The Pryor Mountains rise to the southeast, with prominent cliffs and dip slopes of Madison Limestone deposited in shallow seas more than 300 million years ago, while younger tan Tensleep sandstone and brick-red Chugwater Formation crop out along the mountain margins and canyons expose still older rocks, including Precambrian basement rock more than two billion years old. The crustal blocks that form the range were uplifted along reverse faults about 65-55 million years ago, and over millions of years water dissolved deep caves in the limestone, some opening vertically to the surface so cold dense air sinks underground and, insulated by the rock, creates ice caves such as Big Ice Cave, while other caves contain stalagmites and stalactites. About 115 million years ago, a 10-foot-long dinosaur with razor-sharp claws and teeth and retractable slashing talons on its hind feet hunted in packs here; lightly built, agile, and quick, it could run up to 25 miles per hour. In 1964, on a windswept hill in the nearby badlands, Yale professor John Ostrom discovered fossil remains of this animal, Deinonychus, or "Terrible Claw," a find that revolutionized paleontology and changed understanding of dinosaurs. Because of its skeletal structure, Deinonychus appeared to have more in common with birds than with reptiles, and its high energy needs led some to believe it was warm-blooded. Deinonychus helped lead paleontologists to view dinosaurs as agile, intelligent, and social animals rather than slow-moving and dull-witted. The Pryor Mountains are named for Sergeant Nathaniel Pryor of the Lewis and Clark Expedition of 1804-1806, and the Chugwater Formation takes its name from the Wyoming town and is recognizable for its brick-red color caused by oxidation of iron minerals in the rock.