Devil's Postpile National Monument preserves one of the finest examples of columnar basalt in the world. A lava flow that flooded the San Joaquin River valley roughly 100,000 years ago cooled so uniformly that it cracked into near-perfect hexagonal columns, some reaching 60 feet in height. The formation is a short walk from the shuttle stop, and the scale of it — a solid wall of geometric stone — is genuinely arresting.
The top of the Postpile is equally worth seeing: glacial action polished the column tops flat, creating a tiled floor effect that shows the hexagonal cross-sections clearly. From the monument, the trail continues to Rainbow Falls, a 101-foot drop into a red-rock canyon where sunlight refracts into a rainbow in the mist. In summer, the monument is accessible by shuttle from the Mammoth Mountain Inn — private vehicles are not permitted on the access road during peak season.