Date of Fossil Sediments:
About Two Medicine Formation:
The area to the east of the Rocky Mountains is especially rich in fossils from the Mesozoic Era. A good example is the Two Medicine Formation of northwestern Montana, which has yielded a diverse array of hadrosaurs, tyrannosaurs and raptors, all dating from a dozen-million-year stretch of the late Cretaceous period. If Two Medicine Formation isn't quite as famous as other North American fossil sites like Hell Creek or Dinosaur Provincial Park, that may be because it's more spread out, and thus less of an attraction for dinosaur buffs.
The most famous fossil site at Two Medicine Formation is Egg Mountain, discovered by paleontologist Jack Horner in 1979. This site contains the fossilized remains of adult, juvenile and infant Maiasaura, as well as numerous eggs laid by this gentle, duck-billed dinosaur (not to mention the occasional Troodon or Orodromeus fossil). What made Egg Mountain such a spectacular find is that it offered the first solid evidence of dinosaur child-rearing: Maiasaura mothers clearly cared for their young, rather than abandoning them at birth.
By the way, the origin of the odd name "Two Medicine" is buried in history. One story says there were once two "medicine lodges" (i.e., clinics) on either side of the river, but it's also possible that "two medicine" is an Anglicization of the Native American name for the area.