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."
Building Materials (in Del Norte County).
"The local demand for building stone and brick is so small in the county that this industry has not been developed to any appreciable degree. A sandstone suitable for building purposes, and a clay suitable for making brick, are found 2 miles east of Crescent City. There is a good clay in Elk Valley, and Benjamin Howland is manufacturing brick. He supplies the local demand at $14 per thousand. There is also a deposit of good pottery clay in Elk Valley, owned by George Turner, but it is not developed (circa 1913)."
Area: 1,024 square miles.
Population: 2,759 (1920 census).
Location: Extreme northwest corner of state.
Transportation: Wagon and mule back; steamer from Crescent City.
“Del Norte rivals Alpine County in regard to inaccessibility. Like the latter county also, given transportation and kindred facilities, this portion of the state presents a wide field for development along mining lines especially. Its chief mineral resources, largely untouched, are chromite, copper, gems, gold, iron, platinum minerals, silver, and miscellaneous stone. The decrease in 1919 from the 1918 figure of $371,675 was due to chromite.
“Commercial production for 1919, giving it fifty-fifth place, was as follows:”
(Headings for the information below are: Substance, Amount, and Value.)
Gold, ---, $500 (estimated)
Silver, ---, $4 (estimated)
Stone, miscellaneous, ---, $6,300
Other minerals, ---, $67
(Total value) $6,871
(* Please note this list does not include sand or gravel quarries.)
Crescent City, Del Norte 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. 167.
“Located in Sec. 22, T. 16 N., R. 1 E., H. M., one and one half miles northeast from Crescent City, is a large deposit of superior sandstone suitable for building purposes; it has been worked to a limited extent, furnishing stone for local use. No work is being done at present on the property.”
Mine name: Lopez Rock Quarry; Operator: Reservation Ranch; Address & County: P.O. Box 75, Smith River, CA 95567, Del Norte County; Phone: (707) 487-3516; Latitude: 41.96, Longitude: -124.20, and Mine location number: Map No. 53; Mineral commodity: Rock.