The "narrow escape" here is the one avenue of escape surface water has out of the Basin, a narrow passage in a landform feature called "The Window", through which you can look out over the Chihuahua desert to the northwest from an elevation of over 3000 feet above the desert floor. People gather at a viewpoint near the lodging area in the Basin to enjoy and photograph the spectacular summer sunsets seen through the Window.
The "Window frames" are Amon Carter Peak to the left (as seen from the viewpoint) and Vernon Bailey Peak to the right. Oak Creek flows (or, usually, not) down to a slender, water-carved slot between these two peaks, which are eroded igneous intrusions. Water in Oak Creek is relatively rare, typically occuring only after an infrequent desert thunderstorm. However, spring 2007 was unusually wet in West Texas, and water was still flowing in Oak Creek in June, despite no rain having fallen for several weeks.
The trail down to the Window pouroff (a pouroff is a usually dry desert waterfall) eventually follows Oak Creek. When water is present, the creek flows through the narrow passage in the rock at the top of the pouroff and falls 200 feet. Creeks both erode and deposit sediment, and in the following picture you see some of the alluvium (water-deposited sediment) that had previously been deposited in this drainage, now exposed by current erosion. The rounded clasts (the geologists' general word for silt, sand, gravel, etc.) in this outcrop indicate it was worked over by running water for an extended period before deposition. Big Bend was wetter during the last glacial period, according to several lines of evidence.
There was no water in Oak Creek at this location, and it was not clear we (my brother Randy and I) would encounter any. Actually, since it had been dry for weeks, we had no expectation of finding running water, and, at this point along the trail, we saw what we thought we would - a dry creek bed.
There are many spectacular pinnacles on either side of the trail, eroded from the igneous rock, as you make your way to the Window pouroff. Below you see a view toward Vernon Bailey Peak.
Eventually, the rock becomes more and more devoid of soil cover as slickened and steep rock outcrops crowd the trail. Because of the popularity of this hike, the park service has actually paved and put steps in places where the footing would be difficult. This is seen in the following picture as Randy takes a moment to soak in the scenery.
Just when it looked like Oak Creek was going to be bone dry all the way to the pouroff (we did hold out hope for some remnant pools), we come across a beautiful stream of water cascading down the rocky creek bed.
Just where did this water come from? It certainly wasn't present in the bed upstream. It had to come from underground in a process called "base flow", where groundwater percolates out of the ground and into a stream. This occurs where the local water table is higher than the stream bed. In spite of weeks of no rain, there was still plenty of groundwater to supply Oak Creek just above the pouroff.
And in the water were some tadpoles, hanging on in hopes of becoming frogs as the water flow decreased day to day. Even after running water disappeared, there would be some pools left for many of these amphibians to complete their life cycle.
At last you come upon the pouroff itself. An otherwise spectacular view is marred somewhat by spurs of Amon Carter Peak that block much of what you would otherwise see. I'm standing about as close as I dare to the precipice in order to take a picture (which didn't turn out so great). The rock is extremely slick and covered with white deposits from thousands of years of occasional running water. One slip on the uneven footing near the precipice would land you dead on the rocks 200 feet below.
The following image may give you and idea of how slick the rocks are at the pouroff.
The best view is the following telephoto, taken by Randy. The knob-like feature casting a shadow just above the center of the image is Gano Peak, a small igneous intrusion. To its left and just a bit higher in the image is a small mesa, on the other side of which is the junction of the West Road with the Maxwell Scenic Drive. At the extreme left, left of the mesa, is the northern margin of Burro Mesa. Slickrock Mountain is just out of view to the right. All you can see is its southern shoulder.
Heading back to the lodging area, you get a spectacular view to the east of Casa Grande in the afternoon sun. It would be verging on sacrilege, therefore, not to include an iconic Big Bend picture featuring this mountain and a flowering agave.
Geologists have to dig beneath the breathtaking beauty of the scenery to find out why it is there, so I close with the following image of the volcanic stratigraphy of Casa Grande. This was not an easy task because the several authors I consulted have slightly different interpretations. McGookey (Geologic Wonders of West Texas) identifies what appears to me to be the Bee Mountain Basalt (34-30 million years in age, consisting of a number of individual lava flows) as part of the Pine Canyon Rhyolite. McCloud (Big Bend Vistas) divides the cliff face of Casa Grande into two halves with upper half being ash flow tuff and bottom half the Lost Mine Rhyolite. Spearing (Roadside Geology of Texas) identifies layers only as either rhyolite or basalt, with no names.
Basalt is an iron-rich lava, typically very "runny" (low viscosity), wheras rhyolite is richer in silicon and oxygen than basalt with relatively little iron. It is very viscous and doesn't flow easily. Tuff is formed when volcanic ash hardens into rock. You can refer to the following image to better understand the following argument.
Some 34 to 30 million years old Chisos volcanics lie between the Bee Mountain Basalt and the Pine Canyon Rhyolite in the park, and not only does the bed I have identified above as the Bee Mountain Basalt rather than the Pine Canyon Rhyolite (according to McGookey) look like basalt (a dark-colored rock due to its iron content) but also there is material (look on the left of the image) between it and what McGookey also identifies as the Pine Canyon Rhyolite. I suspect this material is Chisos volcanics. As far as the cliff face is concerned, I see no visual reason to divide it in two halves as was done by McCloud.
The Bee Mountain Basalt was among the last of the Chisos volcanics to be erupted from sources to the west of the high Chisos mountains. It includes a number of flows, ranging in age, according to radiometric dating, from 34 to 30 million years. The Pine Canyon Rhyolite (33-32 million years old) was erupted as violent volcanic flows called nuées ardente from a caldera, the remnants of which are on the northwest side of the Chisos, in the Pine Canyon area. These volcanic ("pyroclastic") flows result from explosive release of hot gases trapped inside the viscous rhyolite, and travel rapidly as a mixture of hot gases, ash, and rock fragments. They harden into a type of rock called ignimbrite. Other eruptions followed this preliminary one, creating the Boot Rock Formation (flows of rhyolite and ignimbrite 32 million years ago) and the Emory Peak Rhyolite, consisting of rhyolitic ash-flow tuffs with an age of 32 million years. My interpretation above is tentative, pending a trip up the side of Casa Grande to look at the rocks myself, something to look forward to.
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