Devils Postpile National Monument
|
|
Devils Postpile is a dark cliff of columnar basalt near Mammoth Mountain 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 originated 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 and flowed to the site of the Postpile where it was impounded by a moraine and reached a thickness of 400 (newer estimate) to 600 (older estimate) feet. Either way the lava that now makes up the Postpile was near the bottom of this mass.
Because of its unusual 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 Park, but discovery of gold near Mammoth Lakes, California 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 John Muir, persuaded the federal government to stop the demolition and in 1911 President Howard Taft made the area into a United States National Monument. The John Muir Trail and Pacific Crest Trail pass through the monument.
The Postpile's columns average 2 feet in diameter, with the largest being 3.5 feet and many are up to 60 feet long. Together they look like tall posts stacked in a pile, hence the feature's name. In a perfect world all columnar joints would create hexagon-shaped columns, however nature isn't perfect; a count of 400 of the Postpile's columns gave; 44.5% 6-sided, 37.5 % 5-sided, 9.5 % 4-sided, 8.0 % 7-sided, and 0.5 % 3-sided. However, compared with other examples of columnar jointing, the Postpile has more 6-sided columns. Another thing that places the Postpile in a special category is the lack of horizontal jointing.
| Contents |
Other postpiles
Smaller versions of the postpile can be glimpsed along US Highway 395 in the Owens Valley. See also Devils Tower National Monument and the Giant's Causeway, in County Antrim, United Kingdom.
Other images
    |
Reference
- Roadside Geology of Northern and Central California, Alt, Hyndman (Mountain Press Publishing Company, Missoula; 2000) ISBN 0-87842-409-1
External links
Template:Geolinks-US-streetscale Template:Geolinks-US-surrounds Template:Geolinks-US-loc