Why do paleontologists care about rocks? Rock formations house the secrets of the past! Let’s take a walk through time, starting with the oldest formation preserved at our field site, the Bearpaw Shale, and work our way forward through the Fox Hills Sandstone, to the Hell Creek and Tullock formations, the two formations that are the focus of our DIG research.
The lowest (and oldest) formation exposed near the DIG field camp is called the Bearpaw Shale (or Bearpaw Formation). This formation formed ~74-70 mya (million years ago) as a fine-grained layered mudstone (or shale) in a shallow sea, the Western Interior Seaway, that ran through the United States from the Gulf of Mexico to Canada. This formation was deposited just before the sea began to recede near the end of the Cretaceous Period. Can you believe there was once a sea cutting North America in half, and living in this seaway were sharks, giant marine reptiles like the long-necked plesiosaurs, and extinct molluscs called ammonites? All of these creatures (and others!) went extinct with the dinosaurs 66 mya.
Fox Hills Sandstone
As the Western Interior Seaway receded, eastern Montana went from being covered by a shallow sea to being near shore and beach, with the seaway still present but located further to the south and east. The sediments that preserve this ancient beach make up the Fox Hills Sandstone Formation, and were deposited 70-68 mya. These yellow/tan, plain sandstone beds are very thick and contain few fossils in our field area, so we most often use the resistant, ledge-forming sediments at the top of this formation as a “marker bed” to help identify the overlying Hell Creek Formation. Scientists think the Fox Hills Sandstone would have been home to a community of dinosaurs, mammals, reptiles, and early birds that would have come down to the shores of the shallow sea to drink and feed.
Hell Creek Formation
The Hell Creek Formation overlies the Fox Hills Sandstone, and is one of the two focus formations for the DIG researchers. This is one of the more famous and widely exposed formations from the Mesozoic Era in the state of Montana. This formation was deposited 68-66 mya and is primarily composed of “drab” and “somber” colored beds of tan sandstones, gray siltstones, and purple mudstones, with little to no coal. These sediments were deposited by freshwater and brackish rivers flowing from the proto-Rocky Mountains into the Western Interior Seaway. Paleontologists have used these sediments to infer an environment that looked something like the picture below. During this time, the environment was composed of large rivers that had rocky shores. Fossils from many animals are found here including invertebrates (like clams and snails), fishes, amphibians, mammals, turtles, crocodiles and dinosaurs. In fact, the first T. rex skeleton, discovered in 1902 by Barnum Brown, was found in the exact region of the Hell Creek Formation that the DIG Field School takes place!
The second focus formation, and the highest we find at the DIG Field school preserves the first Paleogene sediments, and was formed just after the K/Pg mass extinction event. This earliest Paleogene formation was deposited 66-64 mya and is known as the Tullock Formation (also known as the Tullock Member of the Fort Union Formation in some areas). This formation consists of thinner, vibrant and colorful beds with yellow, orange, and tan sandstones, siltstones, and mudstones, and lots of large coal seams (low-grade coals known as “lignites”). These beds are so thin, that from far away the different sediments look like stripes on the outcrop, and they have been dubbed “pajama beds” by someone who must have had striped pj’s! During this Era, rivers carried sediment from the mountains to the inland sea causing a swampy vegetative environment. Here, we find remnants of the mammals, reptiles, amphibians, fish, and birds that succeeded the dinosaurs, some of whom survived, and others who immigrated to the area shortly after the mass extinction.
When you compare the deposits of the Hell Creek and Tullock formations, they look totally different, as do the inferred landscapes they represent! Can you differentiate the drab, somber, mudstones of the Hell Creek Formation from the more finely striped beds of the coal-bearing Tullock Formation in the photo below? Knowing where you are in time when you’re standing on the outcrop is a critical paleontological skill.
Next week we DIG into the fossils we will find during the DIG Field School!