Cody Formation (Kco): Gray to dark gray shale with some sandstone beds. Eldridge Creek Member: greenish gray, glauconitic, fine-grained sandstone. Marine. Thickness as much as 465 m (1,526 ft). Frontier Formation (Kf): Western Montana: gray, fine- to medium-grained, locally conglomeratic sandstone with interbeds of dark gray carbonaceous shale. Local conglomerate, porcellanite, and coal. Central Montana: dark gray, carbonaceous shale interbedded with siltstone and sandstone, chert-pebble conglomerate, and coal. Big Elk Sandstone Member: light gray, chert-rich sandstone commonly stained dark red, interbedded with thin, dark gray to black, clayey shale. Boulder River Sandstone Member: gray to greenish gray, very fine-grained to conglomeratic, glauconitic sandstone. Brackish to nonmarine. Thickness exceeds 2,135 m (7,005 ft) in Lima Peaks region, and exceeds 915m in the Greenhorn, Snowcrest, Gravelly, and Pioneer Mountains of southwestern Montana. Elsewhere thickness as much as 215 m (705 ft).
Vuke, S.M., Porter, K.W., Lonn, J.D., and Lopez, D.A., 2007, Geologic Map of Montana - Compact Disc: Montana Bureau of Mines and Geology: Geologic Map 62-C, 73 p., 2 sheets, scale 1:500,000. This map was digitized in 2012 as a result of a contract between the U.S. Geological Survey and the Montana Bureau of Mines and Geology.