Fewer than 100,000 years ago, basalt lava erupted two miles upstream from today's postpile. The lava flowed into the valley and pooled to a depth of 400 feet. The mass of molten lava then began to cool uniformly from top to bottom.
As it cooled and contracted, stresses built up in the basalt rock causing it to fracture. Each crack branched when it reached a length of about 10 inches, joining other cracks to form a pattern on the surface of the flow. Under ideal conditions, surface cracks deepened to create the vertical, hexagonal columns you see today.
Some 10,000 years ago a glacier flowed down the Middle Fork of the San Joaquin River and overrode the Postpile formation. The moving ice quarried away one side of the postpile, exposing a sheer wall of columns 60 feet high. Evidence of the glacier - the polishing and scratches of glacial ice - remains atop the postpile.
Source: National Park Service
Source URL: http://www.nps.gov/depo/depomain.htm
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