Over one hundred years ago, J.S. Diller (1902) published his classic work, Topographic Development of the Klamath Mountains, in which he proposed that a regional lowland erosion surface, the “Klamath peneplain,” formed in northwestern California during the late Miocene. W.P. Irwin's (1997)  Preliminary Map of Selected Post-Nevadan Geologic Features of the Klamath Mountains and Adjacent Areas clearly delineates the extent of this erosion surface and provides detailed mapping and age determinations for overlap deposits in both the Klamath and Coast Range provinces. Among these deposits, basal late Miocene Wildcat Group, Saint George Formation, and Wimer Formation sediments are littoral. Rapid submergence of northwestern California to bathyal depths occurred, following initiation of Wildcat–Saint George–Wimer sedimentation. By 5 Ma, the Klamath peneplain was at sea level, based upon Wimer Formation overlap. A ⁴⁰Ar/³⁹Ar laser probe analysis of detrital micas indicates that Wildcat Group marine sediments are, in part, derived from the Idaho batholith. Sediment progradation and shoaling over a wide coastal shelf, onset of deposition of locally derived fluvial sediments in the Coast Ranges, and regional uplift during the middle Pleistocene may relate to the decreasing age of the subducting oceanic plate, decreasing dip angle of subduction, translation of the Sierra Nevada microplate northwestward as it impinges upon the Klamath block, and/or migration of the Blanco fracture zone northward along the continental margin.