The Next Layer, A Sampling Column Story

The Next Layer, A Sampling Column Story

By Kelton Meyer

The inherently destructive process of excavation means that archaeologists must devise effective measures to capture as much useful data as possible as deposits are destroyed. One of the most important aspects of our excavations in Eagle Cave is the process by which we sample intact stratigraphy. By carefully exposing intact layers in profile, spending much needed time defining strats and sampling with diligence, we are able to gain high resolution views into the lives of the prehistoric Native Americans who frequented Eagle Cave.

I’m Kelton Meyer, an intern with the ASWT project and soon-to-be graduate student at Colorado State University. I’m writing this post to share my experience in excavating Profile Section 25 (PS025), and to take you into the trench where we are working hard to tell Eagle Cave’s stratigraphic story.

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Excavating my sampling column, PS025

Exposing Profile Section 25

The first step in the process of sampling intact stratigraphy is to create a clean profile to clearly expose the layering. We do this by excavating in fairly traditional excavation units to create a “wall” (AKA profile) in an undisturbed context. In the case of PS025, two units were excavated to create the profile, Units 76 and 85. The unit configurations are not necessarily governed by size, shape, or grid orientation, but are dependent on identifying the dividing line between intact and disturbed deposits. When we first started excavating in the main trench last spring, we had to remove lots of disturbed fill from the face of the trench prior to placing an excavation unit. However, as we have continued deeper this season, we have been able to trace the intact deposits easier and more confidently as we have worked our way down into the trench.

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Unit 76 (left) excavated in May 2015 and Unit 85 excavated in February 2016. The south walls of these two units combine to create PS025.

Our excavation units typically consist of anywhere from 1-10 layers, and these layers are defined by factors like changes in sediment color, consistency, artifact density, or the indication of a possible cultural feature. Without a profile to guide our excavation, it is often very difficult to excavate these layers following natural stratigraphy. To aid us in assigning stratigraphic provenience to any artifacts, we take sets of SfM photos (see Archaeology in a Whole New Dimension)  to build 3D models of our units each night in our digital field lab. Using these 3D models we can link our “traditional” units with the stratigraphy we record in profile. Once the necessary excavation units have been completed the newly created profile wall is ready for cleaning, SfM photogrammetry, and field annotations.

Profile Section 25

PS025 is one of the larger profiles, located towards the rear of the rockshelter and in the approximate middle of the overall vertical stratigraphy of the site. It is representative of different occupational zones, and varying episodes of earth oven dismantling and refuse.

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An orthomosaic of the south trench and profile sections with PS025 highlighted in red.

 

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Orthomosaic of Profile Section 25

What We Can See

After we’ve built a 3D model of the profile in the lab we print out an orthomosaic (a profile view) into the field for annotation. With a conveniently sized paper copy of the profile in hand, we can sketch stratigraphic changes, assign numbers to the strats, and take any other notes that we deem necessary. For PS025, the field copy was especially handy due to the varying effects of sunlight upon the lightly colored sediment, and the broken characteristics of the strats.  Once again, the Eagle Cave stratigraphy bears little resemblance to textbook layer cake simplicity.

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My PS025 field annotation.  The numerous hatchered areas are rodent bioturbated.

When determining stratigraphic changes in profile, several factors are taken into consideration. We first identify any visible disturbed contexts, such as rodent burrows. In  PS025, evidence of rodent bioturbation was obvious. Large pockets of mottled sediment intruded into most of the intact stratigraphic layers (strats), bringing fecal matter, grasses, and other debris into the profile wall. We then assign strats from top to bottom,  according to the superposition of the layers. Strats can vary in color, texture, and consistency of sediment. Some strats extend across fairly large zones, while others are small, thin, and broken in profile view. Once the profile has been fully annotated and the strat information has been entered into the database, each individual strat is ready for direct sampling.

I found the annotation of PS025 to be an enjoyable experience, and it allowed for some artistic expression. Bioturbation often presents annotation challenge, but it sharpened my archaeological skills as I traced and separated the intact from the disturbed.

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Here I am concentrating on my profile annotation

Taking from the Wall

In Eagle Cave it is important to record the provenience of all aspects in excavation, and especially in sampling. A midpoint for each strat is “shot in” using a TDS (Total Data Station) and the precise location of each subsequent sample we take from the profile must also be shot in as well. We collect spot samples, geo-matrix samples, and 14C samples. A spot sample is a small bag of undisturbed strat sediment. A geo-matrix sample is a somewhat larger bag of sediment that includes rock and pebble constituents to allow the geoarchaeologists to characterize sediment size and texture.  A 14C sample can be a collective variety of botanical remains like charcoal, seeds, or leaves that come from unquestionably intact areas within each strat. These point-provenienced samples will allow the analytical team to review sediment characteristics, analyze the geo-archaeological properties of individual strat matrix, and, potentially, to obtain a targeted date from each strat.

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TDS points of all samples and strat definitions taken from PS025

The collection process from the wall requires expert troweling and methodical strategy. A reduction in the size of trowels, pans, and brushes is absolutely necessary! When choosing to collect samples from a profile, it is best to begin from the bottom and move upwards so that the next strat is not contaminated by sediment spills. Sometimes, the strats in profile are so small or intermittent that it is not possible to collect all three sample types. Priority is given to spot samples where adequately sized geo-matrix sampling is not possible, and 14C samples are collected when the appropriate material is visible in the profile (e.g., a charred cut leaf base). Additionally, artifacts that have been left in situ  as the profile wall is cleaned and examined are shot in, photographed, and collected. When samples of each strat have been removed, it is time to choose where to place a sampling column.

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Collecting samples from the profile

Collecting from PS025 was at first a heartbreaking experience. Much time was expended in making the wall an appealing example of visual stratigraphy.  I’m trying to say it was tedious work and often frustrating when seemingly intact proved to be rodent-churned. However, understanding the importance of the samples I was taking made it all worthwhile.

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Distal portion of a chert biface in profile

The Sampling Column

A sampling column is a specific type of unit used in our excavations at Eagle Cave. The principle goal of these units is to provide an in-depth look into the stratigraphy of a profile section by isolating intact strats and collecting sizable matrix samples. Choosing where to place sampling columns depends entirely on the characteristics of individual profiles and factors like stratigraphic density, artifact density, feature locations, etc. The evolving research goals of the project dictate where columns are placed. Eagle Cave field director Charles Koenig consults with the excavator(s) most familiar with each profile section and makes the call. Sampling columns may be placed in areas of the profile that favor exposed features, and this may result in some relatively minor strats not being sampled. This is why it is important to collect the initial samples before the sampling column is placed! Some profile sections receive more than one sampling column, for instance exposures with nicely intact stratigraphy and excellent organic preservation like latrine deposits.

The columns are typically small rectangular areas measuring 20 to 40 cm in each dimension.  Sampling columns are excavated strat by strat, and in the more dense stratigraphic areas of the site they can be consist of 20 plus strats. The proper excavation of a sampling column is very detailed and careful work. All of the undisturbed strat matrix is collected for later processing and curation, and all artifacts encountered during excavation are shot in, photographed, and collected. Additionally, we carryout Rock Sort data collection for each strat. The crew must constantly refer to their field annotations to ensure that the excavated strat layers do not cut into new strats, or involve previously sampled strats as sediment is removed.

At the end of each completed strat, SfM photogrammetry is performed. In many cases, new stratigraphic layers are encountered and identified as the sampling column comes down in the profile. These new strats must be annotated, sampled, plotted, and collected. Sometimes, strats that existed in profile may not continue far behind the profile wall, and thus must be sampled even more carefully to preserve at least some data given the paucity of sediment matrix.

The column for PS025 was strategically placed to sample a feature visible in profile.  The defining characteristics of Feature 11 were the large boulder-like burned rock protruding from the wall and the surrounding pattern of compacted ash, charcoal, and fire-cracked rock. In total, the sampling column consisted of 14 excavated strats, with one being identified mid-excavation. Most of the strats consisted of ashy gray/white sediment having a very fine texture and containing compacted charcoal.  Some strats produced burned and unburned fiber, other botanical remains (e.g., charred seeds), animal bone, chipped stone tools, and other types of artifacts shot in with the TDS.

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Location of sampling column

 

Lab Processing

After each strat is sampled and collected, matrix is brought back to the field lab for processing. The collected matrix from each strat is weighed and quantified, and then screened through a 1/2” sieve to remove large artifacts and rocks. Artifacts collected from the screen are cleaned, weighed, analyzed, and set aside for curation. The remainder of the matrix is bagged and cataloged, awaiting further analysis. The screened matrix is also curated and given a specimen number for our database, so that the provenience of each sample is thoroughly recorded in our system.

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Justin Ayers sieving dusty matrix

As work continues in Eagle Cave and more data is collected, the process of curation becomes increasingly important. The variety of artifacts and samples we collect will provide answers to many of our research questions regarding the lifeways of the prehistoric occupants of Eagle Cave. Samples for macrobotanical data, faunal identification, lithic reduction strategies, tool analysis, archaeoentomology of human fecal matter, and even phytoliths, are awaiting for the analytical team to decipher as we work towards understanding natural and cultural formation processes, ecology, climatic conditions, cultural patterns and much more from this awe-inspiring rockshelter in the Lower Pecos Canyonlands.  I’m proud to be able to contribute the next layer in the Eagle Cave sampling column story.

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Elton Prewitt and I examine an artifact I exposed while excavating my sampling column.

Experimental Gauntlet: Replicate This!

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“Butted knife”  41VV2239  FN50156

By Steve Black

As the principal investigator of Ancient Southwest Texas (ASWT) and as faculty sponsor of the Texas State Experimental Archaeology Club, I hereby challenge the 2016 ASWT crew and Club members to convincingly replicate the use wear pattern(s) apparent on the recently recovered biface pictured above.

This distinctive artifact was found in situ on 3/28/2016 at Sayles Adobe (41VV2239) by ASWT 2016 Intern Kelton Meyer working under Victoria Pagano who is directing the Sayles investigation for her thesis research. The artifact was found about a meter below the surface of this terrace site amid scattered FCR (fire-cracked rocks) that I would guess represent the upper and outer part of an earth oven facility where desert succulents like sotol and lechuguilla were baked. (It could be the edge of a buried burned rock midden, perhaps an incipient one?)

In our previous six seasons in the Lower Pecos Canyonlands (LPC), ASWT has found no other example of this quite formal artifact type, but several were found during the Amistad salvage era at the Nopal Terrace and Devil’s Mouth sites.  They occur fairly often in the Kerrville area and in the western Balcones Canyonlands.

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What kind of a “fist axe” would have such a delicate blade, a butter axe?

These unusual artifacts have been given many names.  What is in a name? Some have called them fist axes or hand axes because of their somewhat similar appearance to Old World artifacts, most of which date tens or hundreds of thousands of years earlier.  There is a simple morphological reason why these Old World terms seem functionally inappropriate – what kind of axe would have such a delicate cutting edge? (A butter axe?)  The latest edition of Turner and Hester (but cf. earlier editions) uses the type name Kerrville biface, which is geographically appropriate, if dissatisfying to some. Typological maven Elton Prewitt prefers the descriptively appropriate term butted biface. I prefer the functionally appropriate term butted knife; that these are some sort of cutting/slicing tool seems obvious.  Consensus, however, has yet to emerge on either the name or the specific purpose(s) of these tools.

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Even though I pride myself on not being “artifact-oriented,” this unexpected new find in excellent context has me in a dither. When I initially looked at the artifact in our field lab the evening it was found, it set my intellectual juices flowing and I harkened back several decades ago.  Then, as is still true, I was convinced that I knew exactly what butted knives were characteristically used as: sotol trimming knives (plus Agave lechuguilla in Lower Pecos?).

I recall that I once envisioned experimentally replicating the striking use wear that most butted knives display:  remarkably bright “silica polish” rather evenly distributed across both faces of almost the entire blade of intact examples. I even took several modest steps in the experimental direction.  For instance, Glenn Goode kindly made a fine replica to be used experimentally.  But alas, I failed to follow through with the hard work that a rigorous replication project would entail and my Goode-made biface sits gathering dust in my TxState office on my show-and-tell shelf.

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Butted biface replica made by Glenn Goode out of Georgetown flint.

Within an hour of seeing the Sayles Adobe specimen, I took several pictures of it and sent one to my former mentor and long-time boss, Dr. Thomas R. Hester, UT-Austin professor emeritus and former director of both the Texas Archeological Research Laboratory at UT-Austin and the Center for Archaeological Research at UTSA.  I also sent one to Elton, who most of us Texan archaeologists regard as a stone-tool typological guru of the first water (most of us think the same about Tom Hester). The subject line of my email was “Butted agave knife” and I closed both email messages with “I knew you would appreciate!”  Sure enough, they both replied right away and here are tidbits.

Hester:  “We call them Kerrville “bifaces” because the polish/wear has never been satisfactorily replicated.  But, I’ve long thought they were plant-working tools, not necessarily slicing and dicing, but mebbe chopping/hacking into an agave or some other soft plant where the distal got “imbedded” and the polish eventually appeared.”

Prewitt: “Nice butted biface. I do not like the term “Kerrville Biface” since it has never been appropriately defined as far as I am aware (I could very well be wrong, but …). And, yes, we do have good ideas about their uses.”

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These are called “butted” because the thick, proximal end of the tool is typically the outer cortex-covered surface of the original chert nodule. It was obviously made this way so the usually rounded butt fits in your hand with the blade tip (distal end) pointing out ready for action. The Sayles Adobe artifact seems to lean to the right in this photo because it is resting on its rather flat butt.  This artifact appears to be made on ledge chert, thus the flat, thin cortex rind you can glimpse.  High quality ledge chert is rare in the Lower Pecos, but often occurs in  in the Kerrville area.

Intrigued?  So here is my challenge.  I think it would be a most worthy project to (1) design a rigorous experimental program to convincingly replicate the telling use wear pattern(s) of a call-it-what-you-will; and (2) successfully follow through with such a program.  Here are some considerations and suggestions.

Before picking up the gauntlet, you will want to do your homework and do your best to read everything ever written about the subject.  These butted things have been admired by many, often speculated and reasoned about in print, studied under the microscope, and studied experimentally (if inconclusively). Doubtlessly more so than I can recall.

This will not be easy.  If the use wear could have been easily and convincingly replicated it would have already been done. And it is entirely possible these artifacts were sometimes used on more than one material and/or in more than one motion. I venture to say that such a project will almost certainly take many months of concerted replicative effort and likely several years to see through to peer-reviewed publication, which should be the end goal.

With that in mind, I recommend that the ASWT crew and the Club talk amongst yourselves and consider forming a leadership team of three to guide the effort.  You will need competent, motivated decision makers and with three, you would always have a tie-breaker (as Dan Potter, Kevin Jolly and I learned on the Higgins Experiment in 1993).  And you will need diverse skills and continuity.  I recommend that the three project leaders include TxState students or former students of varying levels of experience including several who aren’t graduating anytime soon.

But it will likely take far more than three of you to get it done.  You will likely want to try more than one contact material (sotol/lechuguilla, meat, and grass all come to mind).  You will likely need many hundreds of strokes in said materials to create patterned wear.  You will want to properly document each step, photographically, metrically, and so on.  I’d think it would make a fine experimental project.

You would be wise to consult others.  I would put Professor Hester at the top of your list.  I’ll bet he has seen more than one student paper on the subject, he has sure as heck seen many more of things than me, he has published on them, and I know he shares my abiding curiosity.  Elton is always a go-to source for informed typological opinion regarding lithics.   Professor Britt Bousman teaches the graduate seminar in lithic technology at TxState, and he might even let you look at and document butted things (perhaps before and after replication?) using TxState’s fancy use wear microscope. Dr. Mike Collins of TxState is unsurpassed in his knowledge of lithic technology and he once dug a site near Kerrville.  Dr. Marilyn Shoberg (of Austin) might well have looked at some of these things under a scope. Dr. Todd Alhman might have archaeological examples at CAS (ask about the Tom Miller Collection).  Chris Ringstaff or Glenn Goode might be willing to make several freshly chipped stone replicas to be used in experiments.  Ken Lawrence would certainly approve of experimental work of this sort being done out at Professor Grady Early’s place in the phosphate-sampled area.  I’m certain inquiry will lead you to others who would be worth consulting and might lend a hand.

But do not make the common mistake of uncritically accepting dogma.  Challenge assumption, question authority, and think for yourselves.  I, for one, could well be wrong about some of my claims in this piece as well as those I make in classes and in print. (Say it ain’t so, Shoeless Blackie, say it ain’t so.)

Should a group of you rise to the occasion and accept the challenge, don’t do so lightly.  I won’t hold it against you if you don’t pick up the gauntlet.  But I might if you accept the challenge and fail to follow through.  If I’ve piqued your interest, start with doing your homework and decide whether to go forward.  Then craft a proper research design.  If I approve your plan I will endeavor to support your effort in multiple ways. I think this could be fun learning exercise and make a useful contribution to Texas archaeology.

Yours in the experimental cause, SLB

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.

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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.

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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!

 

Journey to the Center of Sayles

By Justin Ayers

Howdy! Justin Ayers here, excited to explain the ongoing bucket auger testing at Sayles Adobe. You may ask what exactly is an auger? The auger is a simple but effective tool for collecting/sampling sediment below the surface without opening excavation units. It is comprised of a lengthy pole with a helical bit (and cylindrical bucket) at the end, which is designed to twist through the earth easily, while the bucket simultaneously collects the sediment displaced by the twisting bit. After one and a half turns of the auger handle, the auger bucket is filled with compressed dirt, and the device is gently pulled up out of the hole and the bucket load is dumped into a screen, examined, and recorded. The process is repeated over and over up to a depth of 3 meters (about 10 feet!).

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Me and the auger.

Conducting a bucket auger survey of Sayles is important because it is allowing us to relatively quickly sample the stratigraphy across the terrace, a task that would take months to accomplish using hand excavation units. Further, by combining the different sets of auger data we are able to map out the subsurface deposits (both natural and cultural) of Sayles. These data will contribute to Tori Pagano’s ongoing thesis research (see Tales of Sayles Adobe), as she aims to define and sample the natural and cultural deposits of the terrace.

Bucket Auger Process

Tori’s plan is to build on the ground-penetrating radar (GPR) survey conducted at the site in January by Tiffany Osburn. We wanted to follow the GPR grid with our bucket augering as closely as possible so we could investigate several interesting radar anomalies seen at certain locations and depths within the terrace.  Happily, our collaborator geoarchaeologist Ken Lawrence kindly allowed us to borrow his bucket auger!

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East-west GPR transect (top) and west-east transect (bottom). The yellow arrows point to the same anomaly in both transects, indicated by the upside-down V-shape.  This anomaly is approximately 1 meter (~3 feet) below the ground surface. These are the types of anomalies we wanted to hit with the auger testing.

 

We wanted to use the bucket auger tests to begin to “ground-truth” the GPR anomalies (i.e., determine what the anomalies are caused by).  We also wanted to compare the sediment sampled by the auger to the exposed stratigraphy in the “Borrow Pit” excavation area.  The auger testing is showing us that the stratigraphy of Sayles Adobe is more complex (and interesting) than that seen so far in our initial excavation exposure in the Borrow Pit.

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You can see the bucket auger on the left, leaning against a tree. To the right you can see the rest of the equipment  we used during auger testing: Munsell color chart, 2mm geologic sieve, 5-gallon bucket, and lots of recording forms!

Augering Procedures

Step 1: Stake out targeted locations of auger columns.

  • Transect sampling intervals – 4 meters apart
  • Shoot in surface points with TDS (Total Data Station)
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Plan map of Sayles Adobe showing east-west bucket auger transect. The red dots are the auger tests (laid out 4 meters apart) that have been completed thus far.

Step 2: Begin sampling

  • The auger excavates a 10cm diameter hole, and each bucket full of dirt is ~8cm deep. Once a bucket of sediment is collected, the auger is pulled up to the surface.
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From left to right: auger in the hole, removing the auger, the auger fully out of the ground, and finally dumping the sediment into the sieve.

Step 3: Sieve the sediment.

  • Once the bucket is pulled out of the auger hole, it is dumped into a 2mm sieve. Only the fine sand/silt mixture will pass through the screen, leaving snail shells, roots, FCR (Fire-Cracked Rock), and debitage on the screen.
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Dumping the sediment into the sieve. The bucket auger bit is clearly visible.

Step 4: Documentation

  • A metric tape (or “pocket stadia”) is used to measure the depth of the hole after each bucket auger sample was removed. This allows us to monitor our progress and make the auger data comparable from one auger column to the next.
  • The sediment texture varies from a fine sand to a compact clay-silt.
  • Color is recorded using a Musell color chart. We found that the 10YR color sheet works best for us.
  • Lastly, general notes are made of the sediment, e.g., organic materials if present, FCR, charcoal, & ped toughness (Wikipedia: ped = a unit of soil structure such as an aggregate, … block, or granule, formed by natural processes.)

Step 5: Homogenize and Sample

The sediment that falls through the screen is homogenized (stirred around) in the bucket before we collect a small representative sample. After the field season Tori, under the tutelage of geoarchaeologists Dr. Charles Frederick and Ken Lawrence, will process the samples to determine grain size and test for magnetic susceptibility (among other things). Tori plans to use the auger column data to create composite stratigraphic profiles (sections) across the terrace that will be integrated with GPR and other lines of evidence.

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A “cube” from the auger testing of Sayles Adobe awaiting further geoarchaeological analysis.

I should note that while the bucket auger does its job very well, it mixes the sediment in each bucket load together such that thin layers only a few cm thick are hard to detect.  And even though we are careful to insert and remove the auger gently, inevitably there is some admixture of sediment from the walls of the hole.  In other words, we end up with somewhat averaged samples in 8cm increments. We homogenize each sample to make sure it truly represents an average of each increment. By carrying out each auger test in exactly the same systematic way,  we can compare apples to apples with our auger samples and differentiate significant stratigraphic changes.

Luckily Sayles Adobe has very nice silty and sandy deposits (as opposed to compact clay or gravel) and testing has gone without a hitch … for the most part. Sometimes we are impeded by rocks and roots, forcing us to move our auger test column to a new location. If we encounter something hard that we can’t get through, we simply move the auger hole 30-40cm over from the original location and try again. Overall, we able to get down to the full 3-meter depth in only about 50% of our tests. In the other half we are stopped by burned rocks or other obstructions (like large mesquite roots) before reaching maximum depth.

What Are We Finding?

Based on our initial excavation in the Borrow Pit area along with the GPR survey findings, we suspected that we would hit the upper mud drape (I will explain why I say “upper” in a bit) around the same depth at multiple points on the terrace (see below profile of the Borrow Pit in Sayles).

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The east profile of the “Borrow Pit” in Sayles. The “upper” mud drape is noted as S003,  just to the left of the orange tag.

So far we have found that the mud drape is not a perfectly flat horizontal layer, it tends to follow the topography of the deposits it covers. What I mean is that in places there may have been exposed rocks or humps in the ground that the mud drape settled over during the flood event, which may be the AD 1340 flood that we documented at nearby Skiles Shelter and Kelley Cave.  Many (all?) of these rocks are FCR that were capped by the mud drape. The upper mud drape is thin and hard (clay-silt), compared to the fine sand and sandy silt that makes up the majority of the site’s deposits. In essence, the mud drape seals the cultural material that lies beneath it. 

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Bryan standing in the “Sandbox” area at Sayles looking at FCR that he is just beginning to expoose. The mud drape in this unit (denoted by yellow arrow) is very thin compared to that seen in the Borrow Pit. Several adjacent auger columns in this area were terminated by hitting rocks and we suspect that some massive FCR accumulation must be present.

Since the mud drape covers the top of the uppermost cultural layer at Sayles Adobe, when we reach the upper mud drape the cultural layer should be directly beneath it. However,  this cultural layer contains many FCR and often the bucket auger is stopped by the rocks. The auger will dig through most Sayles Adobe sediment quickly, but when you hit a sizable rock … everything stops. This is not considered a bad thing though. Usually when we hit a rock, we bring up chips of said rock that reveal if it is FCR. The FCR fragments tended to smell of sulfur when broken by the auger bit. The FCR encountered  about one meter below the surface tells us that we are hitting the upper cultural layer … jackpot!

So far we have not created schematic stratigraphic profiles for all the auger tests, but we created a preliminary illustration based on a test from the east side of the site. The stratigraphic patterns in this auger test compare well to the stratigraphy observed in the Borrow Pit and Sandbox excavation, so we are anxious to complete augering of the entire site so we can get a better map of the stratigraphy!

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Stratigraphic profile created using data collected from an auger test. This profile matches nicely with the exposed stratigraphy in both the Borrow Pit and Sandbox areas of the site.

Looking back at the GPR research done in January, the upper anomalies seen appear to have been the upper cultural layer. The FCR are in a large enough concentration to show significant feedback from the GPR. Three of our west terrace auger tests were stopped at a depth between 80cm-1m when we hit rock—usually FCR, which the auger could just not dig through.  In fact, the Sandbox excavation area was laid out as the result of our first east-west bucket auger transect.  Next week we should expose the concentrated FCR and see why so many burned rocks are piled up in one area.

Plans for the Future

Auger testing at Sayles is an ongoing process, with more sampling columns on the way. So far, only an east to west transect has been completed. We just started on our north to south transect, targeting more anomalies from the GPR survey. If additional FCR/cultural layers are encountered, it is likely that more units will be opened up for further research. The emerging picture from the bucket auger data is proving to be quite informative and tantalizing.  Hint, hint:  we have encountered deeper layers of mud drape silt and of cultural material yet to be exposed!

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Tori and I very excited about a flake from an auger test!

Mortar She Wrote

By Amanda Castañeda

Most of the previous ASWT blog posts have focused on our ongoing excavations in Eagle Nest Canyon, with a few thrown in about earth ovens and our undying love for burned rock. So I thought it was time for a little change of pace! This post highlights another very common archaeological feature found in many of the sites within Eagle Nest Canyon and elsewhere across the Lower Pecos Canyonlands —ground stone bedrock features. Bedrock features, as I will often refer to them, include “slicked” areas, shallow grinding basins, deep mortar holes, and everything else in between.

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Amanda cleaning off bedrock features.

I first became interested in bedrock features during my tenure at Shumla Archaeological Research and Education Center. We went to rockshelters across the region to record rock art, and bedrock features were a common occurrence in many sites. Most intriguing were bedrock features that were over 50 cm deep. I thought to myself, what on earth were the Lower Pecos inhabitants doing with these features when you can barely touch the bottom with your fingertips? Further, despite the ubiquitous nature of bedrock features in the Lower Pecos, they represent a largely understudied part of the archaeological record. In part, this is why I chose to explore this prehistoric technology for my recently completed Master’s thesis at Texas State University.

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A volunteer demonstrating the great depth of some of these bedrock features. Neither hand can touch the bottom of these two bedrock mortar features.

Bedrock Features 101

Archaeologists typically categorize these features by morphology and the perceived type of activity (e.g., pounding, reciprocal grinding, circular grinding, etc.). For example, grinding facets are shallow basins likely used to grind foods with a back and forth or circular grinding motion. Mortars are deep holes that were utilized for crushing or pounding, probably using straight up and down motions or possibly rotary or circular motions in some instances. Lastly, “slicked” areas are flat surfaces that have a shiny, smooth appearance and their function is unknown. The highly polished surface could be the result of multiple activities such as polishing hides or another activity that might include oily substances. Ethnographically, bedrock features of all shapes and depths were used for a variety of activities, mostly related to food-processing.

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Three deep “mortars” are surrounded by much shallower “grinding facets.”

While generic terms, such as the ones listed above, have been used in previous bedrock feature research around the world, prior to my study there had not been any bedrock feature research completed in the Lower Pecos.

Therefore, I was most interested in creating a baseline data set of the morphological variation of these features. In other words, are we able to pick out any unique “types” of features and how are these morphologies distributed across the landscape? Further, using other lines of evidence such as use-wear patterns, can we determine what kinds of foods were being processed in these features? Essentially, I wanted to take a broad approach to my research and try to gain a better understanding of how these features were utilized by the hunting and gathering peoples of the Lower Pecos.

Recording Bedrock Features in the Lower Pecos Canyonlands

Data Collection

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Amanda recording bedrock features at Eagle Cave, and undoubtedly suffering from “bedrock butt.”

For my research I recorded morphological attributes (e.g., shape/size), use-wear characteristics (e.g., wear patterns left behind on the rock from different processing activities), and metric data (e.g., measurements) for 824 individual bedrock features at 10 sites using a combination of Structure from Motion (SfM) Photogrammetry (see SfM Revolution) and traditional field documentation methods. I recorded morphological attributes and macroscopic use-wear patterns using the form below.

BRF Attribute Form

I used SfM as the primary mapping and documentation method for each of the bedrock features I recorded. From the 3D data, I was able to create high resolution feature maps, and gather measurements for each feature in the mapping software ArcGIS.

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Examples of feature maps created via SfM and ArcGIS: a) orthophoto of surface; b) a digital elevation model (DEM) of the same surface; and c), a slope model of the same surface derived from the DEM.

Statistical Data Analysis

I focused the majority of my analyses on three different measurements: maximum depth of each feature and two axes across the opening of each feature. To summarize my results, I completed a cluster analysis using the metric data, which resulted in four distinct groups of bedrock features. The majority of the features (97%) fell into one very large, closely related group. I should note that Cluster 1 was comprised of four smaller subgroups, but there is only so much you can do for a M.A. thesis and still finish in decent time (as my committee wisely advised me!). The remainder of the bedrock features formed three smaller groups.

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Looking at the graphic below, it’s easy to see how some of these groups were formed. Cluster 4 features are all extremely deep, between 40 and 60 cm. Cluster 2 features are moderately deep, between 20 and 30 cm. However, there are pieces of the puzzle the graphic doesn’t show. For example, the majority of Cluster 4 features have completely straight/vertical walls, while Cluster 2 features have sloping walls that form an overall conical shape. These types of observations were exciting because they are diagnostic attributes of these clusters that are independent of the cluster analysis. Said differently, they support the distinction of two different cluster which also had two different activities going on.

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As for Clusters 1 and 3, they seem to blur together in the above chart. The reason for this is that Cluster 3 features are defined by at least one considerably long axis at the opening of the feature. Looking strictly at the length of the axis measurements, Cluster 3 separates itself from the rest of the sample. Cluster 1, in both graphics, is all over the map. This cluster contains a very wide range of variation in all aspects of the feature, depth, length, and width.

Axis v Axis

So essentially what the cluster analysis had shown me is that there are distinct morphological groupings of features, and each had a diagnostic characteristic that defined the group (except Cluster 1). The next thing I wanted to determine was if there were any other attributes of these clusters that further support distinguishing them from one another?

Bedrock Features on the Lower Pecos Landscape

First I looked at location. How are these clusters distributed across the landscape? Unsurprisingly, all ten sites have features that are included in the Cluster 1 group. In fact, four of ten sites have features that only fall into Cluster 1. The other three clusters are more restricted in their distributions. Cluster 2 occurs at three sites, Cluster 3 is present at five sites, and Cluster 4 only occurs at two sites. These data suggest that across the region, the majority of the food-processing that occurred could be completed in a non-specialized, Cluster 1-type feature. This could be due to the relatively small amounts of food being processed in most features or to the predominance of certain foods that did not need a specialized surface.

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Use-wear Patterns of Bedrock Feature Clusters

The next characteristic I wanted to examine were the use-wear patterns. Did each cluster have distinctive or diagnostic wear patterns that might help me interpret the types of food that were processed in those features? Before I start throwing terms at you, let me first explain a little bit about use-wear studies on ground stone surfaces. In regards to ground stone bedrock features, differential use-wear across the surface of a feature can show what type of activity happened most recently. Is the surface pecked and rugged, or is it completely smooth to the touch? These conditions tell different stories about what happened last with that particular feature. When making use-wear observations, the objective is to observe traits about the macrotopography, or the high and low points.

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Illustrations of ground stone use-wear patterns. Redrawn from Dubreuil (2004:Figure 1).

Now going back to my data, the use-wear patterns further distinguished the Clusters from one another. Clusters 1 and 3 had very similar use-wear- they had rugged or pecked surfaces with either leveled or rounded high points. Cluster 1 feature walls was a rugged surface with either levelled high points or rounded high points. This suggests the area was first pecked to roughen the surface, and then different activities occurred to produce the modification on the high points of the feature. Levelled high points could have resulted from significant amounts of stone on stone contact (e.g., during fiber extraction), or if the processed material was hard in nature (e.g., seeds). In the instances with rounded high points, the surfaces were initially pecked, and then some sort of “soft” material was processed that smoothed the highs and lows of the peck marks. As the substance moved across the surface and around the high points, the surfaces became rounded. Softer materials potentially include a variety of plants (e.g., baked agave or sotol, nut meats, fruits) and animal tissue.

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Amanda contemplating use-wear.

Cluster 2 features, the somewhat deep ones, had mostly leveled surfaces on the walls with some gradual, smooth rounded high points. This suggests that materials being processed in these relatively deep features were somewhat abrasive nature, and that the individuals using these features did not feel the need to re-peck the sides of the shaft to roughen the surface. The intensive levelling of the walls also supports the probability of a rotary motion being used, as to increase the contact between walls and the material being processed.

The most common use-wear pattern on the walls of Cluster 4 features are rugged upper walls and mostly leveled lower walls. This pattern suggests the upper walls did not come into contact with either the processing implement or the material being processed. Similar to the walls in Cluster 2 features, the lower half of these features must have been relatively full of semi-abrasive materials. This also suggests a pounding motion was utilized rather than a rotary or gyratory motion since the upper walls showed little signs of wear. However, two of the features in Cluster 4 are leveled on all portions of the walls throughout the shaft, suggesting a rotary motion may have caused the leveling.

Now for the Hole Story

So to sum it all up, there are definitely different morphological types of bedrock features in the Lower Pecos. One of my original goals was to put forth a regional typology of bedrock features. Although the cluster analysis resulted in four highly different morphological groups, Cluster 1 includes an incredibly large range of feature sizes and makes up the majority of the data set. Until Cluster 1 is examined more thoroughly for intra-cluster pattering, I think it is premature to create a formal typology. Clusters 1 and 3 are both highly variable and elude a classification that can encompass all of the morphological and metric variation. Other groups (Cluster 2 and 4) are less variable and likely represent a true morphological and functional type. At this time, I will tentatively classify features in Cluster 1 and 3 as general grinding surfaces, features in Cluster 2 as conical mortars, and features in Cluster 4 as cylindrical mortars.

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Examples of bedrock features within each cluster: a) Cluster 1 features; b) Cluster 2 features circled in red; c) a Cluster 3 feature; and 4), Cluster 4 features circled in red.

Behaviorally, the overwhelming presence of generalized features such as the ones in Clusters 1 and 3 makes sense for a mobile, foraging group. These features take very little time to create, and they were likely used to process many different foods, whatever the group could find on any given day. Other feature types (e.g., Cluster 2, conical mortars; and Cluster 4, cylindrical mortars) were highly specialized and only occurred at certain sites. This pattern could have implications about general lifeways for Lower Pecos hunter-gatherers. Perhaps these foraging peoples were using the many sites with general purpose features for a majority of the year, but sites with specialty features could signal use during certain times, such as a harvest or large social gathering. These theoretical ideas along with experimental work can help archaeologists push our interpretations of ground stone bedrock feature technology past just food processing and into theories regarding site reuse and optimal technological adaptations.

Before I began my research, I expected the results to show many more distinguishable groups or morphological types. Undoubtedly, further analysis of the data will yield a greater insight into what morphological groups may be hiding in Cluster 1, which might change how I’ve interpreted the data thus far! There is so much more to learn, these types or groups are not set in stone…well they are, but you know what I mean. There are also other avenues to explore such as residue studies, which could give us an even better understanding of exactly what was processed in a feature. There are experiments to be done to figure out how long it takes to create a feature 50 cm deep in limestone! We’ve only just begun to peck the surface. But isn’t that the most exciting thing about archaeology? The more we learn, the more questions we have, and the whole process begins again.

This blog post is meant to be a simplified summary, there is much more to this story! If you want to learn more about bedrock features in the Lower Pecos, you can download my full thesis from the Texas State Library webpage here.

 

References Cited

Dubreuil, Laure                                                                                                                                              2004    Long-term Trends in Natufian Subsistence: a Use-Wear Analysis of Ground                       Stone Tools. Journal of Archaeological Science 31:1613-1629.

The Developing Tales of Sayles Adobe

By Victoria Pagano

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E.B. Sayles 1932 sketch map of Eagle Nest Canyon.  Note the area labeled “Sandy Adobe.” Courtesy Texas Archeological Research Laboratory, UT Austin.

Tori here, former 2015 ASWT intern and current Texas State anthropology graduate student. This season I am back leading excavations at the Sayles Adobe site as I collect data for my master’s thesis.

The terrace site of Sayles Adobe (41VV2239) sits just within the mouth of Eagle Nest Canyon (ENC), a short distance up the canyon from its confluence  with the Rio Grande. On E.B. Sayles’ 1932 sketch map of the canyon the site area is indicated as “sandy adobe,” with no mention of cultural material. Apparently, it was considered to be just a natural terrace formation.

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1958 photographs showing the site area looking toward the mouth of the canyon (above);  Skiles Shelter as viewed while standing on Sayles’ “sandy adobe” (below). Courtesy Texas Archeological Research Laboratory, UT-Austin.

 

 

During the Ancient Southwest Texas Project’s 2014 field season, the massive June 21st flood (see The Canyon Runs Deep) deposited a thick layer of flotsam atop the back dirt pile for the Skiles Shelter excavations. In order to finish filling in the open excavation units, the crew decided to take fill  from the alluvial terrace deposit nearby. This seemed like a good choice until the crew encountered fire-cracked rocks (FCR) about a meter below a thick bed of sandy Rio Grande alluvium. Digging stopped immediately and the area remained untouched (by archaeologists) until late 2015.

 

 

And so it begins..

In December 2015, a crew of five: Drs. Steve Black and Charles Frederick, Charles Koenig, Amanda Castaneda, and myself, carried out a three-day reconnaissance of the alluvial terrace and 2014 borrow pit with the idea that this as-yet-unrecorded site might make a good thesis research project.

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Discovery borrow pit, pre-clearing and post-clearing, December, 2015.

My initial observations were limited.  It was clear that the locality was an alluvial terrace and that the burned rocks that the 2014 crew had encountered were not likely just discard washing down from Skiles Shelter. But vegetation across the terrace made any sort of determination on the full extent of the site difficult. We cleared just enough vegetation to get a better idea about the morphology of the terrace. The most promising formation model is that large limestone boulders in the canyon bottom and the canyon wall created a catchment for alluvial sediment during back flooding from the Rio Grande. Repeat flood deposits created an open terrace just a few meters downstream from

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Current views of Sayles Adobe from the canyon rim looking toward the mouth of the canyon (above) and (below) the large boulders in the canyon bottom that seem to have formed a massive sediment trap. Skiles Shelter that could be used for certain activities between deposition events. This formation process would make Sayles Adobe a bit different in comparison to other investigated terraces in the Lower Pecos.

During this visit we worked to clear vegetation from and around the initial exposure to facilitate testing at the site. A quick surface reconnaissance (mostly on hands and knees) revealed scattered FCR on the surface at multiple locations across the terrace. Frederick and I cleaned and squared off two exposed faces of the borrow pit to examine the stratigraphy. I soon discovered a thin, compact layer of very fine silt, directly above (covering) several burned rocks amid carbon-stained matrix. Frederick recognized the silt layer as a flood (mud) drape.  The stratigraphic sequence looked very promising.

 

 

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Extent of the flood drape exposure at the end of the December 2015 visit. You can clearly see the color and textural changes.  From top to bottom: light brown sandy loam, tan silt flood drape, and burned rock and carbon-stained matrix below.

 

Understanding the Geoarcheaology of the Canyon

The excavation and analysis of Sayles Adobe is being conducted as part of my Master’s thesis research in order to reconstruct and understand the natural formation of the terrace and document the prehistoric uses of the locale. I hope to be able to address four main research questions:

  • What is the nature and timing of flood events during the human history at Sayles Adobe?
  • What can the Sayles Adobe terrace deposits tell us about the climatic and environmental conditions at the time the formed?
  • Do flood deposits at Sayles Adobe correlate to other flood deposits seen in shelters in the canyon?
  • How do site use behaviors seen at Sayles Adobe relate to other sites in the canyon?

The primary focus of our excavations will be to collect data aimed towards the cultural and natural formation processes witnessed at the Sayles Adobe terrace. This will form the foundation for my interpretation and analysis of behavioral patterns witnessed at the site.  The cultural materials and geoarchaeological samples will be analyzed and compared to the other sites within Eagle Nest Canyon.

2016 Sayles Adobe Investigations

Our first session focused on testing the Borrow Pit area that was exposed in 2014 and cleaned up in December 2015. During this time myself, Spencer Lodge, and Kelton Meyer, worked to carefully peel away the mud drape in the north to south profile (PS01) and reveal as much stratigraphy as we could.

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The mud drape could easily be “peeled” off in chunks from the charcoal stained sediment below.

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Annotated Profile Section 01 (PS01).

 

 

 

 

 

 

 

 

 

To add greater resolution to the Sayles Adobe tale, Tiffany Osburn of the Texas Historical Commission visited Sayles and carried out a ground penetrating radar (GPR) survey across the terrace. Using two different range antennas, she made multiple passes in a grid and in transects to provide us with an idea of what might be under the surface that we wouldn’t know without completely excavating the site. The GPR results  will aid in the interpretation of (and guide) further excavations at the site.

A priority of our second session at the site was to continue work in the Borrow Pit area. The ultimate goal of the work here is to create a deep profile that can be used to document and sample the natural and cultural stratigraphy of the site.

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Partially annotated results of the 270Hz GPR survey. Courtesy of Tiffany Osburn.

Using GPR data, we began working to ground-truth these results by conducting bucket auger tests that reach up to 3m deep along the East-West and North-South axis of the site.  (More on our auger testing in a few weeks, when intern Justin Ayers takes on the Sayles auger survey).

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Ongoing investigations at Sayles Adobe.

Some readers might ask, why even bother with a site like Sayles Adobe in a canyon that has such culturally rich rockshelters?  In comparison to those sites, Sayles may seem like a pipsqueak with not much to offer.  But, in truth Sayles is no less exciting or enriching than the sheltered sites. Sayles provides an opportunity to see what other activities were taking place in Eagle Nest Canyon that we can’t see in the shelters or along the canyon rim.  Our initial work shows that we have at least one sealed cultural layer that has not been disturbed by later occupation. In contrast, hunter-gatherers returned to the rockshelters time and time again, with the remains of each visit co-mingled with that of the last and mixed through pit digging, plant baking, and many other activities. As ASWT has documented in high resolution, the rockshelters have palimpsest deposits with complicated stratigraphy and few, if any, expansive areas of sealed cultural deposits.  Sayles Adobe has the potential to add a new level of understanding to human behaviors and natural formation processes of deposits in the canyon.

Our goal isn’t just to put together a chronology of the use of the canyon over the past few thousand years, it is to weave together an understanding of the people and the natural world in which they lived.  Stay tuned for further developments.

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A Sabinal arrow point has just come to light about 10 cm below the flood drape.

Rocky Midden High

By Bryan Heisinger

There is no denying that fire cracked rock (FCR) has a heavy presence in the research of the Ancient Southwest Texas Project. On a day to day basis, we sit on, trowel through, trip over, and often smell (strange.. I know) the enormous pile of FCR that fills Eagle Cave. Anyone who has worked with us long enough knows that we take our rocks seriously — and for good reason! By studying the FCR from our excavations we hope to address research questions about earth oven use and intensification in and around Eagle Nest Canyon.

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Charles loves his burned rocks.

FCR is the by-product of rocks that have been used for cooking and heating purposes. A rock becomes “fire cracked” after it is exposed to intense heating/cooling and reuse in an earth oven or other thermal environments. Continuous episodes of thermal cycling cause the rocks to fracture into smaller, angular shaped pieces and once the rocks become too small to retain heat for cooking they are discarded in favor of newer/larger rocks. The accumulation of tossed FCR typically form in the shape of ring around the oven pit and in the case of rock shelters, they begin to form talus slopes. Ultimately, this ring or discard zone is categorized archaeologically as a burned rock midden.

Rocksort

In order to effectively study FCR and burned rock middens, reliable methods needed to be established for quantifying and categorizing the rock that we find during excavations. The Rocksort recording procedure was created as a way to document FCR using known size and attribute divisions that are common among earth oven literature and experimental studies. The size of FCR can tell us some information about the use-life of that rock and approximately how many times it was used for cooking purposes before it was discarded. The attributes of that particular rock (e.g. pitted limestone, roof spall, igneous/metamorphic rock) can help us determine the general source of the rock (e.g., uplands vs. canyon bottoms vs. within rockshelters).

It is important to note that we do not collect and record every rock that we encounter during excavation. Such a process would be extremely time consuming and would produce lots of repetitive data. Rather, we have been collecting and weighing FCR through selective column samples along our exposed profiles and other areas that we deem necessary or informative at our excavation sites. Through this selective Rocksort documentation, we will gain a representative sample of the varying densities and sizes of rock that are occurring at the sites we are investigating in and around Eagle Nest Canyon.

During the Rocksort process we split the FCR from a particular layer/strat or feature on a grid board (lines at 7.5 cm) into the following categories based on the maximum dimension : < 7.5 cm, between 7.5 -11 cm, between 11-15 cm, and > 15 cm (Fig. 1.). These rock size categories are based on the 20,000-odd FCR that were counted and measured as part of excavations at the Higgins site in San Antonio, directed by our very own Dr. Steve Black (see Higgins BRM).

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Bryan and ASWT volunteer Kris Bobbit rocksorting an excavation layer in Eagle Cave.

Furthermore, this separation allows us to identify the stages of thermal fracture in the FCR and whether or not that particular layer/strat or feature being excavated is related to a discard event (e.g., small rocks <11 cm), a cooking event (e.g., larger rocks >11 cm), or some combination thereof.

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A.) Predominately larger FCR >11  cm were found within cooking features; B.) Predominately smaller FCR 11< cm were found within discard zones.

 

Each square on the Rocksort board measures 7.5 cm and provides a speedy method for quickly measuring and grouping the hundreds of rocks that need to be sorted. After the rocks are sorted, they are photographed on the board and weighed according to their size and attribute class (Fig 2.).  This data is then entered on the excavation form and the sorted FCR is dumped into the back dirt piles.

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Fig. 2: The ASWT Rocksort table (above) and the always famous Rocksort board (below).

 

The Big Picture

As mentioned earlier, the ultimate goal of our FCR documentation is to measure the amount of earth oven cooking that took place in Eagle Nest Canyon and other rockshelters and open sites in the Lower Pecos Region. However, you may be wondering how in the name of Einstein do we siphon our Rocksort data into something understandable?

Sparing you the math, we can take this data and calculate the approximate volume, FCR mass, and FCR density  of the burned rock middens in and around Eagle Nest Canyon. This data – with the help of radiocarbon dating – can tell us when and how much earth oven cooking took place at each site in the canyon over time. Additionally, we can compare the Eagle Nest Canyon FCR data with that from previous ASWT projects along the Devils River to give us an idea of the amount of earth oven intensification that occurred across the Lower Pecos landscape over time. Pretty cool right?

What it all boils down to is having the ability in the future to be able to estimate the number of earth ovens at different sites with minimal excavation. We hope to be able to not only compare earth oven features across the Lower Pecos but possibly North America.