From Lajitas to the Cuevas Amarillas Locale, Part 10: Shelter from the Sun | Big Bend Ranch State Park, Texas, USA

Detail of a cliff overhang at the Cuevas Amarillas site. Approximately 2.8 road mi / 4.5 km to the southeast of the turnoff to the Rancho Viejo Campground.

It makes eminent sense that a place like this would be a happy hunting ground for archaeologists. According to the first reference cited at the bottom of this essay, this is a locale that Late Archaic Paleoindians began to occupy periodically about 1.6 ka ago. And there was “more intensive occupation” from 1.1 to 0.9 ka. Excavations conducted back in 1991 and 1992 uncovered a collection of significant artifacts that were good indicators of these phases of habitation.

However, to the nonarchaeological visitor of my sort, the truly obvious sign of habitation consists of a series of bedrock mortars—deep, circular pits excavated into the stone floor of this shelter. These were used, along with some sort of pestle, to grind up acorns, grains, or other foodstuffs. Indeed, this would have been a very good place to engage in that kind of laborious activity, out of the glare of the desert sun.

Of course, the geologist in me wants to know the exact identity of the bedrock that holds the mortars. I’m quite sure that the yellow stone of the roof and wall sides is the Rancho Viejo Tuff Member, fused pyroclastic material deposited about 27.09 Ma ago. The unit mapped directly below it is a clastic deposit, conglomerate and coarse sandstone of the Segundo Lavas Member. Both of these belong to the Oligocene Rawls Formation. But is this notch in the cliff the contact between them? Or is everything visible here just the Rancho Viejo Tuff?

I venture to think that this is in fact the contact between the Rancho Viejo and the clastics. I base this primarily on the next photo in this series, a close-up of some of the mortars. It shows that the rock they’re in has the pebbly texture characteristic of conglomerate. And anyway, if one simply goes by the color contrast between upper and lower portions shown in the photo above, there really do seem to be two different rock types visible here.

While the Rancho Viejo Tuff above may not be as archaeologically significant as the conglomerate below, it does exhibit some wonderful geology in the form of desert varnish. That’s the dark-brown staining present on the ledge in broad bands.

Desert varnish consists of a sequence of super-thin layers of particles of the clay minerals smectite and illite that are deposited by the wind. They also contain other substances, most notably manganese oxide or iron oxide that serve as darkening agents. For decades, geologists and biologists have debated whether the varnish is precipitated by microbial action or is a completely abiotic process. The remains of microorganisms have been found embedded in it, but whether they just got trapped there or were active participants in making the rock coating, is still a matter of considerable contention.

The pattern of dark bands alluded to above is most likely an example of rain stripes. These zones form on the rock surface where precipitation drips down. The desert varnish tends to build up there, where the surface has been hydrated periodically.

Main References for This Essay

- Gibbs, Tim L. “Site Evaluation and Monitoring along the Rancherias Loop Trail, Big Bend Ranch State Park, Presidio and Brewster Counties.” In Annual Texas Parks and Wildlife Department Report to the Texas Historical Commission. Terlingua, TX: Texas Parks and Wildlife Department, 2018.

- Henry, Christopher D. Geologic Map of Big Bend Ranch State Park, Texas. Austin, TX: Texas Bureau of Economic Geology, n.d.

- Dickerson, Robert. “Desert Varnish—Nature’s Smallest Sedimentary Formation.” Geology Today, November 2011.

To see the other photos and descriptions in this series, visit my From Lajitas to the Cuevas Amarillas Locale album.

Taken in: United States / Texas / Papalote Colorado (show map)

Taken on: March 12, 2002

Tags:

archaeology

architectural_geology

nature

parks

stone

rocks

stonework

Texas

Trans-Pecos Texas

Big Bend Ranch State Park

bedrock mortars

conglomerate

tuff

Segundo Lavas Member

Rawls Formation

Rancho Viejo Tuff Member

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Sets appears in: • From Lajitas to the Cuevas Amarillas Locale | Big Bend Ranch State Park, Texas • Homage to the Basin & Range Province more » • Natural History: Big Bend Region & All of Trans-Pecos Texas • Rock Forms, Rock Formations « less

Camera: MAKER NAME 96660, f/1.8, 1/59 sec, 2.8mm, ISO 50 (more info) (hide info)

Camera:	MAKER NAME 96660
Aperture:	f/1.8
Exposure Time:	0.017 sec (1/59)
Focal Length:	2.8mm
ISO:	50
Exposure Bias:	0 EV
Flash:	Auto, Did not fire
Exposure Program:	Program AE
Metering Mode:	Average
White Balance:	Auto
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