Philip Thomas Tyson (1799–1877) toured privately through central California, from San Francisco through the Sierra Nevada foothills gold prospects in 1849, to assess their potential and the general geology of the region. He produced the first regional map with geologic notations and several rough topographic/geologic cross-sections. He described Coast Range basement rocks, now described as Franciscan Complex mélange and broken formation, the stratigraphic configuration of the Great Valley, and general geology of the Sierra Nevada foothills. He recognized that the older Coast Range and Sierran basement were deformed prior to recent volcanism and extensive terrestrial fluvial sedimentation, likely Neogene in age.

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