“Stanislaus County, which comprises 951,000 acres of land, extends from the eastern foothills of the Coast Range on the southwest and runs across the San Joaquin Valley in a northeasterly direction to the western foothills of the Sierra Nevada mountains. Most of the land is under cultivation and very little mineral is found within its boundary. In the southwestern part of the county, beginning at Mt. Boardman in the Coast Range, some quicksilver, manganese and magnesite are found, and a little further east silica, sand and clays are found. The central portion of the county is devoted exclusively to farming, fruit raising, and stock raising, and no mineral is encountered until the northeastern part of the county is reached in the Sierra Nevada foothills. Gravel from the Stanislaus River at Oakdale is utilized for road purposes, and at Knights Ferry, on the Stanislaus River, some ochre is shipped each year. The principal mining operations in the county are at La Grange, where gold dredging is carried on with success. There are five irrigation projects in the county which give an ample supply of water for all the land not served by wells.”
Area: 1,450 square miles.
Population: 43,557 (1920 census).
Location: Center of state, bounded on south by Merced County.
“Gold has usually been the chief mineral product of Stanislaus County, but it was exceeded in 1918-1919 by manganese. Brick, clay, gypsum, iron, mineral paint, quicksilver, and silver are found here to some extent as well. This county, for 1919 ranks twenty-fifth in the state in regard to value of minerals, with an output of $590,326 as compared with $453,913 in 1918, the increase being due mainly to gold and manganese. Gold, platinum, and silver are obtained mainly by dredging.
“Commercial production for 1919 was as follows:
(Headings for the information below are: Substance, Amount, and Value.)
Magnesite, 2,031 tons, $20,831
Manganese, 8,921 tons, $374,584
Stone, miscellaneous, $28,922
Other minerals,* —, $165,989
(Total value) $590,326
(* Includes gold (estimated), mineral paint, platinum, and silver (estimated).)
Mines and Mineral Resources of Stanislaus County, California, by Abbott Charles, Assistant Mining Engineer, California State Division of Mines. Manuscript submitted for publication March, 1947.
Geography (of Stanislaus County)
“Stanislaus County is located at the northern end of the San Joaquin Valley, approximately in the center of California. It is bounded on the west by Santa Clara County, on the north by San Joaquin County, on the south by Merced County, and on the east by Calaveras and Tuolumne Counties. The land area of 1,506 square miles is populated by 74,866 people (1940 census). The population is estimated to have increased 51 percent from 1940 to 1946. The greater portion of the acreage is arable, accounting for a total agricultural and stock production of $77,339,239, in 1945. Of the land area 71.5 percent is in farms, putting this county in ninth place in richness of agricultural production in the United States. The yearly average rainfall for a 50-year period amounts to 12 inches. The wet season extends from October through March.
“The eastern half of Stanislaus County is cut by the drainages of the Stanislaus and the Tuolumne Rivers, which flow westward. The San Joaquin River, flowing in a northwesterly direction, receives water from the other two drainages. These rivers supply the major part of the irrigation water for the county. Wells with an average depth of 215 feet are used to augment the water supply and to control the ground-water level.
“Many good oil-surfaced roads traverse Stanislaus County. The unsurfaced dirt roads are kept in good condition during the dry season. Truck and scheduled bus lines serve the county. United Air Lines has scheduled daily stops at Modesto. Rail facilities are supplied in this county by two Southern Pacific main lines, the ‘west side’ line which runs from Tracy through Patterson and Newman, and the ‘east side’ line which runs from Manteca through Modesto and Turlock; and a branch line that runs from Stockton through Oakdale and Waterford, ending at Montpelier. The Atchison, Topeka & Santa Fe Railway Company has a mainline track that runs through Riverbank, Hughson, and Denair, and has a branch line from Riverbank to Oakdale. The Tidewater Southern Railway Company has a line from Stockton, San Joaquin County, to Hilmar, Merced County, and Turlock, Stanislaus County. The Modesto and Empire Traction Company has a rail line from Modesto to Empire, where it connects with the Santa Fe system. The Sierra Railroad company has a line from Oakdale to Tuolumne, Tuolumne County. At Oakdale this line connects with either the Santa Fe or the Southern Pacific systems.
“The Modesto and Turlock Irrigation Districts furnish electrical power to their districts. The remaining portions of the county are serviced by the Pacific Gas and Electric Company….”
Geology (of Stanislaus County)
“The major portion of Stanislaus County is covered by the recent sediments of the San Joaquin Valley. The Diablo Range, a part of the Coast Ranges, forms the western boundary of the northern half of the San Joaquin Valley. Eocene and Miocene sediments are exposed about 6 miles west of Patterson and Newman, and the Jurassic-Franciscan group comprises the summit of the Diablo Range, the western boundary of the county. The Franciscan formation is composed of many varieties of sedimentary, igneous, and metamorphic rocks. The sediments include arkose sandstone, argillaceous shale, chert, siliceous shale, some conglomerate, and metamorphosed arenaceous and argillaceous materials. Igneous occurrences are principally basic, but stone acidic porphyrites are found. In general, the intrusives have been altered to serpentine. The Franciscan formation is differentiated from the overlying cretaceous strata mainly by its intense alteration. Lower Cretaceous sediments of the Shasta formation have been mapped by N. L. Taliaferro* in the southwestern portion of the county.*
(* Page 86, footnote 1: Taliaferro, N. L., Geologic history and structure of the central Coast Ranges of California; California Division of Mines Bull. 118, pl. 1, 1943.)
“In most of this area the Panoche formation (Upper Cretaceous) rests unconformably upon the Franciscan complex. The Upper Cretaceous marine sediments of the Pacheco and Asuncion groups are composed mainly of gray and black clay-shalres, often with lenses of shaly limestone, sandstone, and conglomerate. N. L. Taliaferro** states, ‘In northwestern Stanislaus County, west of Ingram Creek, the Mount Oso anticline causes a local thinning in the Panoche sandstones but not in the Moreno shales. This appears to have been another irregularity in the Upper Cretaceous sea. The buttressing effect of the Cretaceous Mount Oso anticline had a pronounced influence on the structures form in the late Tertiary.’
(** Page 86, footnote 2: Taliaferro, N. L., Geologic history and structure of the central Coast Ranges of California; California Division of Mines Bull. 118, p. 133, 1943.)
“The Tejon formation of upper Eocene age is exposed from west of Patterson to the south beyond the county line. The Tejon is composed of fine-grained gray clayey sandstone with interstratified beds of fine-grained light-brown shale and gray clay. Layers of coal occur in the lower portion of these marine sediments. Upper Miocene marine sediments of the San Pablo formation can be traced along the base of the Diablo Range. The Kreyenhagen shale (upper Eocene) overlies the Tejon formation west of Crows Landing and extends southward to the county line.
“Foothills of the Sierra Nevada expose upper Miocene sediments as far west as Denair, Waterford, and Valley Home. These are non-marine sediments of the Mehrten formation including sandstone, laminated siltstone, conglomerate, and andesitic breccia and tuff. The Valley Springs formation that underlies the Mehrten is composed principally of fragmental and glassy products from Miocene rhyolites. The Valley springs formation extends the length of the eastern flank of Stanislaus County. Jurassic porphyrites closely follow the eastern edge of the county for most of its length. This porphyrite is an altered lava with the composition of andesite. In the Knights Ferry area, the Ione formation (upper Eocene) is locally exposed. Although the Ione group is exposed over a small area here, it is important economically owing to its clay and ochre content. A ‘gongue’ of Upper Jurassic Mariposa formation extends from the southeast corner of the county to La Grange. The strata are mainly clay-slate with local sandy conglomerates and tuffs. Upper Eocene sediments composed mainly of light-colored tuffaceous beds exposed from Cooperstown south to the county line are no longer considered part of the Ione formation in view of the study by Allen.*
(* Page 87, footnote 3: Allen, V. T., The Ione formation of California: University California, Dept. Geol. Sci. Bull, vol. 18, pp. 347-448, pls. 24-37, 1929.)
Mineral Resources (in Stanislaus County)
“Gold has been the chief mineral product of Stanislaus County. Miscellaneous stone products hold second place in dollar value produced….”
“Clay, chromite, coal, copper, gold, lead, magnesite, manganese, miscellaneous rock products, natural gas, (yellow) ochre), platinum and associated metals, silica, and silver comprise the real and potential natural resources of this county.”
(* Please note this list does not include sand or gravel quarries.)
“Settled by William Knight in 1848, this town on the Stanislaus River soon became a center for local mining and an important point on the route to the Mother Lode….Knight was succeeded by the Dent brothers (brothers-in-law to U.S. Grant), who built the Tullock Mill (Fig. 37) at the east end of town near the famous covered bridge. The brick one-story warehouse was raised in 1852-58 and the stone grist mill buildings were erected in 1852 by T. Vinson, an English stone mason. Vinson built the piers and abutments of the covered bridge in 1862-63. The brown and pink stone used in the Tullock Mill is local Ione sandstone quarried in the nearby slopes; other stones in the main east and south walls are from granite and conglomerate river boulders.”
“La Grange is located south of Highway 120 and west of Coulterville…”
“In La Grange proper is a beautifully preserved adobe brick building….Opposite the building is a pair of stone buildings complete with iron doors…They are made of light tan-and-pink Ione sandstone which occurs just west of La Grange.
“Two miles south of La Grange on the road to Coulterville is a stone-walled ditch used in earlier times for mining operations. It is neatly made of dry-laid flat schist slabs gathered from the immediate vicinity."
“West of Mountain Pass on a knoll on the south side of the road, the ruins of a house, made of serpentine blocks set in lime mortar, can be seen…The outcrop is nearby and shows signs of workings. Close by on the same knoll is another and similar stone ruin.”
“Toni Francisco of Crows Landing produces an unscreened aggregate from Orestimba Creek dry wash in sec. 3, T. 7 S., R. 8 E., M. D. He uses a small gasoline-driven truck loader and operates intermittently.”
“J. C. Scanlon, Elfers Avenue, Patterson, produces road material in the dry wash of Orestimba Creek in sec. 10, T. 7 S., R. 8 E., M. D. A scraper conveyor is used to feed a hopper fitted with a grizzly at the top. The aggregate is carried by a conveyor belt to a dry trommel which has 1-inch holes. The minus 1-inch material is transported to a metal loading bin by means of a small bucket elevator.”
“Wright Ranch, in Sec. 21, T. 1 S., R. 12 E.; G. W. Wright, Knight’s Ferry, owner. Stone has been quarried from this locality at different times for the last fifty years, furnishing material for all the principal buildings in the vicinity of Knight’s Ferry. The entire hill is composed of bedded sandstone, the beds ranging in thickness from a few inches to several feet, and dip slightly to the southwest. The stone is a light buff-colored sandstone, and contains some mica flakes. No regular quarry has been developed, but stone has been taken out at intervals along the roadside. Much waste rock covers the small faces and only a small amount of dimension stone is exposed at present, the result of very careless quarrying by divers persons.”