This area (Pl. 9), often called “the caprock,” is an elevated plateau which rises above the rolling plains which surround it. The High Plains are bounded by the Pecos River valley on the south, southeast, and west and by the North-Central Plains on the east. The surface of the High Plains is very flat and characterized by a sparse cover of grasses and few trees. The surface strata consist largely of unconsolidated deposits of sands and gravels of Quaternary and Tertiary age, with remnants of Lower Cretaceous limestones along the southern margin. The rocks of the High Plains are mostly unfossiliferous, but mammalian remains have been found at several localities. [Illustration: Plate 9 Physiographic map of Texas.] HIGH PLAINS NORTH-CENTRAL PLAINS GRAND PRAIRIE TRANS-PECOS TEXAS VAN HORN UPLIFT THE BIG BEND AREA SOLITARIO UPLIFT MARATHON UPLIFT EDWARDS PLATEAU LLANO UPLIFT BALCONES FAULT ZONE GULF COASTAL PLAIN North-Central Plains Surface strata of the North-Central Plains (Pl. 9) are westward-dipping Pennsylvanian, Permian, and Triassic rocks. Present also are extensive exposures of Quaternary sands and gravels which trend north-south across the central portion of the region. The area is bounded on the west by the High Plains, on the east by the Grand Prairie, and on the south by the Edwards Plateau and Llano uplift. Many vertebrate fossils have been collected from the Permian and Triassic rocks of this area. There are also many excellent outcrops of fossiliferous Pennsylvanian formations in the North-Central Plains region. Edwards Plateau The Edwards Plateau (Pl. 9) is located in south-central Texas and is bounded on the south by the Balcones fault zone and on the north by the North-Central Plains. The surface of the area is typically flat with a gentle slope to the south. The rocks of the Edwards Plateau consist primarily of Lower Cretaceous limestones and shales, many of which are very fossiliferous. Grand Prairie This area (Pl. 9) has a relatively flat surface but there are areas of gently rolling hills. The eastern boundary of the Grand Prairie is marked partly by the Balcones fault zone. North of McLennan County, however, the Balcones fault zone is not expressed at the surface and in this area the eastern boundary is defined by the western edge of the Woodbine exposures. Upper and Lower Cretaceous rocks occur at the surface and dip to the southeast; many of these rocks contain a large number of invertebrate fossils. Llano Uplift