Devils Postpile National Monument

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Devils Postpile National Monument is a National Monument located near Mammoth Mountain in eastern California. The monument protects Devils Postpile, an unusual rock formation of columnar basalt. It encompasses 798 acres (323 ha) and includes two main tourist attractions: the Devils Postpile formation and Rainbow Falls, a waterfall on the Middle Fork of the San Joaquin River. In addition, the John Muir Trail and Pacific Crest Trail merge into one trail as they pass through the monument.[3] Excluding a small developed area containing the monument headquarters, visitor center and a campground, the National Monument lies within the borders of the Ansel Adams Wilderness.

The most common method for accessing Devils Postpile is via the mandatory shuttle bus operated by Eastern Sierra Transit Authority in the summer months, followed by a 1/4 mile walk. The shuttle route begins at Mammoth Mountain Ski Area's Adventure Center and makes several stops throughout the valley and begins operating when the Reds Meadow Road opens in the summer, and continues through Labor Day weekend.

The name "Devils Postpile" refers to a dark cliff of columnar basalt. Radiometric dating indicates the formation was created by a lava flow at some time less than 100,000 years ago. 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. Estimates of the formation's thickness range from 400 feet (120 m) to 600 feet (180 m). 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 contract while cooling.

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

Ref : https://en.wikipedia.org/wiki/Devils_Postpile_National_Monument

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