Transverse ranges

The Transverse Ranges are a prominent east-west-trending mountain chain that forms a spectacular backdrop behind the most populated region in California. The range is unique in several ways: The east-west orientation departs markedly from California's usual north-south trend of other geologic structural blocks. There is a greater variety of unusual rocks and structures in the range than elsewhere in the state, and the range has important mineral and energy resources.

Location and Characteristics

The Transverse Ranges are a narrow mountainous belt in California that extends from Point Arguello, 80 kilometers west of Santa Barbara, eastward for more than 450 kilometers to the Eagle Mountains in the Mojave Desert; they are from 16 to 80 kilometers wide. The Transverse Ranges are a distinctive, if not unique, geologic province of California because of an east-west orientation (which is rare in North America and unique in California), a greater range of rock types and geologic structures than any other region in California, the oldest plutonic and metamorphic basement rocks in California, and the largest oil fields in the state outside the San Joaquin Valley and Los Angeles County.

Initial observations of the overall alignment of the Transverse Ranges might suggest that the province constitutes a single, apparently homogeneous east-west-trending mountainous belt. However, that is not the case, as closer examination reveals that the Transverse Ranges comprise three unrelated segments, each with a widely differing geologic history and origin, brought into alignment coincidentally by the large-scale tectonic regime peculiar to Southern California. Based on geologic differences, the three segments of the Transverse Ranges may be subdivided into the eastern segment, which includes the San Bernardino, Little San Bernardino, Hexie, Pinto, and Eagle Mountains; the central segment, the major portion of which is the San Gabriel Mountains but which also includes the lesser Verdugo, Sierra Pelona, Liebre, and Sawmill Mountains; and the western segment, which consists of the Santa Susana, Santa Monica, Topatopa, and Santa Ynez Mountains and the northern four Channel Islands.

The mountains of the eastern segment of the Transverse Ranges are bounded largely by faults, of which the San Andreas fault forms the southern margin. The eastern segment is about 250 kilometers long and includes the highest elevation in Southern California at San Gorgonio Mountain, 3,505 meters above sea level, in the San Bernardino Mountains. Rocks underlying the eastern segment are predominantly Mesozoic granitic rocks, gneiss and schist of probable Precambrian age, and lesser amounts of fossil-bearing Late Paleozoic metamorphosed marine sedimentary rocks. Most of the summit region of the San Bernardino Mountains is a plateau-like old erosion surface of moderate relief that developed before uplift. This surface now stands at elevations of about 1,950 to 2,290 meters. The preservation of the old erosion surface is explained by the comparatively recent and rapid uplift of the range, which is thought to have occurred within only the last 2 million years. A second, elevated, plateau-like old surface exists in the Little San Bernardino Mountains. This surface has been eroded into the picturesque, bold rock formations for which Joshua Tree National Monument is famous.

The central segment of the Transverse Ranges is dominated by the San Gabriel Mountains, a high, rugged upland about 100 kilometers long and as much as 38 kilometers wide. The range is bordered on the north and east by the San Andreas fault zone and by reverse faults on the south and southwest. The range consists mainly of a crystalline basement complex of intrusive igneous rocks and a variety of metamorphic rocks. Within the rock complex are two generations of early Precambrian gneisses (1,045 million and 1,700 million years old, respectively), the oldest rocks in California. Summit elevations of the peaks in the range vary from about 1,950 to 2,450 meters, the highest being Mount San Antonio (Mount Baldy) at 3,072 meters.

There are many faults within the central segment of the Transverse Ranges, some of which have right-slip movement of several tens of kilometers. The range has been uplifted essentially as a unit by broad arching across its northern margins and central part and by reverse faulting along its southern margin. Most of the uplift has occurred within the last 15 to 20 million years. Because the uplifting of the ranges of the central segment began long before that of the eastern segment, erosion has had greater time to wear away any uplifted old erosion surfaces. Consequently, no erosion surfaces exist in the San Gabriel Mountains, in contrast to those present in the more recently elevated ranges of the eastern segment. A thick sequence of Cenozoic fossiliferous nonmarine sedimentary rocks and some volcanic rocks occur around the base of the central segment. These rocks have been eroded into the scenic landforms at Devil's Punchbowl County Park near Pearblossom, Vasquez Rocks County Park on State Highway 14, and Cajon Pass.

Unlike the segments of the Transverse Ranges to the east, the mountains of the western segment are composed almost entirely of marine sedimentary rocks of the late Mesozoic and Cenozoic Age. Locally within the sedimentary rocks are extensive volcanic rocks, produced by submarine eruptions. The ranges of the western segment have a combined length of about 190 kilometers. Summit elevations of the mountains are modest, reaching their highest elevation of 2,050 meters at Hines Peak in the Santa Ynez Range.

Part of the western Transverse Ranges is underlain by a large syncline about 190 kilometers long, named the Soledad Basin in the east and the Ventura Basin in the west, where it is contiguous with the Santa Barbara Channel farther to the west. The Ventura Basin, filled with primarily marine sedimentary rocks, merges eastward with the nonmarine sediments of Soledad Basin. The Santa Susana Mountains, South Mountain, Oak Ridge, Ventura Anticline, and Sulphur Mountain are prominent anticlinal hills formed in the last 200,000 to 300,000 years within the Ventura Basin. The Ventura Basin is geologically famous for its immense combined thickness of late Mesozoic and Cenozoic marine sedimentary rocks totaling more than 17,600 meters, the recency of its deformation, and for its great petroleum production.

Study of Transverse Ranges

Geologists use their full armamentarium of devices and techniques to study the geology of the Transverse Ranges. The most important technique has been field mapping, the on-site research on foot, by car, or airplane to locate and plot the various rock units, faults, and folds on aerial photographs and topographic base maps. Rock and fossil samples are collected and labeled in the field. These samples are taken into the laboratory for detailed examination and identification and compared with samples from other areas of the same or similar geology that have been studied previously. Paleontologists, experts in fossils, may be consulted to assist in fossil identification. Correlation (the determination of age relationships between rock units or geologic events in separate areas) of rocks and fossils in exposures on opposite sides of faults allows the determination of the amount and direction of movement along the faults, assuming that rocks with identical properties and fossils were once a single body.

Added to the work of field geologists is subsurface information obtained from oil wells: drilling logs, electrical resistivity and self-potential logs, gamma ray-neutron logs, cores, and side-wall samples. This subsurface information is immensely useful to geoscientists, enabling them to “see” the geology underground. Other techniques to discover what is underground are seismic reflection and seismic refraction profiling. Seismic reflection profiling is based on the travel time of sound waves excited by precisely timed “thumping” of the ground reflected off buried rock layers or fault surfaces. Seismic refraction profiling is based on the travel time of sound waves generated by a small, controlled explosion. Data generated by these techniques are plotted on graphs, from which the depths and orientations of the various layers and structures in the subsurface geology may be interpreted.

Supplementary subsurface information important in the study of the Transverse Ranges is obtained by a geophysical technique called gravimetry. By using gravimeters, sensitive instruments that can precisely measure very slight differences in the Earth's gravitational field, it is possible to deduce much about the physical characteristics of the rocks underlying the surface. For example, gravimetry reveals that the San Gabriel Mountains lack “roots,” lower-density rocks such as granite, which usually exist beneath mountain ranges, extending into the mantle for tens of kilometers. Such underlying roots provide the buoyant lifting force that supports mountain ranges as elevated masses above the average elevation of the Earth's surface. Thus, most of the world's mountains float on the mantle buoyed by their roots, much like the hidden keel of an iceberg that supports its visible portion. Another geoscientific discipline, geodetic surveying, explains how the high-standing San Gabriel Mountains can exist without such roots.

Geodetic surveying is a branch of civil engineering that requires consideration of the Earth's curvature. Instrumental techniques used in geodetic surveying incorporate laser optical devices, pulsed infrared lightwave, and microwave electronic surveying equipment. These instrumental techniques produce extremely accurate measurements of distances and angles to high precision. Precision surveys conducted since the 1970s reveal strains in the Earth's crust across the San Gabriel Mountains that cause the range to be compressed in a north-south direction. Thus, the San Gabriels are supported essentially by being squeezed upward by crustal compression, a process much different from that supporting most of the world's mountain ranges.

Radioactive age dating, which determines rocks' absolute ages, is another technique used in the study of the Transverse Ranges. Knowing the rate of decay of radioactive elements allows geochronologists (geoscientists who specialize in radioisotopic age dating of rocks) to determine rocks' ages in years. The technique involves a laboratory procedure requiring a mass spectrometer and similar specialized equipment. Radioactive age dating is especially useful in determining the ages of igneous and metamorphic rocks.

Environmental Impacts and Resources

Scientists and others must understand the relationship between rock types, slope angles, and watershed management in the Transverse Ranges to protect the lowland communities from flooding during the episodic winters of high precipitation. A second concern with the Transverse Ranges is their role as a watershed that provides runoff water used by the residents of the foothill communities that border those ranges.

The mineral wealth of the eastern and central segments of the Transverse Ranges is economically important, with numerous mines and mining prospects scattered throughout the province. Mineral deposits in the central segment of the Transverse Ranges have been exploited for almost 150 years. The first gold discovery in California was made in the San Gabriel Mountains at Placerita Canyon in 1842. Exploration during the fifty years following the discovery produced important gold discoveries in several areas of the San Gabriel and San Bernardino Mountains and some of the lesser ranges of the eastern segment. Iron ore was mined for many years in the Eagle Mountains. Silver, copper, lead, tungsten, and numerous nonmetallic minerals also have been sought and mined occasionally in the central and eastern segments. Limestone deposits in Lower Cushenbury Canyon on the north side of the San Gabriel Mountains are the basis for the current mining activity in the Lucerne Valley limestone district and the existence of the largest cement plant in the world.

Probably even more significant is the great petroleum production from the western segment of the Transverse Ranges. The marine sedimentary rocks making up the western segment, in concert with the faulting, folding, and rapid uplift of the rocks during the last 1 million years, not only have created the impressive modern landscape but also have formed the geologic structures that are responsible for the numerous oil fields of the region. Of historical importance is that the first petroleum production in California, in 1857, came from oil seeps near Ventura and that the first oil well drilled in California and put into serious commercial production was completed near Ojai in 1866. This early well is located in what is now known as the Silverthread area of the Ojai field. Even as late as the 1980s, the production from the Silverthread area was still economically feasible, yielding upwards of 1 million barrels of oil per year.

Understanding and protecting the environments adjacent to California’s Transverse Ranges are essential in the twenty-first century and intrinsically tied to global climate change. Changes in temperature and precipitation can impact the geological and topographical qualities of the Ranges. Climate change can threaten its biodiversity and also have negative hydrological impacts. 

Principal Terms

basement: an underlying complex generally of igneous and metamorphic rocks

reverse fault: a steeply to moderately inclined fault in which the overlying block of rock has moved upward over the underlying block

right-slip: sideways motion along a steep fault in which the block across the fault appears displaced to the right; left-slip faults are the opposite

tectonics: a branch of geology that deals with the study of regional large-scale structural or deformational features and their origins, mutual relations, and evolution

thrust fault: a variety of a reverse fault in which the inclination of the fault plane is at a low angle to horizontal

Bibliography

Carballo, Rebecca. "California Faces Heavy Rain and Snow From Storm." The New York Times, 1 Apr. 2024, www.nytimes.com/2024/03/30/us/california-storm-flooding-snow-forecast.html. Accessed 28 July 2024.

Ehlig, P. L. “Origin and Tectonic History of the Basement Terrane of the San Gabriel Mountains, Central Transverse Ranges.” The Geotectonic Development of California: Rubey, vol. 1, edited by W. G. Ernst. Englewood Cliffs, N.J.: Prentice-Hall, 1981.

Fife, D. L., and J. A. Minch, editors. Geology and Mineral Wealth of the California Transverse Ranges. Santa Ana, Calif.: South Coast Geological Society, 1982.

Gamache, Tom, and Matthew Jaffe. The Santa Monica Mountains: Range on the Edge. Los Angeles: Angel City Press, 2006.

Hatcher, Robert D., Jr. Structural Geology: Principles, Concepts, and Problems. 2d ed., Englewood Cliffs, N.J.: Prentice-Hall, 1995.

Grotzinger, John, and Tom Jordan. Understanding Earth. 6th ed., New York: W. H. Freeman, 2009.

“Mount Baldy Mining Area, Los Angeles and San Bernardino Counties, California.” California Geology, vol. 39, Aug. 1986, pp. 179-184.

Onderdonk, N., Garcia, A. F., Kelty, C., Farris, A., & Tyler, E. "Topographic Development of a Compressional Mountain Range, the Western Transverse Ranges of California, USA, Resulted from Localized Uplift along Individual Structures and Regional Uplift from Deeper Shortening." Geosphere, vol. 18, no. 6, 2022, pp. 1804-1830, doi:10.1130/GES02505.1. Accessed 28 July 2024.

Plummer, Charles C., Diane H. Carlson, and Lisa Hammersley. Physical Geology. 13th ed., New York: McGraw-Hill Science, 2009.

Salcedo, Chris. San Bernardino Mountain Trails. Berkeley: Wilderness Press, 2003. San Gabriel Mountains. 5th ed., Berkeley, Calif.: Wilderness Press, 2005.

Trent, D. D. “Geology of Joshua Tree National Monument, Riverside and San Bernardino Counties.” California Geology, vol. 37, Apr. 1984, pp. 75-86.