Posted 19 April 2016 by A.G. Sylvester ©
CA 76 goes up the flood plain of the San Luis Rey River from Oceanside past the airport to Mission San Luis Rey de Francia (PM 4.9). The mission is situated on river terrace overlooking the flood plain.
The small rounded hills a short distance beyond and north of the mission are Miocene volcanic plugs, locally called the “Sleeping Indian Hills.” Morro Hill (elev. 922 feet), the prominent hill on the skyline, is a volcanic neck.
Bonsall (SD PM 11.8)
CA 76 penetrates well into the Peninsular Ranges batholith. One of the main components of the western series of plutonic rocks is Bonsall tonalite, which is a specific form of granitic rock that consists mainly of plagioclase feldspar. The remaining minerals comprise quartz, biotite, and hornblende. The dark gray blobs in the granite, called mafic enclaves, are typical in the Bonsall tonalite as is its “boulder pile” mode of outcrop. You will see good examples of both in the steeper hills around Pala Mesa (SD PM 14.9) and where CA 76 crosses I-15 (SD CB 16.5). Many of the stream courses you cross between here and Lake Henshaw are choked with such big boulders, suggesting an immature landscape evolution. A nick point in San Luis Rey River between Harrah’s Rincon Casino and the La Jolla Indian Reservation is yet another line of evidence of immature landscape development.
Pala (SD PM 23)
The hills north of the community of Pala are laced with haul roads leading to gem-rich pegmatite dikes. Since the early 1900s these quarries have yielded incredible specimens of both pink and green tourmaline, kunzite, topaz, beryl, and other semi-precious gems. Most of the mines are inaccessible because they are on private property.
Look for good views of Palomar Mountain off to the east as you drive from Pala to Pauma Valley where the highway rises steeply out of the San Luis Rey River flood plain. The Elsinore fault strikes along the steep, southwest face of the mountain, which is famous for its observatory with the 200-inch Hale Telescope, once the largest in the world. With its clear air and position 6,100 feet above sea level, Palomar Mountain was a splendid site for the telescope, but it is now hampered by light pollution from San Diego.
CA 76 practically traces the Elsinore fault as does the course of the San Luis Rey River as far as Lake Henshaw in Warner basin. The lake is an enlarged sag pond along the Elsinore fault zone which lies east of the Henshaw Ranger Station, the store, and the restaurant. In fact, the San Luis Rey picnic area lies squarely upon the fault. The roadcut on the north side of the highway is strongly fractured and contorted as a result of deformation in the fault zone.
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Roadside Geology of Southern California 