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The San Bernardino and San Gabriel Mountains provide an excellent setting for exploring the evolution and diversity of crystalline rocks in California. The oldest rock-forming events which can be explored in these ranges involved episodic Paleoproterozoic magmatism and orogenesis extending from 1.81 to 1.65 Ga. Rock units of this age are widespread both east and west of the San Andreas fault. This Paleoproterozoic tectonism was followed by intrusion of younger Mesoproterozoic anorogenic igneous rocks that are areally limited, but well exposed in the San Gabriel Mountains as 1.19 Ga gabbro, anorthosite, and syenite. Proterozoic igneous activity and tectonism in southwest North America was followed by rifting during the Neoproterozoic, which led to development of the Cordilleran geosynclinal belts. Belts of rocks within the geosyncline in southern California trend northeast-southwest, with deeper water rocks to the northwest, and Neoproterozoic and Paleozoic metasedimentary rocks in the San Bernardino Mountains belong to the transition zone between the cratonal and deeper water miogeoclinal sequences. Passive margin sedimentation ended with initiation of arc magmatism oriented along a northwest to southeast trend in Late Permian time. A diverse group of Mesozoic plutons and dike swarms as young as Late Cretaceous in age characterize the crystalline terranes of both the San Bernardino and San Gabriel Mountains, culminating in emplacement of large calc-alkalic intrusive suites in both ranges about 78 Ma. The diversity of ages and types of crystalline rocks makes a field trip through either or both of these ranges a great opportunity to engage students in active learning while linking petrology and historical geology course content in a field context. Students can utilize rock identification skills learned in the laboratory, and with knowledge of available geochronologic data, can construct a more detailed geologic time scale for the region. Here we will provide an example of a one-day trip to examine Proterozoic metamorphic and Mesozoic intrusive igneous rocks that are easily accessible in roadcuts and on short field traverses along National Forest roads. The trip is adapted from more detailed field guides and road logs for this region (principally Barth et al., 2001), with a focus on undergraduate learning.
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