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Why Dirt Rocks

Updated: Apr 19

Author: Sarah Meinekat.


Usually, when trying to explain my job as a geoarchaeologist, I start like this: Think about an archaeological site, and about all the amazing finds and artefacts that are contained within that site. Now, take all of those things away! What you are left with is what I look at:


The dirt.


hand holds knife in front of dirt wall
Stratigraphy at the Cuncaicha rock shelter.

Welcome to Geoarchaeology!


Geoarchaeology in general is a very broad field that tries to answer archaeological questions by using methods from the geosciences.


Using geo-spatial data to create landscape scale predictive models for past human settlement patterns? Geoarchaeology. Linking landslide events to past human actions in a specific area? Geoarchaeology. Identifying a specific mineral that can tell us bones were dissolved and that is why they are missing from the archaeological record? Geoarchaeology. It can really be anything - from interregional scale to a few microns under the microscope.


The core of geoarchaeology is that we study how some kind of geo-data relates with past humans. My working group, The Geoarchaeology Working Group at the University of Tübingen, is focused on applying microanalytic methods directly to archaeological sites.


Geoarch in Peru


When travelling to Peru for the first time to work at Cuncaicha in 2015, I was working on my Masters of Science at the Institute for Archaeological Sciences at Tübingen. Part of the work at Cuncaicha was planned as my master’s project. In addition to helping with different steps of the archaeological excavation, I was there to collect data and samples for my studies.


This meant going on hikes through the Puna landscape with my advisors and mentors, Christopher Miller and Susan Mentzer, taking notes on rock formations and geomorphological features, and collecting reference samples for off-site comparisons.


two people stand in an arid high-elevation desert
Christopher Miller and Susan Mentzer on a hike in the high-altitude Puna landscape.

It also meant having a daily "Strat Chat" with Principal Investigator, Kurt Rademaker, to discuss site stratigraphy while excavating. This allowed us to adapt excavation strategies and methods to the information we gained each day. The Strat Chat is now a tradition that we maintain at all sites we have worked on since.

Finally, the most important aspect of my MSc. research meant taking sediment samples from the site. Our strategy involved collecting a combination of many small baggies of loose sediment for pedological and mineralogical analyses, and obtaining in situ blocks of sediment for micromorphological, or "Micromorph", analysis.


About Micromorph


As the main methodological focus of my work at Cuncaicha, micromorph is the study of oriented, intact sediment samples that are processed into thin sections (resin-indurated, slide-mounted slices of intact sediments ground to 30µm thickness) and analyzed under the microscope.


hands hold a square block of dirt wrapped in plaster
Sarah Meinekat holds a micromorph block sample from the site of Cuncaicha.

Now, while the analysis of loose samples can give us valuable information about the single components of the sediment at a site, micromorphology is special because it provides contextual information on a micro-scale. Each intact block of sediment preserves the relationships between the components of a site.


At Cuncaicha for example, we identified carbonates using both calcite analysis and microscopy of the thin sections. This dual identification was important because calcite can form in a variety of ways. In our Terminal Pleistocene deposits, most of the calcite derives from tufa formation– a natural, geogenic, process. In the later strata, most of the calcite derives mostly from plant ashes, specifically, from fire made by humans – it is therefore an anthropogenic component. This is an important distinction that we can identify using micromorphology, and something that remains undetected with other methods.


On the thin sections we can also run other, micro-contextual, analyses for things like elemental or mineral identification. At Cuncaicha, we used µFTIR analyses to identify specific minerals to illuminate possible diagenetic (weathering) processes.


So Why Geoarch at Cuncaicha?


Some might argue that every archaeological excavation should include geoarchaeological investigations as, “every archaeological problem starts as a problem in geoarchaeology” (Renfrew 1976). Our research questions can extend from very broad questions of site formation (where do the sediments come from?), to questions about what happens to sediment – and artefacts! – after disposal (e.g. post-depositional processes), to excavation-specific questions (what is the black layer?). So, really, geoarchaeology links to everything!


At Cuncaicha we targeted a combination of these questions. We wanted to understand the general site formation and post-depositional processes but, of course, during excavation some more specific questions also arose. As Cuncaicha dates back to at least 12,300 years ago, we were especially interested in looking at the integrity of the oldest deposits at the site.


Five people stand in a rock shelter with buckets and coffee presses
From left to right: PI, Kurt Rademaker, Judith Beier, Gregory Hodgins, Sarah Meinekat, and Christopher Miller stand in the Cuncaicha rock shelter where Sarah collected her samples.

An archaeological site is an interesting and complicated system. It is at the interface between human and environmental factors. Not only can we gain insights into past human behavior, but a site –a rock shelter in our case– can also provide a long record of environmental signals. A record of how the landscapes that humans inhabited and migrated through have changed through time.

Anthropogenic (human-made), geogenic (earth processes), and biogenic (living organisms) components all contribute to archaeological deposits. The interplay of these different depositional agents tells us a fascinating story…a story that goes far beyond our normal understanding of dirt.


To read more about our field observations, lab results, and analytical interpretations, access our recent research article in the journal Geoarchaeology here: http://doi.org/10.1002/gea.21889

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