University of California Press Geology of the Pinnacles National Monument

The oldest rocks in the region belong to the Sur series,⁸ of which the Gavilan Limestone⁹ is a member. These ancient sediments are often called the Santa Lucia series,¹⁰ together with the quartz diorite and granite which intruded and metamorphosed them. Both are pre-Franciscan.

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⁸Trask, P. D., Univ. of Calif. Publ., Bull. Dept. Geol. Sci., vol. 16, p. 127, 1926.

⁹Becker, G. F., U. S. Geol. Surv., Monograph 13, p. 181, 1888.

¹⁰Willis, B., Science, n. s., vol. 11, p. 221, 1900.

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The absence of Cretaceous and early Tertiary sediments in this part of the Gavilan Range indicates that the area was either a land surface during this period or that subsequent erosion has removed any such deposits. Furthermore, the granite was almost completely unroofed before Miocene time. In an epoch considered to be early middle Miocene, rhyolitic magma was forced through fissures in the granitic mass. Later activity developed central vents along a zone trending north and south, and explosions from these built up a vast thickness of pyroclastics above the earlier lavas. The action of erosion on these pyroclastics has given rise to the characteristic "pinnacles." As Miocene time progressed immense deposits of fanglomerate composed of both rhyolitic and granitic detritus, were built up along the flanks of the Gavilan Range. Farther from the source, the fanglomerates graded laterally into continental and shallow marine arkosic gravels which were in part interbedded with and overlain by diatomaceous shales of considerable thickness The rather abrupt change from gravel to diatomaceous shale suggests a lowering of the land mass and consequent reduction in the amount of detritus entering the sea. The volcanic rocks of the Pinnacles were probably never submerged.

Fig. 4. View north toward South Chalone Peak, showing the central mass of volcanic rocks superimposed upon the Santa Lucia quartz diorite and granite. Pleistocene terrace deposits are visible in the left center of the picture.

Pliocene sediments are absent from the area, but in a late epoch (? Pleistocene) terrace gravels were deposited about the edges of the central volcanic mass. Subsequent readjustments, with elevation at the end of Pleistocene time, have caused these terraces to be dissected, an action still proceeding.

The area lies only 5 miles to the southwest of the great San Andreas fault, and it is therefore not surprising that minor and roughly parallel faulting, showing considerable movement since Monterey time, occurs within this region.