Devils Postpile National Park Geologic Story

The Postpile is Exposed

Exposed ends of columns on top of the Postpile.

The glaciers that flowed down the valley of the Middle Fork of the San Joaquin River eroded most of the lava flows from the area. Although these slow-moving bodies of ice were large and powerful, they did leave a few remnants of the original lava flows. On the remnant known as the Devils Postpile, the glaciers cut a vertical face, exposing the interior of the flow and the sides of the columns. In the 10,000 years since the last glacier left, numerous columns have spalled from the face of the cliff, creating a talus pile of broken posts.

Other lava columns, less spectacular than those of the Devils Postpile, are found in several places within or near the Monument. Most of the exposures clearly indicate the direction of ice flow since they have steep downstream- that is, down-glacier -slopes with gentle upstream slopes. When glaciers move over fractured rock, the plucking action on the downstream side of the rock outcrop is much greater than the grinding action of ice overriding the upstream side, so that the downstream slopes are commonly precipitous.

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