Devils Postpile
NATIONAL MONUMENT • CALIFORNIA
DEVILS POSTPILE NATIONAL MONUMENT
Along the picturesque Middle Fork of the San Joaquin River at 2,300 meters (7,600 feet) on the western slopes of the Sierra Nevada lies Devils Postpile National Monument. The 324-hectare (800-acre) monument near the resort community of Mammoth Lakes includes one dominant feature: a formation of columnar basalt known as Devils Postpile. Another important feature in the area is Rainbow Falls. Pumice is the dominant rock-type encountered, but basalt, andesite, rhyodacite, and granite lie just under the surface and crop out in many places. Plants and animals are typical of lodgepole pine and red fir forests. To see the features of the monument one must walk.
DEVILS POSTPILE
The formation of Devils Postpile began when basalt lava erupted in the valley of the Middle Fork of the San Joaquin River. As lava flowed from the vent, it filled the valley near the Postpile to a depth of 122 meters (400 feet). Recent radiometric dating of rocks thought to correlate with basalt of Devils Postpile suggest an age of less than 100,000 years.
Surface cracks formed when tensions caused by the shrinkage of the cooling lava were greater than the strength of the lava itself. Each crack branched when obtaining a critical length and together with other cracks formed a pattern on the surfaces of the flow. Ideal conditions allowed surface cracks to deepen, resulting in the formation of long columns.
Approximately 10,000 years ago, glaciers flowed down the Middle Fork of the San Joaquin River. This moving ice easily overrode the fractured mass of lava and quarried away one side of the Postpile, exposing a sheer wall of columns 18 meters (60 feet) high. Many fallen columns now lie fragmented on the talus slope below.
A hike to the top of the Postpile reveals not only a cross section of the posts, but the most interesting effect of the ice—polished tops of the basalt columns. Here, the column ends are exposed like a tiled floor and exhibit parallel striations where the glacier dragged rocks across them.
Even though Devils Postpile is among the world’s finest examples of columnar-jointed basalt, it is not unique. Giant’s Causeway in Ireland and Fingal’s Cave in Scotland are similar formations.