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Facts: Devils Postpile National Monument Origins of the Postpile 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. |
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