Burro Mountain

In this region there is a relatively small, 3-square-kilometer body of unusually fresh ultramafic rock. The body is composed of an alpine-type peridotite, primarily harzburgite and dunite, which has been nearly completely serpentinized around the edges. This serpentinization decreases toward the center of the body, where the freshest rock is found.

The shell of the body is a sheared serpentine zone, 10 - 15 meters (30 - 50 feet) thick. The shearing appears to have been caused by the emplacement of the body as a cold, solid mass into the surrounding Franciscan melange; thus the contacts are of a "fault" type rather than igneous. Los Burros Creek cuts through the body and here in the gorge is the best exposure of any ultramafic body in the State. Many unusual features of the body are clearly visible, including ghost banding, various contacts, dikes, and other structures within the peridotite.

Vegetation in the area includes chaparral, mixed conifer, and riparian. The rare Galium hardhamae is found in the vicinity.

Integrity: Pristine

Use: Education, research, observational.

Ref: Loney, R. A. et al, 1971. Structure and Petrology of the Alpine Type Peridotite at Burro Mountain, California. Jour, of Petrology 12 (2), pp. 245-309.
Burch, Stephen H. 1968. Tectonic Emplacement of the Burro Mountain Ultramafic Body, Santa Lucia Range, California. Geol. Soc. Amer. Bull. 79, pp. 527-544.

January 1976