Devils Postpile National Monument, Est. 1911, 798 acres (3.23 km2). Devils Postpile, a cliff of long, symmetric columnar basalt created by a lava flow sometime between less than 100,000 to 700,000 years ago (depending on dating method) and exposed through glacier movement. Columns average 2 feet (0.61 m) in diameter, largest being 3.5 feet (1.1 m), many are up to 60 feet (18 m) long. A survey of 400 columns found 44.5% were 6-sided, 37.5% 5-sided, 9.5% 4-sided, 8.0% 7-sided, and 0.5% 3-sided. Inyo County, CA.