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San Luis Obispo County

  • San Luis Obispo County – Dacite Granophyre, Andesite Granophyre, and Tuff (historical times through circa 1906) – Excerpts from The Structural and Industrial Materials of California, Bulletin No. 38, California, State Mining Bureau, San Francisco, California, 1906.

    “A line of buttes of igneous rock runs in a northern direction west of the Santa Lucia range from San Luis Obispo to Morro. These rocks are of a grayish color, and have been classified by Mr. H. W. Fairbanks, Journal of Geology, Vol. VI, page 567, as dacite granophyre and andesite granophyre. These buttes afford very good quarry sites.

    “Another belt of igneous rock (tuff) is found in the southwestern part of the county, running from near Arroyo Grande in a southeastern direction past Los Berros Creek. The belt is over a mile wide, and the tuff has been quarried in several places.”

  • San Luis Obispo County, by C. A. Logan (circa 1915) – Excerpts from Report XV of the State Mineralogist, Mines and Mineral Resources of Portions of California, Chapters of State Mineralogist's Report Biennial Period 1915-1916, Part V. Monterey, San Benito, San Luis Obispo, Santa Barbara, Ventura counties, Field Assistant. Field Work in November, 1915, California State Mining Bureau, 1919, pp. 595-769.)

    History and Topography (of San Luis Obispo County)

    "In September, 1772, the Mission of San Luis Obispo de Tolosa was established by the Catholic Fathers on the county seat, and the village which grew up about it took the same name, which was also bestowed on the county when it was established as one of the original twenty-seven, shortly after the state joined the Union. While known principally as a cattle-raising and farming district, the county has also yielded a fair tribute to the miner's enterprise.

    "Bounded by Monterey County on the north, Kern on the east, Santa Barbara on the south and the Pacific Ocean on the west, the county comprises alternate mountain chains and narrow valleys which strike northwest and blend together at the southeast, as would the sections of an open fan. In the southwestern part of the county lies the valley of the Santa Maria River and the smaller troughs which flank the San Luis Range. These Mountains meet the Pacific at Point Buchon and form the southern coast line. The higher peaks of the chain are in the northwest and are hardly 1900 feet in elevation. North and east lie the fertile valleys of Los Osos and San Luis, which are separated by a scarcely perceptible divide. Los Osos Valley in its northwestern part lies submerged in Morrow Bay and the free sweep given to moist winds and fogs over these valleys helps farming greatly by lessening the need for irrigation…."

    Transportation (in San Luis Obispo County)

    "The Southern Pacific Railway traverses the county from north to south, crossing the Sierra Santa Lucia through Cuesta Pass, and going nearly south through San Luis Obispo, and thence to the coast near Pismo, giving rail connection with San Francisco and Los Angeles.

    "From San Luis Obispo the Pacific Coast Railroad (narrow gauge) has a line to Port San Luis on deep water, and another which serves the district lying between the county seat and Santa Maria.

    "The rest of the county not adjacent to the railroads relies on auto stages for mail and passenger service. Two such lines operate daily over easy grades from San Luis Obispo to Morrow, Cayucos, Cambria and San Simeon. Pozo and the La Panza country are reached through Santa Margarita by triweekly trips. The eastern plains region communicates with the railroad with McKittrick. Freight service is given to the upper coast towns by the steamers of the Pacific Coast Steamship Company.

    Geology (of San Luis Obispo County)

    "The geology of that portion of the county from the southern boundary to latitude 35 30' N. and from the coast to longitude 120 30' W. has been covered in Folio 101 of the United States Geological Survey. For the geology of the oil districts in the county, the reader should consult Bulletin No. 69, recently issued by the California State Mining Bureau…Features of greatest interest are the thermal sulphur springs and wells, the bituminous sandstones and the deposits of chromite and quicksilver.

    Mineral Resources (in San Luis Obispo County)

    "Gold production began before 1850 and copper was mined in small quantity in the early '60s. Antimony and manganese occur, but have so far yielded no ore in commercial quantities. Chromic iron or chromite has been mined along the entire length of the Sierra Santa Lucia, and the quicksilver's chief ore, cinnabar, is reported over an equally wide range, though developed chiefly in the northwest. In the south and southwest, bituminous rock and asphaltum have been worked a good deal, and some heavy oils developed. Here, also, as well as around Paso Robles, are many thermal sulphur springs and wells. Granophyres, sandstone, volcanic ash and California onyx have been used for building purposes, but granite, although plentiful, is so far unutilized. Bricks have been made in considerable quantity. There are also found in the county deposits of diatomaceous earth, pumice, gypsum, limestone and some asbestos of uncertain quality, on which little or no development work has been done. A small annual output of mineral water has been reported for several years…."

    Limestone (in San Luis Obispo County)

    "Many deposits of limestone of commercial size and good quality are found in various parts of the county. Most of these are at present too remote from transportation to be available except for restricted local use. Lime has been burned in small quantity in the Adelaide district in Secs. 18 and 19, T. 26 S., R. 10 E.; in Sec. 35, T. 32 S., R. 14 E., and on the Newson Estate, 2 ½ miles east of Arroyo Grande. Other unexploited deposits are mentioned in former reports, which are listed in the bibliography.

    "Specimens of crystallized limestone of brownish color and apparently very pure, were shown to the writer by J. W. Hobson of Santa Margarita, who says that an extensive deposit of the material crops over a width of 20' for a distance of about one mile, running northwest, 2 to 3 miles north of Santa Margarita.

    Bibl.: State Mining Bureau, R. VIII, p. 532; R. X, p. 584; Bull. 38, p. 79."

    Stone Industry (in San Luis Obispo County)

    "Many varieties of stone have been used in the county for building purposes and road construction, harbor work and culverts. A large tonnage was required for use on the state highway recently constructed, and the search for material near at hand led to the opening of several new plants within the past year to supply crushed rock, sand and gravel. While the region has a plentiful supply of stone suitable for cutting and polishing, the distance of the deposits from transportation and the lack of a nearby market of any importance, render the possibility of their exploitation very doubtful."

    Granite (in San Luis Obispo County)

    "Part of the Salinas River cañon is cut through granite, and this deep-seated rock is uncovered over a large area in the eastern and northeastern part of the county; but is too remotely situated to be of any economic value at present (circa 1915-1916).

    Sandstone (in San Luis Obispo County)

    "Sandstone of good quality for building construction occurs at several places in the county but has never been utilized except for making railroad culverts. The best material is found between Cambria and Cayucos on the coast and close to deep water transportation."

  • San Luis Obispo County Mineral Industry (circa 1919) – Excerpt from California Mineral Production for 1919, Bulletin No. 88, by Walter W. Bradley, California State Mining Bureau, 1920.

    Area: 3,334 square miles.
    Population: 21,893 (1920 census)
    Location: Bordered by Kern County on the east and the Pacific Ocean on the west.

    "The total value of the mineral production of San Luis Obispo County in 1919 was $212,430, as compared with the 1918 output, worth $858,679, the decrease being due to chromite and manganese. Among its mineral resources both developed and undeveloped, are: Asphalt, bituminous rock, brick, chromite, coal, copper, gypsum, infusorial earth, iron, limestone, marble, mineral water, onyx, petroleum, quicksilver, and miscellaneous stone.

    "In thirty-eighty place, commercial production for 1919 was as follows:

    (Headings for the information below are: Substance, Amount, and Value.)

    Chromite, 1,158 tons, $26,431
    Petroleum, 31,656 bbls., $32,922
    Stone, miscellaneous, ---, $20,300
    Other minerals,* ---, $132,777
    (Total value) $212,430

    (* Includes bituminous rock, manganese, quicksilver, sandstone, and soda.)

    San Luis Obispo County, 1916 Map, from California Mineral Production for 1919 (with County Maps), Bulletin No. 88, by Walter W. Bradley, California State Mining Bureau, San Francisco: California State Printing Office, 1920, pp. 196. San Luis Obispo County, 1916 Map
  • San Luis Obispo County Limestone Industry and Deposits (up to 1947) – Excerpts from "Limestone in California," by Clarence A. Logan, California Journal of Mines and Geology, Vol. 43, No. 3, July 1947, California Division of Mines, San Francisco, California, pp. 175-357. Used with permission, California Department of Conservation, California Geological Survey.)

    "This county is midway between Los Angeles and San Francisco and is traversed from north to south by a main state highway and the Southern Pacific Railroad. The population is small, and the local demand for lime in earlier days was supplied by a few small kilns, none active in the past 40 years. The only recorded production of limestone in the past 10 years has been from a large deposit on Lime Mountain in the northwestern part of the county. This has been used for beet-sugar refining.

    "The limestone seen in surface outcrops is generally buff, dust colored, or off white. That on the top of Lime Mountain shows abundant shell remains, passing downward into a dense, hard and compact, crystalline stone of medium crystal size, and fetid. Some shells found here indicated the Santa Margarita formation (upper Miocene age) as do specimens of Ostrea titan found in other beds nearer the coast. In the San Luis folio of the U. S. Geological Survey covering the southwestern part of the county, H. W. Fairbanks reported limestone derived from Formaminifera and attributed it to the Monterey (Miocene)."

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