Geologic units in Wibaux county, Montana

Tongue River Member of Fort Union Formation (Tertiary) at surface, covers 70 % of this area

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

Hell Creek Formation (Cretaceous) at surface, covers 9 % 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).

Ludlow Member of Fort Union Formation (Tertiary) at surface, covers 9 % 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).

Pierre Formation (Cretaceous) at surface, covers 4 % of this area

Dark gray, partly silty shale with abundant bentonite beds and zones of gray, calcareous concretions. Marine. Thickness as much as650 m (2,133 ft). Only upper 50 m (164 ft) exposed.

Alluvium (Quaternary) at surface, covers 3 % of this area

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

Colgate Member of Fox Hills Formation (Cretaceous) at surface, covers 2 % of this area

White to yellowish, fine- to medium-grained, porous sandstone. Brackish to marine shoreline. Only present near Glendive (eastern Montana) and in several other isolated areas. Thickness 0–40 m (130 ft).

Timber Lake and Trail City Members of Fox Hills Formation (Cretaceous) at surface, covers 2 % of this area

Timber Lake Member (Kftt): Yellowish orange to gray, fine- to medium grained, noncalcareous, hummocky-bedded sandstone. Thickness 15–22 m (50–72 ft). Trail City Member (Kftc): Yellowish orange, wavy-bedded siltstone and black shale with calcareous concretion zone. Thickness 10 m (33 ft).

Gravel (Quaternary) at surface, covers 0.4 % 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.

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).

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).

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

Grayish-brown and yellowish-brown silt, sand, clay, sandstone, and lignite; river, lake, and swamp sediment; as thick as 100 metres (300 feet).