Middle Canyon Formation: Dark gray, silty limestone with chert beds and nodules. Shallow marine. Thickness at type section 335 m (1,100 ft). McGowan Creek Formation: Upper part: dark gray and pale yellowish brown, thin-bedded, calcareous siltstone interbedded with dark gray, silty limestone. Lower part: dark gray, carbonaceous, thin-bedded argillite with interbeds of dark gray siltite, medium gray, fine-grained, conglomeratic sandstone and quartzite, and dark gray, silty limestone. Marine. Thickness as much as 61 m (200 ft). Three Forks Formation (MDt): Sappington Member (upper part of formation):yellowish orange and yellowish gray siltstone. Trident Member (middle part of formation): greenish gray and light olive gray, fossiliferous, calcareous shale that contains interbeds and nodules of fossiliferous, argillaceous limestone. Logan Gulch Member (lower part of formation): yellowish gray and grayish red, argillaceous limestone breccia and shale breccia that may include dolomitic siltstone. Marine to restricted marine with evaporite basins. Thickness as much as 185 m (607 ft). Jefferson Formation (Dj): Birdbear Member (upper part of formation): light to medium gray, sucrosic dolomite. Lower part of formation: grayish black, commonly petroliferous dolomite or limestone that may be interbedded with light gray quartzite. Marine. Thickness as much as 520 m (1,706 ft). Kinnikinic Formation (Ok): Very light gray, massive quartzite with local lenses of dolomite and dolomitic shale. Marine shelf. Thickness as much as 245 m (804 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.