There is no evidence that Jurassic sediments were ever deposited in the region. Cretaceous rocks are also missing in this area although water-worn fossil oysters occur in the gravel at the base of the overlying Ogallala. These fossils indicate that marine Cretaceous sediments were deposited nearby and possibly covered the Triassic deposits in the region. The rocks were then eroded away some time between the end of the Cretaceous Period and the beginning of the Pliocene Epoch, a span of about 50 million years. During the Pliocene Epoch, approximately 2-10 million years ago, the Rocky Mountains were again uplifted. Sediments in streams and floodplains were deposited on the erosional surface of the Trujillo Formation (Figure 5). These stream-deposited sediments are the Ogallala Formation. The Ogallala Formation, which forms the upper part of the sequence of rocks exposed in the canyon, is present throughout most of the Panhandle. The formation is important as it is the principal aquifer of the Panhandle and supplies many farms and cities in the region with water. The Ogallala is a siltstone and sandstone that has, in places, been cemented by silica which came from groundwater. The formation contains many pockets of common opal and the basal part is in many places almost a chert. There are also some thin gray shale lenses. [Illustration: Figure 5. Paleogeographic map of the Pliocene (10 m.y.)] Scattered over the Ogallala are Late Pliocene and Pleistocene playa lake deposits up to 3 million years old. Some of these are fresh water lake deposits of silt, limestone, and wind-transported sediments or loess. Below these sediments is a layer of caliche which was deposited by evaporation of groundwater rich in calcium carbonate during Late Pliocene and Pleistocene time. Less than one million years ago, during the Pleistocene Epoch of the Quaternary Period, the Prairie Dog Town Fork of the Red River began eroding headward into the Llano Estacado (Figure 6). The caprock escarpment is the result of differing resistance to erosion. The faster erosion of softer layers under the more resistant Ogallala and Trujillo formations forms the steep slopes of the escarpment. The Prairie Dog Town Fork of the Red River is the small stream flowing in Palo Duro Canyon. Throughout the past million years it has been slowly excavating the canyon. The rate of change has been slow but continuous, carving the steep colorful walls of Palo Duro Canyon, an area of geologic interest and great scenic beauty. PALEONTOLOGY