"The highway rose for miles then dropped into wacky Texas Canyon, an abrupt and peculiar piling of boulders, which looked as if hoisted into strange angles and points of balance. Nature in a zany mood had stacked up the rounded rocks in whimsical and impossible ways, trying out new principles of design, experimenting with old laws of gravity, putting theorems of the physicists to the tests. But beyond Texas Canyon, the terrain was once more logical and mundane right angles, everything flat or straight up." -- William Least Heat Moon, Blue Highways
Years later, well after William Least Heat Moon described it much better than I (Christian, the male third of hzoi) ever could, I remain struck by my own first impression of Texas Canyon. I was on my way to report to Fort Huachuca (not far south of here), my first duty station at the very beginning of my Army career. And though I'd seen other parts of the Southwest and the desert, I was taken aback by how different the landscape here in Texas Canyon was. Literally, nothing I'd ever seen before compared to it, and few places I've seen since have come close.
To this day, the only memory I have of that long road trip from Georgia to Arizona was the moment I arrived in Texas Canyon. It was early morning, not long after sunrise, and as I passed, I saw the warm rising sun light up this giant jumble of rock in a warm glow. So, I was very excited to try to share some of that feeling through this earthcache. I find I'm still excited now, as I add this paragraph to the cache listing in 2023, ten years after this cache was published. If you come out of Texas Canyon with even a tiny bit of the awe I felt all those years ago, and still feel today, then I will be happy that you've met my expectations.
All that said, this is an earthcache, and we do have a bit of light earth science (and bonus astrogeology) to discuss. So let's get to it. :)
FORMING THE MOUNTAINS AROUND YOU
The Little Dragoon Mountains are much younger than the bedrock around the mountains. Based on current scientific theory, around 50 million years ago, magma pushed up or intruded through the earth's surface. But this bubble of hot liquid rock did not break the surface of the earth like a volcano. This forceful intrusion of magma caused the earth around the Little Dragoons to fold and fault. When the magma cooled, it crystallized into a material called quartz monzonite. This rock is often mistaken for granite, and for good reason -- it looks similar, and it is even made of the same substances, quartz (the whitish crystal rock) and biotite (the darker rock). Granite, however, has more quartz content (20% or more) than quartz monzonite (5 to 20% quartz).
Intrusive igneous rock like granite and monzonite cool slowly, since they are underground and well insulated from the atmosphere. As a result, the crystals the rock forms are larger. Extrusive igneous rock, like basalt, cools much more quickly when it is exposed to air, so it has much finer grain.
Some of the boulders in Texas Canyon have stripes, called aplites. Aplites are the last part of the magma to solidify. They are made up of the same materials as the rest of the rock, but because they are the last to cool, they contain different concentrations of the minerals making up the surrounding quartz monzonite. Aplite veins (thinner stripes) and dykes (thicker) in Texas Canyon quartz monzonite consist mostly of quartz, feldspar, and muscovite. In some aplites, you can see small amounts of the darker biotite.
You will also see smaller inclusions, small pebble sized crystals in the rock. These formed in the same manner as the aplites, just on a smaller scale. Inclusions are also called "xenoliths," Greek for "foreign rocks."
"PRECARIOUSLY BALANCED ROCKS"
Many of the rocks in Texas Canyon are balanced on top of one another. This happened through a gradual, two-stage process called spherical weathering, which involves both chemical and physical weathering. When igneous rocks like quartz monzonite form underground, they are stable, kept in a pressurized, airtight environment. But when they are exposed to the atmosphere, they are subject to physical or chemical weathering.
Physical weathering just breaks rocks down into smaller pieces; chemically, it's the same rock. But chemical weathering, whether by oxidization or dissolution, actually changes the chemistry of the rock and turns it into something else. Over time, joints or cracks formed in the Texas Canyon quartz monzonite, and chemical weathering spread through these cracks, weakening the rocks. Physical weathering then causes the rock to wear away and become rounded along the surfaces.
Eventually, weathering caused what was once a solid bubble of magma into a pike of round boulders. Geologists call these boulders "precariously balanced rocks." And they are definitely precarious, because weathering continues to shape this landscape. The balanced rocks in Texas Canyon can become unbalanced and topple, either through seismic activity (tremors) or just weathering. Human activity (vandalism) can cause these rocks to topple as well by pushing them over. As the signs indicate, vandalizing these rocks is a crime, so please look but don't shove!
A MARS CONNECTION?
On June 12, 2022, NASA's Perseverance rover spotted a precariously balanced rock in the Jezero crater, where Perseverance and its Ingenuity helicopter companion landed in February 2021. Without more study, it's hard to know how this Mars boulder came to be.
LOGGING THIS EARTHCACHE
To log this earthcache, email us or send us a message and copy and paste these questions, along with your answers. Please do not post the answers in your log, even if encrypted. There's no need to wait for confirmation from us before you log, but we will email you back if you include your email address in the message. Group answers are fine; just let us know who was with you.
1. The name of this earthcache: Texas Canyon Rest Area: I-10 Rocks!
2. Were you on the eastbound or westbound side of I-10?
3. Describe the color and texture of the rock face.
4. Look for inclusions. This should not be hard, the rock should be full of them.
a. Describe the inclusions.
b. How do they appear different from the rock around them?
c. Which appears more resistant to weathering, the inclusion or the rock around it? Why do you think that?
(Optional) How do the balanced rocks in front of you compare to the photo of the Mars precariously balanced rock? Do you think they formed similarly or not?
Photos of your visit are always appreciated, but please don't post any close-up photos of the rock surface.
SOURCES
John R. Cooper, "Some Geologic Features of the Dragoon Quadrangle, Arizona." Southern Arizona Guidebook II, 1959, L.A. Heindl, ed., Arizona Geological Society.
David E. Haddad and J. Ramón Arrowsmith, "Geologic and Geomorphic Characterization of Precariously Balanced Rocks." Contributed Report CR-11-B, Arizona Geological Society, August 2011.
John R. Cooper and Leon T. Silver, "Geology and Ore Deposits of the Dragoon Quadrangle, Cochise County, Arizona." Geological Survey Professional Paper 416, U.S.G.S., 1964.
Richard F. Marvin et al., "Tabulation of radiometric ages, including unpublished K-Ar and fission-track ages, for rocks in southeastern Arizona and southwestern New Mexico." New Mexico Geological Society Guidebook, 29th Field Conf., Land of Cochise, 1978.
Mike Wall, "Perseverance Mars rover spots weird snake-head rock and balancing boulder (photo)." Space.com, 2022.