Pinnacles National Monument

A myriad of spires, rocky crags and pinnacles, eroded remnants of early Miocene volcanic activity, makes this one of the most unusual landscapes in the State.

Lava flows, primarily of rhyolite but later including some andesitic and basaltic magmas, piled up to a considerable thickness. During the later stages, the lava became quite viscous, plugging the main fissure and leaving only five or so north-south trending vents from which solid and liquid material spewed forth. Eventually a large, steep-sided, elongated volcanic dome, perhaps 2,400 meters (8,000 feet) high, was formed. These vents can be identified, as the explosions filled them with layered masses of rhyolitic tuff in contrast with the surrounding massive rhyolite. These vent fillings have been carved by erosion into conical pinnacles.

The lava flows and the greater thickness of erupted material were cemented into volcanic breccias in which the main erosional features were sculptured after the rock had been broken by a great number of prominent joints. Weathering along these fractures has further enlarged the joints, creating a complex of unusual land forms. Further, disposition of water-borne silica hardened some of the breccias causing them to become more erosion-resistant than the softer, adjacent rocks.

Talus caves are present on both branches of Chalone Creek, the only permanent waterflow in the Monument. By scouring the less resistant rock, the creek excavations have been so extensive in places that they have undermined the rock, causing large masses to slump down.

The Pinnacles were formed 304 kilometers (190 miles) to the south, southeast in an area bisected by the San Andreas Fault. Lying west of the fault, the Pinnacles have moved northward. The rest of the formation, the Neenach Formation, has remained in place and the volcanic rocks are buried under smoothly rounded hills.

The dominant vegetation in the Monument is chaparral, with chamise, Adenostoma fasciculatum, buck brush, Ceanothus cuneatus, toyon, Heteromeles arbutifolia, and mountain mahogany, Cercocarpus betuloides, the dominants. Some foothill woodland is found here, and along the creeks there is the typical Coast Range riparian association. A rare buckwheat, Eriogonum nortonii, has been reported from several localities in the Monument.

Animal life is typical of the South Coast Range.

Integrity: Protected since 1906, the Pinnacles were designated a National Monument in 1908. Fire suppression has created a dense growth of chaparral that yearly becomes more inadequate as browse and cover for animals adapted to the habitat. There are three campgrounds, some 29 kilometers (18 miles) of trails, 8 kilometers (5 miles) of roads, and various service buildings and areas.

Use: Research, educational, observational, light recreational.

Ref: Andrews, Philip. 1936. Geology of the Pinnacles National Monument. University of California Dept. Geol. Sci. Bull. No. 24, pp. 1-38.
Webb, Ralph C. 1969. Natural History of the Pinnacles National Monument, San Benito County, Calif. Pinnacles Natural History Association, pp. 1-72.

October 1975