Dating Eagle Cave

As followers of this blog know well, the Ancient Southwest Texas research team has been investigating this Eagle Nest Canyon and Eagle Cave since 2013.   Following a cutting-edge “High Resolution, Low Impact” excavation strategy, we have carefully exposed, documented, and sampled literally hundreds of stratigraphic layers at Eagle Cave, some pencil-thin and some thick and massive.  The deposits in this dry rockshelter are complex – nothing like the flat, layer-cake examples found in archaeology textbooks.

Instead we encounter twisting and turning layer upon layer often cutting through one another.  This intricate layering is the result of daily life in the shelter on and off over thousands of years as the ancient inhabitants dug cooking pits, baked desert plants in earth-covered ovens, and carried out myriad other activities.  They often used their abandoned cooking pits as convenient trash dumps where spent cooking debris, worn-out fiber sandals, fire-cracked cooking rocks, and much more were discarded. The Eagle Cave deposits may be complicated, but the preservation is incredible, and we are recovering an amazing variety of scientific data from uncharred plant remains, wooden artifacts, and woven mats to animal bones, insects and human coprolites.

To allow us to properly and thoroughly date the Eagle Cave deposits and critical analytic samples, we have embarked on a crowdfunding campaign and are Texas State University’s inaugural guinea pig.  Most of this post is taken from our Dating Eagle Cave campaign page.  Check it out and be sure and see the video as you consider helping support our goal of making Eagle Cave the best dated and most thoroughly studied site in the Lower Pecos Canyonlands of southwest Texas.

Eagle-trench-2015-dates-unknown

Eagle Cave Main Trench Section as it looked at end of 2015 season showing mid-points of calibrated radiocarbon dates (yellow) and lots of questions about as-yet undated deposits.

Eagle Cave Challenge

The last major excavation of a dry rockshelter in the Lower Pecos Canyonlands took place back in the 1970s, when archaeologists from Texas A&M investigated Hinds Cave about 10 miles from here.  The ecologically oriented Hinds Cave dig recovered hundreds of coprolites which have been studied by graduate students and specialists ever since.  Truly, Hinds Cave has proven to be a scientific treasure (see http://www.texasbeyondhistory.net/hinds/.  We believe that Eagle Cave has the potential to build on and expand this legacy in many important ways.  Herein lies the challenge.

Major archaeological investigations of dry rockshelters with outstanding organic preservation, like Hinds and Eagle Caves, take many years of concerted research effort.  The actual digging is completed in a few years, but thoroughly analyzing the resulting data and fully realizing its scientific potential takes considerable research time and funding.  And it takes numerous carbon-dated, stratigraphically controlled samples.   We have already collected many more samples at Eagle Cave than were obtained at Hinds Cave and we have much better scientific control.

Because we are taking advantage of 21st century digital technologies, our documentation system at Eagle Cave is sophisticated and precise.  Every sample is assigned a unique code linked to a database that tells us precisely where it came from, usually within a few centimeters.  For every surface we expose – horizontal and vertical – we systematically take dozens of overlapping photograph.  Each night in our digital field lab we use special software to “stitch” these images together to form seamless three-dimensional models.  In other words, we can digitally reconstruct almost everything we excavate.  Scientifically speaking, this makes our samples extremely valuable.

OLYMPUS DIGITAL CAMERA

UT-North profiles from 2014 excavations being studied by Tina Nielsen for her M.A. thesis. In yellow are the calibrated midpoints of radiocarbon dates we obtained last year and the red question marks highlight yet-to-be dated areas.

Radiocarbon Dating

For us to achieve the scientific potential of such materials we must precisely date the Eagle Cave layers and special samples. Knowing exactly where something came from and what it was found with is only part of the challenge – we also need excellent chronological control – how long ago was a given layer created?  To figure this out, archaeologists use a combination of understanding the stratigraphy (layering), formation processes (how layers formed), and (radio)carbon dating (how old it is).  Any organic material can be used for carbon dating, and Eagle Cave has an abundance of organic material in virtually all of the layers. We prefer to date plant remains: charred wood, uncharred plant leaves, seeds, fiber artifacts and so on.   Using modern radiocarbon dating techniques all that is needed is a very small sample.  Specially equipped labs can measure the ratio of carbon isotopes, and calculate age based on the ratio of carbon-12 to radioactive carbon-14 (which has a half-life of 5730 years).  Do the math, and you can determine about how long ago the once-living plants died and ceased to accumulate carbon-14.

To confidently date a site with complex stratitgraphy, many dates are needed.  For instance, over fifty radiocarbon dates have been obtained on samples from Hinds Cave.  Thus far we have less than half that many for Eagle Cave. This isn’t a matter of one-upmanship, it’s a matter of scientific need.  We must know the absolute dates of key stratigraphic layers and critical samples through a concerted, multiphase program of radiocarbon dating in order to make our hundreds of samples scientifically valuable.  Securely dating key layers will in turn give us approximate (relative) dates for the many more “in between” layers.

What We Need

So far we have 18 radiocarbon dates from our 2014-2015 work.  For the next phase of dating we are seeking funding for 20 more dates.  Dating a complex site like Eagle Cave is an “iterative” process, meaning that the results from one round of dating helps us see the gaps and fine-tune the next round.  Radiocarbon dating is expensive with the going commercial rate for the most precise dating method (AMS dating) is $600 per sample.  Fortunately, we are working with a radiocarbon scientist at another university in a collaborative arrangement that allows us to get dates for less than half that rate.  Add in the need to get expert identification of the plant remains we are dating and 20 more dates will cost us about $300 each for a total of $6000, our campaign goal.  If we are very fortunate and exceed our goal, we will be able to get started on the following phase.

 Please consider helping to support our Dating Eagle Cave campaign!

 

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