Dark, manganese-rich rock varnish of probable Miocene age is exhumed in a road cut in the drainage of Davidson Canyon, southern Arizona. Its structure and inorganic chemistry is similar to modern arid and semiarid rock varnishes. The δ13C content of organic matter in the varnish is consistent with a C4 photosynthetic pathway for adjacent plants. Organic and inorganic chemical analyses of paleovarnishes can be used to interpret past subaerial environments.

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