Carboniferous rock units on the eastern flank of the Klamath Mountains, northern California, include the upper Mississippian Bragdon Formation, upper Mississippian and Pennsylvanian Baird Formation, and possibly the basal part of the predominantly lower Permian McCloud Limestone. The Bragdon is dominantly argillite and wacke, with local limestone and calcilutite containing two kinds of brachiopod associations and a coral association. The Baird is a mixture of argillite, siltstone, tuff, volcanic-rich siltstone and arenite, and numerous minor rock types. It contains in its lower part one low- and one high-diversity mollusk association, and one coral association. The Hirz Mountain Limestone Member (new name) of the Baird Formation, of late Pennsylvanian or early Permian age contains an additional brachiopod association and a coral association, and is approximately correlative with the oldest part of the McCloud Limestone, which contains a brachiopod-fusulinid association. Brachiopod associations indicate shallow marine waters of relatively constant depth and salinity from late Mississippian through early Permian times; sedimentary deposition was complex and highly variable.