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Devils Postpile National Monument: Map

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The longer fragments of basalt at the base of the cliff are much larger than a person.
Devils Postpile is a dark cliff of columnar basalt near Mammoth Mountainmarker in extreme northeastern Madera Countymarker in eastern California. The postpile was created by a lava flow sometime between less than 100,000 years ago (according to current potassium-argon dating) to 700,000 years ago (according to other dating methods). The source of the lava is thought to have been somewhere near Upper Soda Springs campground at the north end of Pumice Flat on the floor of the Middle Fork of the San Joaquin River, from where it flowed to the site of the Postpile, was impounded by a moraine, and reached a thickness of 400 feet (newer estimate) to 600 feet (older estimate). In any event, the lava that now makes up the Postpile was near the bottom of this mass.

Because of its great thickness, much of the mass of pooled lava cooled slowly and evenly, which is why the columns are so long and so symmetrical. Columnar jointing occurs when certain types of lava cool; the joints develop when the lava contracts during the cooling process.

A glacier later removed much of this mass of rock and left a nicely polished surface on top of the Postpile with very noticeable glacial striations and glacial polish.

Devils Postpile was once part of Yosemite National Parkmarker, but discovery of gold near Mammoth Lakesmarker prompted a boundary change that left the Postpile on adjacent public land. A proposal to build a hydroelectric dam later called for blasting the Postpile into the river. Influential Californians, including Walter L. Huber, persuaded the federal government to stop the demolition and in 1911, President William Howard Taft made the area into a United States National Monument. The John Muir Trail and Pacific Crest Trail pass through the monument.

Basalt column on side
The Postpile's columns average in diameter, the largest being , and many are up to long. Together they look like tall posts stacked in a pile, hence the feature's name. If the lava had cooled perfectly evenly, all of the columns would be expected to be hexagonal, but some of the columns have different polygonal cross-sections on account of variations in cooling. A survey of 400 of the Postpile's columns found that 44.5% were 6-sided, 37.5% 5-sided, 9.5% 4-sided, 8.0% 7-sided, and 0.5% 3-sided. Compared with other examples of columnar jointing, the Postpile has more hexagonal columns. Another thing that places the Postpile in a special category is the lack of horizontal jointing.

Several stones from the Devil's Postpile can be seen at the entrance to the United States Geological Survey headquarters lot in Reston, Virginiamarker.

Similar structures

Although the basaltic columns are impressive, they are not unique. Basalt columns are a common volcanic feature, and they occur on many scales (faster cooling produces smaller columns). Other notable sites include Giant's Causewaymarker in Northern Irelandmarker, Fingal's Cavemarker in Scotland, the Garni gorgemarker in Armeniamarker, the Cyclopean Isles near Sicily, Devils Tower National Monumentmarker in Wyomingmarker, Sheepeater Cliffmarker at Yellowstone National Parkmarker in Wyomingmarker, Basalt Prisms in Hidalgomarker, Mexicomarker, the "Organ Pipes" formation on Mount Cargillmarker in New Zealandmarker, Gilbert Hillmarker in Mumbaimarker, Organ Pipes National Parkmarker in Australia and the "Columnar Cape" (Russian: Mis Stolbchaty) on Kunashir, the southernmost of the Kurile Islandsmarker in Russiamarker.

Ancient columnar basalt can be seen in a high desert dry river falls area just north of Lajitas, Texas. The columns are accessible on horseback, on foot, or mountain bike via trail .

See also

Rainbow fall at Devils Postpile National Monument


Bibliography

  • Roadside Geology of Northern and Central California, Alt, Hyndman (Mountain Press Publishing Company, Missoula; 2000) ISBN 0-87842-409-1


References



External links

  • Official NPS Site
  • The Devil's Post Pile, Popular Science monthly, Feb 1916, page 178, Scanned by Google Books: http://books.google.com/books?id=iSYDAAAAMBAJ&printsec=frontcover&source=gbs_summary_r&cad=0_2#PPA178,M1



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