Devils Postpile National Monument. In June we visited Mammoth, California gateway to Devils Postpile. I had always wanted to see this place as the basalt columns had fascinated me. Having seen Devils Tower in Wyoming in 1998 I expected a similar experience. As the postpile is smaller (60 to 100 feet) it wasn't quite the same. However, the valley in which the postpile is located, east of Yosemite, is very lovely.