According to the National Park Service pamphlet, about 82,000 years ago basalt lava flowed here from an unknown source. This particular lava flow was thick, with a consistent mineral composition, and it cooled slowly and evenly - apparently ideal for column formation. As the lava cooled it contracted and split into the symmetrical, vertical, hexagonal columns that now make up Devils Postpile. About 20,000 to 12,000 years ago a glacier flowed down the Middle Fork of the San Joaquin River - the moving ice carved away one side of the postpile, exposing a shear wall of columns 60 feet high. Erosion and earthquakes later caused many of the columns to fall, and they now lie fragmented on the talus slope below the postpile.
Click here to see a few more images from Devils Postpile and some other things we saw on our recent road trip to Mammoth Lakes, California.
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