
Exposed ends of columns on top of the Postpile.
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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. |