The geologic history of our earth has been recorded primarily in marine sedimentary rocks, and this record indicates that our earth is very old and that life has been present for many millions of years. The earth is not only extremely old (more than 3½ billion years of age), but it has also undergone many changes which have taken place slowly but steadily and have greatly affected both the earth and its inhabitants. The earth’s physical features have not always been as they are seen today. Geologic research has shown that mountains now occupy the sites of ancient seas, and that coal is being mined where swamps existed millions of years ago. Furthermore, there is much evidence to indicate that plants and animals have also undergone great change. The trend of this organic change is, in general, toward more complex and advanced forms of life, but some forms have remained virtually unchanged and others have become extinct. In order to interpret geologic history, the earth scientist must attempt to gather evidence of the great changes in climate, geography, and life that took place in the geologic past. The record of these changes can be found in the rocks, and here is found the story of the various events in earth history. GEOLOGIC COLUMN AND TIME SCALEIn order to discuss fossils and the age of the rocks containing them, it is necessary to become familiar with the geologic column and the geologic time scale (Pl. 1). The geologic column refers to the total succession of rocks, from the oldest to most recent, that are found either locally or in the entire earth. Thus, the geologic column of Texas includes all The geologic time scale is composed of units which represent intervals of geologic time, during which were deposited the rocks represented in the geologic column. These time units are used by the geologist to date the events that have taken place in the geologic past. The largest unit of geologic time is an era, and each era is divided into smaller time units called periods. A There are five eras of geologic time, and each has been given a name that is descriptive of the degree of life development that characterizes that era. Hence, The eras, a guide to their pronunciation, and the literal translation of each name is shown below.
The oldest era is at the bottom of the list because this part of geologic time transpired first and was then followed by the successively younger eras which are placed above it. Therefore, the geologic time scale is always read from the bottom of the chart upward. This is, of course, the order in which the various portions of geologic time occurred and during which the corresponding rocks were formed. As mentioned above, each of the eras has been divided into periods, and most of these periods derive their names from the regions in which the rocks of each were first studied. For example, the The
The Carboniferous The periods of the
The
While the units discussed above are the major divisions of geologic time, the geologist usually works with smaller units of rocks called formations. A geologic THE GEOLOGY OF TEXASThe geologic history of Texas, like the geologic history of the rest of the earth, is recorded primarily in marine sedimentary rocks. These rocks provide some knowledge of the early geography and the first inhabitants of what is now the State of Texas. Most of these rocks were formed By studying these rocks and their relations to each other, geologists have established a geologic column for Texas. PhysiographyIn 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 REGIONThe 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 Numerous invertebrate fossils occur in the TEXAS PLAINSThe 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 PlainsThis 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 Plate 9
North-Central PlainsSurface strata of the North-Central Plains (Pl. 9) are westward-dipping Edwards PlateauThe Edwards Plateau (Pl. 9) is located in south-central Texas and is bounded on the south by the Balcones Grand PrairieThis 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 Llano UpliftThe Llano uplift (Pl. 9) is located in the central part of the State where GULF COASTAL PLAINThe Gulf Coastal Plain (Pl. 9) is composed of The region has broad river valleys and uplands of low relief, but there is an increase in relief toward the interior of the State. The surface of the area slopes gradually toward the Gulf and successively younger formations are encountered gulfward. The rocks of the Texas Gulf Coastal Plain are relatively unfossiliferous, but many of the Upper GeologyGeologic studies of the State of Texas have indicated the presence of rocks formed during every era and Plate 10 One of the best ways to become acquainted with the geology of Texas is to study the |