Montana group, undifferentiated

Montana group, undifferentiated: mainly sandstone and shale, soft and in part poorly consolidated; some coal; shown undivided only in areas where detailed data are lacking and in these areas some rocks not of Montana age may have been included. In particular, rocks designated Km in southwestern Montana may include some Tertiary strata.
State Montana
Name Montana group, undifferentiated
Geologic age Phanerozoic | Mesozoic | Cretaceous-Late
Primary rock type fine-grained mixed clastic
Secondary rock type coal
Other rock types
Lithologic constituents
Sedimentary > Clastic > Mixed-clastic > Sandstone-Mudstone (Bed)
Sedimentary > Coal (Bed)
Map references
Ross, P.R., Andrews, D.A., and Witkind, I.J., 1955, Geologic map of Montana: U.S. Geological Survey, 2 sheets, scale 1:500,000.
Unit references
Larsen, E.S., 1940, The petrographic province of central Montana: Geological Society of America Bulletin, v. 51, p. 887-948.
Viele, G.W. and Harris, F.G., III, 1965, Montana Group stratigraphy, Lewis and Clark County, Montana: American Association of Petroleum Geologists Bulletin, v. 49, no. 4, p. 379-417.
Stebinger, E., 1914, Titaniferous magnetite beds on the Blackfeet Indian Reservation, Montana: U.S. Geological Survey Bulletin 540, p. 329-337.
Gill, J.R. and Cobban, W.A., 1973, Stratigraphy and geologic history of Montana Group and equivalent rocks, Montana, Wyoming, and North and South Dakota: U.S. Geological Survey Professional Paper 776, 37 p.
Merewether, E.A., 1996, Stratigraphy and tectonic implications of Upper Cretaceous rocks in the Powder River basin, northeastern Wyoming and southeastern Montana, in Evolution of sedimentary basins--Powder River basin: U.S. Geological Survey Bul
Counties Beaverhead - Blaine - Chouteau - Lewis and Clark - Madison - Park - Stillwater - Sweet Grass - Teton