Harlowton lies in the Fort Union Geological Formation, created about 65 million years ago shortly after the dinosaur extinction, when rivers from the southwestern mountain slopes of Montana carried sediment from swampy, subtropical eastern coastal plains into a shallow inland sea. As those rivers shifted, swamp vegetation and peat were buried beneath thick sand, silt, and clay that later formed coal, while the deposits compacted into sandstone, siltstone, and mudstone that erode easily and leave few resistant outcrops. The area is also known for Paleocene fossils: in 1902, Princeton University paleontologist Earl Douglass and local homesteader and self-taught paleontologist Albert Silberling discovered primitive mammal remains in a quarry southwest of Harlowton. Over the next four decades, paleontologists excavated that quarry and three others, yielding remains of at least 23 species, including the squirrel-like Ptilodus, which first appeared in the middle Jurassic about 160 million years ago and became extinct about 35 million years ago. It lived alongside sauropods such as Apatosaurus and Diplodous and the meat-eating Allosaurus, and in Montana's warm, humid, possibly densely forested setting it was a good climber with dexterous feet, legs, and tail, a well-developed sense of smell, and herbivorous teeth. Other fossils found by Douglass and Silberling included a primitive ungulate called Phenacodus and a primate-like animal called Plesiadapis, which co-existed with Ptilodus and were preyed upon by Creodonts, the dominant carnivorous animal of the time. Silberling's excavation of thousands of fossils from quarries in the Harlowton area greatly contributed to scientific knowledge about life in the Paleozoic Era.