History of Alameda County, California : including its geology, topography, soil, and productions, Part 3

Author: Munro-Fraser, J. P
Publication date: 1883
Publisher: Oakland, Calif. : M.W. Wood
Number of Pages: 1206

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To the north of San Pablo are low hills of very recent strata, which are nearly horizontal, and which rest unconformably on the edges of the tertiary. Whether these beds contain any extinct species of shells has not yet been determined ; at all events, they are no older than the post-pliocene.

In the valleys between San Pablo and Walnut Creek, many sections made by the rains of 1861-62 in the superficial detritus are observed. The beds are horizontally stratified, and made up of light and darker colored materials, the lighter ones being darker near the upper surfaces, and growing lighter downwards to the depth of from six to twelve inches, as beds usually do when acquiring a color from decaying vegeta- ble substances. This would indicate that the rate of deposition of this detritus has been exceedingly irregular, long periods having sometimes elapsed without much addition to the detrital deposits, and then, again, a heavy mass of materials being suddenly

* The, quantity of oil obtained seems to have been too small to pay, as the work was not profitable, and had been discontinued previous to the oil excitement of 1865. Whether resumed between that time and the present, 1883, we have been unable to discover.

Robert Livermore evermore

GEOGRAPHY, AREA, GEOLOGY, MINERALOGY, ETC.

spread over the surface, just as takes place at present during a winter of extraordinary storms, like those of 1861-62. The appearances indicate sometimes a heavy deposit during one year only ; at others a succession of them for several years. The same or similar facts are observed at many points in the coast ranges.

The whole range under consideration is divided into a great number of hills and valleys, the latter running parallel with the strike of the strata. The valleys are exca- vated in the softer materials, and are frequently drained by streams running in two opposite directions, which connect at their sources by very low divides, so that one hardly recognizes the fact that he is passing over them. When streams cut across the strike of the strata, as they occasionally do, the valleys become mere cañons, or nar- row rocky defiles.

To the southeast of Martinez there is a good exhibition of the folding of the strata, exhibiting in synclinal axis, which runs from a point one mile north of Pacheco southwest to the Cañada del Hambre, a distance of about four miles.

Walnut Creek (Arroyo de las Nueces) heads in the divide between the valley of this name and that of the San Ramon ; it separates the Contra Costa Hills from the Monte Diablo Group proper. High hills of tertiary sandstone rise to the west of it, attaining an altitude of from eighteen hundred to two thousand feet. The high group of hills north of the head of the San Ramon is also of sandstone, and has about the same elevation. The strike of the strata here is about N. 50° W. to N. 55° W., and the dip 65° to the southwest. The San Ramon, heading in this group of hills, runs southeast, then turns and runs parallel with its former course in the opposite direction, having a high and steep range of fossiliferous sandstones between the two parallel portions.

The foot-hills along the eastern base of these higher ridges are of strata very much broken, with every possible dip and strike, the latter frequently at right angles to that of the strata in the main ridge, and standing vertical. There are indications of a line of quite recent disturbances of the rocks through the San Ramon and El Hambre Creeks, which line crosses the general direction of the stratification at an angle of 35°. There are fissures in the soil along the west side of the San Ramon Valley, which were formed during the earthquake of June, 1861, and which may be considered as strengthening the probability of the recent formation of this valley. That extensive disturbances have taken place in the Monte Diablo chain within the most recent geological epoch will be seen farther on.

Near the head-waters of the San Ramon, the hills of tertiary sandstone rise to the height of about two thousand feet ; the strata having a strike of about N. 39º to 41° W .; and they have a high dip to the southwest. The same strata, as followed along a few miles farther to the northwest, near Moraga Valley, become more nearly vertical, and the strike curves around more to the west. The same belt of rocks extends southeast from the head of the San Ramon, through the range of hills west of Amador Valley, and they have a lower and more uniform northwesterly dip. These hills sink into the plain near the eastern end of the pass leading from Haywards to Amador Valley.

Near the " Walnut Creek House," a small patch of cretaceous occurs, extending over a few acres, from which the overlying tertiary, forming the crown of a low anticlinal, has been denuded.

HISTORY OF ALAMEDA COUNTY, CALIFORNIA.

A belt of metamorphic rock may be traced along the western side of the Contra Costa Hills, beginning near San Pablo, thence following the west side of Wild Cat Creek, and appearing in a southeast direction along the foot-hills of the range, for a distance of about thirty-five miles. It generally forms a narrow belt, not over two miles wide, and often not half that; but in some places there is more or less meta- morphic action observable over a width of four miles. The northwestern portion of this band of altered rock curves to the northwest, and seems to form the isolated metamorphic hills lying near the bay, apparently connecting with the range of high hills which run out at Point San Pedro and extend back to San Rafael.

Near San Pablo a great variety of the results of metamorphic action may be observed; as, for instance, in following a line extending from the house of V. Castro back to the top of the ridge. The original rock seems to have been a more or less bituminous slate or shale, and patches of it have almost entirely escaped metamor- phism, while others in the immediate vicinity are very much altered and converted even into mica slate. The dip of the strata, when it could be made out, was to the northeast, 30° at the base of the hill and gradually getting higher towards the crest of the ridge, where the metamorphism is most complete. Here the rock is traversed by small quartz veins, and has evidently been acted on by water containing silica in solu- tion, as it is, to a large extent, converted into that mixture of ferruginous, jaspery, and chalcedonic material, which is so well known as frequently containing cinnabar, that we have become accustomed to call it the " quicksilver rock." Considerable masses of actinolite have been found lying on the surface in this vicinity, evidently derived from the rocks of this ridge. The specimens resemble exactly those obtained from the very much older metamorphic rocks of New England.

The widest and highest portion of this metamorphic belt lies near the pass leading from Oakland to Lafayette, the summit of which is thirteen hundred and eleven feet above high tide. About a hundred rods west of the summit metamorphic slates stand vertical, having a close lithological resemblance to rocks elsewhere known to belong to the cretaceous system ; a short distance northwest they have a high dip to the northeast. A sharp ridge, half a mile in a direction N. 32° W. from the Summit House, is of hard metamorphic sandstone, of which the strike is N. 64° W., but curv- ing more to the south as we go southward; the dip is to the northeast, about 70° in amount. Hand specimens of this rock have a very trappean look, but they appear to be of metamorphic origin.

About one mile farther north is the highest point north of the pass, called " Rocky Mound"; it is nineteen hundred and twenty-one feet high, forming a rounded hill, having a distinct stratification, although very trappean in its appearance, and a dip to the northeast. Between this point and the ridge spoken of in the last paragraph, there is a mass of trappean rock, finely crystalline and very hard, in which no planes of stratification can be observed. On the northeast of San Pablo, the unaltered strata rest on these metamorphic rocks and dip northeast.

The ridge between Wild Cat and San Pablo Creeks is made up of strata dipping northeast from 30° to 35°, and having a strike of about N. 52° W. The north end of this ridge is of quite unaltered strata, while the southern portion is highly meta- morphosed.

GEOGRAPHY, AREA, GEOLOGY, MINERALOGY, ETC.

On the east side of Carlisle Creek, a metamorphic limestone occurs, in which all traces of stratification have been obliterated, the mass of the rock being traversed by veins of quartz, resembling semi-opal in appearance.

South of the pass from Oakland to Lafayette, several high, dome-shaped hills rise, having an elevation of about eighteen hundred and fifty feet, made up of highly metamorphic rock, having a trappean aspect, but stratified and dipping northeast. Intruded in this are masses of rock which appear to be of decidedly eruptive origin, as the metamorphic strata are displaced in the vicinity. Here, as in many other locali- ties in California, it is difficult to draw the line between eruptive and sedimentary, as both have undergone extensive metamorphism since their formation.

A short distance south of the pass the metamorphic strata suddenly contract to about one and one-half miles in width, an arm of unaltered sandstone and slates extending up between two branches of the metamorphic. In this region the slates are little metamorphosed, appearing white and easily decomposed, although much con- torted. Portions are highly silicious, but soft and friable, and, under the name of " Kaolin," are used to mix with clay in making pottery at San Antonio. This belt of slates and shales may be traced southeast as far as Suñol Valley, beneath which they dip, rising again probably and appearing in a highly metamorphic form in the mass of the Mount Hamilton Group. In the places where they are not metamorphic they have all the lithological character of the strata known to be of cretaceous age, which have been described as occurring near Martinez, and which will be noticed farther on as so well developed near Monte Diablo.

Lying to the west of this are massive sandstones, entirely unaltered, which, as yet, have furnished no fossils, but which are believed to be of cretaceous age. They form an elevated ridge, of which a culminating point is Redwood Peak, sixteen hun- dred and thirty-five feet above the level of the bay. The strike of these sandstones at this point is about N. 69° W., but they curve more to the south on the southeastern side of the ridge. Their usual dip is to the northeast, but near Redwood Point the strata are much broken, and three miles southeast they sometimes stand vertically or have a very high dip to the northeast.

Beneath this mass of sandstones, and extending to the southwest, there is a body of coarse conglomerate, forming a series of ridges of considerable altitude. Northeast of San Leandro it appears in the range of hills forming the eastern boundary of the San Antonio Ranch. Ten or twelve miles farther to the southeast it appears in Suñol Peak, which rises to an elevation of over two thousand feet, on the southeast side of which it dips to the southwest. It passes through the Su ol Valley and becomes a portion of the great metamorphic belt of the Mount Hamilton Range.

Although no fossils have been found in place in the belt of slates and shales alluded to above as exhibiting so well-marked a resemblance to rocks elsewhere determined to be of cretaceous age, yet a few boulders have been picked up which contained shells undoubtedly of this epoch. A more careful search will hardly fail to furnish some farther evidence on this point. One of these boulders was found near the entrance of Suñol Valley, in a locality where it is hardly possible that it should have come from any other belt of rocks than that indicated above.

The metamorphic band, before alluded to as beginning near San Pablo, after

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HISTORY OF ALAMEDA COUNTY, CALIFORNIA.

narrowing near Redwood Peak, extends along the western slope of the hills, forming the lower ridges at their base. It does, not, however, form a well-defined belt parallel with the strike of the strata, nor does it appear to represent an axis of elevation. In a section examined from San Leandro across the summit of Monte Diablo, it was seen conformably underlying the conglomerates and sandstones before spoken of; but far- ther south its relations to the adjacent rocks become very obscure, owing to the almost entire obliteration of the lines of stratification consequent on the increased metamor- phism of the mass. As observed in the foot-hills of the range between San Antonio and Alameda Creek, this metamorphic belt has all the characters which are so often exhibited by the altered cretaceous rocks. Serpentine is abundant in it in large irreg- ular masses, and jaspery slates, like those of Monte Diablo. East of San Antonio large patches are to be seen, having all the characters of the quicksilver-bearing rock of New Almaden and New Idria, exactly like those noticed as occurring near San Pablo. Considerable masses of chromic iron occur in this position, one of which was formerly worked to some extent. Stains of copper are not unfrequent, and have led to several attempts at mining, none of which have proved successful, or are likely to repay the labor and capital invested.

In the neighborhood of Alameda Caóon this metamorphic belt appears to be almost lost, but traces of chemical action commenced and partially completed, are exhib- ited in narrow streaks visible among the highly inclined and broken strata; these, how- ever, do not appear to connect through with the metamorphic mass of Mount Hamilton.

Monte Diablo Group as Regards Alameda County .- After describing the Monte Diablo Group as it ranges through the county of Contra Costa, with variations in geology and vast coal-beds, Professor Davidson proceeds with his interesting description in these words :-

The pass called after Mr. Livermore, an old settler in the valley, is on the west side of the eastern division of the Monte Diablo Group, and about sixteen miles south- east of the summit of the mountain, and here the chain is more completely broken through than at any other point in its whole extent; this pass has, for that reason, been selected for the passage of a railroad from San José to Stocktoh, for which the pre- liminary surveys have been made, and which is now in process of construction. The western division of the chain, the Contra Costa Hills, already described in this chapter, are entirely broken through by the cañon through which the Alameda Creek, which drains a large region in the interior of the range, finds its way to the Bay of San Francisco. This caùon, therefore, in connection with the Livermore Pass, furnishes a good and easy route for a railroad entirely across from the bay to the plains of the San Joaquin; it is, indeed, the only feasible one. As we issue from the Ala- meda Cañon, going towards Livermore Pass, we come into a valley or plain some twelve miles in length, but of irregular width, which extends to the western entrance of the pass. The western portion of this is called "Amador Valley"; the eastern, "Livermore Valley." These valleys are formed by the sinking down of the tertiary ranges of hills which lie along the southeastern slope of Monte Diablo. The portion of the range, which continues to the southward, and connects the Monte Diablo Group with the Mount Hamilton Group, and over which the Livermore and Corral Hollow

GEOGRAPHY, AREA, GEOLOGY, MINERALOGY, ETC.

Passes cross, is made up of a very irregular belt of hills about ten or twelve miles wide, most elaborately wrought out by denudation into a labyrinth of ridges and cañons, the minute exhibition of whose details would require a map on a very large scale. These ridges are nearly destitute of trees, with but a scanty supply of feed, in the shape of grass or forage, and poorly provided with water, what there is being generally alkaline. The hills in the vicinity of the pass are seldom over twelve hundred feet high, but they rise higher a little farther north, and "Bushy Knob," or "Las Cuevas," may be considered as the culminating point of this division of the group; it is one thousand seven hundred and forty-two feet above tide-water.

The pass itself is, according to the Pacific Railroad surveys, four hundred and eighty-one feet high at its western entrance, and six hundred and eighty-six feet at its summit; by our measurement, the eastern entrance, at "Zimmerman's Mountain House," is two hundred and twenty-three feet above the sea.

The rocks near the pass are sandstones, which are soft and easily disintegrated. They are of tertiary age, but contain few fossils. Along the eastern side of the range the dip is to the northeast, and on the western side it is in an opposite direction, there being a low axis running through the region from southeast to northwest, and passing a little to the east of Bushy Knob. The dip in both directions is quite small, in keeping with the comparatively small elevation of this portion of the chain. At Bushy Knob it was only 15°, the direction of the strata being N. 69° W. ·

South of Livermore Valley the hills rise in altitude, but exhibit in the main the same features as on the other side, until we approach Corral Hollow. They are rounded in outline, and the rock is rarely seen, except in the cañons. As we reach Corral Hollow we find the strata more disturbed, and all the indications of an approach to another great metamorphic center.

The pass, or cañon, usually known as Corral Hollow, extends back from the San Joaquin Plain and opens into Livermore Valley; it crosses the hills about ten miles south of the Livermore Pass, but is more elevated than that, and traverses a greater variety of rocks, as it intersects both the altered and the unaltered cretaceous strata in its upper portion.

This upper region of the pass, which has a northwest and southeast direction, is a deep, precipitous cañon, extending across to the Livermore Valley from the bend in Corral Hollow Creek. This creek, as is so often the case with the streams in this region, runs towards the northwest, parallel with the stratification, for a considerable distance, and then turns suddenly, and, crossing the strata, runs at right angles to its former course. The change in direction is just at the point where the unaltered strata are intersected. The general strike of the strata in this region is nearly .mag- netic. east and west, or N. 70° to 75° W .; the dip is usually to the north, at a pretty high angle, but very variable. The metamorphic region in which Corral Hollow Creek heads will be noticed farther on, as it forms a part of the Mount Hamilton Group. The division between the metamorphic and unaltered rocks in the angle of the creek is well marked and easily recognized at a distance, from the varied character which the different rocks give to the landscape. The metamorphic hills are covered with a darker and thinner soil, and are more bountifully supplied with trees, especially the Quercus Agrifolia and Quercus Garryana, while the outlines of the

HISTORY OF ALAMEDA COUNTY, CALIFORNIA.

ridges are sharper and the outcrops of rock more numerous. Near the junction with the unaltered strata the metamorphosed beds preserve their original lines of stratifi- cation, and are perfectly conformable with the overlying beds of rock, which have undergone no change. These metamorphic rocks are of cretaceous age, and are identical in appearance with the jaspery rocks noticed as occurring so abundantly near Monte Diablo. The jasper bands are from a fourth of an inch to several inches in thickness, sometimes very much contorted and of various colors-red, rose-colored, green, gray, and white ; the whole presenting, especially on the weathered surfaces, the most brilliant and beautiful appearance. There are also reticulations of quartz cross- ing the mass in small, irregular veins and threads, such as have already been described as occurring near Monte Diablo, forming the peculiar rock which we have followed from this region north as far as Clear Lake. Serpentine is also abundantly distrib- uted through this metamorphic region.

Between the metamorphic and the tertiary there is a narrow belt of unaltered rock, of cretaceous age, the metamorphic action not having penetrated through the whole mass of the strata. But few fossils were found in these unaltered rocks, and these were distinct from any obtained elsewhere in the cretaceous, to which formation, however, they were referred, with some doubt, by Mr. Gabb, who described two species from this locality, namely, Cyprinella tenuis and Carbula primosa ; besides these, Anomia and Mytilus were observed, as also three other bivalves, too imperfect to be referred with certainty to any genus. Mr. Gabb considered that these fossils, which overlie rocks known to be of cretaceous age, and which dip under the miocene tertiary may probably prove to belong to a brackish water deposit of the cretaceous, although it is possible that the formation in which they occur may represent the eocine division of the tertiary.

The dip and strike of these unaltered rocks are variable, but they usually incline at a pretty high angle to the north. Two sections were made across the valley in this region, which will serve to show the position of the strata and also the coal-beds, which are found here, and which have been worked to some extent. The first crosses Corral Hollow at the "Pacific Mine" and runs N. 20° E. [See Whitney's Geological Survey of California, p. 36, vol. on Geology]; it represents a length of five miles, and is on an equal scale of horizontal and vertical distances. The other one crosses at the Almaden Mine [See p. 37, Ibid.], and is of about the same length as the other. On comparing these two sections it will be seen at once how great a disturbance of the rocks has taken place in this region, within so short a distance, as the whole series of sandstones on the north side of the hollow has in one section a posi- tion exactly the reverse of that in the other. In the section at the Pacific Mine the mass of unaltered strata north of the hollow lies nearly conformable with the metamorphic; and, like that, dips to the north for a distance of a mile and a half ; then we have a mass of disturbed strata and a reversal of the dip. In the Almaden Mine section the reversal of the dip takes place in the strata immediately connected with the coal, which of course has an unfavorable influence on the working of the bed. Indeed, the disturbances in this district are so extensive that it is to be feared that these coal- beds will not be made available ; and up to the present time, at least, they have not been, although the quality of the coal is good as compared with other Pacific Coast

GEOGRAPHY, AREA, GEOLOGY, MINERALOGY, ETC.,

coals, and the, thickness of the beds sufficient for profitable working, if they had not been so much disturbed by the movements of the strata.

An analysis of the Corral Hollow coal gives : Water, 20.53 ; bituminous sub- stances, 35.62 ; fixed carbon, 36.35 ; ash, 7.50.

The Pacific Mine is situated near the upper curve of the stream, on the south side, about nine miles from the mouth of the valley, and nine hundred and fifty feet above the sea. The stratum of coal is in sandstones and shales ; the strike is N. 80° W., and the dip 50° to 70° to the north. The strata are much broken, and frequent slips and faults occur, while the dip is quite variable. A drift cuts the bed from the lower side and follows it for a distance of three or four hun- dred feet. The bed shows at its eastern end a thickness of fifty inches of worka- ble coal. It was stated that, up to October 15, 1861, from two hundred to four hun- dred tons had been mined here. When visited again in 1862, but little further work had been done here. The drift had been extended a few feet, and the thickness of the coal was found to be sixty-six inches. Many facts were noticed, showing how great a disturbance had taken place here. In the strata beneath the coal are many angular fragments of the coal itself, which appear to have slipped out of place and to have been caught between the strata while undergoing these convulsions, which in one place have cut the seam entirely off and brought it squarely up against the broken edge of the sandstone.

The Coast Range Company was the first one to mine in this district. Their shaft is one and a half miles below that of the Pacific Company, in the bottom of the valley, on the south side. The coal seam was perpendicular, and a shaft was sunk in it to the depth of one hundred feet; but little good coal was found, and it is now abandoned.

Some explorations and attempts at mining at the Alameda Company's mine, on the south side of the cañon, gave equally discouraging results, the strata being much broken, so that the bed could not be followed continuously in the shafts, one of which had been sunk to the depth of one hundred and fifty feet in 1861. In this shaft the strata dip to the south at a high angle, there being a complete overturn of the strata here. The bed of coal exposed in this shaft was about two feet thick, and of good quality, although much crushed, and therefore easily disintegrated. As far as can be ascertained, no coal has been shipped from these mines of late, and they are probably abandoned. Unless the quality and quantity of the Corral Hollow coal were quite superior, it would be impossible for it to come in competition with the product of the Monte Diablo mines, the latter being so much more favorably situated with reference to a market.

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History of Alameda County, California : including its geology, topography, soil, and productions, Part 3

Author: Munro-Fraser, J. P Publication date: 1883 Publisher: Oakland, Calif. : M.W. Wood Number of Pages: 1206

Author: Munro-Fraser, J. P
Publication date: 1883
Publisher: Oakland, Calif. : M.W. Wood
Number of Pages: 1206