Mineral Resources On-Line Spatial Data
Mineral Resources > Online Spatial Data > Geology > by state > Montana Geology
Geologic units in Prairie county, Montana
- Fort Union formation (Phanerozoic | Cenozoic | Tertiary Cretaceous-Late | Paleocene) at surface, covers 93 % of this area
Fort Union formation: Clay shale, siltstone, and sandstone; local lenses of impure limestone, and numerous lignitic beds; contains Tertiary plant and animal fossils but no dinosaurs; base generally placed at the lowest of the succession of lignite beds within it; includes the Tongue River member, Lebo shale member, and Tullock member.
- Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers 5 % of this area
Alluvium: mainly valley fill consisting of silt, sand, and gravel; includes some terrace deposits and glacial drift of Pleistocene age in some areas; locally includes hot spring tufa. The older part of the alluvium, where present, is probably of Pliocene age.
- Fox Hill sandstone (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.4 % of this area
Fox Hills sandstone: Typically shaly sandstone grading upward into massive brownish sandstone with white sandstone of the Colgate member locally at top.
- Flaxville gravel (Phanerozoic | Cenozoic | Tertiary) at surface, covers < 0.1 % of this area
Flaxville gravel: Brown, yellow, and gray gravel, sand, and silt with marl and volcanic ash locally.
- Pierre shale (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.6 % of this area
Pierre shale: dark-gray clay shale with calcareous and ferruginous concretions and sandy members.
- Hell Creek formation (Phanerozoic | Mesozoic Cenozoic | Cretaceous-Late Tertiary | Paleocene) at surface, covers 0.2 % of this area
Hell Creek formation: somber-gray sandstone and greenish shaly clay and mudstone containing dinosaur bones; a few thin lignite and subbituminous coal beds.