Devils Postpile: Columns of Cooled Lava
Near Mammoth Lakes, California
In front of you rises one of the strangest rock formations in the Sierra Nevada, a wall of tall stone columns packed together like a bundle of pencils stood on end. This is Devils Postpile. Somewhere between eighty thousand and one hundred thousand years ago, molten lava flowed down this valley and pooled here to an unusual depth, perhaps four hundred feet thick.
Because the lava was so deep and so uniform, it cooled slowly and evenly. As it cooled it shrank, and the shrinking cracked the rock into tall columns, many of them with six sides, meeting at angles of about one hundred twenty degrees, the same geometry you see in dried mud or cracked pavement. Later, glaciers ground down the valley and sheared off the tops of the columns.
If you hike to the top, you can see the flat, polished column ends gleaming like a tile floor, scratched by the passing ice. In nineteen ten a water company applied to blast these columns into rubble for a dam. The naturalist John Muir and his allies at the Sierra Club fought the plan, and in nineteen eleven President William Howard Taft signed the proclamation that made this a national monument, protecting it forever.
