The Devils Postpile is a striking geological formation in the Sierra Nevada mountains of California, characterized by a sheer wall of dark, multi-sided columns and found within the Middle Fork of the San Joaquin River valley. This unique structure is a textbook example of columnar basalt. Its formation resulted from a precise sequence of volcanic events, a specific cooling process, and subsequent glacial action. Understanding this symmetrical monument requires examining the geological timeline, starting with molten rock and ending with the abrasive power of ice.
The Initial Volcanic Eruption and Lava Flow
The geological genesis of the Devils Postpile began approximately 80,000 to 100,000 years ago with a massive volcanic event. A vent near the present-day monument spewed a large volume of highly fluid basaltic lava. Basalt is rich in iron and magnesium, making it less viscous than other lava types, which allowed it to flow rapidly across the landscape.
The molten rock poured into the valley of the Middle Fork of the San Joaquin River, moving downstream for at least five kilometers. This lava was eventually blocked by a natural topographic obstruction, likely a glacial moraine, causing it to pool and create a massive, insulated lava lake.
The thickness of this lava pool reached depths of up to 122 meters (400 feet) in some areas. This volume ensured the interior remained hot and insulated for a prolonged period, setting the stage for the slow, uniform cooling process required for the postpile’s unique structure.
The Unique Cooling Process Creating Columnar Basalt
Once the flow had stopped, the deep pool of basaltic lava began the slow process of solidification, a phase known as columnar jointing. Cooling occurred from the top surface exposed to the air, the sides in contact with the valley walls, and the bottom resting on cooler bedrock. As the lava lost heat and changed from a liquid to a solid state, it underwent significant thermal contraction.
This shrinkage generated immense tensile stresses within the newly formed, solid rock. To relieve this internal stress, the rock fractured, with the cracks propagating inward and downward from the cooling surfaces. The pattern of these fractures is determined by the most efficient way to release the accumulated strain, which results in a polygonal geometry.
The most energy-efficient pattern for stress relief is a network where cracks meet at 120-degree angles, which produces a six-sided, or hexagonal, column. While the columns at the Postpile exhibit a range of sides, the formation is recognized because an unusually high percentage, about 55%, are perfect hexagons. The impressive length of the columns, reaching up to 18 meters (60 feet), is a direct consequence of the lava’s remarkable depth and the slow, uniform rate of cooling and fracture propagation.
Glacial Sculpting and Exposure
Long after the columns had solidified, the final stage of the monument’s formation began with the arrival of massive alpine glaciers during the Pleistocene Ice Age. Beginning around 10,000 to 12,000 years ago, a river of ice flowed through the valley, acting as a powerful agent of erosion. The moving glacier easily stripped away the surrounding, less-resistant rock and sediment.
This abrasive action excavated much of the original lava flow, exposing the hard basalt columns hidden within the interior of the solidified mass. The glacier cut a vertical face into the formation, revealing the sheer wall of columns that stands today.
The top surface of the Postpile bears the clearest evidence of this glacial activity. Fine silt and sand embedded in the moving ice functioned like a massive natural scouring pad, polishing the column tops to a smooth, flat surface known as glacial polish. Coarser debris dragged across the surface, leaving behind parallel grooves, or striations, which show the direction of the ice flow.