By Christina Nielsen, Charles Frederick, and Ken Lawrence
**This post is the second of several that give additional details regarding some of the different analyses that are currently being conducted with material from Eagle Nest Canyon.**
Eagle Cave (41VV167) is a large dry rockshelter with deeply stratified deposits spanning the Early Archaic through Late Prehistoric periods. My thesis research focuses on deposits in the northern sector of the shelter sampled during 1963 excavations by UT-Austin and again a half century later by Texas State University in 2014. My goal is to use multiple lines of evidence to evaluate the natural and cultural formation processes that resulted in the complexly stratified, culturally rich deposits present in Eagle Cave.
Our ongoing analysis involves a robust geoarchaeological sampling strategy that included the collection of micromorphological (micromorph) samples from Profile Sections (PS) 3 and 4 in Eagle Cave. This poster highlights the benefits and difficulties of collecting micromorph samples from fragile rockshelter deposits and shows how the analysis of the resulting slabbed samples and thin sections can aid in evaluating site formation processes.
- Section of profile cut back to expose block of matrix
- Block carefully removed, wrapped in toilet paper, tightly wrapped in tape, and labelled with provenience information,orientation, and north arrow
- Sample placed in plastic Tupperware or sturdy container
- Sample impregnated with polyester resin made from polyester, styrene, and methyl ethyl ketone peroxide (MEKP)
*The micromorph samples were carefully carried out of the canyon and up to Jack Skiles’ shed located a few hundred feet from Eagle Cave. Had the impregnation happened further from the shelter, these friable samples would have been far less successfully embedded.
- After completely solidified, sample removed from container and north orientation notched in block
- Outer casing removed using oil-based rock saw to expose intact soil block
- Each side of block is scanned using high resolution
- Block is cut into 1cm slabs for thin section production, curation, and macroscopic analysis
- 4 x 6cm sections cut from slabs to be sent to Spectrum Petrographics, Inc. to be made into thin section slide
The sampling strategy was fairly simple: capture as many stratigraphic layers (strats) as possible within the PS3 and PS4 profiles. As archaeological and geoarchaeological sampling had already occurred prior to the micromorph collection, some strats identified during the initial profile recordation were no longer visible in the profile.
Specific types of strats that were especially important to capture in the micromorphs included microstratigraphy such as thin lamina and lenses as well as strats that were associated with cultural features. Using this strategy, the 13 relevant micromorph samples captured approximately 27 of the 85 total stratigraphic layers identified in the field. A total of 22 thin section slides were made then from the 13 micromorph samples.
The deposits in Eagle Cave, like many other Lower Pecos rockshelters, are very dry and have a loose consistency. This posed many challenges during initial recordation of strats and with the subsequent geoarchaeological sampling. Profile Section walls became enveloped in a film of dust with the slightest breeze or movement. Despite efforts to clean walls prior to all documentation, observations during strat recording were somewhat hindered by the persistent dust. The collection and analysis of micromorph samples, however, allows for a clearer examination of stratigraphy and the relationships between various deposits. Characteristics that aid in deciphering formations processes, such as boundaries between strats, are especially difficult to determine when obstructed by dust. Thin sections made from the micromorph samples can provide information crucial to the study of formations processes such as the size, orientation, sorting, and mineral composition of grains, organics, and artifacts as well as post-depositional disturbances of sediments.
Since the micromorph samples in PS3 and PS4 were collected after all other sampling had been completed, it was sometimes difficult to correlate the samples with the strats initially identified in the field. In hindsight, ideally the micromorph samples should have been collected immediately after the strats were identified and documented to allow for a more accurate correlation.
Field collection is also not always successful in loose deposits such as these and many first (and second) attempts at collection failed. Patience and perseverance are necessary qualities to have in this type of setting. The entire micromorph process, from collection to analysis, is a lengthy one but the potential information that can be obtained from this type of analysis greatly outweighs the challenges you may face along the way.
Long inhabited limestone rockshelters with deeply stratified deposits, such as Eagle Cave, can be difficult for an archaeologist to interpret. The natural degradation of the shelter itself, combined with human modification and natural forces create often complicated stratigraphic deposits. My thesis research involves a multidisciplinary approach to evaluate the formation processes evident in PS 3 and 4. The ongoing analysis of the micromorph slabs and thin sections from this sector of the shelter will help elucidate some of these complex processes and contribute to the overall analysis of formation processes in this sector of the shelter.
**A PDF version of this poster is available here: Nielsen_Micromorphs_TAS2015_FINAL