Closeup view of an Oligocene-Miocene Vasquez Formation conglomerate. The angularity of the cobbles and boulders indicates a short distance of transport, and the reddish color is due to nonmarine deposition. The cobbles and boulders are made of anorthosite, gabbro, quartzite and other igneous and metamorphic rocks, some having come from the San Gabriel Mountains and others from farther away. The above features all point to deposition on a rather steep alluvial fan that existed here during the Oligocene and Miocene Epochs. A further conclusion is that the San Gabriel Mountains existed during the Oligocene and that they had already been eroded deeply into the plutonic igneous rocks.
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