University of California Press Geology of the Pinnacles National Monument

GEOLOGY OF THE PINNACLES NATIONAL MONUMENT
BY
PHILIP ANDREWS

THE PINNACLES NATIONAL MONUMENT was first set aside as one of the nation's playgrounds by proclamation of President Theodore Roosevelt on January 16, 1908. Additional land grants in the years 1923 and 1924 increased the Park's area to a total of 2980 acres. In 1931, San Benito County purchased additional land from private holders for park use which brought the total area of the Monument to 4609 acres, and this area was practically doubled in July, 1933, by the acquisition of the Chalone Peaks. There still remain 160 acres of privately owned land in the northern part of this tract.

A ranger station is maintained at the Pinnacles Monument and camping facilities, guide service, entrance and other privileges are provided free of charge. Cabins and meals may be had at nominal prices.

Since wild life has been protected by State laws as well as by Federal authority, black-tailed deer have taken advantage of the refuge and are frequently seen in rather large bands. Quail are also numerous, especially along stream courses. More than forty-four species of wild birds have been observed in a single day and more than two hundred species of wild flowers¹ find their natural habitat within the boundaries of the Monument. Besides chamiso, scrub oak, and manzanita, occurring as a thick chaparral on many of the dry hillsides, there are also sycamore, live oak, willows, and alders in the better-watered areas. Yellow pines are sprinkled over the elevations and digger pines occur in considerable numbers. The Pinnacles region is important as one of the last strongholds and breeding places of the California condor, the largest and one of the most characteristic birds of the State.²

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¹Schilling, Hugh, Pinnacles ranger, communication.

²U. S. Dept. Interior, Glimpses of Our National Monuments, 1930.

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Through the construction of new roads, the Pinnacles are now easily accessible, and in the year 1932 about fifteen thousand tourists enjoyed their natural beauty and grandeur. Excellent trails have been constructed over Hawkins' Peak, making the heart of the Pinnacles readily accessible to hikers.

PLATE 1.—Aerial view of the Pinnacles, looking toward the east.

The unusual scenic beauty of the Pinnacles was recognized as long ago as 1794, when Captain George Vancouver visited them and wrote about them. David Starr Jordan visited the area on numerous occasions before it was set aside for public use, and he was instrumental in its selection as a national monument. Tiburcio Vasquez, a daring and notorious bandit, is credited with finding refuge among the caves and crags of the Pinnacles in the latter part of the nineteenth century, before he was finally brought to justice.

The purpose of the survey for this report was to discover the nature and origin of the rock formations composing the Pinnacles. Field work was pursued during intervals of the winter of 1932-1933 and spring of 1933. To facilitate an understanding of the geology of the Monument proper, a surrounding area of approximately fifty square miles was studied in considerable detail. The location of the area may be determined from the index map (fig. 1, below), but may be briefly stated as 135 miles south of San Francisco and Oakland in the Gavilan³ Range, between the Salinas and San Benito valleys. It lies 35 miles south of Hollister and about the same distance north of King City. At present, it is best reached from the east side of the range, through Bear Valley.

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³Modern Spanish orthography, as well as early literature, makes this spelling, the Spanish word for Sparrow Hawk, preferable to "Gabilan," which is often used.

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Fig. 1. Index map of part of California, showing the location of the Pinnacles National Monument.

True to its name, the Pinnacles National Monument is an area rich in rock spires, crags, and other points of sharp relief. From elevations of less than 1000 feet in Chalone Valley, one passes rapidly to an elevation of 3287 feet at the summit of North Chalone Peak. Hawkins' Peak, which is composed largely of "pinnacles," is more than 2600 feet in elevation. These characteristic features are the result of erosion of fragmental volcanic rocks.

Vertical jointing has assisted in producing spires by opening the massive beds of breccia to the action of erosional agencies, and local silicification has promoted differential weathering. In many places, the rhyolitic lavas are so shattered that they behave like the fragmental volcanics in forming pinnacles.

Atmospheric agencies acting upon the breccias tend to harden the fine matrix into durable cement so that the upper, more exposed parts of the surface become more resistant. Superficially, many of the rocks are pinkish, owing to the iron oxide content, although the apparent color of the rocks at a distance is controlled in large part by the firmly adhering lichens, which vary in shades of yellow, red, green, and brown.

Fig. 2. Balanced rock (in the southwest of section 27 and one-half mile north of the Pinnacles). Erosion has removed an intermediate bed of volcanic ash faster than the volcanic breccia which occurs both above and below.

Natural caves of rather large size occur along both branches of Chalone Creek where streams have cut under the massive beds of fragmental volcanic rocks. Stream action has not only carried away the softer material of these beds to form part of the natural cavities, but, even more important, it has removed material which supported the overlying rocks and permitted slumping of huge blocks. A short distance west of the Pinnacles Ranger Station, a subterranean room, nearly 100 feet across and in total darkness except when illuminated by artificial light, is formed by a single massive block of cemented breccia, supported by minor blocks about the edges. Rocks have also rolled downhill into the sharp canyon, thus assisting in the formation of these caves.

Streams are intermittent with the exception of a part of Chalone Creek, and carry water only after rainstorms. All drainage eventually reaches the Salinas Valley, either by way of Chalone Creek or by a more direct route.

The arid climate tends to preserve the Pinnacles and prevent a thick mantle of soil. Springtime at this National Monument is its most attractive season, owing to equitable temperatures and blooming flowers, but winters are mild and summers are not unbearably hot, so that tourists find it an attractive retreat throughout the year.

Special thanks are due to Dr. G. D. Louderback, who first suggested the problem of the Pinnacles National Monument and who also made valuable suggestions and criticisms. Dr. Charles A. Anderson kindly permitted access to his manuscript on the Tuscan Formation. I am also indebted to Professor N. E. A. Hinds, Professor N. L. Taliaferro, the late Ranger Hugh Schilling, the Hollister Chamber of Commerce, the Federal Park Service, and ranchers in the vicinity, who have all shown much interest in the work and have cooperated in every way.

The first published mention of the Pinnacles was probably made by the explorer, Captain George Vancouver.⁴ While his three small sailing vessels were being repaired and reprovisioned in the port of Monterey, he made a mule trip into the interior with inhabitants of the mission, and was much impressed by an "unusual mountain." On Wednesday November 19, 1794, he entered a detailed description of this "unusual mountain" in his diary:

On one side it presented the appearance of a sumptuous edifice fallen into decay; the columns, which looked as if they had been raised with much labour and industry, were of great magnitude, seemed to be of elegant form, and to be composed of the same cream colored stone, of which I have before made mention.

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⁴Vancouver, Captain George, A Voyage of Discovery to the North Pacific Ocean and Around the World, vol. 3, p. 334, 1794.

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Whitney⁵discusses the Gavilan Range and mentions "Chelone" Peak, but says nothing either about volcanic rocks at the south end of the range or about the Pinnacles.

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⁵California Second Geological Survey, vol. 1, pp. 159-160, 1865.

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Fairbanks made a reconnaissance of the Pinnacles region in 1894, and writes:

The Chalone Peaks form the southern terminations of the Gavilan range. The south peak lies at the western extremity of a body of both massive and banded liparite which stretches north for about 6 miles with a width of 2-1/2 miles. . . . Altho the whole body seems to belong to the acid series, yet there is a great variety of crystallization products. Obsidian is abundant. . . . This liparite is particularly interesting on account of containing gold, apparently as an original constituent. There are no signs of a crater, for the amount of erosion since the flow took place must have been enormous. . . . An immense mass of volcanic tufa stretches along the western side of the solid lavas. . . . The tufa dips west at an angle varying from 30 to 60 degrees. The northern portion has been eroded into deep cañons and high jagged pinnacles, rising hundreds of feet. . . . The total thickness of the tufas cannot be less than 6000 feet. They are made up wholly, except at the base, where some granitic gravels are mixed with the volcanic, of more or less rounded bowlders of the lava cemented by sand of the same material.⁶

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⁶Fairbanks, H. W., 12th Ann. Rept. of the State Mineralogist, California, 1894.

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Kerr and Schenck⁷ considered the lavas in the vicinity of Willow Creek, 6 miles north of the Pinnacles, to be a continuation of the rhyolite that occurs typically in the Pinnacles National Monument.

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⁷Kerr, P. F., and Schenck, H. G., Geol. Soc. America, Bull., vol. 36, pp. 465-494, 1925.

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Fig. 3. Pinnacles: erosional features in volcanic breccia on the north side of Hawkins' Peak.