Devils Postpile Formation
The formation of Devils Postpile began
when basalt lava erupted 100,000 years ago 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, possibly pooling behind a natural obstruction.
The mass of molten lava then began to cool slowly and evenly. To
release the stress that built up in the rock, it fractured at
the surface of the flow. These cracks tend to reach ten inches in length and
branch off. The next crack will do the same, creating a series of 120 degree angles.
Eventually these various cracks unite, forming a polygonal pattern along the surface of
the cooling lava. Once the pattern is established, the cracks begin to extend down
vertically towards the interior of the flow. This completes the process, creating
the vertical, six-sided columns you see today.
This geologic formation, called columnar basalt, will only form under ideal conditions.
This requires a slow cooling time and an even mineral composition to the lava.
These conditions were met at Devils Postpile, resulting in the unusual symmetry of the outcrop.
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| Glacial polish is evident on the top of the postpile columns. Glacial ice carrying silt created this polish effect as it scraped across these columns.
© Photo by Tom Warner. |
Some 20,000 to 12,000 years ago, glaciers flowed down the valley of the Middle Fork
of the San Joaquin River and eroded most of the lava flows from
the area. The slow-moving ice quarried away one side of the Postpile, exposing
a sheer wall of columns sixty feet high. On the tops of the columns there is evidence that a glacier
cut and smoothed
off a large portion of the upper part of the lava flow. This cross-section
reveals glacially-polished column tops as well as the striations left behind by glaciers.
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