Formation · Cretaceous
Two Medicine Formation
Late Cretaceous unit, Montana
The Two Medicine Formation of northwestern Montana is the Campanian unit that produced the Maiasaura nesting colonies at Egg Mountain and the horned dinosaur Einiosaurus. Dealers see hadrosaur bone, eggshell fragments and shed teeth of Troodon and tyrannosaurids. Because so much of the outcrop is reservation or research land, provenance paperwork matters more here than for Hell Creek.
| Country | United States |
|---|---|
| Region | Montana |
| Age | Campanian, Late Cretaceous |
| Age range | ~83–74 Ma |
| Macrostrat rank | Fm· Macrostrat |
| Macrostrat age | 83.9–67.5 Ma· Macrostrat |
| Lithology | shale, sandstone, volcanic, conglomerate, siltstone· Macrostrat |
| Environment | fluvial braided, non-marine, delta plain, channel, fluvial indet.· Macrostrat |
| Macrostrat columns | 4 column(s) sampled from 4 unit(s)· Macrostrat |
| Group (Macrostrat) | Montana· Macrostrat |
| PBDB collections | 90 collections, 290 occurrences· Paleobiology Database |
Collecting and legality
Much of the outcrop lies on the Blackfeet Reservation, where collecting requires tribal permission, or on private ranches near Choteau and Bynum. Egg and nest material from Egg Mountain is under research control and should not appear in trade without a clear private-land provenance.
Lithology and setting
Alluvial-plain mudstones and sandstones shed eastward from the rising Cordillera, with abundant paleosols and eggshell horizons.
Commonly recorded taxa (PBDB)
Genera most often recorded from this unit in the Paleobiology Database (first 200 occurrences): Hypacrosaurus (14), Hadrosauridae (9), Theropoda (9), Troodon (8), Lambeosaurinae (7), Orodromeus (7), Daspletosaurus (6), Ankylosauridae (5), Maiasaura (5), Dinosauria (4), Nodosauridae (4), Saurornitholestes (4), Spheroolithus (4), Centrosaurinae (3), Cerasinops (3).
Source: Paleobiology Database
Collecting rules
Elsewhere
- Macrostrat: 2164
- macrostratConcept: 10693
- PBDB: strata:Two Medicine
- Wikidata: Q3054650
- Macrostrat stratigraphic name
- PBDB occurrences for this unit
- Wikipedia: Two Medicine Formation
- Mindat: Two Medicine Formation
Cite this article
Stratum Study. "Two Medicine Formation" (Formation). Stratum Study pack 2026.09, as of 2026-09-19. https://stratum.black/study/formations/two-medicine-formation. Accessed 2026-09-20.
@misc{stratum-two-medicine-formation,
title = {Two Medicine Formation},
author = {{Stratum Study}},
howpublished = {\url{https://stratum.black/study/formations/two-medicine-formation}},
year = {2026},
note = {Formation article, Stratum Study pack 2026.09, as of 2026-09-19; sources: stratum, macrostrat, pbdb, wikidata, commons. Accessed 2026-09-20.}
} Citations carry the pack version and the article's as-of date so they resolve to the same text later. The facts themselves belong to the sources listed below; cite those for the science.