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.

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Eagle Cave-Where Context is Crucial

By Brooke Bonorden, Bryan Heisinger, and Charles Koenig

Eagle Cave has been the scene of previous archaeological investigation (e.g., E.B. Sayles and J. Charles Kelley as well as the Witte Museum in the 1930s followed by the work by the University of Texas in the 1960s).  Looking at the surface of the shelter today, one of the first things people notice are the numerous unnatural depressions and mounds—vestiges of the decades of shelter excavation.  In fact, upon our initial arrival at Eagle Cave this past winter one of the first things that caught my (Brooke’s) eye in regards to the shelter’s topography was a large sloping depression in essentially the center of the rockshelter, running from the back wall to the drip line. It is massive! If I stand at the lowest point I can’t see over the present ground surface!  Now, several months later, it has become evident the modified landscape left behind by previous expeditions to Eagle Cave have created a challenge for our ongoing excavations in Eagle Cave: searching for intact deposits in their original context.

Profile Section 2 (PS02) is located near the rear wall of Eagle Cave.  The intact stratigraphy is visible as white bands in  both photographs.  Where Brooke is sitting in the top photograph, and Jake in the bottom photograph, is the disturbed fill we had to remove before we exposed the intact stratigraphy.  This fill is full of goat and sheep poop that has mixed in with the sediment when previous excavation trenches collapsed and filled in.

Profile Section 2 (PS02) is located near the rear wall of Eagle Cave. The intact stratigraphy is visible as white bands in both photographs. Where Brooke is sitting in the top photograph, and Jake in the bottom photograph, is the disturbed fill we had to remove before we exposed the intact stratigraphy. This fill is full of goat and sheep poop that has mixed in with the sediment when previous excavation trenches collapsed and filled in.

Let’s take a pair of scissors for example. When found in association with crayons, glue, and paper, a pair of scissors could be described as used for crafts. However, when the same pair of scissors is found in association with a comb, shampoo, and blow dryer, the scissors could be described as used for cutting/styling hair. Depending upon what an object is found in association with can completely change its interpreted use. The age of an artifact can also be determined by the other objects it is found in association with. In essence, context is crucial to understanding an artifact’s function within an archaeological site.

Depending on where an artifact is recovered from can drastically change the archaeological interpretation of that artifact.  Images from Google.

Depending on where an artifact is recovered from can drastically change the archaeological interpretation of that artifact–much like in Hollywood. Images from Google.

As archaeological methodologies have improved, the ways in which sites are recorded/mapped has drastically changed since the initial investigations at Eagle Cave in the 1930s. After reading the accounts of our predecessors’ work at the site, we knew the approximate locations where previous work had occurred, but we were unsure as to which areas of the site were still intact.  We set out to do so by exposing the vertical profile of a small area near the back wall of the shelter. In doing so, we soon discovered much of what we were excavating had previously been exhumed, and was in fact mainly backfilled sediment. We were able to identify the sediment as backfill because several feet of soil were completely homogenous in color and loaded with sheep and goat pellets.  We proceeded to clean more of the profile through the disturbed deposits until we exposed what we believed to be intact/unexcavated strata, and we are now excavating and sampling these intact layers systematically and recording the context where all the samples and artifacts are collected.

This is a composite image of eastern profile of PS02 showing the intact stratigraphy.

This is a composite image of eastern profile of PS02 showing the intact stratigraphy.

While we were excavating the disturbed fill from the profile we recovered dozens of artifacts—projectile points, flakes, fragments of bone, fiber—but none of these artifacts are in their original context.  Although some useful data can be collected from the artifacts themselves (e.g., projectile point styles), we cannot learn as much from many objects without knowing their exact archaeological context. This begs the question of how might we utilize artifacts and ecofacts (e.g., plant remains) with no known provenience?

This is a sample of artifacts recovered from disturbed context (from left to right): pebbles with paint, projectile points, bone tool, quid (chewed lechuguilla or sotol leaf), and a fragment of cordage.

This is a sample of artifacts recovered from disturbed context (from left to right): pebbles with paint, projectile points, bone tool, quid (chewed lechuguilla or sotol leaf), and a fragment of cordage.

ASWT has teamed up once again with Shumla, and we are taking some of the material recovered from disturbed contexts and creating an educational collection students can use for hands on activities.  Eventually, we hope we can bring these samples to classrooms not only in the Comstock/Del Rio area, but also to San Marcos and beyond.  The first group of students to study samples of the materials from disturbed contexts are part of the Shumla Scholars program.  The Shumla Scholars is a semester-long program for high school students at Comstock ISD.  Students explore research design by examining previous and ongoing projects, and by planning and implementing a project of their own. The main objective for the current class is to map and record the Comstock Cemetery, but another part will be to develop hypotheses relating to the artifacts recovered from Eagle Cave.  The students will be given several different artifact “packets” collected from disturbed contexts and posed the general questions: what are these artifacts and ecofacts and what can they tell us about the native peoples who occupied Eagle Cave?  Hands-on experiential learning instead of looking at pictures and drawings.

 

Students from Comstock High School screening disturbed fill at Eagle Cave.  These students will be the first to use the artifacts from Eagle Cave in hands-on experiential learning with SHUMLA.

Students from Comstock High School screening disturbed fill at Eagle Cave under the supervision of Vicky Munoz and Jeremy Freeman. These students will be the first to use the artifacts from Eagle Cave in hands-on experiential learning with SHUMLA.

We look forward to the Comstock students writing a follow-up blog post about what they learned from the Eagle Cave artifacts!

Low Impact-High Resolution: Using the Strat System in Eagle Cave

By Charles Koenig

Within archaeology, and certainly in several of our blog posts, you will hear the term stratigraphy.  So, what exactly do we mean by stratigraphy?  Within any archaeological site there are different layers of stuff—artifacts, rocks, different kinds of dirt or sediment—and each of these layers may contain different things from the layers above or below it.  We consider each layer to be a stratigraphic unit, or strat for short.  In some cases these strats can be the result of human activity (e.g., making fires) or when a river floods and leaves behind a deposit of sand and clay.  All things being equal, over time strats build upward and older strats are continuously covered by younger strats.

A photograph of a layer cake borrowed from Google (right), and a hungry archaeologist's version of the same cake (right).

A photograph of a layer cake borrowed from Google (left), and a hungry archaeologist’s version of the same cake (right).

In other words, imagine a triple-decker cake as a substitute for an archaeological site.  This cake has chocolate cake on the bottom, strawberry in the middle, and white cake on the top.  Between each level of the cake are layers of frosting.  When you cut into this hypothetical cake, you will be able to see all the different layers, and to an archaeologist each layer of this cake (e.g., the chocolate, strawberry, and white cake as well as the different frosting) can be considered a unique strat.  Actually, archaeologists even refer to very distinct ideal layering within a site as “layer-cake stratigraphy” (a classic example of “layer-cake” stratigraphy from an archaeological site in the Lower Pecos is Arenosa Shelter, which you can learn more about here: www.texasbeyondhistory.net/arenosa).

When you study the different strats within the cake, you can start determining the order in which the cake was constructed.  For example, the chocolate cake layer at the bottom is the oldest part of the cake, and then the strawberry frosting was spread on, and so on).  As you study and record how the different cake and frosting layers were placed onto the cake, you begin to understand how the complete cake was created.  Understanding how a site (or cake) formed is what archaeologists call site formation processes.

Now, to jump back to stratigraphy in Eagle Cave.  One of the tragedies about Eagle Cave is when the University of Texas wrapped up their excavations in 1963, they left all of their excavation units open (as did the Witte Museum in 1936).  Over the past 50 years all the walls have collapsed, but there are still large depressions where the units once were.  When we (Steve and Charles) began writing the research design for Eagle Cave, we knew we would be able to take advantage of the existing units to expose any intact stratigraphy.  However, we knew from previous excavations at Eagle Cave the stratigraphy is anything but layer-cake—dozens of thin, hard to see strats mixed in with thick, rocky strats all churned up by rodent burrows and human-dug pits—in short, extremely difficult stratigraphy to carefully document and understand.  So, the big challenge became: once we expose the stratigraphy, how should we document and sample the complex layering within the site while minimizing the overall impact our project has on the remaining intact deposits?

What we decided upon was to use a version of Steve’s Strat System from his work in Belize in the early 1980s, and came up with the motto, “Low Impact-High Resolution,” meaning we want to learn as much as we can while doing the least damage to the site.  To go back to our layer-cake analogy, imagine you are a food critic and you have 10 of your closest food critic friends coming to sample every layer within the cake tomorrow.  But, because the cake is so delicious you need to save enough cake for 10 more food critics to sample the cake every 50 years for the next 10,000 years (fortunately this hypothetical cake never rots).

Over the next 6 months (and likely on and off for several years) we will be documenting the stratigraphy of Eagle Cave at dozens of different spots.  All of these exposures will be in areas which have been previously excavated or dug into by critters or artifact collectors so we are only damaging small portions of the intact stratigraphy.  We are calling these Profile Sections.

Steve and Tina Exposing and Cleaning PS001 prior to strat identification and subsequent sampling

Steve and Tina Exposing and Cleaning PS001 prior to strat identification and subsequent sampling

Yesterday we documented and began to record our first profile section: PS001.  The location of PS001 was selected because there was a large animal burrow and adjacent backdirt pile, and we could just make out what appeared to be intact stratigraphy above the burrow.  The burrow angled downward into the deposits (and can be seen toward the bottom of the profile), but by cutting back only a few centimeters above the burrow we were able to expose ~60 cm tall profile section. We were very surprised to encounter intact stratigraphy only a few centimeters below the ground surface, so we are very encouraged to begin exposing and documenting additional Profile Sections.

Image of PS001 exported from ArcGIS. A 3D model of 60 photographs was used to produce the image.

Today we began documenting the different strats in PS001, but those descriptions are incomplete.  When we begin our sampling we will likely take small matrix samples out of each identified strat, and screen the material through nested geologic sieves.  We will then be able to coordinate with our collaborators (like Phil Dering and Charles Frederick, among others) to begin analyzing all the different material in each strat – sediment, plant parts, animal bones and so on.  This is where High-Resolution comes into play.  We want to systemically sample each strat individually so we can gain a more detailed understanding of when and how the site formed over time, how the site was used during different time periods, and use the data to address many other questions.

Feel free to check out the 3D PDF of PS001 at the Link below!

3D PDF of PS001