The Geology of the Jurupa Mountains, San Bernardino and Riverside Counties, California

Citation

MacKevett, Edward Malcolm (1950) The Geology of the Jurupa Mountains, San Bernardino and Riverside Counties, California. Master's thesis, California Institute of Technology. doi:10.7907/Y1V7-CG63. https://resolver.caltech.edu/CaltechTHESIS:04012010-082544565

Abstract

The Jurupa Mountains are in the western parts of San Bernardino and Riverside Counties, California. The geographical center of the Jurupa Mountains is about 44 miles east of Los Angeles and 7 miles northwest of Riverside. This report describes an area of approximately 15 square miles. Crystalline rocks comprise the bulk of the Jurupa Mountains and probably are the northernmost exposures of the rocks typical of the Southern California batholith. The oldest rooks in the Jurupa Mountains are a series of pre-batholithic metasedimentary rooks which are questionably Triassic in age. This series is composed of quartz-biotite gneiss, impure quartzite, biotite-quartz schist, marble, calc-silicate contact rocks, and amphibole schist, listed in order of abundance. Characteristically, these rocks occur as septa between the intrusive plutons. The largest deposits of marble and contact rocks are at Jensen quarry, a locality renowned for its rare minerals. San Marcos gabbro occurs in a few hornblende-rich island like bodies within the later intrusive looks, and it is the oldest of the batholithic rocks. Bonsall tonalite is the most widespread rock in the Jurupa Mountains. It contains abundant inclusions near its contacts, and commonly has good foliation and lineation. The Woodson Mountain granodiorite is a leucocratic rock that has several textural variations. It generally crops out in large boulder-like masses and is one of the most erosion-resistant rocks in the area. Pegmatite dikes are abundant in the Jurupa Mountains and are characterized by bold, rib-like outcrops. Alluvium, mainly in the form of fan material that is locally overlain by aeolian sand, practically surrounds the Jurupa Mountains. The Jurupa Mountains are in the northern part of the Perris fault block. No definitely-known faults occur in the Jurupa Mountains. The batholithic rocks of the Jurupa Mountains were probably emplaced as a sequence of plutons by a combination of processes of which stoping was the most important. The economic resources of the Jurupa Mountains are marble, for making cement, and granodiorite and tonalite, mainly for rip-rap. In addition, several abandoned gold prospects are present. Most of these are on quartz stringers within the gneiss or schist.

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