Geology & Fossils
A souvenir of the inland seas that washed across Montana during the Cretaceous Period, this ammonite (Placenticeras) was taken from the Bear Paw Shale of eastern Montana.
Nowhere do the Great Plains crash into the Rocky Mountains with more grandeur than in our fourth largest state, Montana. But the beauty of the Big Sky Country, as Montana is nicknamed, doesn’t divert people’s attention from the wealth that lies under their feet.
A hammer and pick and the motto Oro y Plata (“Gold and Silver”) are depicted on Montana’s state seal. These symbolize gold and silver rushes, so important in early Montana history.
Today, Montana’s mining industry is based on its wealth of fossil fuels. The evidence includes counties, communities, and landforms named for petroleum or coal, such as “Carbon” and “Carbonado.” Where there’s coal, there are other fossils, and Montana boasts a mother lode.
Fossils are scattered across Montana, from Tyrannosaurus skeletons entombed in the Hells Creek Formation under eastern grasslands to billion-year-old stromatolites high in the Rocky Mountains in the west.
Stromatolites, or “layer stones” can be seen in Glacier National Park. They’re thought to be the fossilized remains of algae, among the earliest known life forms. Algal mats grow along coasts even today. But Montana’s stromatolite fossils were carried skyward when the Rocky Mountains were pushed up during the Cretaceous Period, over 65 million years ago.
Fossils of mammals and plants from the early Cenozoic Era are found in mountain valleys. Here they were buried under sediments eroded from mountains and volcanic ash, including a recent dusting from Mt. St. Helens, far to the west in Washington State.
Montanasaurs!
What was Montana like between ancient mammals and even more ancient stromatolites? Mesozoic fossils are most common east of the Rocky Mountains. The Triassic and Jurassic periods left fossils of marine animals, as well as coal, a souvenir of lush lowland forests.
Montana’s most exciting fossils represent the last chapter of the Mesozoic, the Cretaceous Period. A great sea stretched from the Gulf of Mexico to the Arctic Ocean, splitting North America in two. The sea might have covered all of Montana, were it not for the slowly rising Rocky Mountains in the west.
In 1806, the explorers Lewis and Clark found what they imagined to be the rib of a giant fish near a now famous Montana landmark called Pompey’s Pillar. It was probably a giant Cretaceous marine reptile, whose remains are buried in the area.
One of North America’s earliest dinosaur discoveries was a tooth found in Montana in 1854. Some teeth found in Montana in 1855 became the first dinosaur fossils in the Western Hemisphere to be given a scientific name — Troodon (“wounding tooth”).
But Montana is best known for its fossils of Triceratops, Tyrannosaurus and hadrosaurs, or duck-billed dinosaurs. Found across Montana, hadrosaur fossils seem to indicate that they were replacing the long-necked sauropods that were so diverse during the Jurassic Period. Montana’s state fossil is the hadrosaur Maiasaura peeblesorum.
Hell Creek Formation is the name given to a blanket of Cretaceous rocks that stretches from eastern Montana into the Dakotas. The formation is exposed in badlands, some of which are protected in Hell Creek State Park, near Fort Peck, Montana.
Hell Creek fossils tell us the area’s ancient forests and wetlands were home to a great variety of turtles and primitive mammals. There were also a surprising variety of small dinosaurs, including the raptor Troodon. Its brain was similar to a bird’s in relative size, one of the largest known dinosaur brains. (Calling a dinosaur a “bird-brain” would be a compliment!) Also bird-like were Troodon ’s large eyes (was it nocturnal?). Its eyes also faced forward somewhat, perhaps giving it depth perception, similar to humans.
But the most famous Hell Creek dinosaurs are its abundant Triceratops and the super-predator Tyrannosaurus. These were among the last dinosaurs to stalk the land.
The rising Rocky Mountains and ebbing seas changed Montana’s climate. Things were changing around the world, for dinosaurs and marine reptiles became extinct, along with many other creatures.
Since the extinction of the dinosaurs, lush forests, deserts, and Ice Age glaciers have all left their mark on Montana. Today, fossil hunters can find Tyrannosaurus bones in Montana badlands, while glacial drift may hide bison bones and arrowheads left 10,000 years ago, or a couple centuries ago.
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