Monday, February 2, 2015

Three days in Teotihuacan

Big pyramid, little pyramid
I am posting from the Arizona State University archaeology facility in San Juan Teotihuacan, Mexico. I've just spent three days at the site of Teotihuacan, exploring the outer neighborhoods of the ancient city and learning about geophysical techniques applied to archaeology. I am considering using geophysics at Teotihuacan, so I came down to see Luis Barba and his team at work. Luis is collaborating with David Carballo, who has been working in the southern Teotihuacan neighborhood called Tlajinga, exposing apartment compounds by both excavation and geophysical prospecting. Check out his project website.

Geophysical prospecting is one of the "magical" tools available to archaeologists. We can see what lies beneath the ground surface without excavating.
Luis Barba with the gradiometer
Luis Barba (Instituto de Investigaciones Anthropológicas, Universidad Nacional Autónoma de México) is the leading archaoemetry specialist in Mexico (I can't get blogger to insert accents! Aaarghhh). He does geophysical research, archaeological chemistry of soils and artifacts, and other scientific studies. He and David spent the whole week doing geophysical work in the Tlajinga area, and I arrived Wednesday to see the fieldwork first-hand. Luis's team includes two members from his lab at UNAM: Jorge Blancas and Agustín Ortiz. Also, Meztli Hernandez Grajales of the Escuela Nacional de Antropologia e Historia, and Ashley Krauss of Yale University joined the crew for the week.The goal was to locate the outlines and some inner walls of apartment compounds in the Tlajinga area.

Me measuring electrical resistivity
Luis uses three main methods. The fastest method is magnetometry (see photo). They grid off a 20 by 20 meter square, and carry the instrument down the grid lines, taking four readings on the gradiometer per meter. This generates quantitative measurements of the magnetic properties of the subsoil, which is useful for archaeologists because the magnetic signatures of buried structures and features often contrast with the surrounding soil. The second method is electrical resistivity. Two probes are placed in the ground away from the sampling area, and then two metal probes are inserted into the ground at set intervals, again following the grid lines. Luis let me try this for a few passes (see photo). For these techniques, the data are uploaded, and maps can be generated very quickly to serve as guides to further steps.
Jorge Blancas & Ashley Krauss with the GPR unit

The most sophisticated and complicated geophysical technique is ground-penetrating radar ("GPR"). The device is dragged along the ground (the orange box in the photo), and a wheel records the distance traveled. The instrument sends radar waves into the ground, and registers the waves when they are reflected back up. GPR is far more sensitive and precise than the other methods, but it requires a large amount of processing and analysis after fieldwork is complete. While we could look at the day's magnetometry maps each evening, it takes much longer to make the GPR results available.

David, me, and Jorge with the GPS
The 20-meter survey blocks are aligned with the Teotihuacan grid, which means that they are aligned with the orientation of the ancient structures. The team uses a differential GPS unit to fix the precise location of the survey blocks. See the photo, with David and me standing around while Jorge works the unit.

I have always been fascinated by geophysical prospecting, but I haven't had the opportunity to use the methods in my research, and I didn't have up-close experience with the fieldwork. This was a great experience, and I am considering a possible fieldwork project with Luis and David, to expand their approach to other parts of Teotihuacan.
Street of the Dead in Tlajinga

Close-up showing the Moon pyramid
The Tlajinga neighborhood includes the central avenue at Teotihuacan, the Street of the Dead. In the main INAH archaeological zone, which is open to tourists, the street is paved, and lined with ceremonial structures. Down in Tlajinga it was evidently not paved, and was in fact a channel dug out of the underlying tepetate bedrock. This photo is looking north up the Street of the Dead from the Tlajinga area. You can just see the Moon Pyramid at the upper end of the street; I include a close-up from the center part of this photo with arrows pointing to the outline of the pyramid.

Reconstructed murals from Tetila

I also saw the excellent new museum,the Beatriz de la Fuente Museum of Teotihuacan Mural Paintings. In addition to having many of the actual wall paintings on display, several entire painted rooms are reconstructed at the museum. Also, the museum is integrated with a 3-temple group that was excavated a number of years ago (see the photo at the top of the post). These were probably neighborhood temples. The photo shows one of them, with the huge Pyramid of the Moon in the background.

I have been staying at the ASU archaeology facility in the town of San Juan Teotihuacan. This is a major storage facility, with many important artifact collections from Teotihuacan and nearby sites. There is space for artifact analysis, and living quarters for people using the lab or doing fieldwork.
Arizona State University lab facility at Teotihuacan

In the photo the two-story storage/analysis building is at the left. On the right are three small cabins with beds and bathrooms (I am staying in one of these). The doorway at the end of the driveway leads to the "Casa antigua," an old house that has bedrooms, a kitchen and common space, in addition to technical workspace and some storage. The tree behind the doorway is in the yard of the house.

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