The Santa Rosa Mountains and adjacent San Jacinto range are uplifted blocks of igneous and metamorphic rocks lying between two major fault zones, the San Andreas and the San Jacinto. The Santa Rosa range and the two faults all trend northwest-southeast. The Santa Rosas contain abundant shear zones that testify to the intense faulting in this tectonically-active region. The Santa Rosa Mountains consist of a core of early Mesozoic granitic and metamorphic rocks intruded by somewhat younger Mesozoic granitic rocks ranging in composition from granite to granodiorite to tonalite. Tonalites are common along the southwest flank of the range. Several large exposures of sheared Pre-Cretaceous metamorphic rocks occur in the north-central part of the Santa Rosas; smaller exposures are found in the southern part of the range (especially in the Rattlesnake Canyon/Palo Verde Canyon area). This intensely sheared zone of early Mesozoic metamorphic rocks is a source of asbestos (near Asbestos Mountain).
The contact between the early Mesozoic "country rock" and the younger granitic intrusions is sometimes sharp, sometimes gradational, and occasionally fault-bounded. Rockhouse Canyon is located between the northwest-southeast trending San Jacinto Fault (a major fault in the area) and the Buck Ridge Fault (an off-shoot of the San Jacinto). The canyon cuts through the southern spur of Buck Ridge and runs nearly perpendicular to the two faults. The rock types in Rockhouse Canyon are the same as those described above. The western entrance to Rockhouse Canyon lies immediately adjacent to the San Jacinto Fault.
The early-Mesozoic sheared zone is host to several small bodies of slightly younger Mesozoic ultrabasic intrusive rocks. These intrusives are located just north of Pines Palms Highway (Highway 74) and south-southwest of Asbestos Mountain.
Beryl, the principal ore mineral of beryllium, occurs in a variety of colors, each with its own distinctive name. Gem-quality blue to blue-green beryls are known as aquamarine, yellow beryl is called heliodor, pink is known as morganite, and the exquisite green variety is called emerald. Emeralds occur in a variety of geologic settings. They are occasionally found in association with granites and related pegmatite bodies. Many times, emeralds are found in biotite schists that have been intruded by pegmatites. Sometimes, they occur in altered marble or in calcite veins formed by metamorphic segregation.
The Santa Rosa Mountains consist of a core of Mesozoic granitic rocks and older metamorphics intruded by younger Mesozoic granites. This is certainly a favorable environment for the formation of beryl. Many of the granitic plutons and batholiths of southern California are known to harbor pegmatite bodies, some of which contain exotic minerals like beryl and tourmaline. It is entirely
feasible that similar deposits occur in the Santa Rosas. Prospectors may want to concentrate on these pegmatite bodies, especially those hosted in sheared metamorphic rock. Beryl is highly resistant to weathering due to its great hardness and chemical inertness. It tends to remain unaltered as it is transported downslope. Consequently, beryl shows up in the sands and gravels that accumulate at base of the deposit. Careful mapping and tracing of any beryl float may prove to be fruitful.
Some portions of the Santa Rosas are off limits to prospecting. In the northern portion of the range, some areas lie within the Santa Rosa Mountains State Game Refuge and Anza Borrego Desert State Park. Several sections of the northern part of the range also lie within the Santa Rosa Indian Reservation. Parts of the southern Santa Rosas also lie within Anza Borrego Desert State Park. More recently, the entire range has been designated a National Monument.