If you want to examine the innards of a thrust fault, Dog Canyon is the place to go. The images seen here were taken during June, 2007, on a relatively mild day for the northmost end of Big Bend National Park. However, "mild" is a relative term. The temperature was above 100 - for most of us this would be a "dog day" in Dog Canyon. Fortunately, Dog Canyon is only a stroll of a few miles along a level surface that trails downstream beside Nine Point Draw. You still want to take plenty of water.

Below you see Dog Canyon in the distance, the notch in the Santiago Mountains that appears near the center of the image. This picture was taken roughly halfway down the trail. Already apparent are beds of sedimentary rock going in different directions, indicating something very unusual occurred here long ago. Note the plants. Those familiar with Big Bend will recognize they are greener than you would expect for June. My brother, Randy, and I took this hike after a particularly rainy spring in West Texas.

Setting the scene of the hike are the following couple of pictures taken on the route to Dog Canyon. The first is of the Rosillos Mountains to the southwest. In the middle distance, a mile or so away, are a series of low cuestas. These are due to the beds of the Cretaceous Pen Formation, mostly a limy clay, which dips gently to the north. Cuestas like these are formed when weathering and erosion eat into the more exposed parts of dipping beds, creating relatively steep slopes, which then slowly retreat down-dip (north in this case) under the attack of mass-wasting processes, such as creep, slides, and rock falls. The Rosillos Mountains consist of intrusions of quartz trachyte magma, which is similar to andesite, but richer in the alkali elements (sodium and potassium). The highest point is Rosillos Peak at 5445 feet.

You are looking across a down-dropped block between here and the Rosillos Mountains called a graben, which was filled in with sediment, resulting in a flat plain. This particular graben is called the Chalk Draw Graben. On the left of the image, at the same level as the cuestas, you can just make out the hills in front of the Rosillos Mountains. These hills are made of the same stuff as the mountains but have been down-faulted by the Chalk Draw Fault, which serves as the western boundary of the graben.

Turning around and looking to the north, you see the Santiago Mountains, through which Dog Canyon was cut. In the middle distance is a small cuesta, apparently consisting of the Cretaceous Boquillas formation. The Boquillas consists of limestone and shale deposited in a shallow sea. The Santiago Mountains are on the boundary of the graben opposite the Rosillos Mountains and consist of Cretaceous limestones riddled with thrust faults.

As you approach the canyon hiking down Nine Point Draw, there are some interesting fluvial features. I suspect you don't usually see water in the draw this time of year - the rainy season generally starts later in the summer. The water shown below was in a depression hollowed out during times of heavy stream flow, the last of which likely occurred sometime during the previous two months. Water flowing over the limestone on the left carried clay, silt, and sand, polishing the limestone. An eddy forming behind the limestone excavated the hollow. Sand has trapped in this hollow but was carried away where the flow was swifter, leaving behind gravel.

The scooped pattern in the rock in the next image appears to be a ventifact, a feature created by the abrasive action of sand driven by the wind. However, the presence of these features in the limestone of the stream bed suggests they were created, at least to some extent, by the abrasion of water-driven sand. Additionally, once a small hollow forms in limestone, water that lingers there can facilitate the dissolution of the limestone, which is soluble in water that is even slightly acidic.

I've been told butterflies need minerals they can't get from nectar, so they obtain them from mud. Here are some of the little mudders doing just that, taking advantage of the recent rains. Other than being a type of sulphur butterfly, I don't know what species it is, but it may be a mimosa yellow, whose range includes southwestern Texas.

Back to Dog Canyon, the image below is the same one shown earlier, doctored with lines indicating the position of the thrust faults. (These lines are based on the interpretation presented by Ross Maxwell in the book The Big Bend of the Rio Grande.) Thrust faults are low angle faults where one block of rock has been shoved up over another. I have heard that so-called "young Earth creationists" reject the existence of thrust faults, since they are often responsible for older rock being emplaced over younger rock. These creationists would rather see older rock over younger rock as proof geologists don't know what they are talking about. A trip through Dog Canyon should put to rest all doubts that thrust faults exist.

All the units named in the image are Cretaceous rocks. The cliff-forming Buda Limestone (pronounced "byoo-dah", not "Buddha" - this here's a Christian state, y'all) on the right (south) side of the canyon is younger than the Del Rio Formation (mostly claystone) on which it rests, so no problem here. Nor is there a problem with the Boquillas on top of the Buda. However, what's the Buda doing, appearing again above both the younger Boquillas and itself? And what's with the even older Glen Rose Formation sitting on top of them all? If that weren't enough, you find the Santa Elena Limestone over the much younger Buda and Del Rio on both sides of the canyon. The lines indicate the locations of a pair of thrust faults, dashed where the faults are covered with weathered debris, responsible for these apparent contradictions. Note the stepped pattern in the Glen Rose. This is due to it being composed of alternating beds of weathering-resistant limestone and softer marl, which is essentially a dirty limestone.

Below you see the entrance to Dog Canyon. As you enter you are struck by rocks falling on your head. Just kidding. You are actually struck by the fact that the beds where you are standing are horizontal while those in the distance are vertical. (Being struck by a fact is usually less painful than being struck by a rock.) A little observation reveals the beds are folded sharply upward in the distance. (See the line in the image below. My brother on the lower right helps set the scale. If you were not so lucky, you in fact could be struck on the head by a rock from high on the steep cliffs.)

It was at about this point we became aware of an increasingly deafening roar approaching from the east. We had no idea what was going on. Big Bend is supposed to be this refuge from the assaults of civilization (so called). It was only a few seconds until the source of the incredible noise made its presence. A military jet, flying low just above the canyon, flashed above our heads. The sound, reverberating between the canyon walls, was incredible.

Now, I believe whole-heartedly that a ready military force is a necessity. However, I don't think it likely that Dog Canyon is a target of al-Qaida. Nor do I believe there is something about Big Bend National Park in general or Dog Canyon in particular that is vital to the training of military jet pilots. I really doubt this behavior would be tolerated in Yosemite. All our national parks should be respected. This is no way to treat a National Park.

Getting back to geology, here's a closer view (looking upward at the nearly shear cliffs) of the fold, a drag fold created in a fault zone by friction as the rock on one side of a fault pulls on the rock on the opposite side as it is displaced along the fault. In this case there was a thrust fault, whose plane used to be above the rocks shown. It has been eroded away at this location.

However, walk a little farther down the canyon and you come face to face with the plane of the fault. In the fault zone area the rocks are bent vertically and even beyond the vertical. The image below was taken on the south side of the canyon in the vicinity of the fault zone. The next image was taken on the north side. Rock to east of the fault zone (on the left of the first image and on the right of the second) - the Glen Rose Formation - was shoved over the rock to the west during the Laramide mountain-building event.

On the way out, we pause to examine the limestone sides of the canyon wall that have been smoothed by the abrasion of thousands of years of flash floods, rushing down Nine Point Draw and through the canyon.

Another interesting feature formed by water is seen at the entrance to the canyon. Rainfall flows over the tops of the walls in places, resulting in a fluted design. As mentioned earlier, limestone is soluble in acid, and rain is slightly acidic (unless you live in New England, where the rain is so acidic the trees have to take Tums).

On the north side of Dog Canyon, there is a major rockfall on the west side of the Santiago Mountains. This was described in an earlier chapter of this field trip. From Nine Point Draw you can get a better view. Some of the boulders are as large as houses.

Finally, this is the view of the north side of Dagger Mountain, south of Dog Canyon, as you make your way back to the trailhead. The cliff-forming part of the Buda extends from the lower right upward at a slight angle into the mountain. It appears again as a nearly horizontal, partially covered outcrop above itself - the result of thrust faulting. The top of Dagger Mountain is composed of the Glen Rose Formation, above another thrust fault. Dagger Mountain was apparently named for the abundant Spanish Daggers (yucca). One account says Dog Canyon was named by a traveler who found an ox-drawn cart protected by a dog. The owner of the oxen, in this account, was never found. Another account says that the canyon was long known as "Canon del Perro" (Dog Canyon in Spanish).

FORWARD to Persimmon Gap

BACKWARD to Burro Pouroff

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