University of California Press University of California Press
Geology of the Pinnacles National Monument


Unmetamorphosed epiclastic sediments of the area mapped include:

1. Gravel of undetermined age, resting upon the granitic complex
2. Fanglomerate and gravel deposits of pre-Monterey age (Temblor)
3. Terrace deposits of probable Pleistocene age
4. Recent sands, gravel, and alluvium

The gravel, resting upon the granitic complex, and underlying the volcanic rocks, is exposed in a small area 2 miles due east of North Chalone Peak along the granite-rhyolite contact. Well-rounded quartz pebbles are also found at several points along this contact at the extreme north as well as along the rhyolite-granite contact at the extreme northwest corner of the National Monument.

Fig. 5. Arkosic gravels, dipping 10 degrees to the southeast (in the northeast corner of section 7 and east of Chalone Creek). These beds conformably underlie diatomaceous shales and are upper Temblor in age. Fig. 6. View of a composite dike (east of Chalone Creek, in the north center of section 18), showing, in the white band just below the hammer, a fragment of rhyolite which has been included in later dacite.

East of Chalone Peaks is found the only important exposure of these basal conglomerates. The outcrop shows a massively bedded deposit at least 75 feet in thickness, composed of partly consolidated, coarse, arkosic gravel, with a liberal sprinkling of well-rounded quartz, granitic, and some other igneous pebbles, averaging from 1 to 2 inches in diameter. The gravel was apparently laid down in a depression or valley on the old granite surface. Bedding planes dip 30 degrees toward the northwest, but this inclination probably results from faulting. The arkose is composed of angular, white quartz, glassy feldspar, and biotite flakes clearly of granitic origin, without long transportation or any great amount of weathering.

Lack of fossils makes age determination impossible, but the absence of typical Pinnacles rhyolite fragments points toward deposition during pre-volcanic times. Conglomeratic beds are found in the Vaqueros of Salinas Valley concerning which Reed notes that they are composed largely of granitic sand and conglomerate with pebbles of igneous rocks.17

17Reed, R. D., Geology of California, p. 166, 1933.

Trask also notes18 conglomeratic sands in the Temblor of the Point Sur Quadrangle. Granitic conglomerate cannot be used as a definite means of correlation, although the occurrence of these beds beneath the volcanic rocks places them earlier than upper Temblor. The time interval between the volcanic rocks and these pebbly arkose beds was probably not great and it seems reasonable to postulate either lower Temblor or Vaqueros as their age. The following discussion will make this determination more evident.

18Trask, op. cit., p. 144, 1926.


East of the Chalone Creek fault, thick, land-laid deposits grade laterally away from the Gavilan Range into arkosic gravels, which in turn conformably underlie white, chalky, diatomaceous shales. Just east of Chalone Creek and northwest of lower Bear Valley are exposed 1000 feet of these land-laid deposits which are considered typical fanglomerates.

The deposit is cross-bedded and very poorly consolidated. Coarse fragments composed of angular blocks of granite, pink to gray rhyolite, lapilli-tuff, and metamorphic rocks occur in beds of coarse arkose which alternate with beds of finer material. Although usually angular, the larger fragments may be waterworn. Boulders less than 30 inches in diameter are the rule although blocks from 6 to 8 feet on a side are not rare. The granite is coarse, approaching pegmatite, and usually much decomposed, and the volcanic fragments are typical of the Pinnacles series. Attitudes vary from nearly horizontal to a dip of 10 degrees to the southwest, that is, essentially the same as in the diatomaceous shales.

The fragments are not well rounded, and have suffered little from chemical weathering. Colors of the deposits are commonly dull tans, yellows, grays, greenish grays, or brown, although pinks and lavender shades are not unusual. From the higher elevations of the Pinnacles region these gravels cannot always be differentiated from the granite areas on the basis of color alone, although erosion is characteristic, but the white diatomaceous shale which occurs farther to the east stands out in bold contrast in a long line of white outcrops. This prominent band of white shale lies somewhat nearer the rift zone of the San Andreas fault, which is also clearly visible from Chalone Peaks, Hawkins' Peak, and other high points.

These land-laid deposits are in fault contact with the rhyolite and with the granite farther to the south, so that their thickness cannot be determined, but 1000 feet of coarse detritus is exposed in Chalone Valley. They are clearly derived from the Gavilan Range, and the size of the fragments indicates a near-by source of considerable relief. According to drilling records,19 land-laid facies thin rapidly toward the east and south and almost disappear to the south of Topo Ranch (6 miles southeast of the Pinnacles).

19U. S. Geol. Surv. Bull. 581—d, p. 153, 1914.

The topographic features of the fanglomerate and arkosic material are striking. Locally, the deposits form vertical cliffs several hundred feet high. Erosion is extremely rapid, as is evidenced by the large amount of material carried out into Chalone Creek and Bear Valley by even the very smallest of tributaries.

Lack of fossils in these arkosic gravels and land-laid breccias make age determination difficult, but their conformable position beneath the diatomaceous shales of Bear Valley and Horse Valley is significant. The presence of Pinnacles rhyolite, as transported fragments for the most part, places the gravels later than these volcanics.

In describing the Point Sur Quadrangle, Trask notes that

The Temblor is largely of continental origin. Most of the deposits seem to be fanglomerates. A few marine intercalations occur in the upper part of the formation. . . . The angularity of the constituents and the large size of the fragments, some of which are five feet in diameter, indicate the short distance of transport and the moderate to high relief of the region from which they came.20

20Trask, op. cit., p. 148 and p. 150, 1926.

Also, Kerr and Schenck describe the Temblor as interbedded diatomaceous shale and coarse arkosic sandstone with a poorly sorted breccia toward the base, carrying granitic boulders. Further, they note that

Monterey Miocene, diatomaceous shale and interbedded arkose sandstone is the only marine formation in this area and is best exposed in Bear Valley. The shale is fairly rich in the diatom, Coscinodiscus incretus A. Schmidt. Other fossils of a fragmentary nature were collected; there are Arca sp., an echinoid, fish bones and scales. These Monterey shales are probably Middle Miocene in age.21

21Kerr and Schenck, Geol. Soc. America, Bull., vol. 36, p. 473, 1925.

The absence of siliceous fragments and the conformable occurrence of these land-laid deposits beneath typical Monterey diatomaceous shale precludes the possibility of calling these beds Paso Robles as suggested by R. Willis.22 The gradation upward from arkosic gravels through sandy shales to diatomaceous shales is gradual, indicating continuous deposition about a sinking land mass.

22Willis, Robin, Geol. Soc. America, Bull., vol. 36, p. 674, 1925.


Terrace deposits composed chiefly of fragments derived from volcanic rocks occur at several places about the edges of the central volcanic mass of the Pinnacles National Monument. These deposits arc poorly stratified although there is a general alignment of pebbles, and they vary from a feather edge to 100 or more feet in thickness. Dips average from 3 to 5 degrees away from the central mass and they were apparently deposited on a partly peneplained surface.

Fragments varying from clay particles to blocks and boulders of from 2 to 3 feet in length are chiefly rhyolitic, but granitic material also occurs. Just over the granite surface a layer of residual granitic detritus is usually present. Iron cement binds the terrace deposits and frequently lends a red color to them.

The terrace deposits unconformably overlie both granite and rhyolite surfaces and clearly are derived from the central volcanic mass. Their age is later than the rhyolitic intrusion. These beds are also younger than much of the faulting which has dropped this central mass, as proved by the minor displacement of these deposits where cut by the Pinnacles Fault. Erosion subsequent to their deposition has cut rather deep canyons in both granite and rhyolite so that at least a Pleistocene age is postulated. They may be equivalent in age to the San Benito gravels described by Lawson23 as belonging to the Pliocene period but considered to be Pleistocene by Kerr and Schenck24 Pleistocene age of the San Benito gravels was determined through the discovery of camel remains and a horse type very close to Equus occidentalis.

23Lawson, A.C., Univ. Calif. Bull. Dept. Geol., vol. 1, p. 152, 1893.

24Kerr and Schenck, op. cit., p. 476, 1925.

The surface beds of rhyolitic outwash material to the north are also correlated with these deposits although their nature is less well defined.


Recent material includes sands and gravels of Chalone Creek and tributaries as well as alluvium covering parts of the area. These are of minor importance.

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Last Updated: 8-Jan-2007