Devils Postpile National Monument is a volcanic formation in the Sierra Nevada Mountains of California, near Mammoth Lakes. Viewed from the side
the Postpile appears as an outcropping of dark, tightly packed vertical or angled columns, some 60 feet high; from above, the rock, worn flat by glaciers in
places, bears networks of cracks forming hexagons, pentagons, and other polygons averaging about 2 feet in diameter. At the base of the columns is a
talus slope consisting of many pieces of the columns which have broken off over time.
The Postpile is a fine example of columnar jointing in a basaltic lava flow. The mass of lava that became the Postpile was unusually uniform in composition and was deposited under conditions which allowed it to cool slowly and evenly. The rock contracted as it cooled and the cracks formed incrementally to relieve the resulting stress. Since the rock was so homogenous the cracks formed along
geometrically efficient lines.
Examples of columnar jointing of lava flows can be found around the world, notably at Devils Tower National Monument in Wyoming, Giant's Causeway in Ireland, and Kirkjubaejarklaustur in southern Iceland. The common attribution of these formations to the
handiwork of mythical beings, as reflected in their popular names, is quite understandable.
Thanks to: "Geology Underfoot in Death Valley and Owens Valley" by Robert P. Sharp and Allen F. Glazner, Mountain Press Publishing Co., 1997 | 
