The Maxwell Scenic Highway extends down the west side of the Chisos all the way to Santa Elena Canyon. Named for Ross Maxwell, a geologist and the Park's first administrator, it is replete with remarkable views and even more remarkable geologic features. Our first stop is a syncline exposed in the Chisos Group of rocks in the Sierra Quemada ("Burned Mountains"). (The image says "Chisos Formation", because it was created before I knew of the upgrade of the Chisos rocks from "formation" to "group".

A syncline is a geologic fold where beds of rock have been bent downwards near the center of the fold relative to the sides. A fold in geology can be one of many different types, but the two most common are syncline and anticline, where an anticline is folded up in the middle, rather than down. Anticlines and synclines are divided into two halves, called "limbs", by an imaginary plane called the axial plane (sort of like how the Earth is divided into two hemispheres by the equatorial plane). Note the gully running up the hill on the right side. It marks a fault that cuts through the syncline. The fault is a zone where fractured rock makes the job of erosive forces easier, hence the formation of the gully. The rock has been faulted down on the right (south) side of the fault.

From Sotol Vista, a turnout and viewpoint about seven miles south of the turnoff onto Maxwell Scenic Highway, you get the expansive view seen below. Goat Mountain is on the left and Kit Mountain center right. Both are topped with Burro Mesa Rhyolite, a volcanic rock high in silica that is the chemical equivalent of granite. The difference is the magma that becomes granite never reaches the surface and so cools and solidifies underground. Were it to reach the surface, it would be rhyolite. In the distance you can see the 1400-foot scarp marking the eastern edge of Mesa de Anguila, and just visible in the scarp over Kit Mountain, is the notch that marks the position of Santa Elena Canyon. From this distance the canyon seems puny.

One of the most interesting geological features along the Maxwell highway is Goat Mountain, pictured below. The stratigraphic interpretation of the mountain is in the image after that. The southwest side of the mountain is an exposure of a volcanic vent, uncovered by erosion. I have found two different interpretations of this vent. Geologists Maxwell (The Big Bend of the Rio Grande) and Darwin Spearing (Roadside Geology of Texas), write that the eruption occurred in a pre-existing canyon, whereas an interpretive exhibit at the Goat Mountain turnout and William MacLeod (Big Bend Vistas) claim the "canyon" is actually the result of the eruption itself. I need to check the latest scientific research on the mountain, but I bet the latter interpretation is probably the currently accepted one, as it is a newer interpretation. Note, however, that I haven't yet changed my interpretative slide below. (I've heard that slothfulness is one of the seven deadly sins, but I haven't bothered to check if that's true - well, I've at least added a question mark to the slide.)

Below the floor of the "canyon" the top three units of the Eocene Chisos Group are exposed. From younger to older they are: the 32-million-year-old Tule Mountain Trachyandesite, the 33-million-year-old Mule Ear Spring Tuff, and the Bee Mountain Basalt, which was erupted over a period stretching from 34 to 30 million years ago. Since the basalt shown here is beneath the other two units, its age would be 33-34 million years. Andesite is a volcanic rock with an amount of silica intermediate between that of rhyolite and basalt. Trachyandesite is an andesitic rock enriched in potassium and sodium with little or no quartz. Basalt is an iron-rich volcanic rock with relatively little silica.

Cutting through the Chisos rocks is an intrusion of rhyolite, marking the site of a volcanic vent. The intrusion represents some of the last of the magma that didn't make it out, and now it's a volcanic plug. The eruption resulted in the Wasp Springs Tuff and, on top of that, the Burro Mesa Rhyolite. Both units were apparently erupted from a number of volcanic vents on the west side of the Chisos Mountains 29 million years ago.

The next picture is of one of the most famous landmarks in the park. From a distance you would swear there was this gigantic animal crouching down behind a ridge with just his ears and the top of his head showing. The ears look very mule-like, hence the name "Mule Ear Peaks". They are actually the eroded remnants of a pair of dikes.

As you get closer to Mesa de Anguila (called Sierra Ponce on the Mexican side), you drive by Cerro Castelan (below), an erosional remnant that, due to its rather isolated position, stands out prominently. The Wasp Springs Tuff, with a wee cap of Burro Mesa Rhyolite, protect the rocks below from erosion. Beneath the Wasp Springs is another tuff, which, like that at Goat Mountain, probably belongs to the Mule Ear Spring Tuff formation. Below that you see the Bee Mountain Basalt and various beds of ash and tuff.

Below is a view of Cerro Castelan from the side. You can see the small remnant of Burro Mesa Rhyolite lying conformably (no significant time break) above the Wasp Springs Tuff. However, below the Wasp Springs there is a time break, called an unconformity, during which erosion took place. It appears what you are looking at here is what is left of one side of a valley into which the Wasp Springs Tuff and subsequently the Burro Mountain Rhyolite were deposited. The unconformity may mark what used to be part of the valley floor. Also note the talus (debris) from rock falls of the Wasp Springs Tuff. This is a reminder that erosion is still at work, and, geologically speaking, Cerro Castelan is not long for this world.

Continuing along Maxwell Scenic Highway toward the Rio Grande, you cross a landscape carved into the volcanism of the past by erosive forces. You see lava, tuff, ash, and, in the following image, a remarkably tiny volcanic plug (or, neck). The flow of lava through a tube-like opening in the earth created a texture where the lava was in contact with the walls of the tube, making the tube look for all the world like a petrified tree stump. It even has a knot. No wonder this is what many thought it was. However, the "stump" is composed of solidified lava, and the knot is probably where some piece of rock caught up in the flow later weathered away, creating a hole.

As you approach the Rio Grande, Mesa de Anguila (actually, Sierra Ponce, as it is known in Mexico) looms up before you, 1400 feet above the river. This side of the mesa is a fault scarp - the rock across the Rio has been uplifted 3000 feet relative to that on this side, forming the western boundary of the down-faulted crustal block upon which most of Big Bend National Park sits. The east side of the block is bounded by the fault that created the Sierra del Carmen escarpment. As mentioned in other episodes of this virtual field trip, the Chisos Mountains are high, not because they have been lifted up, but because the land around them has been eroded down. The Chisos owe their existence to the very resistant rock that caps the mountains.

In the image below you see that another fault has cut the mesa escarpment, down on the left and up on the right. Also, the rock layers are labeled. On top is the massive Santa Elena Limestone, over 700 feet thick. Below it is the Sue Peaks Formation, a blend of shale, marl, and limestone. Much less resistant than either the Santa Elena above it or the Del Carmen Limestone below it, it tends to form slopes rather than cliffs. The 150-foot thick Del Carmen is relatively resistant like the Santa Elena and is a ledge-forming unit. Beneath that is the relatively easily eroded Telephone Canyon Formation, consisting of 25 feet of limestone and marl.

The Rio Grande (out of sight at the base of the escarpment) is continually undercutting the cliffs such that from time to time they give way. The result is a landslide or rockfall. The collapse you see below appears to be a bit of both. Although it appears to have been a somewhat chaotic collapse, which would make it a rockfall, there are also preserved bedding planes and an upright block of Del Carmen Limestone, which imply a more coherent collapse, consistent with a landslide. And, possibly, more than one event led to what is seen here.

The canyon which was almost invisible from Sotol Vista (and is invisible on days of poor air quality), and even if visible appears to be a tiny chink in Mesa de Anguila (which itself looks like a something you could mount with a couple of steps), now appears as a huge narrow gash in the mesa, with sheer walls that tower well over 1000 feet above you. The canyon has frustrated many a traveler trying to move up the river. There is no trail that continues all the way through the canyon. The cliffs of Mesa de Anguila (and Sierra Ponce) are insurmountable. The currents in the canyon are swift and difficult to fight.

You can, however, cross the (usually) dry bed of Terlingua Creek, up and over a shoulder of rock, and down to a riverside trail that leads up the canyon, gradually playing out. The view below is from the shoulder looking down the trace of the Terlingua Fault, which is the name of the major fault that formed the western boundary of the downthrown Chisos block and the eastern boundary of Mesa de Anguila. The Del Carmen Limestone is in the foreground. The Rio follows the fault for a while before turning east. In the distance you can see in the profile of the escarpment how the Santa Elena forms steep cliffs, the Sue Peaks forms slopes, and the Del Carmen forms steep slopes but not as prominent as the Santa Elena.

The view below is toward the end of the trail into the canyon. Note how the rock beds dip slightly to the west. Those who claim the Earth is young would have you believe the only way a canyon like this could form is by floodwaters rushing through soft material, rapidly cutting vertically downward. There are a couple of problems (at least) with this interpretation. One is that these rocks are limestone, which is not known to be deposited quickly in thick, soft beds. On the contrary, limestones are largely made up of the calcareous remains of marine organisms that accumulate over millions of years. Limestones are therefore hard rock rather early in their history. Another problem is more general. Even if the rock was actually soft material when the waters cut down into it, soft material does not form cliffs. The poorly consolidated material would have collapsed under its own weight.

Geologists have deciphered the story of the canyon as follows. Several million years ago the ground surface was much higher and the Santa Elena was deeply buried under material that has since been removed by erosion. The Rio Grande was confined to places farther west and the Rio Concho, which now flows into the Rio Grande at Presidio, Texas, had its bed well above the surface that exists today. Eventually, the Rio Grande broke through and joined the Rio Concho and continued the erosion once performed solely by the Concho. Its bed was set when it encountered the resistant Santa Elena, and it began to eat down into the hard rock by abrasion - dragging sand and gravel along its bed in the swift current, a process that is accelerated during times of flood.

In addition to the major Terlingua Fault, there are numerous smaller faults that parallel it and can be seen in the canyon walls. Below you can see a normal fault on the left and a reverse fault on the right. A normal fault occurs where the block above the fault plane slips down with respect to the block under the fault plane. A reverse fault is just the - ahem - reverse of this. A normal fault is created by tension in the crust (or, actually, less compression in one direction than in the other) and is associated with crustal distension. However, a reverse fault is thought to be created by compression and is associated with crustal shortening. How come you see both types of faults together here?

Since the fault planes are parallel, it is unlikely they were created by two separate events. Most likely the crustal extension that created the (normal) Terlingua fault also created all the faults seen below. It's just that block to the right of the reverse fault slipped down more than the block to the left, making it appear to be a compressional feature. Actually, the reverse fault is due to an episode of random motion in an otherwise extensional process.

Finally, below is the view as you emerge from the canyon. Once again it is taken from the shoulder of rock at the entrance. If you read the episode involving the trip to Boquillas Canyon, you may remember the old river terraces seen there. Once again, here are terraces which mark previous levels of the river valley. There are at least two, maybe three or more - hard to tell because of the perspective. The current flood plain is covered with vegetation. Also in view are the Chisos Mountains and Cerro Castelan (beneath "a" in "Mountains". As the water emerges from the canyon, it spreads out and slows down, depositing sand bars.

FORWARD to Burro Pouroff

BACKWARD to The Road to Terlingua

ALL THE WAY BACK to the Contents