Postpile is a compound word. Think post, then pile—a pile of vertical posts. Only here, instead of wood, the posts are hundreds of columns of cooled basalt lava. Perhaps the best example is in Devils Postpile National Monument in California (on early maps, it still shows up as Devils Woodpile, the name the U.S. Cavalry buffalo soldiers patrolling Yosemite gave it, as had the Basque sheepherders before them), where the cooling conditions for a four-hundred-foot-deep lava flow were ideal enough that the cracks on the surface—the columnar jointing resulting from stresses of cooling—were able to branch down, forming long, delicate columns, or posts, each one having from three to seven sides. While all lava flows cool and crack, not much cools so regularly from top to bottom as the flow that became Devils Postpile. Seen from the side, where the formation was sheared off by a glacier and the columns revealed, the terms postpile and woodpile make sense. Standing on top of it, however, Devils Postpile looks like nothing so much as a polished tile floor.

Stephen Graham Jones