This bluff, like the neighboring Wildcat Hills, Chimney Rock, and Courthouse Rock, is an erosional remnant of the ancient Great Plains. These plains were formed by the deposit of gravel, sand, and silt brought down by rivers from the Rocky Mountains after they were uplifted about 60 million years ago. At intervals, 30 to 40 million years ago, volcanoes to the west also added great quantities of ash and dust deposits. When the process of deposit slowed, erosion gained headway, cutting new river valleys in the Plains. High tablelands were left on both sides of the North Platte Valley. Landmarks such as Scotts Bluff and Chimney Rock have hard rock caps that protect them from erosion. The lower two-thirds of the bluff consists of Brule clay, an Oligocene deposit of buff-colored, soft-textured sandy clay. The badlands formation at the foot of the bluff dramatically demonstrates the rapid erosion of the Brule clay when unprotected by cap rock. The upper one-third of the bluff consists of the Arickaree formation, gray sand beds of the Miocene Epoch. The uppermost Arickaree beds form the top surface of the bluff. These are laced with hard tubular concretions which help protect the bluff and add to the resistance of the beds to weathering. All the formations exposed in the walls of the bluff are interspersed with thin layers of pinkish volcanic ash. _Paleontology._ The Badlands regions of western Nebraska and South Dakota have become world famous for their extensive deposits of fossils of mammals and other animals that lived during the Oligocene and Miocene Epochs of the Cenozoic Era. The Brule clay formation of Scotts Bluff and vicinity is particularly rich in fossil remains of extinct animals of Oligocene age, 30 to 40 million years ago. These exposed remains gave rise to many Indian legends and were frequently noted by curious emigrants. Scientists from many institutions of learning have continued to explore and examine these fossils since 1847, when Dr. Hiram Prout of St. Louis described a jawbone brought to him by a fur trader as that of a _Titanothere_, a giant rhinoceros-like creature. Among the most common fossils of the Scotts Bluff vicinity, now in the collections of the Oregon Trail Museum, are giant turtles, pig-like _Oreodonts_ ancient forms of rhinoceroses, saber-toothed tigers, dogs, deer, camels, and rodents. The horse family is represented here by _Mesohippus_, a three-toed creature about 18 inches in height. _Plantlife._