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Geology
Joshua Tree National Park Geology

The Final Polish

Nearly all rock surfaces in the high desert show some degree of desert varnish, a usually thin patina of insoluble clay, iron, and manganese oxides. In some cases the surface impregnation of varnish is so deep into the partially decomposed rock that it binds the material together and produces a dark-brown, metallic looking rind called case hardening. The Pinto Gneiss and the monzogranite which crops out at Indian Cove and along the Fortynine Palms Oasis trail have good examples of desert varnish (Photo 17).

Photo 17. Inselberg with desert varnish and old soil line (arrow) revealing geologically recent exhumation of the pediment by rejuvenation of drainage across the pediment. The inselberg is located on the nature trail at the west end of the Indian Cove Group Campground.

Varnish is not unique to the desert, but is best revealed there. Two current hypotheses for the origin of desert varnish are: (1) a microbial origin in which bacteria concentrate manganese oxides (Oberlander and Dorn, 1981), and (2) an inorganic origin in which clay and iron and manganese oxides that are derived from air borne dust and other sources form thin layers on the rock surfaces (Potter and Rossman, 1977; Allen, 1978).

Acknowledgements

My appreciation is extended to hundreds of students who over the years have accompanied me on field trips to Joshua Tree National Park. Their comments and questions have sharpened my perception and improved my explanations. Thanks are due also to my field assistants, my wife Patricia, and Walter R. Stephens.

References

Allen. C.C., 1978, Desert varnish of the Sonoran Desert—optical and election probe micro analysis: Journal of Geology. v. 86, no. 6, p. 743-752.
Axelrod, D.I., 1950, Studies in late Tertiary paleobotany: Carnegie Institution of Washington, Publication 590, Washington. D.C., 323 p.
Axelrod, D.I., 1958, Evolution of the macro-Tertiary geoflora: Botanical Review, v. 24. no. 7, p. 433-509.
Bateman. P., 1981, Geologic and geophysical constraints on models for the origin of the Sierra Nevada batholith, California, in Ernst, W.G., editor. The geotectonic development of California: Prentice-Hall, Inc.. p. 71-86.
Bradshaw, M.J., Abbott, A.J., and Gelthorpe, A.P., 1978, The Earth's changing surface: Halsted Press, John Wiley and Sons, 336 p.
Brand, J.H., and Anderson, J.L., 1982, Mesozoic alkalic monzonites and peraluminous adamellites of the Joshua Tree National Monument, southern California (abstract): Geological Society of America Abstracts with Programs. v. 14, no. 4, p. 151-152.
Dibblee, T.W., Jr., 1967, Geologic map of the Joshua Tree quadrangle, San Bernardino and Riverside Counties, California: U.S. Geological Survey Miscellaneous Geologic Investigation Map I-516, scale 1:62,500.
Dibblee, T.W., Jr., 1968, Geologic map of the Twentynine Palms quadrangle, San Bernardino and Riverside Counties, California: U.S. Geological Survey Miscellaneous Geologic Investigation Map I-581, scale 1:62,500.
Ernst. W.G., 1981, Summary of the geotectonic development of California, in Ernst, W.G., editor, The geotectonic development of California: Prentice-Hall, Inc,. p. 601-613
Flint, R.F., 1971, Glacial and Quaternary geology: John Wiley and Sons, 892 p.
Garner, H.F., 1974, The origin of landscapes—a synthesis of geomorphology: Oxford University Press, 734 p.
Joshua Tree Natural History Association, 1974, The natural landscape of Joshua Tree National Monument, a geologic resume: Twentynine Palms, Joshua Tree Natural History Association. 8 p.
Lanphere, M.A., 1964, Geochronologic studies in the eastern Mojave Desert, California: Journal of Geology, v. 72, p. 381-399.
Miller, C.F., 1977, Early alkalic plutonism in the calc-alkalic batholithic belt of California: Geology, v. 5, p. 685-688.
Miller, W.J., 1938, Precambrian and associated rocks near Twentynine Palms, California: Geological Society of America Bulletin, v. 49, p. 417-446.
National Park Service, 1980, Twentynine Palms Weather, typed M.S., 1 p.
Oberlander, T.M., 1972, Morphogenesis of granitic boulder slopes in the Mojave Desert, California: Journal of Geology, v. 80, no. 1, p. 1-20.
Oberlander, T.M., and Dorn, R.I., 1981, Microbial origin of desert varnish: Science, v. 213, p. 1245-1247.
Ollier, C.D., 1975, Weathering: Oliver and Boyd, London, 384 p.
Potter, R.M., and Rossman, G.R., 1977, Desert varnish: the importance of clay minerals: Science, v. 196, p. 1446-1448.
Powell, R.E., 1962, Crystalline basement terranes in the southern eastern Transverse Ranges, California in Cooper, J.D., editor, Geologic excursions in the Transverse Ranges: Geological Society of American Cordilleran Section, 78th annual meeting, Anaheim, California, p. 109-136.
Rogers, J.J.W., 1954, Geology of a portion of Joshua Tree National Monument, Riverside County, California: Map Sheet 24 in Jahns, R.H., editor, 1964, Geology of Southern California: California Division of Mines, Bulletin 170.
Rogers, J.J.W., 1961. Igneous and metamorphic rocks of the western portion of the Joshua Tree National Monument Riverside and San Bernardino Counties, California: California Division of Mines and Geology, Special Report 68, 26 p.
Small, R.J., 1972, The study of landforms: Cambridge University Press, 466 p.
Streckeisen, A.L., 1973, Classification and nomenclature recommended by the I.U.G.S. Subcommission on systematics: Geotimes, v. 18, p. 28-30.

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