Devils Postpile National Monument

The Devils Postpile is the best example of columnar jointing in the State. The Postpile is composed of andesite which had its primary source in the Mammoth Mountain-Mammoth Pass area some 5 kilometers (3 miles) to the east, though there probably were smaller nearby vents. Volcanic eruptions in the area had occurred in the late Pliocene, but the andesite of the Post-pile is considerably younger, probably about 600,000 years old, though possibly it is of even more recent origin.

Within the Monument, three jointing types are found. The most common is the orthogonal type which gives outcrops a blocky appearance. Near Rainbow Falls there are horizontal platy joints. Least common is the non-orthogonal polygonal jointing of the Postpile.

Most of the columns, which may reach 20 meters (60 feet) in length, are hexagonal or pentagonal, the former occurring in slightly greater numbers. About 15% show other forms. Cracking probably occurred more or less instantaneously when the uniformly cooling mass reached the jointing stress point. Many of the columns are tilted or curved and this may be due to the irregularities of the isotherms during the cooling, caused, in part, by irregularities in the underlying bedrock.

Evidence of glaciation is strikingly shown by the glacial striation and remnants of glacial polish atop the columns. The latter is beginning to peel away by exfoliation.

Vegetation is primarily a coniferous forest with red fir, Abies magnifica, Western white pine, Pinus monticola, lodgepole pine, Pinus contorta ssp. murrayana, and mountain hemlock, Tsuga mertensiana.

Integrity: There are trails and other facilities within the Monument, but the geological features are virtually undisturbed.

Use: Research, educational, observational, present.

Ref: Huber, N. K. and C. D. Rinehart, 1967. Cenozoic Volcanic Rocks of the Devils Postpile Quadrangle, Eastern Sierra Nevada, California. U.S. Geo. Sur. Prof. Paper No. 554-D, 21 pp.

February 1978