The Santa Rosa Plateau is part of the southern portion of the Santa Ana Mountain Range, and the Santa Ana Mountain Range is part of the Peninsular Range, which extends 930 miles (1500 km) from Southern California to the tip of Baja California. The Plateau averages about 2,000’ in elevation and covers about 20,000 acres; about 8,300 acres of it are preserved as the Santa Rosa Plateau Ecological Reserve. 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. From GZ, look East and you’ll see two mesas separated by a slight dip that rangers call Lost Mesa and Mesa Sin Nombre, or No Name mesa.
To understand how the Santa Rosa Plateau was formed we must go back about 10 million years when most of Southern California was relatively flat, close to sea level, due to a lack of earthquakes to uplift the area. The change began occurring 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. About nine million years ago the Santa Rosa Basalt erupted on the surface and deposited a thick layer of dark red-brown basalt. Eruptions of basaltic lava flows were deposited onto the surface of what is now the Santa Rosa Plateau area. Estimates are that the lava covered an area 20 miles across, roughly centered on the Mesa de Colorado. Continued readjustment of Southern California due to the stresses that produced the San Andreas Fault, the entire Santa Ana Mountain Range was uplifted. 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 wear 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.
About 18,000 years ago Earth was in an ice age and the Plateau may have received twice the rain as now. The Plateau would have look much lusher with plants that survived with that amount of water. As the amount of rainfall decreased new plants that could survive with half the rainfall took hold. Water is very important to flora and fauna and the Plateau has two unique water features, vernal pools and tenajas; both are important to provide water year-round to wildlife and plants (to learn more see EC GC39FRY).
TO LOG THIS CACHE: 1. Looking East you will see two mesas with a gap in between. What do you think caused this gap? 2. Looking north to south, do you think you see the entire Plateau? If not, about what percentage do you believe that you are able to see? 3. At GZ there are a group of large rocks of two dominant colors. The majority are reddish, what color is the other? 4. The reddish rocks are what type of rock?
Santa Rosa Plateau Geology http://tchester.org/srp/geology/overview.html
https://brightstarswildomar.blogspot.com/search?q=Santa+Rosa+Plateau