Piegan group (MTpCApi?;0)

Piegan group: most of the Piegan group is subdivided into formations that are in part equivalent to each other. Where correlations are relatively doubtful the group designation is retained. Carbonate-bearing rocks predominate in the group, but the proportions and character of the impurities in these rocks and the relations to non-carbonate-bearing rocks vary from place to place.

State	Montana
Name	Piegan group
Geologic age	Proterozoic \| Mesoproterozoic
Original map label	pCpi?
Comments	pCpi? is not a unit shown on the source paper map, but was assigned by Raines and Johnson (1995) for unlabeled polygons thought to be this unit. This unit nomenclature is now obsolete, replaced on more modern geologic maps by individually mapped formations of the Belt Supergroup. As mapped, probably consists mostly of the Helena Formation.
Primary rock type	carbonate
Secondary rock type	meta-argillite
Other rock types	quartzite
Lithologic constituents
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	Raines, G.L. and Johnson, B.R., 1995, Digital representation of the Montana stage geologic map: a contribution to the Interior Columbia River Basin Ecosystem Management Project: U.S. Geological Survey Open-File Report 95-691, 21 p.; http://pubs.usgs.gov/ [http://pubs.er.usgs.gov/publication/ofr95691] [http://pubs.usgs.gov/]
Geographic coverage	Beaverhead