Piegan Group: Helena and Wallace Formations

Helena Formation (Yh): Northwestern Montana: (formerly “lower Wallace”): cycles of basal white quartzite or intraclast beds overlain by couplets of green siltite and argillite, capped by dolomite beds. Calcite pods and ribbons (molar tooth structure) common. West-central Montana: gray to dark gray limestone and dolomitic limestone with siltite partings. Thickness as much as 2,000 m (6,562). Wallace Formation: Tan-weathering, dolomitic quartzite and siltite, and black argillite with calcite ribbons (molar tooth structure) in graded pinch-and-swell couples and couplets. Thickness as much as 2,500 m (8,202 ft).
State Montana
Name Piegan Group: Helena and Wallace Formations
Geologic age Middle Proterozoic
Lithologic constituents
Metamorphic > Metasedimentary > Metaclastic > Argillite
Metamorphic > Metasedimentary > Metaclastic > Quartzite
Metamorphic > Metasedimentary > Metaclastic > Metasiltstonesiltite
Sedimentary > Carbonate > Limestone (Bed)
Sedimentary > Carbonate > Dolostone (Bed)

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.

NGMDB product
Counties Beaverhead - Broadwater - Deer Lodge - Flathead - Glacier - Granite - Jefferson - Lake - Lewis and Clark - Lincoln - Mineral - Missoula - Powell - Ravalli - Sanders