Geologic units in Sheridan county, Montana

Tongue River Member of Fort Union Formation (Tertiary) at surface, covers 74 % of this area

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

Lebo Member of Fort Union Formation (Tertiary) at surface, covers 15 % 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 5 % of this area

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

Gravel (Tertiary) at surface, covers 3 % 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.

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).

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

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

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).