As I understand it, degradation of this area by humans eliminated all the grapevines that gave these hills their name. This is, unfortunately, a common situation in the Big Bend area, which apparently used to be much more amenable to human habitation. The problem seems to have been mostly overgrazing by early ranchers. Fortunately, the rocks remain, and in these hills erosion has created a truly grotesque landscape - a terrain that should be on a planet other than earth. The Native American legend that Big Bend was the dumping ground for what was left over after the creation of the world can almost be believed when you take this hike.
Before you hike (virtually speaking) up the draw the trail follows, check out the view northwestward toward the Rosillos Mountains (below).
In the distance you see the south side of Tornillo Creek. The layered structure in the side of the creek shows its banks consist of previously deposited sediments derived from the Rosillos Mountains to the north. This sediment (alluvium) was deposited over the past several thousand years, but now, possibly due to either a decrease in rainfall or an increase in overall elevation (or both), it is being worn away. The dominant regime of the Big Bend area is one of erosion, so the deposition indicated in the arroyo's banks is but a glitch in the overall trend to wear down the landscape. Also seen in this picture is a small igneous intrusion, probably just the tip of a larger body of intrusive igneous rock that resides mostly subsurface.
As you stroll up the easy trail you are increasingly surrounded by these odd roundish oblong rocks that extend all the way up the ridges on either side. It is almost like being in a stadium or coliseum with all these rock spectators watching you make your way up to the Head Rock (who lives at the end of the trail - more about him later). Below is a view seen early on in the hike.
What is responsible for these strange rock shapes? First of all, the Grapevine Hills is an igneous laccolith (a mushroom-shaped body of igneous rock) exposed by erosion due to its greater resistance to weathering than the surrounding rock. Secondly, the rock was broken up into compact-car sized (more or less) chunky pieces as it cooled and shrank. (A basic thermal property of most solids is that they expand when heated and shrink when cooled.) The shrinkage caused the rock to pull apart into those chunky pieces, which have weathered into the shapes seen.
In the above picture you can see the fracturing patterns that have allowed access by water and air into the rock, accelerating the weathering. These fractures are called joints. No relative motion has occurred on either side of these fractures. Otherwise, they would be faults.
The draw up which you are walking sees occasional flash floods due to desert downbursts. These floodwaters carry along sand and gravel which acts like an abrasive to wear away the bedrock. However, in many places there is something quite bizarre about the exposed bedrock. I'm talking about strange patterns like the ones in the image below, which was taken from above, standing on a tall boulder.
It took a minute or two of thought, but the origin of these patterns quickly became clear. You are looking where the erosional surface has cut through the bedrock, exposing the jointing pattern. The discoloration is from water and the chemicals dissolved in it seeping into the joints and reacting chemically with the rock. This is the first step in producing the "rock spectators" that line the trail.
A bit farther up the trail, the same patterns are seen in the rocks on the hillsides (below).
This type of weathering, where water seeps down into rock and weathers it into rounded shapes is called spheroidal weathering, and the Grapevine Hills are a veritable celebration of this process. Not in person nor in pictures have I ever seen spheroidal weathering so clearly exposed and on such a scale. Below are three more pictures of spheroidal weathering.
The first photograph is a closeup of the weathering pattern exposed on a boulder. The second shows the process of exfoliation, giving birth to new "spectators". (Exfoliation is where layers of weathered rock peel off, sort of like peeling an onion.) Toward the top of the picture you see a boulder pattern we dubbed "The Foot". The third is a closer picture of the exfoliation process. (This last picture was actually taken near the end of the trail.)
I haven't mentioned the type of igneous rock that makes up the Grapevine Hills, although it is in several of the images. Syenite is a fairly rare type of igneous rock, related to granite. However, in spite of the close resemblance, you shouldn't take it for granite (ha! ha!), because syenite, unlike granite, contains little or no quartz. (Not realizing the rock was syenite during the hike, I mistook the veins of whitish minerals for quartz until I put my finger on a vein and had the crystals cleave off into my hand. At that point I knew they were not quartz, since quartz does not exhibit cleavage. A little dilute HCl proved the crystals were calcite, which probably crystallized out of groundwater in the joints.) Syenite is found in continental settings where the magma is high in potassium, other alkali metals, and aluminum; in syenite the silica (silicon and oxygen) gets used up in making feldspars, primarily potassium feldspar (also known as orthoclase), such that little is left over to form quartz.
At this point you have gone far enough up the trail that you might pause and look back over the draw up which you came.
In the distance you see the Rosillos Mountains. There really isn't anything in this picture to give a reason for the existence of the draw at this location, but a northwest-southeast trending fault does run through the Grapevine Hills laccolith at this point - one of several parallel faults that do so. It is along this fault that erosion has been accelerated, forming the draw. Later in this virtual hike, I'll show you some evidence of the existence of this fault. Note that the motion has been up on the southwest and down on the northeast (as claimed by the geological map of the hills and confirmed by an observation to be discussed later).
The goal of the hike, for many, is reaching Balanced Rock, which resides on a ridge at the end of the trail. This rock, like an Egyptian sovereign carried aloft by his Nubian slaves, casts his gaze over the trail you followed in your long pilgrimage to this august setting. (How's this for B-grade prose?)
The image above indicates a possible (but unlikely) hypothesis for the formation of Balanced Rock. More likely it is an erosional remnant where the rock in the "doorway" of the feature weathered away. In fact, a great example of spheroidal exfoliation, which my brother is looking at, is found right before you reach the rock. Also check out the boulder emerging from spheroidal weathering just behind him.
The view above is to the south from Balanced Rock. Note the twin Spanish daggers that serve as orientation between this picture and the one to follow. Directly below the "d" in the word "Balanced" in the picture, you might be able to make out a sort of miniature "Baby Balanced Rock" down the ridge on the left. Visible in the distance to the left is an igneous dike, a sheet-shaped body of igneous rock that solidified in a volcanic fissure. In the far distance is Nugent Mountain, a large igneous intrusion exposed by erosion. Just to the right of Nugent Mountain you see the eastern edge of the main body of the Chisos Mountains. In the foreground the jointing pattern responsible for allowing such extraordinary terrain to be created is well-displayed.
"Baby Balanced Rock" is only about six or so feet long but large enough to stand on and take the above picture of Balanced Rock from the south.
After a good morning's hike, you have returned to the beginning of the trail, and, as you come out of the draw, you observe the trend of the fault that led to its creation (below). (We clambered up the boulders for this view and rested unseen while watching late-arriving hikers make their way up the trail.)
In the distance you can see that the trend of the fault runs off into the surrounding alluvium. The geological map indicates a contact might be found between two types of rock across the fault, if you can find a location where the alluvium has been removed by erosion. On the up-faulted side you should find igneous rock that has been exposed by uplift and erosion; on the down-faulted side Cretaceous rock preserved where erosion has not yet eaten down all the way to the igneous rock.
The trace of the fault is seen above in a small arroyo into which the rare flash-flood waters drain from the draw. On the left (south side) the uplifted igneous rock is seen, whereas on the right you have sedimentary rock with the igneous intrusive rock presumably at some depth below the surface.
Update: I just went back over this part of this field trip and was embarrassed to see that I had inadvertantly written "Ahuga Formation". What?! Maybe I was thinking of the sound old car horns make! I wrote:
The map shows the Ahuga Formation, described in the geologic literature as sandstone and clay, should be exposed here, but the rock appeared to be quite limy. Unfortunately, I didn't have my HCl acid bottle with me at this point to help determine the rock type. (Limy rock fizzes when you put a drop of dilute hydrochloric acid on it.)
The rock mapped here is actually the Aguja Formation, and another reference I have consulted says that it can be calcareous in part, so the map may well be correct.
FORWARD to the South Rim Trail, Basin
BACKWARD to the Marathon Uplift
ALL THE WAY BACK to the Contents