Plaza of the Columns Complex

Analytical science: straight from the ground and to the laboratory

Soil samples saved from the field are analyzed in the laboratory in order to recover plant and seed remains. The procedure for separating those tiny organic remains from the soil is called “flotation,” that is, where dried soil is gently sieved through a wire mesh with the help of some water. With this method, organic vegetable remains (including ancient ones that are usually carbonized or charred) float up to the water’s surface (light fraction) while the remaining material sinks to the bottom of the container (heavy fraction).

While the light fraction is analyzed by the paleoethnobotanist to identify the remains of ancient plants and seeds, the heavy fraction is analyzed by trained technicians who recover assorted tiny fragments of ceramic, obsidian, flint, slate, green stone, pyrite, mica, pigment, bone, shell, eggshell, among others. Each type of material is then placed in its own labeled bag with its data recorded.

Although material recovered from the heavy fraction may appear less significant, the study of bone, for example, offer a different picture with the introduction and identification of several animal species, some even needing the help of a microscope. Due to their size, these tiny bones would have otherwise been impossible to find during excavation. Such is the case with the discovery of sea urchin, fish, reptiles, ducks and small birds like hummingbirds and quail. Fauna diversity like these have been very revealing for the study of animals that were used and consumed by the ancient inhabitants of Teotihuacan.

By Alexis Bridges, Tanya Catignani y Ariel Texis Muñoz

At Teotihuacan the use of detailed satellite imagery and LiDAR technology has allowed for remote detection of archaeological features which are often impossible to see at ground level. It affirms that we can neither escape the legacy of the past nor the influences that it has on our present.

One of the goals of the Project Plaza of the Columns Complex this season has been to determine how much of the present-day Teotihuacan Valley was influenced by the ancient alignment of 15 degrees east of true north. Our team tracked this alignment by digitizing modern features in ArcGIS Online (Figure 1). Because the Avenue of the Dead is so central to the city, it seemed logical that nearby modern structures would be aligned in the same configuration, and areas farther away from the city center would less likely display this pattern. Our results abiding by the strictest of calculations revealed that more than 30% of the region does match this traditional alignment, even areas that are far away from the city center. One theory for this is that ancient structures, long crumbled and buried over the centuries since their initial construction, may have  influenced contemporary building and agricultural decisions by raising complications of digging and plowing around these archaeological features.

One town in particular that drew our attention lies to the west of the city center (see Figure 1). Nearly all of the town is aligned, resulting in a massive hotspot of digitized features on the map. However, the LiDAR and satellite maps did not reveal any obvious archaeological features in the area. After studying old archaeological reports, we found that this location had, indeed, been previously excavated. In the 1960s William T. Sanders discovered an apartment compound capable of housing hundreds of people at its peak occupation and was inhabited at least until the Colonial period (Figure 2). Although Sanders’ team identified this site as TC-8, the eighth site associated with the Teotihuacan Classic period, his site map lacked identifiable features that could have led us to the excavation site.

Despite this, Sanders created a second map that charted the entire valley, fortunately safeguarding sites that may have disappeared over time. A rough location of the site was found by georeferencing what streets and towns still existed. From there, a rock alignment could be seen on the LiDAR map as well as very, very slight mound formations that closely matched with the ones identified on Sanders’ map (Figure 3).

Preliminary ground truthing has yielded promising results of pottery sherds and shell fragments  ̶  an unusual find for an inland area. Using a similar process, another site in the southwest known as TC-21 was also located with similar finds of ceramic sherds. Although these preliminary results are not confirmation, they do indicate that our locations may be these previously forgotten Sanders’ sites.            

This experience highlights the power of combining modern technology with historic data. Technology without the analog aspects of archaeology cannot show us everything, and relying entirely on technology will create a loss of data. The re-discovery of TC-8 and TC-21 only shows that archaeology is, and will likely remain, a historical science at its foundation.

LiDAR technology (Light Detection and Ranging/ Laser Imaging Detection and Ranging), is a remote sensing method that uses a device that measures different surfaces of the earth, to analyze various forms of geographical structures and architectural structures. Emitting a laser that measures and records the distance between the emitting device and the object that reflects the emission, LiDAR technology creates three-dimensional information about the shapes and surfaces from scanning the cultural and natural features. The device works as an airborne scanner, firing thousands of laser emissions per second and recording “returns” to the device; creating a “point cloud” that allows for the analysis of the data points and is also geographically referenced.

The data points obtained from LiDAR technology create a three-dimensional model of the earth’s surface but also the various man made and natural features that cover the land that it hovers over: including but not limited to viewing  density of vegetation, agricultural features, ancient structures, modern structures, and land use. One of the great advantages of this technique is that the data processing makes it possible to “eliminate” the vegetation layers (virtually) that covers the land and to obtain a detailed “portrait” of the land surface, creating a Digital Elevation Model (DEM). A Digital Elevation Model (DEM) is a three-dimensional model of the surface that is formed from the elevation data, obtained by LiDAR technology in this case. The degree of detail that these types of digital maps and images have is greater than that which could be obtained by traditional surveying, in addition to saving a lot of time and work force. LiDAR was first used by scientists and mapping professionals, but now a days has been incorporated into the research of other disciplines as well.

Recently, LiDAR technology  has been incorporated into archaeological research complementing traditional methods of surveying lands such as aerial photography and simply walking/recording sites manually. The LiDAR data is referenced to the Global Positioning System (GPS) that collected data and information can be geographically referenced. Technological advancement has allowed for high resolution images to be created and has reduced degrees of error, that now the registration of the features or elements detected on the LiDAR map are calculated in centimeters.

At the Teotihuacan Valley, the Project Plaza of the Columns Complex generated a LiDAR map in 2015. The map covers an area of ​​approximately 165 km². The local vegetation does not interfere with the surface of the investigated area, instead modern structures (urban areas) dominate the valley today covering ancient remains.  Instead of working through vegetation, the challenge is to know what is ancient and what is modern. The LiDAR map is unique in the region and presents a portrait of the valley in 2015, where the centuries of changes to its surface are being be recorded and compared to older surveys in the area.

In 2017, a special team was appointed to work on surface exploration. The objective of the team has been to ground truth the Teotihuacan Valley, to search and verify each of the data sets recorded on the LiDAR map of 2015. The team has concentrated on walking surveys, taking samples of the materials visible on the surface and verify the accuracy of the LiDAR map, to distinguish between what may be ancient and modern elements. Establishing relationships with local communities has been essential to understand how the modern areas of occupation have formed.

The surface survey has two teams, one that works in the laboratory and one that works in the field.. First, members of the team analyze the LiDAR map to detect possible ancient features and  name them as potential elements in the laboratory. The potential elements include features such as depressions, mounds,  plazas, structures, or terraces. Second, members of the team are responsible for verifying the potential elements in the field, a survey method known as ground truthing. With the help of geographical positioning devices (GPS), the team go to the precisely marked geographic coordinates on the map to collect materials and samples on the surface and record the areas. The members of the team cross compare the material in the field with the map, to infer ancient vs. modern occupancy.

In this case, a specific area is delimited, collecting ceramic fragments, obsidian, clay figurines and polished stones such as metates and manos. Furthermore, a detailed record of the elements, on the present vegetation, the current land use, and the presence of other materials are taken carefully. Everything that is collected is labeled and taken to the laboratory, where it is cleaned, washed, and marked by specialists. Ceramics and lithics specialists analyze and prepare materials and samples for preservation.

This type of work carried out in the Teotihuacan Valley would not be possible without the support of the local communities, they are the ones that give us permission to walk on their fields, agricultural lands, “ejidos” and other properties. Communication is vital, as we are all united in the effort to preserve and study cultural heritage and history. In addition to the permits, provided by the government, to carry out the research. Local communities and residents provide personal knowledge about their land; narrating stories about past and changes that have occurred recently, and/or the old names of some places (such as the different hills that surround the valley). Communicating with local communities and residents provided us with information beyond the digital maps alone. We thank the following communities and residents for their unconditional support of our research:

• Atlatongo (Palomar)
• Barrio de Purificación
• Belén
• Cozotlán
• Cuautlazingo
• El Saltito
• Ixtlahuaca
• Maquixco
• Oxtotipac
• San Agustín Actipac
• San Antonio de las Palmas
• San Francisco Mazapa
• San Isidro
• San Lorenzo Tlalmimilolpan
• San Lucas Tepango
• San Pedro Tepetitlán
• San Sebastián Xolalpan
• Santa María Palapa
• Santiago Tolman
• Santiago Tepetitlán
• Tlachinolpa
• Tlajinga
• Xometla

Furthermore, we want to thank the municipalities (Acolman, San Martín de las Pirámides, Teotihuacan de Arista, and Otumba), “Reino Animal” Conservation Center, and the Texcoco Delegation of the Secretary of Environment of the State of Mexico. We thank you for the attention you have given, the collaborative effort in understanding the history of the area,  the permission to grant permits, and ongoing support for this research project.