Geologic units in Richland county, Montana

Tongue River Member of Fort Union Formation (Tertiary) at surface, covers 71 % of this area

Yellowish orange sandstone, sandy and silty carbonaceous shale, and coal. Alluvial plain. Thickness as much as 300 m (984 ft).

Gravel (Quaternary) at surface, covers 8 % of this area

Variable deposits that range from pebble to boulder size and include sand, silt, and clay. Dominantly alluvial terrace, abandoned channel and floodplain, remnant alluvial fan, and local glacial outwash.

Lebo Member of Fort Union Formation (Tertiary) at surface, covers 8 % of this area

Dark gray carbonaceous shale, bentonitic claystone, sandstone, and coal. Alluvial plain. Thickness as much as 185 m (607 ft).

Alluvium (Quaternary) at surface, covers 8 % of this area

Gravel, sand, silt, and clay deposits of stream and river channels, and floodplains.

Tullock Member of Fort Union Formation (Tertiary) at surface, covers 2 % of this area

Yellow sandstone interbedded with subordinate grayish brown and black shale and thin beds of coal. Alluvial plain. Thickness as much as 180 m (590 ft).

Hell Creek Formation (Cretaceous) at surface, covers 1 % of this area

Light gray, bentonitic claystone that alternates with gray to brown sandstone interbedded with carbonaceous shale. Laterally equivalent to Lance Formation. Fluvial and flood plain. Thickness as much as 335 m (1,100 ft).

Gravel (Quaternary and Tertiary) at surface, covers 1 % of this area

Variable deposits that range from pebble to boulder size and include sand, silt, and clay. Dominantly alluvial terrace, abandoned channel and floodplain, remnant alluvial fan, and local glacial outwash.

Bearpaw Formation (Cretaceous) at surface, covers 0.8 % of this area

Dark gray shale with several zones of calcareous concretions, a basal zone of ferruginous concretions, and numerous thin bentonite beds. Marine. Thickness as much as 300 m (984 ft).

Fox Hills Formation (Cretaceous) at surface, covers 0.5 % of this area

Yellowish orange to gray, fine- to medium-grained, non-calcareous sandstone in upper part, and interbedded sandstone, siltstone, and black shale with calcareous concretion zone in lower part. Marginal marine. Thickness 30–45 m (98–148 ft).

Bullion Creek Formation (Paleocene) at surface, covers 0.1 % of this area

Yellowish-brown silt, sand, clay, sandstone, and lignite; river, lake, and swamp sediment; as thick as 200 metres (600 feet).

Ludlow Member of Fort Union Formation (Tertiary) at surface, covers < 0.1 % of this area

Gray and brown shale, siltstone, silty or bentonitic claystone, sandstone, and coal. Alluvial plain with marine-influenced tongues. Thickness as much as 230 m (755 ft).

Gravel (Tertiary) at surface, covers < 0.1 % of this area

Variable deposits that range from pebble to boulder size and include sand, silt, and clay. Dominantly alluvial terrace, abandoned channel and floodplain, remnant alluvial fan, and local glacial outwash.

Sentinel Butte Member of Fort Union Formation (Tertiary) at surface, covers < 0.1 % of this area

Dark gray shale with interbedded lignite and gray sandstone. Nonmarine. Thickness 200 m (656 ft).

Sentinel Butte Formation (Paleocene) at surface, covers < 0.1 % of this area

Grayish-brown silt, sand, clay, sandstone, and lignite; river, lake, and swamp sediment; as thick as 200 metres (600 feet).