Geologic units in Custer county, Montana

Lebo Member of Fort Union Formation (Tertiary) at surface, covers 33 % of this area

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

Tongue River Member of Fort Union Formation (Tertiary) at surface, covers 29 % of this area

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

Tullock Member of Fort Union Formation (Tertiary) at surface, covers 21 % 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).

Alluvium (Quaternary) at surface, covers 6 % of this area

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

Gravel (Quaternary) at surface, covers 5 % 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.

Ludlow Member of Fort Union Formation (Tertiary) at surface, covers 3 % 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).

Hell Creek Formation (Cretaceous) at surface, covers 3 % 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 < 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.