Devil's Postile is located in Madera County, California. However, it can only be accessed by hiking through the Sierra Nevadas or through Mammoth Lakes, California. This interesting formation of rocks is created by lava flows between 100,000-700,000 years ago (depending which source you look at) which cooled slowly leaving long, smooth, symmetrical columns. The top of the Postpile is smooth, similar to tile, due to glacial movement. Each column is roughly about 2-3 feet in diameter and might be as long as 60 feet in length. Most the columns have a hexagonal shape.
http://en.wikipedia.org/wiki/Devils_Postpile_National_Monument