SANTA CLARA FORMATION 



75 



they readily disintegrate to yield a light- to dark-gray 

 porous soil containing only scattered fragments of 

 rock. In good exposures the shales can commonly be 

 seen to be rhythmically bedded in thin layers, averag- 

 ing about 2 inches and locally exceeding 4 inches in 

 thickness; but in places they form massive exposures 

 several feet thick so dissected by closely spaced ir- 

 regular fractures that it is difficult to distinguish any 

 bedding. Where the shales are well bedded, however, 

 the fractures are generally normal to the bedding 

 planes and thus cause the rock to break out into rec- 

 tangular blocks. Neither cherts, formed by extensive 

 silicification of the siliceous shales, nor opaline con- 

 cretions, both of which are common in other areas, 

 are found in the Monterey in the New Almaden 

 district. 



The siliceous shales of the Monterey in the Xcw 

 Almaden district are light colored, porous, and punky 

 to hard, depending on the amount of siliceous ce- 

 ment. In greater part they are finely laminated and 

 correspond to the "porcelaneous shales'' of Bramlette 

 (1946, p. 15) ; the term "porcelanite" has been ap- 

 plied, however, to rocks that are thin bedded, but not 

 conspicuously laminated (Taliaferro, 1934, p. 196). 

 True diatomites, composed chiefly of the tests of di- 

 atoms, are uncommon in the New Almaden district. 

 The most striking feature of the siliceous rocks is the 

 dazzling-white to pale-cream color they assume on 

 weathering. Their bedding planes, which are sharply 

 defined and smooth, are flecked in places with fish 

 scales, and in many places they show rings and bands 

 of limonitic stain. The texture of the shales is much 

 too fine to show individual grains under a hand lens, 

 but in many specimens Foraminifera, or pores result- 

 ing from the removal of Foraminifera by weathering, 

 are visible. Even in thin sections under the highest 

 power objective, the groundmass of the rock looks like 

 a fine cloudy mud; it consists chiefly of opal in the 

 siliceous rocks and of clay in the more argillaceous 

 shales. Scattered through this groundmass are angu- 

 lar fragments of quartz, small amounts of glauconite 

 and zircon, and a few fragments of fine-grained sili- 

 ceous rock. The fossils observed in the shales include 

 numerous Foraminifera and diatoms, fish scales and 

 bones, the thin-shelled Pecten peckhami, and a few 

 large fragments of silicified vertebrate bones. 



SANTA CLARA FORMATION 



The Santa Clara Valley contains alluvium of at 

 least two ages. The older alluvium is exposed mainly 

 in the northwest corner of the New Almaden district, 

 in and near Los Gatos, but small patches of it are 

 scattered along the northern foothills of the Los 



Capitancillos Eidge as far east as the mouth of Al- 

 maden Canyon. From the area around Los Gatos it 

 may be traced northwestward into the Santa Cruz 

 quadrangle, where it has been mapped as the Santa 

 Clara formation (Branner and others, 1909, p. 6), and 

 although the other scattered patches shown on the 

 map of the New Almaden district may differ a bit in 

 age, all probably were deposited within the deposi- 

 tional interval represented by the Santa Clara forma- 

 tion as mapped elsewhere. The formation has been 

 dated as Pliocene and Pleistocene in age on the basis 

 of a few fresh- water fossils (Branner and others, 

 1909, p. 6), some plant remains (Hannibal, 1911, p. 

 329-342), and a correlation with marine beds of the 

 Merced formation lying south of San Francisco (Law- 

 son, 1914, p. 14) . Although no diagnostic fossils have 

 been found in the formation in the New Almaden dis- 

 trict, this age determination is accepted because it 

 seems consistent with the local physiographic devel- 

 opment of the landforms on the formation. 



The Santa Clara formation in the New Almaden 

 district consists largely of fine to very coarse alluvial 

 material, deposited by streams that differed little from 

 those now eroding the mountains that border the Santa 

 Clara Valley. It is poorly sorted in most places and 

 locally shows irregular bedding due to gullying. The 

 finest material is a coarse silt, and the largest observed 

 boulders are a little more than 2 feet long. In shape 

 the pebbles and boulders are commonly more flat than 

 spherical, and because they lie with their flat surfaces 

 parallel to the bedding, they provide in many places 

 the only means of estimating the attitude of the for- 

 mation. The pebbles and boulders are of rocks found 

 in the adjacent range, and although they are domi- 

 nantly of the harder rocks, they include some pieces 

 of soft siltstone. The formation contains boulders of 

 silica-carbonate rock in several places, and on Blossom 

 Hill, in an exposure almost 400 feet above the present 

 valley floor, it also contains an appreciable amount of 

 detrital cinnabar. Beds of light -brown limestone con- 

 taining abundant fresh-water gastropods were found 

 in the formation at a single locality, south of Los 

 Gatos and half a mile north of St. Josephs Hill. 



The formation as a whole is poorly exposed, except 

 in roadcuts or streambanks, for it rapidly weathers 

 to a reddish bouldery soil. The small patches of 

 Santa Clara sediments lying on spurs of the Los 

 Capitancillos Ridge are recognized only by the abun- 

 dance of rounded boulders of the hardest rocks, such 

 as diabase, gabbro, silica-carbonate rock, and chert, 

 strewn over the surface. The attitude of the beds was 

 measured accurately in only a few places, where the 

 steepest recorded dip was 20, and although in some 



