fewer than 100,000 years ago basalt lava erupted two miles upstream from today's postpile. as the lava cooled it contracted and cracked, forming the 60 foot high columns of devils postpile. the vertical and hexagonal columns formed because nature provided ideal conditions -- a slower cooling time and a consistent mineral composition in the lava -- resulting in the symmetry of the lava flow.

beautiful parkland at mammoth mountain. more at nps.gov and google images.