A number of Pliocene vertebrates have been found in the Palo Duro area. Known as the “Age of Mammals,” the Tertiary Period was characterized by mammals as diverse as were the reptiles of the Mesozoic Era. Among these unusual creatures were such now-extinct species as the saber-tooth cat and the elephant-like shovel-jawed mastodon (fig. 18). The remains of these as well as bones of giraffe-like camels, pony-sized horses, and sloths have been found in the vicinity of the canyon. The grassy plains of Pliocene time were also inhabited by large tortoises which reached lengths of up to 3 feet (fig. 19). Dioramas showing how these animals might have looked, as well as their actual remains, are on display in the Hall of Pre-History in the lower floor of the Panhandle-Plains Historical Museum in Canyon, 13 miles west of the park (p. 35). [Illustration: Fig. 18. This life-size model of a shovel-jawed mastodon is typical of the now-extinct, elephant-like creatures that lived in this area during the Pliocene Epoch. (Photograph courtesy Panhandle-Plains Historical Museum.)] [Illustration: Fig. 19. The carapaces of giant tortoises as much as 3 feet long have been collected from Pliocene rocks in the Palo Duro area. (Photograph courtesy Panhandle-Plains Historical Museum.)] Rocks of the Pleistocene.— The youngest rocks in Palo Duro Canyon State Park were formed during the Pleistocene Epoch of the Quaternary Period of the Cenozoic Era (_see_ geologic time scale, p. 11). Pleistocene rocks are rather widespread in much of the Panhandle-Plains area and they are mostly composed of sediments which were deposited in stream valleys, in lakes or ponds, or by the wind. Most of the Pleistocene strata in the park area consist of loose deposits of silt and sand which were deposited by wind action. Known locally as “blow sand,” this reddish-brown, silty sand overlies the Ogallala caliche at most points along the canyon’s rim. HOW THE CANYON WAS CARVED The visitor seeing Palo Duro Canyon for the first time may find it difficult to believe that this yawning chasm began as a simple gully. But to the _geomorphologist_—the geologist who studies the origin and development of landscapes—Palo Duro Canyon is but a gully magnified many times over. This is evident because the shape of the canyon, the nature of its tributaries, and the character of its walls indicate that it has been deepened and lengthened by the downcutting of a stream and widened by other geologic processes. THE GEOLOGIC WORK OF RUNNING WATER