13 Rocks You Won't Believe Aren't Man-Made!

StellarSpindle Updated: 2025-07-05

8. Devils Postpile National Monument

Rising in the Sierra Nevada mountains of eastern California, the geological marvel known as Devil's Postpile National Monument highlights the amazing columnar jointing process in volcanic rocks. With studies showing that this amazing deposit formed less than 100,000 years ago, this old appearance belies comparatively modern geology. Rising to heights of up to 60 feet, the unique hexagonal columns of the monument provide guests with a rare view of the tremendous geological processes sculpting our globe.
The tale of Devils Postpile starts with Pleistocene era volcanic activity in the area. A huge lava flow from a volcano close to the Upper San Joaquin River inundated the valley. Rich in iron and magnesium, this basaltic lava flowed faster than most varieties of lava, forming a lava lake almost 400 feet deep in what is now Reds Meadow Valley. Important elements in the development of the unique columnar structure observed today include the fast flow and high iron concentration of the lava.
The lava lake cooled and solidified at varying rates as well. First the lake's shallow sections, exposed to air, toughened. The contraction of the solid lava brought about by this cooling process stressed the rock body. The cooling lava created a network of fractures, or joints, to help to release this tension. Usually forming in a hexagonal pattern, these joints let the rock relieve strain most effectively via their design. Deepening cracks throughout time produced the tall, multi-sided columns that define Devils Postpile today.
Devil's Postpile developed during an uneven cooling process across the lava lake. Variations in lava composition and cooling speeds produced varyingly sized and shaped columns. Though most of the columns are hexagonal, others have four, five, or even seven sides. This diversity gives geologists important information about the conditions under which the lava cooled and accentuates the visual appeal of the formation.
Further geological and climatic occurrences have resulted in the present condition of Devils Postpile. The columnar structure was revealed in great part by glaciers that passed through the valley during the last ice age. These huge ice sheets destroyed the surrounding rock as they moved down the valley, finally revealing the more durable basalt columns. The passing ice polished the top of the formation, producing a level surface for visitors to now walk on and view the column cross-sections from above.
President William Howard Taft created the Devils Postpile National Monument in 1911 to guard this unusual geological formation from commercial exploitation. Plans to blast the structure to build a dam for hydroelectric electricity have threatened the area. Being a national monument guaranteed the preservation of not just the basalt columns but also the surrounding ecology, which comprises of immaculate alpine meadows, woods, and a section of the San Joaquin River.
Devils Postpile National Monument draws geologists, environmentalists, and inquisitive tourists from all around today. The site presents a unique chance for grand scale observation and investigation of columnar jointing. Though similar structures abound elsewhere, like Fingal's Cave in Scotland and the Giant's Causeway in Northern Ireland, Devil's Postpile is still among the most striking and easily accessible specimens of this geological occurrence in North America.