University of California Press Geology of the Pinnacles National Monument

The oldest rocks of the Pinnacles region are the Gavilan limestone and other sediments of the Sur series which were intruded and metamorphosed to marble, schist, and quartzite by the Santa Lucia granite and quartz diorite.

Elevation of the Gavilan Range started erosion which almost completely unroofed the batholith in the southern end of the range, leaving only sporadic masses of metamorphosed sediments as roof pendants. Erosion in the northern part of the Gavilan Range was much less. Lack of both Cretaceous and early Tertiary sediments in the Pinnacles area as well as the almost complete removal of the Sur series from the batholith indicates a prevalence of land conditions. If Cretaceous or early Tertiary sediments ever existed, they have been removed by subsequent erosion.

The granitic surface under the central body of volcanic rocks is not open to inspection but probably has no major irregularities. The granite surface along the fault to the east of the Pinnacles corresponds generally with the old peneplained surface to the east of Chalone Creek, and although this fault has but moderate displacement, the relationship between volcanics and granite does not change.

Remnants of the old peneplain to the east of Chalone Creek are easily recognized. The nearly plane, though dissected, surface of from 1500 to 1700 feet elevation dips gently to the south and may in large part have been developed in pre-Miocene time, so would also underlie the Pinnacles. Blocks of rhyolite, agglomerate, and volcanic breccia are thickly strewn over this old surface although it is at present separated from the parent volcanic mass by Chalone Valley, which has cut out nearly 1000 feet below this old surface. The presence of these fragments strengthens the opinion that pyroclastic deposits once existed east of the Pinnacles but have since been removed by erosion.

Although less striking, an old surface can be recognized to the west of the Pinnacles which was no doubt a part of the old surface to the east. The western part of the peneplain has, at present, a slightly superior elevation.

In Miocene time, probably Vaqueros or early Temblor, thin beds of well-rounded pebbles and arkosic gravels were deposited along a zone which now lies a little west of Chalone Creek. Somewhat later, rhyolite was injected into tension cracks of the granitic mass. These cracks may have been opened by diastrophic movements at the close of the Vaqueros.⁴⁶

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⁴⁶Reed, R. D., Jour. Geol., vol. 33, no. 5, p. 607, 1925, notes diastrophic movements at the close of the Vaqueros, Monterey, Pancho Rico, and Paso Robles.

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Although most of these fissures were narrow, several must have been of rather large size to permit the outpouring of several cubic miles of viscous, rhyolitic magma, which formed an elongated dome of several thousand feet elevation. After the first lava effusions, explosions occurred from a number of centers and showered vast quantities of fragmental material down the slopes. These explosive eruptions alternated with numerous quiet flows which added to the volume and helped to weld the fragmental eruptives into a solid mass.

Owing to the superior elevation of the volcanic mass, erosion was rapid and large blocks of rhyolite were transported with granitic material, to be deposited as fanglomerates. While these breccias and fanglomerates were collecting close to the land mass, the sea to the east was receiving arkosic gravels in large quantity from the Gavilan Range. These were conformably covered and occasionally interbedded with diatomaceous shales.

Displacement of the volcanic rocks along the faults which bound the Pinnacles area gives evidence of movement since the period of volcanic activity. That this movement closely followed the last eruptions seems probable, when one considers the almost complete removal of volcanic ejecta outside the area bounded by the Pinnacles and Chalone Creek faults.

Gradual settling of the area east of the Chalone Creek fault during deposition of the land-laid deposits (Temblor) seems necessary to account for their vast thickness. It seems probable that the major movement along the Chalone Creek fault took place in Temblor time, with the downthrow side to the east. Settling of the Pinnacles volcanic block as a unit took place a little later.

Pliocene records are no longer recognizable, but Pleistocene history is more definite. The central volcanic mass was above sea level and was of rather deep relief. Angular talus material was carried out and deposited about its periphery, much as we find material being deposited today in arid regions subject to short, sharp storms.

Post-Pleistocene elevation⁴⁷ rejuvenated the region and caused active stream cutting which is still going on. It has dissected these earlier terraces and deeply cut the underlying rocks. This rejuvenation has caused the formation of minor "badlands" in the soft Miocene deposits to the east of the area.

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⁴⁷Lawson, A. C., Post-Pliocene diastrophism of the coast of Southern California, Univ. Calif. Bull. Dept. Geol., vol. 1, no. 4, 1893.

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Robin Willis, in describing the physiographic correlation of the region, notes that

On the east side of the Gabilan Range, near its southern end, there is a valley of mature aspect, developed in the soft Paso Robles gravels below the general level of the second cycle valleys. It is known as Bear Valley. . . . It must belong to the recent cycle because of its position, but there is no stream in it now which could have cut it since this cycle was inaugurated. This is the old course of the San Benito River across the range.⁴⁸

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⁴⁸Willis, Robin, Geol. Soc. America, Bull., vol. 36, p. 674, 1925.

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As previously mentioned, these gravels are Temblor rather than Paso Robles in age, and observations on the rapidity with which these deposits are eroded, especially during periods of storm, would make it seem unnecessary to postulate more than the present drainage area for carving out Bear Valley.

Various attempts at mining have been undertaken, the most extensive of which are those of the Copper Mountain Mining Company. This company opened several exploratory adits in the southern part of section 33 and 1 mile west of the Pinnacles, but never succeeded in developing a mine. A few sulphides appear in the dikes of this area but deposits of economic importance are probably lacking.

Although numerous small prospects have been opened along Miners Gulch, nothing of importance has yet been found. Old residents report a small amount of gold recovered from placer mining some years ago in the gravels of the stream bed.

Iron oxides are common along fissures, in both rhyolite and granite. The mineralization is younger than the rhyolites. Gold assays of these mineralized zones show values ranging from $1 to $2 a ton, but because of their limited extent and low value, profitable mining can hardly be pursued.

Seepages of heavy oil occur 5 miles to the southeast of the Pinnacles in Miocene sediments. Drilling in the vicinity of these seepages has failed to produce oil in economic quantities, partly through lack of favorable structural conditions.

The value of the Pinnacles National Monument as one of the nation's play grounds cannot be calculated in dollars and cents, but the region promises to hold its fascination for the visitor, just as it stirred the enthusiasm and imagination of Captain Vancouver nearly a century and a half ago.

PLATE 2.—Aerial view in the direction of Chalone Mountain, looking toward the north. The belt of west-dipping breccias and tuffs of the Pinnacles formation are seen on the left, and the more massive volcanics on the right.

[NOTE: After this paper was in press, there were received through the kindness of the National Park Service aerial photographs of the Pinnacles region which were of such extraordinary illustrative value that it was decided to add them as frontispiece and final plate, in the absence of the author now in the interior of South America.

As it has not been possible to reach the author, the paper is published without the benefit of author's corrections.—G.D.L.]