoric logs and in place (''

'') upright tree trunks that are in the initial stages of being naturally petrified by

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

. 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.

References

External links

* Anonymous (nd
''Petrified Forest''
National Park Service, Washington DC. * Viney, M (2008

{{Petrified forests Mountains of Wyoming Mountains of Yellowstone National Park Mountains of Park County, Wyoming Petrified forests Tourist attractions in Park County, Wyoming Paleontology in Wyoming Fossil parks in the United States

Geology

Yellowstone Petrified Forest

See also

References

External links