The title of this section is a quote from, I believe, a trail guide in either a book or on the Web. I can't remember where I read it, but I do remember the author saying something to the effect you may as well throw up your hands and admit defeat a priori. You aren't going to make heads or tails of the rocks you see on this trail, so just give up before you even start hiking. Fortunately, my brother, Randy, and I didn't throw in the towel just because someone said we should. What you can find - if you paid attention in your freshman geology class and look carefully - are numerous dramatic geological features.

The trail runs up a drainage through outcrops of Devonian (?) Caballos Novaculite (the age has not been nailed down) and Maravillas Chert, dating to the Ordovician. Both units are resistant to weathering, forming pouroffs (dry waterfalls), such as the one you see below. This one, in Caballos Novaculite was too high and steep to assault head on, so we made our way around it. The picture was taken while ascending the south side of the arroyo, looking down at the sand at the bottom of the pouroff. The Tesnus Formation of interbedded sandstone and shale should sit on top of the Caballos Novaculite, but at the top of the pouroff it appears limestone overlies novaculite, possibly indicating a fault exists here. It turns out faults and folds are ubiquitous along the trail.

Farther along the trail you come across more Paleozoic rocks, such as the ones pictured below. This outcrop features sandstone lying on top of shale and is probably the Tesnus Formation of Late Mississippian to Early Pennsylvanian age. The Tesnus consists of sediments deposited by turbidity currents, which are mixtures of water and sediment (a sort of slurry), that flow downhill off continental shelves under the influence of gravity. Hence, this area, that is now continental desert, was once off the continental shelf

Eventually, we came to an outcrop that really did tempt me to throw away the geology books. At first it was hard to figure out what we were looking at. After some observation and thought, it became clear we were looking at a faulted fold exposed by erosion. The beds above the head of the rock hammer were originally continuous with those running along the bottom of the image. The missing sectons of beds came out from the outcrop just above the rock hammer and were folded into the outcrop below the hammer. The axis (center) of the fold is marked by the dashed line. The outcrop surface cuts across the fold diagonally, which is why you see the fold has a "nose" to the right. Visible across the upper right is the trace of a fault.

Below you get a better view of the fault. Note how the beds on either side are in very different directions. The outcrop here appears to be mostly the Maravillas Chert.

Below you see my interpretation of the faulted fold displayed as a cross-section of the outcrop. In this picture the head of the rock hammer is located right on the axis of the fold, with beds "coming out" of the outcrop above the hammer and "going back into" the outcrop below. Actually, I think the folding is tighter than pictured in this artwork.

Here, a small pouroff in the Maravillas Chert provides shade from the increasingly warm temperatures. (The hike was undertaken in the morning.) Randy provides the scale.

Below is a small fault in the Maravillas Chert with an apparent offset of about one foot. The line marks the trace of the fault. I say "apparent" because there is no guarantee the fault motion was parallel to the ground. All one can say is that the minimum offset was about one foot.

Yet another fault in the Maravillas. This one appears to be a small thrust fault.

Brother Randolph does the scale thing in front of a prominent fault in the Maravillas Chert, subtended by the arrows. When I examined this fault, I concluded it was a reverse fault. However, I was not able to correlate beds on either side of the fault plane well enough to show this to be true beyond a doubt. Although thrust faults in the area prove strong compressive forces once occurred in this region (during the late Paleozoic and again during the Late Cretaceous to Early Cenozoic), there could well be compression in one direction and "tension" (actually, compressional forces in one horizontal direction that are less than those in a perpendicular horizontal direction as well as less than those in the vertical direction) in another. So, even in a compressional environment it would be possible to have a normal fault. If the fault pictured were a reverse fault, the rock above the fault plane would have moved to the left relative to below. The opposite occurs for a normal fault. However, this is assuming the motion along the fault was parallel to the outcrop, which is unlikely to be true, as it is quite probable at least some of the motion was perpendicular to the outcrop. Nevertheless, chances are it is indeed a reverse fault.

Whichever type of fault it is, the fault plane is marked by "gouge", rock broken up into pieces by the faulting action. The arrow points to a significant collection of gouge.

Below: another reverse (?) fault in the Maravillas Chert and more gouge (arrow).

There are a number of major thrust faults in the Persimmon Gap area. The image below is apparently of one of them. The rock above is novaculite (or possibly Maravillas Chert) and the rock below is Tesnus. Since the rock above is much older than that below, indications are there should be a thrust fault here. And the presence of significant fault gouge indeed indicates a thrusting of the older rock over the younger. Additionally, the fault plane here is virtually horizontal, that is, a very low angle reverse (thrust) fault plane.

When Randy and I reached what was supposed to be the end of the hike, a minor dispute broke out. The guide book said that if you reach the "saddle" you get a view few people have ever seen. Randy thought the saddle was where you found yourself between two high points - the position indicated by arrow 1 in the topographic figure below (modified from Topozone). I, on the other hand, saw a saddle-shaped mountain that I was sure was the saddle the guide book meant. However, climbing up to the top would be daunting as it was covered with lots of plants having long, painful spines. Randy was convinced we were at the saddle. I dismissed his assessment and headed up the mountain. Afterward, looking at the topographic maps (which we foolishly didn't have at the time of the hike), I think he may have had a point. However, at that time I headed for the saddle indicated by arrow number 2.

One thing is for sure. You definitely get a much more expansive view from where I ended up, bleeding from numerous cactus punctures. I'll have to say it was worth it. Below you see Persimmon Peak, looking south. Refer to the chapter on the marathon uplift for the geology of Persimmon Peak.

Looking just to the west of Persimmon Peak, your view is across the Chalk Draw Graben. A graben is a down-faulted block of crust that subsequently fills in with sediment. A graben, therefore, is typically a flat area as you see here. The Rosillas Mountains in the distance are composed of igneous intrusions of syenite composition. This is rock a lot like granite, but where the silicon has been used up to form alkali feldspar before much quartz can form.

Turning farther to the west, you can make out Nine Point Mesa, composed of intrusive igneous rocks.

Looking to the west, you see Persimmon Gap where U.S. Highway 385 enters (or leaves, depending on your direction of travel) Big Bend National Park. The gap is in the Santiago Mountains, of which Persimmon Peak is a member.

The Santiago Mountains trend to the WNW, separating the Chalk Draw Graben from the Black Gap Graben. In the picture below you see the Santiago Mountains on the left and in the middle of the image, where a ridge of the Santiago range extends to the east from the main trend. In the far distance on the right appears to be Santiago Peak. Hard to tell from this distance, but it looked like Santiago Peak to me and is in the right direction. Santiago Peak is another of the ubiquitous igneous intrusions in the region.

This next picture is looking north along US Highway 385, across Maravillas Flat in the Black Gap Graben. Chalk Draw Graben is within the down-dropped Chisos block, but Black Gap Graben is to the NE of the block boundary. As I've mentioned before, most of Big Bend resides on a crustal block that has been faulted downward with respect to the crust on either side. The Chisos Mountains have been made, not by being uplifted more than the surrounding terrain, but, on the contrary, by the fact they are made of erosion-resistant material and have fared better against erosion than the rocks on the east and west.

One final view is toward the NE. Looking toward the morning sun degrades the image, but you can make out Stillwell Mountain in the middle distance beyond the shade-darkened ridge on the right. It consists of flows of the Black Gap Basalt dated to 22 million years ago.

Before we go, we can't help but admire the beautiful flowers blooming in the desert. Here is a particularly gorgeous bouquet of brilliantly red Indian Paintbrush. This alone made the difficult climb up to the top of the saddle-shaped mountain worthwhile.

FORWARD to Rancherias Canyon

BACKWARD to Dog Canyon

ALL THE WAY BACK to the Contents