Geology 101
Santa Rosa Plateau
Geology 101
The Santa Rosa Plateau is a part of the Santa Ana Mountain Range. The Santa Ana Mountains extend from State Highway 91 to the Santa Rosa Plateau. The Santa Anas (sometimes referred to as the Ortega Mountains) are a part of the Peninsular Ranges Province, that consists of many mountain ranges stretching nearly 1,000 miles to the tip of Baja California, Mexico. Local mountain ranges of Riverside and San Diego Counties located in this province include the San Jacinto, Palomar, Cuyamaca, and Laguna Mountains.
The Peninsular Mountain Ranges Province runs north - south while southern California’s other major mountains run east - west. The Transverse Mountain Ranges Province that include the Santa Monica, San Gabriel and San Bernardino Mountains are some of the few mountain ranges in the western U.S. that trend east - west.
The Peninsular Ranges Province shares much of its geologic history with the Sierra Nevada Mountains. Rocks of both areas are similar in age, type, and tectonic uplift. The Sierras and the Peninsular Ranges were tilted to the west, leaving a very steep escarpment on the eastern slopes of the mountains.
Although histories are similar, elevations are not. The highest peak in the Peninsular Ranges Province is Mount San Jacinto at 10,874 feet, whereas the highest in the Sierra Nevada Mountains is 14,494 feet in elevation. The Santa Ana Mountains where the Plateau is located rise to a maximum of 5,687 feet on Santiago Peak. The Santa Rosa Plateau’s average elevation is about 2,000 feet.
A plateau is a large, elevated landmass that is fairly flat in relief. The Santa Rosa Plateau is approximately 20,000 acres in size, and is bounded by Los Alamos and San Mateo Canyons in the north, the Santa Margarita Mountains in the west, the Temecula/Murrieta Valley on the east, and Mesa de Burro in the south. The Santa Rosa Plateau Ecological Reserve manages approximately 8,000 acres of the Plateau.
The Plateau’s topography is dominated by six main mesas. A mesa is simply an elevated land mass that is smaller in size than a plateau. Colorado Mesa, Mesa de la Punta, and Mesa de Burro are the mesas within the Reserve, while Avenaloca Mesa, Redonda Mesa and Miller Mountain (Mesa) are found outside of its boundaries. Adjacent to, and separated by, a dip or saddle from, Mesa de Burro is a land form sometimes referred to as No Name Mesa. This is not often included in the total count of Plateau mesas because of its limited size. It may in fact be categorized as a butte (an elevated land form with a flat top and smaller than a mesa).
These mesas, or the processes that created them, are the chief reasons why the Santa Rosa Plateau exists. But to understand the whole story, one must go way back in time.
For millions of years southern California, and much of the western portion of North America, was relatively flat. The lack of geologic activity, other than erosion, made for a very stable and uneventful land mass. The real excitement began approximately 10 million years ago.
At about that time, an oceanic-floor spreading center was over-ridden by the Pacific Plate. The Pacific Plate continued east and ran into the North American Plate, giving birth to the San Andreas Fault. Seven to eight million years ago, molten rock rose out of the buried spreading center and broke through fissures in the earth’s crust and flowed as lava over the flat landscape. A series of lava flows blanketed the entire landscape, covering many square miles.
As the lava cooled quickly, it hardened into a rock called basalt. Basalt is the same type of rock found at the bottom of the Pacific Ocean that comes out of …yep, you guessed it, a spreading center. Basalt is a very dense and heavy kind of rock that is very resistant to erosion.
Although resistant, eight million years is a long time. Most of the basalt has been broken down and washed away by the action of water, wind and time.
The land around the basalt was the first to go. The flat terrain the lava flowed onto eight million years ago was comparatively easy to erode. As that terrain was removed by the process of erosion and lowered in elevation, the resistant basalt remained. Originally at the low points of valley bottoms, the basalt now stands above the surrounding landscape, effectively reversing the landscape of eight million years ago. The basalt-capped mesas of the Plateau and the Hogback Hills in eastern Murrieta are what remain of these ancient lava flows.
Standing on top of the basalt mesas of the Plateau is not only a chance to see rare vernal pools and bunchgrass prairie, it is also a look back in time. The flat expanse of these mesa tops is topographically similar to the landscape that dominated much of western North America for millions of years.
As southern California continued to readjust due to the stresses produced by the San Andreas Fault, the entire Santa Ana Mountain Range was uplifted, and likely continues to be uplifted, earthquake by earthquake. The range is a massive block of rock primarily consisting of ancient seafloor sediments that were laid down 200 to 60 million years ago. The sediments were metamorphosed into the Bedford Canyon Formation 150 to 60 million years ago as the Peninsular Ranges Batholith rose from below. This massive magma batholith was created as the now-gone Farallon Plate was subducted, or forced into Earth’s mantle and melted. As the magma cooled, it formed the granitic rocks that make up the foundation of southern California and Baja Mexico. Although the history of the uplift of this rock foundation is uncertain, the Baja California part of the Peninsular Ranges Province began separating from mainland Mexico 5 million years ago.
As the Santa Ana Mountains were forced up, water and gravity combined to erode most of the uplifting rock mass into the classic mountain topography of peaks and narrow ridges - most, but not all. The land that was forced up in the southern-most part of the range had been covered with the basaltic lava flows a few million years earlier. The basalt continued to resist the powerful forces of erosion to retain its horizontal plain.
In the geology of Earth however, erosion is king. As the powerful combination of water and gravity persisted through time, breaks were created in the basalt and much has been broken down and washed away. The remaining mesas of today are remnants of what once was, and become smaller every year. Storm by storm, crystal by crystal, erosion constantly attacks the edges of the mesas, whittling them down ever further, until one day in the distant future, they’ll be gone.
Perhaps the longest-lasting piece of the Santa Rosa Basalt will be that that cannot be presently seen. Over two thousand feet beneath the valley, where Lake Elsinore, Wildomar and Murrieta lie, the basalt can be found. As the Santa Ana Mountains have risen, the valley (or graben) has dropped. As the graben valley between two branches of the Elsinore Fault dropped, it was covered with sediments from the eroding Santa Ana Mountains. The two thousand feet of sediment atop the basalt also contain animal and plant residents of the past in the form of mastodon, tapir and redwood fossils.
The rocks of the Plateau affect everything you see. The basalt helps create the vernal pools. The clay soils eroded from the basalt retain enough moisture for bunchgrass prairie species. Bedrock outcrops of granite allow water to remain in tenajas along stream beds throughout summer for aquatic and terrestrial species alike.
As you enjoy the Plateau of today, you are also experiencing the past. You can witness a landscape of fairly flat relief that resembles a California prior to its destiny of lofty mountain peaks. You may also experience the ancient interactions of plant and animal life, affected by the history of rock and dependant today on the actions of us.