Brief Geologic Description of Del Norte, Humboldt and Mendocino Counties.
"In discussing the geology of this section of the Coast Ranges, one is confronted with the similarity of the character of the rocks of the different geological ages and the scarcity of fossils by which the different series of stratified rock may be distinguished. The rocks of the different formations have undergone such metamorphism that it is at times difficult to detect the change from one series to another. The Coast Range has been subject to so much disturbance that the rock masses have been crushed and faulted out of their original stratigraphic positions.
"Beginning at the northern boundary of Sonoma County and extending north through Mendocino, Humboldt, and Del Norte counties, the geological structure is very regular. The rocks are mostly of cretaceous age and are often very much altered. Serpentine, jaspers and mica slates are encountered in large quantities and in a very irregular manner. There are but few areas of unaltered strata.
"The general strike of the axis of the Coast Ranges through these three counties is northwest and southeast and the preponderance of dip is toward the southwest, the crest of the range being nearer the eastern slope. The deep valleys have been eroded by the abundance of water and the level valleys of some of the watersheds contain strata of Pliocene age. These strata are shallow, and fossils have been noted in Del Norte and southern Humboldt counties. The Tertiary rocks are not as prevalent as those of the Cretaceous. The latter are to be noted more particularly in the oil field region of southwestern Humboldt County.
"The South Fork of the Trinity River takes the same general northwest direction as the other rivers of the Coast Range. Trinity River changes its direction, flowing nearly west from Weaverville in Trinity County to where it joins the South Fork thence northwest through the mountains to the coast. The Trinity Mountain ranges seems to be the joining strip uniting the main Sierras and the Coast Range. The rocks in this northern section become more crystalline, and the old granites which form the nucleus of the Sierras make their appearance. This granite outcrops north of Humboldt Bay and thence north to the state line.
"From the junction of the Klamath and Trinity rivers, extending northward to the northern end of Del Norte County, the country is very rugged and covered with forests. The rocks resemble those of the Sierras and are auriferous and cupriferous. The gravels of the rivers also carry gold and platinum values. In this northern region, serpentine is the principal rock. Peridotite, the parent rock of serpentine, is found exposed by erosion on Horse Mountain in northeastern Humboldt County.
"It might be said that the greater part of the geological formations of Mendocino, Humboldt and Del Norte counties is composed of sedimentary rocks of Tertiary and Cretaceous age. There does not appear to be a nucleus of igneous rock forming the axis of the range, although granite does outcrop in some parts of this section. There are remains of volcanic activity in the form of volcanic glass and tuff, and solfataric action is still taking place at some of the springs of southern Mendocino County.
"Throughout Mendocino and southern Humboldt counties the Cretaceous sandstones are abundant, being very noticeable at Point Arena and in the oil section of southwestern Humboldt. Organic remains are absent except in a very few instances. The rocks of this age have been altered to a considerable extent and serpentines and mica slates are the alteration products.
"In Del Norte County, granite forms the nucleus of the mountain ranges and over it is a mantle of metamorphic rocks. In the western portion of the country sedimentary rocks prevail. Intrusive serpentine carries the copper and chrome iron deposits. Quartz occurs in small seams and veins. Copper occurs in lenses of a rich concentration, either as free metal or in sulphides. The slates carry many thin seams of quartz, sometimes rich in gold, and no doubt the erosion of these formations is responsible for the gold concentrated in the streams.
"This serpentine belt continues through Humboldt and Mendocino counties. Perhaps the most noticeable occurrence is at Horse Mountain, in Humboldt County. There the copper deposits in serpentine are encountered again. The country has been eroded so extensively that the older crystalline diorites protude (sic) through the most recent rock formations. Other acid rocks, such as quartzite, outcrop in large masses, besides a dike of porphyry which forms a well defined contact with serpentine. Not far from Horse Mountain on Willow Creek a large body of limestone is exposed, having a northwest strike. It resembles that in western Trinity County. another limestone formation north of Humboldt Bay is well exposed. The granite formation also outcrops here.
"Passing south through Humboldt County and across the redwood belt, the later formations of sandstone and shale come in. These carry the oil and gas of this section. The formation is very badly broken up and seepages of oil, and gas emanations are numerous. This formation continues into Mendocino County and is most noticeable on the coast at Point Arena. In the southern portion of Mendocino County, the amount of alteration by the introduction of magnesian combinations is noticeable. This is illustrated by the magnesite deposits and the mineral springs of that section.
"Taking the three counties together, one might say that the geology is complex, the solution of which will take considerable time and much patience to decipher."
Quarries (in Mendocino County)
"There are no quarries operating in the county at the present time (circa 1913). The best quality of road metal being used in Mendocino comes from Petaluma, Sonoma County. The metamorphic rocks of the county are not suitable for road building."
Area: 3,453 square miles.
Population: 24,116 (1920 census).
Location: Joins Humboldt County on the south and bounded by the Pacific Ocean on the west.
"Mendocino's annual mineral production has usually been small, the 1919 output being valued at $14,214, ranking it fifty-first among the counties. That of 1918 was worth $108,388, the decrease being due to chromite and manganese.
"Deposits of, in part undetermined value, of asbestos, chromite, coal, copper, graphite, magnesite, and mineral water have been found, as well as traces of gold and silver.
"Commercial production for 1919 was as follows:"
(Headings for the information below are: Substance, Amount, and Value.)
Stone, miscellaneous, ---, $7,000
Other minerals*, ---, $7,214
(Total value) $14,214
(* Includes chromite and platinum.)
"The larger part of Mendocino County is covered by the Franciscan (Jurassic) rocks, with a wide border of Cretaceous rocks along the coast. Neither of these groups is particularly favorable for furnishing large deposits of limestone in California although both do contain numerous small bodies and veins or stringers of calcite, limestone, and calcareous tufa. The only recorded production of limestone from the county was between 1930 and 1933 from a deposit near Laughlin."
(* Please note this list does not include sand or gravel quarries.)
Onyx marble occurs at Gravelly Valley, Mendocino County.
"Fisher Ranch Deposit. M. H. Fisher, Laytonville, is owner. The deposit is in W ½ sec. 36, T. 22 N., R. 15 W., M.D., 2 miles north of Laytonville and a few yards east of the state highway, at an elevation of 1600 feet.
"The limestone outcrops in two outcrops in two small hills, separated by a saddle of 150 feet with no outcrops. The north hill is about 345 feet long from north to south and the width of outcrop is 105 feet though the hill slopes are wider. The south hill shows an outcrop 85 to 165 feet. The north outcrop is higher, rising 50 feet. No work has been done on the deposit and the owner was not aware of its nature.
"The limestone varies in color from pink to yellowish gray and it is a fine-grained hard, dense stone of sub-conchoidal fracture. Parts of the deposit appear to be quite siliceous. Longvale on the Northwestern Pacific Railroad is 13 miles south by state highway."
Analysis by Abbot A. Hanks, Inc.
Insoluble, 5.56 percent
Ferric and aluminic oxides, 0.69 percent
Calcium carbonate, 93.16 percent
Magnesium carbonate, 0.39 percent
The following is an excerpt from "Mineralogy of the Laytonville Quarry, Mendocino County," California, by Charles W. Chesterman, California Division of Mines and Geology. (The entire article will not be included here.)
"The Laytonville quarry is a locality where an unusual variety of minerals has been found in Franciscan glaucophane schists, including the three recently discovered new species, deerite, howiete, and zussmanite (Agrell, Brown, and McKie, 1965)."
"The Laytonville quarry is in sec. 6, T. 20 N., R. 14 W., M.D.B.M., on the north side of the U.S. Highway 101, about 5 miles south of Laytonville, Mendocino County...It was excavated by the California Division of Highways to supply rock for riprap along the embankment of Long Valley Creek nearby. The quarry was first worked in 1957, and since that time approximately 91,000 tons of rock have been obtained.
"The rocks exposed in the quarry include both metamorphosed sedimentary and volcanic rocks of the Francisco Formation (Jurassic and Cretaceous age). The list of minerals found here is long and impressive; seldom is it possible to find so many of the minerals characteristic of the glaucophane schists at one place.
Photo 1. Massive metabasalt (mb) in fault contact with foliated and thinly banded metasedimentary rocks (ms) at the Laytonville quarry.
Geology (of the Laytonville Quarry Area)
"The area in which the Laytonville quarry is situated is referred to by the California Division of Highways (Clemens, oral communication, 1965) as 'Hell's Half Acre,' a name that is justifiably applied to the area because it is essentially a large landslide. Such slides are common in the California Coast Ranges, and many, like this one, contain large and small masses of metamorphosed and unmetamorphosed Franciscan rocks randomly encased in a slumgullion composed of sheared and crushed Franciscan graywacke, siltstone, shale, chert, and greenstone, accompanied by serpentine.
"The Laytonville quarry was carved in a single gigantic landslide mass of metamophosed Franciscan rocks, which has retained its integrity although it has been somewhat removed from its original site...."
Agrell, S. O., Bown, M. G., and McKie, D., 1965, Deerite, howiete, and zussmanite, three new minerals from the Franciscan of the Laytonville district, Mendocino County, California: Am. Mineralogist, v. 50, nos. 1-2, p. 278.
Bailey, E. H., Irwin, W. P., and Joanes, D. L., 1964, Franciscan and related rocks and their significance in the geology of western California: California Div. Mines and Geology Bull. 183, 171 p.
Coleman, R. G., and Lee, D. E., 1963, Glaucophane-bearing metamorphic rock types of the Cazadero area, California: Jour. Petrology, v. 4, no. 2, p. 260-301.
Mine name: Little Eagle Rock Quarry; Operator: Louisiana-Pacific Corp.; Address & County: P. O. Box 340, Calpella, CA 95418, Mendocino County; Phone: (707) 485-8731; Latitude: 39.17, Longitude: -123.30, and Mine location number: Map No. 345; Mineral commodity: Stone.
"Quinan Ranch deposit is owned by L. S. Quinan, Route 1, Box 377, Healdsburg, California, and is located in sec. 26, T. 17 N., R. 13 W., M.D. Formerly the product was hauled several miles to Laughlin on the Northwestern Pacific Railroad where the crushing plant stood, but the railroad passes within less than half a mile of deposit. The old road from the Quinan Ranch house is passable for an automobile to within half a mile of deposit.
"This is a terrace deposit of travertine and calcareous tufa that was deposited by springs near the top of a ridge at an elevation of about 2000 feet. The springs, which still flow (circa 1947), but do not deposit appreciable amounts of mineral now, issue along a fault zone striking west and dipping 61 S. One terrace, west of the old workings, is 250 feet long by 100 feet wide, at 2000 feet elevation. The lower terrace, where work was done, is about 35 feet below the first, and east of it. It covered an acre or less. A pit 27 by 50 feet and 10 to 15 feet deep produced material used for agricultural purposes. This pit shows the travertine is 6 to 8 feet thick, underlain by soil and angular rock fragments. It was worked between 1930 and 1933 by Northwest Pacific Lime & Sulphur Company. Only a few steel rails remain at the pit.
"All machinery has been removed from the building formerly used as a crushing plant at Laughlin, and the railroad company was planning in July 1945 to tear down the building."
"Analysis by Abbot A. Hanks, Inc.
Insoluble, 3.20 percent
Ferric and aluminic oxide, 0.24 percent
Calcium carbonate, 94.97 percent
Magnesium carbonate, 1.51 percent"
Mendocino County, California - Sandstone Deposit - Excerpt from the Tenth Annual Report of The State Mineralogist For The Year Ending December 1, 1890, California State Mining Bureau, Sacramento: State Printing Office, 1890, pp. 311.
Mendocino County, by Alex. McGregor, Assistant in the Field.
Building Stone (in Mendocino County).
“Located in Sec. 16, T. 15 N., R. 13 W., M.D.M., is a large deposit of good sandstone for building purposes. It is not in use at present, but when its value is known it will be; also, in Sec. 1, T. 14 N., R. 13 W., M.D.M., can be found a similar deposit.”