Texas Bureau of Economic Geology Down to Earth at Tuff Canyon, Big Bend National Park, Texas

As you stand in Big Bend National Park and look around you, the scenery is changing. Gravity, water, and wind are shaping the landscape of the park even as you read this. These are agents of erosion, and they are carving and moving rocks that were formed on the floor and shoreline of an ancient shallow sea, or laid down by streams and windstorms, or erupted from volcanoes, or solidified from hot molten rock (magma) beneath the Earth's surface. In Big Bend National Park you can actually stand on lava from a volcano or on rocks that were laid down on the bottom of an ocean! Many of the common products of volcanic eruptions are easily spotted in Tuff Canyon, a small gorge cut by Blue Creek in its course to the Rio Grande (see the map at the back of the book).

Blue Creek starts high on the western slopes of the Chisos Mountains below Emory Peak and flows southwestward (fig. 1). About 3 miles northeast of its junction with the Rio Grande, Blue Creek has cut Tuff Canyon in a gently southwest sloping surface covered by gravel. This gravel layer, difficult to wash away because the boulders and cobbles are large and tightly interlocked, overlies sedimentary and igneous rocks. Where the creek has cut through the gravel, the underlying rocks are soft and easily eroded by running water, and the smaller particles (silt and sand) are carried away to the Rio Grande. As a result, the stream has cut a narrow, steep-walled valley (fig. 2). For erosion to have created a wide valley with gently sloping sides, much more gravel would have had to be removed. But in this arid climate, there is rarely enough water in Blue Creek to carry away much material.

Figure 1. Map of part of Big Bend National Park, showing Tuff Canyon and other places mentioned in this guidebook. This map will help you find your way around this part of the park. A detailed map of Tuff Canyon is found at the back of this book.

Figure 2. Looking upstream at Tuff Canyon from central observation platform (see map in back of book). This guidebook describes features visible from the three observation platforms, as well as along the canyon floor as you walk toward the dark area in the center of the photo.

Use the map at the back of this book to get started exploring the canyon. The guidebook begins with descriptions of views from the observation platforms above Tuff Canyon that you see on the map. Most of the guidebook, however, describes features you can see on a walk through the canyon, starting with a detailed look at features in the canyon walls below the easternmost observation platform (#1) and continuing roughly 1,300 feet westward to the canyon mouth.