Geology of Big Bend Ranch State Park, Texas

Big Bend Ranch State Park, the largest in the Texas State Parks System, lies in the rugged Big Bend country of West Texas (figs. 1 and 2). The primary attraction of Big Bend country is its scenery, and scenery is geology. The mountains, canyons, plateaus, rivers, waterfalls, and, indeed, every aspect of the natural landscape result directly from geologic processes of rock formation and erosion. In Big Bend country, these processes include mountain building, volcanism, faulting, and erosion. Taken together, they have shaped the land into the rich vistas we enjoy today. Geology is also the dominant influence on many other characteristics of the land and on where and how plants, animals, and humans live. The distribution of rock types determines the location and movement of surface water, recharge, and the occurrence of groundwater and springs. Soil is the weathered product of the underlying rocks, and soil, topography, and availability of water determine the kinds, distribution, and density of vegetation. In turn, these aspects of vegetation determine the distribution and abundance of wildlife. Early human inhabitants lived where they could find shelter, water, and food. Most shelters used by prehistoric people were natural rock enclosures that resulted directly from geologic processes. More recent inhabitants looked for good grazing or agricultural land, as well as for water. Much of the geologic attraction of the park stems from its being a geologic crossroads exposing a great diversity of rocks developed over hundreds of millions of years (fig. 3). Rocks exposed in the Solitario, in the Bofecillos Mountains, and around the Cienega Mountains represent more than 500 million years (Ma) of Earth history. From oldest to youngest, major geologic events in this history include: (1) deposition of a thick sequence of marine sedimentary rocks during Paleozoic time; (2) folding and faulting of these rocks during an intense episode of mountain building near the end of the Paleozoic; (3) deposition of limestone and shale during the Cretaceous; (4) folding and faulting during another mountain-building episode in the early Tertiary; (5) formation of the Solitario dome in the Oligocene during some of the earliest volcanic activity in and around the park; (6) continued volcanism during several episodes in the Oligocene and Miocene; and (7) faulting of the Basin and Range province beginning in the Miocene and continuing to the present. This guidebook attempts to illustrate the long, diverse, and dynamic geologic history of the park and to demonstrate how geology has influenced its landscape, water, habitat, and other features. The report is divided into three parts. This Overview summarizes the geology. The section Geologic Concepts and Processes briefly explains some basic aspects of geology to help readers understand the rest of the guidebook. Most of the book is a detailed discussion of the geologic history of the park. It shows the kinds of evidence that geologists look for in the rocks and landscape and the reasoning they use to explain their origin and evolution. Sites that illustrate prominent and informative geologic features are pointed out where appropriate. Furthermore, each of these geologic events reflects and results from major plate tectonic processes that acted upon large areas of the Earth’s crust. The book therefore attempts to relate many of the geologic events to plate tectonics, as well as to describe the specific events that affected the park. A glossary offers a definition or elaboration of any bold-faced term in the text. The accompanying geologic map is an essential part of the book; the map illustrates the geologic features in a way that words cannot and should be consulted for a full understanding of the geology.