Outline of 2007 Geology Class for Santa Rosa Plateau Docents
Ten million year old drainages seen at the Santa Rosa Plateau
From east to west, the Santa Rosa Plateau is comprised of the following: Mesa de Colorado, Mesa de la Punta and Mesa de Burro are the mesas in the Reserve. Avenaloca Mesa, Redonda Mesa, and Miller Mountain complete the Plateau. Mesa de Burro has an extension to the northwest separated by a slight dip that the rangers call Lost Mesa and is called Mesa Sin Nombre on the map at several trailheads.
The Santa Rosa Plateau is part of the southern portion of the Santa Ana Mountain Range. In turn, the Santa Ana Mountain Range is part of the Peninsular Range, which extends 900 miles (1500 km) from the Los Angeles Basin to the tip of Baja California.
The Peninsular Range contains rocks similar to those of the Sierra Nevada that were formed at roughly the same time in both. Thus, at the time those rocks were formed, the Peninsular Range was the southern extension of the Sierra Nevada. Even the subsequent history of the rocks are similar in the two ranges. During the recent uplift of both ranges, they have been tilted to the west, so that both ranges have their steepest scarps on their eastern sides.
The Transverse Ranges (Santa Monica Mountains, San Gabriel Mountains, San Bernardino Mountains), one of the few areas of the Western United States where the mountain ranges trend east-west, separate the two ranges.
The Plateau exists for several geological reasons:
- Until about 10 million years ago, much of Southern California, including the Santa Rosa Plateau, was a geologically boring area on the surface. Erosion and lack of earthquakes to uplift the area created a fairly flat surface whose elevation was close to sea level.
- The action began about 7-15 million years ago when an oceanic-floor spreading center was overridden by the Pacific plate, and the San Andreas Fault first became active. The dying spreading center pushed up the area that is now the San Onofre and Santa Margarita Mountains, and about nine million years ago erupted lava onto the surface of the Earth in the Santa Rosa Plateau area, which flowed over that ~flat surface. Estimates are that the lava covered an area 20 miles across, roughly centered on the Mesa de Colorado.
The basalt is described by Michael P. Kennedy in Special Report 131, California Division of Mines and Geology, Recency And Character Of Faulting Along The Elsinore Fault Zone In Southern Riverside County, 1977:
The Santa Rosa Basalt is composed mostly of a series of basaltic flows, but volcanic agglomerate and tuffaceous debris occur locally in exposures east of the trough at Vail Mountain. The flows in the type area [SRP] are late Miocene in age, have an average thickness of 2 m and a combined thickness of approximately 30 m, and can be divided into a lower alkalic and an upper theoleiitic series.
Basalt is exposed 3 km west of Vail Lake in the upper reaches of Temecula Creek. Along the south side of the creek in the slopes underlying Vail Mountain, the basalt is offset vertically more than 100 m by small northeast-striking faults. The elevations of the bases of essentially undisturbed flows that cap Vail Mountain are exactly the same as those at Mesa de Burro.
The regional distribution of the basalt remnants suggests that these flows might once have been contiguous over an area of more than 500 km^2.
Potassium-argon analyses of basalt from the southern end of Mesa de Burro suggest an age of 8.3 +- 0.5 my (Hawkins 1970). Subsequent analyses of basalt from Mesa de Burro, the Elsinore trough, Hogbacks, and Vail Mountain yield an average age of 9.6 +- 0.7 my (D. Krummenacher, SDSU).
"Vail Mountain" is called "Oak Mountain" on the USGS 7.5' topo map, and is directly west of Vail Lake at an elevation of ~2000', northwest of Dripping Springs Campground.
The same basalt, with a thickness of 16', has been found nearly a half mile below ground in a well in Wildomar. It is below ground there due to the relative subsidence of the Temecula Valley. See Don Layton's The Temecula Trough for an explanation and a beautiful cross-section showing the distribution of the lava. (But note that the age of the lava, as given above, has been better determined than the 2 million year age quoted by Don Layton.)
An area of >500 km^2 has a circular radius of >12.6 km. A circle centered on the middle of the east-west extent from Miller Mountain to Oak Mountain extends south to the San Luis Rey River in Bonsall and north to the southern edge of Lake Elsinore.
Ted St. John writes thatThere is at least one hill just south of Burro that is also basalt-capped, although it does not have a flat top.
The Murrieta hogbacks are apparently related. The Murrieta hogbacks are the conspicuous ridge east of I-215, south of Los Alamos Road, and north of Murrieta Hot Springs Road. There are houses on top, and a road goes up there from Los Alamos. They also have a similar basalt on top.
Farther east, some of the hills at the west end of Vail lake are also volcanic. To find the Vail Lake site, drive toward the dam along the flood channel of Temecula Creek. In times of low water this is not too hard. Once you work back toward the dam, you can see the basalt, the basalt ridge, and the putative cinder cone.
There is a ridge near the dam that ends in what looks for all the world like a cinder cone. The soil is a very similar clay, and supports Chocolate lilies and other clay soil plants. Again, the basalt appears very similar. The county geologist (Steve Kupferman) told me that someone published a paper a few years ago that tied all of this together.
There is a local rumor that there is a volcano or deep cave of some sort on Punta, but I was up there once and there was no such thing.
- As part of the continuing readjustment of Southern California due to the stresses that produced the San Andreas Fault, the entire Santa Ana Mountain Range was uplifted, and probably continues to be uplifted, earthquake by earthquake. Although we do not know the history of the uplift, the Baja California part of the Peninsular Range began separating from mainland Mexico 5 million years ago.
- With uplift comes increased erosion, as water and gravity have more power at their disposal. The area began to lose its uppermost rocks. Basalt is a relatively hard rock, and thus helped to protect the top of the Santa Rosa Plateau from erosion. The dominant erosion mechanism is to eat into the sides of the lava cap.
- Erosion was able to eat completely through the Plateau in several areas, leaving the remnant mesas we have today. These are the only areas in the Santa Anas where this lava still exists, having been eroded from the other areas between the mesas where it must have originally been as well.
See also Plant Guide to Vernal Pool Trail: Introduction: California 10 Million Years Ago And Today.
For more information, see:
- Geology of California by R.M. Norris and R.W. Webb, John Wiley & Sons, 1976, pp. 169-189. (There is a later edition of this book, but I don't own it.)
- Coastal Southern California by R.P. Sharp, Kendall-Hunt, 1978, p. 197.
- Geological Map of California - Santa Ana Sheet by California Division of Mines and Geology, Fifth printing, 1986.
- California Wild Lands: A Guide to the Nature Conservancy Preserves by D. Holing, Chronicle Books, 1988, pp. 177-181.
Copyright © 1996-2007 by Tom Chester.
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
Comments and feedback: Tom Chester
Updated 23 November 2007 (link to temecula trough webpage updated 20 July 2016)