Devils Postpile

Devils Postpile is a formation of columnar basalt in Devils Postpile National Monument near Mammoth Lakes, California, formed roughly 100,000 years ago when a thick flow of basaltic lava pooled within a valley and cooled slowly and evenly. As it contracted, the lava cracked into tall, mostly hexagonal columns as much as 18 meters high, unusually symmetrical and uniform even by the standards of columnar basalt. During the last ice age, a glacier moved across the top of the formation and planed and polished the broken tops of the columns, exposing a smooth, tightly fitted mosaic of hexagonal cross-sections on its upper surface, a feature that is unusually well preserved and visible to visitors walking across the top of the formation.