Devils Postpile National Monument
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).
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 national 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.