The Two Medicine Formation is a geological formation, or rock body, in northwestern Montana and southern Alberta that was deposited between 82.4 Ma and 74.4 Ma, during Campanian (Late Cretaceous) time. It crops out to the east of the Rocky Mountain Overthrust Belt, and the western portion (about 600 metres or 2,000 feet 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 formation 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 Formation 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 Formation is mostly sandstone, deposited by rivers and deltas.
History of research 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, David 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.
Geology The loosely consolidated fine grain sediments composing the formation allow for fast plant growth in badland areas, limiting the number of exposed outcrops. Paleosols, fluvial deposits and bentonitic layers are common in the Two Medicine Formation.
Age The Two Medicine Formation spans from 82.4 to 74.4 Ma, nearly the entire length of the Campanian stage. The age of the different members of the formation are as follows:
Rock City Member: 82.4-80.8 Ma Shields Crossing Member: 80.8-80.2 Ma Hagans Crossing Member: 80.2-77 Ma Flag Butte Member: 77-74.8 Ma
Equivalents There are several equivalents to the Two Medicine Formation, as with many geologic formations (most of which are named after their type locality). The Sweetgrass Arch in Montana divides the Two Medicine from the Judith River Formation, Bearpaw Shale, Claggett Shale, and Eagle Sandstone. Across the Canada–United States border, the Two Medicine Formation correlates to the Belly River Group in southwest Alberta, and the Pakowki Formation eastward.
Stratigraphy The Two Medicine overlies the Virgelle Sandstone, which formed from the beach sands exposed on northern and western shores of the receding Colorado Sea. A Cretaceous Interior Seaway transgression submerged the area briefly early on in Two Medicine history leaving anomalous paralic sediments and isolated shale bodies about 100 m above the base of the formation. The Middle portion of the two medicine formation is about 225 m thick, deposited while the Clagette Sea was receding and the Bearpaw Sea transgressing. This portion is stratigraphically equivalent to the Judith River Formation and Judith River Group. The sediments are mainly bentonitic siltstones and mudstones with "occasional sandstone lenses." These sediments are thought to be the remains of a coastal plain "far removed" from the interior sea. The upper portion is about one half of the formation. Its sediments are similar to the middle portions but punctuated by extensive red beds and caliche horizons. The uppermost 80 m were deposited after the inundation of the Judith River equivalent sediments by the Bearpaw Sea. They are thought to have been deposited in only 500,000 years. Bentonitic ash is common in the Two Medicine. To the south extrusive volcanic activity occurred in association with the Boulder Batholith collectively called the Elkhorn Volcanics.
Taphonomy Most of the vertebrate fossils are preserved by CaCo3 permineralization. This type of preservation preserves high levels of detail, even down to the microscopic level. However, it also leaves specimens vulnerable to weathering when exposed to the surface.
Paleoenvironment
Climate
The Two Medicine Formation was deposited in a seasonal, semi-arid climate with possible rainshadows from the Cordilleran highlands. This region during the Campanian experienced a long dry season and warm temperatures. Lithologies, invertebrate faunas, and plant and pollen data support the above interpretation. The extensive red beds and caliche horizons of the upper Two Medicine are evidence of at least seasonally arid conditions. Some of the dinosaurs from the formation have been speculated to have shown signs of drought-related death.
Elevation A more upland environment existed in the south of the Two Medicine Formation. Streams had a northeasterly flow away from these southwestern uplands. The southern part of the Two medicine formation grades into brackish water siltstone/sandstone series called the Horsethief Formation. The sediments of the Horsethief represent shallower water deposits than the Bearpaw Shale adding further evidence of higher elevation areas existing in the south.
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