In order to discuss the distribution and exposures of the rocks of Texas, it is helpful to be familiar with the _physiography_ of the State. Physiography deals with the study of the origin and description of land forms, such as mountains, valleys, and plains. Plate 9 is a map of Texas which shows the major physiographic provinces within the State. The majority of the land forms in Texas have been produced by the processes of erosion attacking the structural features of an area. Certain other land forms may be related to the effects of igneous activity which resulted in the accumulation of large masses of igneous rocks. The Davis Mountains are an example of surface features produced in this manner. In discussing the physiography of Texas, three major physiographic provinces will be recognized. These are (1) the Trans-Pecos region, (2) the Texas Plains, and (3) the Gulf Coastal Plain (Pl. 9). TRANS-PECOS REGION The Trans-Pecos region, located in the westernmost part of the State, is an area of mountains and plateaus with broad basins between the major mountain ranges. Many different types of rocks are exposed in Trans-Pecos Texas and these include marine, fresh-water, and terrestrial deposits. In many areas igneous rocks flowed out on the surface and now overlie sedimentary rocks. There are also many places where igneous rocks have been injected into the surrounding rocks, and these igneous rocks have been exposed by later erosion. Included within this area is the Van Horn uplift of southern Hudspeth and Culberson counties, the Solitario uplift of southern Presidio and Brewster counties, and the Marathon uplift of northeast Brewster County. This region also includes the Big Bend area of Texas, a part of which has been set aside as a National Park where many interesting and important geological features may be seen. The Trans-Pecos region is one of rugged topography with elevations as high as 8,700 feet, at Guadalupe Peak in the Guadalupe Mountains of northern Culberson County, and as low as 1,500 feet, in the Rio Grande valley. Numerous invertebrate fossils occur in the Cretaceous limestones and shales of the Trans-Pecos region and in the Paleozoic rocks of the Marathon uplift. The Gaptank formation of Pennsylvanian age and the Permian reef limestones of the Glass Mountains are especially fossiliferous. In addition, many vertebrate fossils have been collected in Trans-Pecos Texas, particularly in and around Big Bend National Park. TEXAS PLAINS The plains of Texas are broad expanses of country with very little surface relief. Most of the plains support grasses and some have wooded areas, particularly along stream valleys. The plains of the northwestern part of the State have been subdivided as follows. High Plains