Columnar basalt FTW! Near Mammoth Lakes CA.

due to a volcanic eruption 30k years ago which pooled in a natural basin. When it cooled it formed cracks to relieve the stresses and they formed vertically, then at the ends, they stretched out at natural 130 degree angles.  eventually these cracks liked to each other forming many hundreds of perfect hexagons and pentagons.  these then were unseen until the last ice age scraped off the top of the volcanic rock with embedded granite and other rock forming a specially named glacial etching pattern on the tops of the remaining columnar obsidian. 

It's really a really awesome place to visit.  $9 per person for the ride, then .4 mile hike to the base, another .2 miles up and then you can go to a waterfall farther down the trail about 2.1 miles.  I live 3 hours from mammoth (and 2 from L.A.) it's about 35 minutes on the bus to the trail head from mammoth village.