In an article titled Good Day at Black Rock: 10 Cool Columnar Basalt Formations Author Steve writes about some of the most amazing columnar basalt formations, which he notes are created by lava cooling and cracking in vertical angles. One of the largest of these formations is Devils Tower in Wyoming that stands 1,267 feet tall. Another formation in Scotland called Fingal’s Cave is a large island made of this molten rock that has a 279 feet deep cave. One such formation is located right here near Mammoth, California. Called the Devil’s postpile, this collection of hexagon shaped lava rocks is thought to be around 100,000 and 700,000 years old. A 600,000 year time gap does not narrow down how old these rocks are but I can take their word for it that these rocks are old. Although many of these rocks are great tourist spots for the kids to climb and people to take pictures, but they are also prime rock climbing destinations.

http://webecoist.com/2010/12/07/good-day-at-black-rock-10-cool-columnar-basalt-formations/
http://www.ram.org/ramblings/other/cctrip.html