This study focuses on the lithology of a Triceratops excavation site near Marmarth, North Dakota. The excavation site, Hell Creek Formation, consists of sedimentary rocks, predominantly clays, sandstones, and mudstones dating to the Late Cretaceous period approximately 66 million years ago. Fossilized dinosaur skeletons and preserved concentrations of microfossils, or fossil micro-sites, can be found in exposures of the Hell Creek Formation throughout Montana and North and South Dakota. Sediment samples were collected directly from and near the Triceratops excavation site. The majority of sediments present at the site are sand and silt-sized, representing floodplains and channel sands, with occasional plant fossils and thin coal layers. The types of sediment found at the site, along with evidence supporting the presence of terrestrial species, supports the inter-pretation that during the Cretaceous Period, the depositional environment of western North Da-kota was warm and wet, likely a paludal or transitional coastal environment similar to North America’s Gulf Coast today.