Egg Mountain
| Two Medicine Formation Stratigraphic range: Campanian, 83.5–74.9 Ma |
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Exposure of the Two Medicine Formation near "Egg Mountain" in northern Montana.
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| Type | Geological formation |
| Underlies | Bearpaw Shale |
| Overlies | Virgelle Sandstone |
| Lithology | |
| Primary | Sandstone |
| Location | |
| Coordinates | 48°04′27″N 112°17′58″W / 48.07417°N 112.29944°W |
| Approximate paleocoordinates | 55°18′N 77°48′W / 55.3°N 77.8°W |
| Region | Montana, Alberta |
| Country | U. S. A., Canada |
The Two Medicine Formation is a geologic formation, or rock body, that was deposited between 83.5 ± 0.7 Ma to 70.6 ± 3.4 Ma (million years ago), during Campanian (Late Cretaceous) time, and is located in northwestern Montana and southern Alberta. It crops out to the east of the Rocky Mountain Overthrust Belt, and the western portion (about 600 m thick) of this formation is folded and faulted while the eastern part, which thins out into the Sweetgrass Arch, is mostly undeformed plains. Below the Two Medicine Fm. are the nearshore (beach and tidal zone) deposits of the Virgelle Sandstone, and above it is the marine Bearpaw Shale. Throughout the Campanian, the Two Medicine Fm. was deposited between the western shoreline of the Late Cretaceous Interior Seaway and the eastward advancing margin of the Cordilleran Overthrust Belt. The Two Medicine Fm. is mostly sandstone, deposited by rivers and deltas.
In 1913, a US Geological Survey crew headed by Eugene Stebinger and a US National Museum crew headed by Charles Gilmore worked together to excavate the first dinosaur of the formation. Stebinger was the first to identify the Two Medicine Formation and formally described the first fossils in a scientific paper published in 1914. Gilmore returned to the Formation in 1928 and 1935. During this time frame only three species were named and of these only Styracosaurus ovatus and Edmontonia rugosidens are still regarded as valid. Barnum Brown prospected the formation in 1933, but found nothing significant. Both of their research were interrupted by World War II. In 1977 Trexler reports finding hadrosaur remains west of Choteau, Montana. During the next year baby hadrosaurs were discovered. In 1979 Horner and Makela referred these hadrosaur bones to Maiasaura peeblesorum. The announcement attracted renewed scientific interest to the formation and many new kinds of dinosaurs were discovered. More nesting sites were discovered later, including the Devil's Coulee site yielding Hypacrosaurus stebingeri in southern Alberta in 1987.
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Wikipedia