GEORGE POST / SCIENCE PHOTO LIBRARY GEORGE POST / SCIENCE PHOTO LIBRARY
Devil's Postpile National Monument. The flattened tops of polygonal basalt columns are seen here. Basalt is an igneous rock formed when lava cools in the air. It forms pentagonal or hexagonal prisms when it cools. The tops of the columns have been smoothed by glacial action. Photographed near Mammoth, California, USA.
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