Devils Postpile National Monument
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 400 feet. Recent radiometric dating of rocks thought to correlate with basalt of Devils Postpile suggest an age of less than 100,000 years.
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 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 on the basalt columns. Here, the column ends are exposed like a tiled floor and exhibit parallel striations where the glacier dragged rocks across them.
A mandatory shuttle bus is required during the busy summer months and departs from the Mammoth Adventure Center and The Village at Mammoth. Closed in the winter.
For more information on Devils Postpile contact the National Park Service / Devils Postpile National Monument Ranger Station in the Reds Meadow Valley
http://www.nps.gov/depo or 760-934-2289.