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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.