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.
[Illustration: uncaptioned]
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.