Two Medicine Formation

The Two Medicine Formation is a rock unit composed primarily of sandstone deposited by ancient rivers and deltas, dating from about 82.4 to 74.4 million years ago during the Campanian stage of the Late Cretaceous, spanning nearly the entire stage. It is found in northwestern Montana and southern Alberta, extending east from the Rocky Mountain Overthrust Belt, with a heavily folded and faulted western portion and a relatively undeformed eastern portion thinning toward the Sweetgrass Arch. The formation sits between the nearshore Virgelle Sandstone below and the marine Bearpaw Shale above, and its loosely consolidated, fine-grained sediments support rapid plant growth that limits exposed badland outcrops. It is renowned for extraordinary dinosaur fossils, most notably at Egg Mountain, where Maiasaura fossils showing cartilaginous, not-yet-mobile hatchlings in dug-out earthen nests provided some of the first direct evidence of parental care in dinosaurs, with more than two hundred Maiasaura specimens recovered ranging from embryo to adult, alongside numerous other dinosaur genera. This entity documents the Two Medicine Formation specifically as a geological formation, a Late Cretaceous fluvial and deltaic sedimentary sequence.