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

To the west you can see pyroclastic layers in the vertical walls of the canyon (fig. 9). The layers tilt gently toward the mouth of the canyon, and farther to the west they are hidden beneath much younger alluvium. The pyroclastic-flow deposit in the opposite wall is lower than it was at the central observation platform because of the westward slope of the layers and because a fault, just west of the central platform, has dropped the rocks down to the west. The pyroclastic-flow deposit is overlain by surge and debris-flow deposits. Channels that were eroded and then filled by debris flows are especially visible in the wall opposite this platform; here, some have cut into the top of the pyroclastic-flow deposits, and other channels are at higher levels, within the surge deposits (fig. 10).

Figure 9. View of Tuff Canyon, looking west from west observation platform. Pyroclastic surge, -flow, and debris-flow deposits slope gently toward the mouth of the canyon.

Figure 10. (a) Channel filled by a debris-flow deposit exposed in cross section in the wall of Tuff Canyon opposite west observation platform. The channel reaches a thickness of about 2 feet. (b) Channel filled by a pyroclastic-surge deposit, underlain and overlain by pyroclastic-flow deposits in a quarry wall, Castel' Ottieri, Italy. The building blocks cut from a pyroclastic-flow deposit and stacked on the quarry floor are about 1.5 feet wide.

To the southwest is Cerro Castellan (fig. 1), a prominent landmark visible from many places in the park. It is a stack of pyroclastic rock layers capped by a rhyolite lava dome. This dome was fed by the dike exposed on the north flank (fig. 11). Other dikes are well exposed east of Ross Maxwell Scenic Drive in the western slopes of the Chisos Mountains, from 13 to 16 miles north of the Tuff Canyon parking area. Look for sharp "walls" as much as 30 feet high and 10 feet thick. The dike rocks resist weathering and erosion much more than do the soft rocks that the dikes cut. By the way, the rock layers at Cerro Castellan are higher than they are at Tuff Canyon because of uplift on a fault between Cerro Castellan and where you are now standing.

Figure 11. Cerro Castellan as seen from west observation platform, Tuff Canyon. The feeder dike is the sunlit fin below the dark cliffs of the lava dome. The lower and smoother slopes are cut in pyroclastic deposits.

Small intrusive rock plugs of rhyolite (including the so-called "petrified tree," fig. 12) poke through white air-fall deposits on the north side of Ross Maxwell Scenic Drive northwest of Cerro Castellan (fig. 1).

Figure 12. The so-called "petrified tree" northwest of Cerro Castellan; actually an intrusive plug formed by rhyolite magma pushing up through white air-fall tuff. The exposed part of the plug is about 20 feet high. The "knot hole" once held a boulder (Maxwell, R. A., 1968, The Big Bend of the Rio Grande: The University of Texas at Austin, Bureau of Economic Geology, Guidebook 7, figure 90).

Now it is time to get a close-up view of the rocks. Take the trail (see map at the back of the book) into Tuff Canyon from the southwest end. After you get down to the canyon floor, you will be walking on fairly level ground, but there is soft sand in places. Give yourself at least 45 minutes for the walk into Tuff Canyon and back. CAUTION: do not enter the canyon if rain is falling anywhere upstream of Tuff Canyon; flash floods can move rapidly through a narrow canyon such as this one.

Once you are in the canyon, note the sand and gravel bars banked against the walls. They are rearranged by every flash flood. Even the boulders on the canyon floor were carried there by running water. Rocks that make up some of these boulders came from miles upstream in the Chisos Mountains, moving perhaps only a short distance during each flood.