R.E. Taylor Student Poster Award at the 84th Society for American Archaeology Annual Meeting, 10th - 14th April 2019, Albuquerque, USA

Eunice Villasenor (Arizona State University) and Rachel Cajigas (University of Arizona) are the honourable mention of the R.E. Taylor Student Poster Award. Here's the summary of their awarding winning research.

The Distribution and Characterization of Agricultural Terraces on Cerro de la Mesa Ahumada, Mexico

Eunice Villasenor Iribe, Christopher T. Morehart, and Andrés Mejia

This poster presents the preliminary results of research conducted on the agricultural landscape modifications of Cerro de la Mesa Ahumada, a medium sized mountain that is located between the northern Basin of Mexico and the southern Mezquital valley. Located on the top of this mesa is the Epiclassic (600-900 AD) archaeological site known as Los Mogotes. Evidence of occupation is present in the form of large residential and ceremonial structures, terraces, and water reservoirs. Some of these features have been excavated as part of the Northern Basin of Mexico Historical Ecology Project being conducted by Dr. Christopher Morehart. 

Prominent features of this mesa are the terraces found throughout the mesa (Fig. 1). Terrace systems are also found throughout the surrounding hillslopes and region. Terraces are used not only for agricultural purposes but are often used to build level platforms for the construction of residential and ceremonial structures. Using ground survey, aerial geographical survey, and excavation it was determined that terraces found within the site were used agriculturally as well as structurally. To better understand the terraces found on this mesa it was important to collect multiple types of data by using different methods. Documenting the extent, distribution, and chronology of the terraces is essential to understanding the connections between anthropogenic landscapes, agricultural production, and demography. 

Figure 1. Terraces in Los Mogotes (taken by Christopher Morehart).

In this poster we present several lines of data to better refine our understanding of the terraces: GIS maps made using satellite data; topographic data collected with total stations; GPS data from ground survey; topographic data produced using drones; and excavation data.  These combined lines of data allow us to propose preliminary interpretations of form, function, distribution, and chronology of the terraces and their role in the ancient economic systems of the hills’ inhabitants. For this poster, we were most interested in determining the characteristics, distribution, and possible productivity of the terraces found within the mesa top. 

To first establish the distribution and characteristics of the terraces, a diverse range of data were used to create a map of the terraces on the mesa. Terraces were mapped (Fig. 2) and analyzed in Arc GIS using .40 cm VHR GeoEye-1 data, GPS data, and topographic data collected. This mapping resulted in 1352 features of which some may be connected to form larger linear terraces. Once terraces were mapped the characteristics of terraces such as slope and aspect were evaluated. Through the use of 5m LiDAR data, it was possible to determine the slope and aspect of terraces. Slopes ranged from 0 -  ̴40 % slope. After determining the slope of the mapped features, we were able to calculate the average % slope of the mapped terraces, which yielded an average of 13.20 % slope (Figure 3). After determining the average slope of mapped terraces, the aspect prevalence was determined. Through this analysis, we discovered that a majority of terraces are constructed with a dominantly easterly prevalence. It is likely that the slope and aspect were important factors in determining where terraces were constructed. If it is accepted that a majority of the terraces mapped were used for agricultural purposes then it is likely that these characteristics impacted productivity in some way. 

Figure 2. Satellite image of Cerro de al Mesa Ahuamada with mapped terraces in white.

The % slope average calculated is similar to the upper limit of 15 - 20 % slope used by Hirth (2000) as the maximum range of where terraces are constructed in Mesoamerica. Due to the large range of slope determined for the terraces on this mesa, we decided to use the average slope increased by one standard deviation (6. 25 % slope) to determine the range of likely agriculturally productive terraces.  Using the distribution map created we determined areas of agriculturally productive terraces. Based on a review of previous estimates of terrace use limits (Hirth 2000), we determined that the range of productive and useful agricultural terraces most likely was constructed in a slope with a range of 0 -   ̴20 % slope (Fig. 3). This range of slope covers the tops of the mesa and results in an area of 402.35 ha of potentially cultivable land. To determine the potential productivity of this area using terraces we conducted calculations based off of previous estimates of productivity of maize for upland rainfed zones in Mexico (Hirth 2000; Sanders 1976).  To simplify this estimate, we decided to limit cultivable crops to just maize. Using the estimates mentioned above to create a calculation of productivity, it was concluded that the potential production is scarcely sustainable for the current population estimates, 750-1000, for the site of Los Mogotes (Parsons 2008). If an annual fallow cycle is considered as part of the production process, then sustainability of maize production for this mesa top is even less likely sustainable for the estimated population. 

Figure 3a. Percent slop distribution of all mapped terraces.

Figure 3b. Percent slope distribution of mesa top terraces with percent slope <20%.

The productivity estimates presented in this poster were greatly simplified and future work will broaden some of the assumptions used. Further floral analysis of the soil samples retrieved from the agricultural terraces will provide a more accurate representation of the agricultural profile. Excavation has occurred for one agricultural terrace (Villasenor Iribe and Morehart 2019), but future excavations are also planned that will provide a better understanding of the construction methods for agricultural terraces. The new data collected will also be useful in creating more accurate estimates for the productivity of terraces. This type of research explores not only the characteristics of agricultural productivity but also the economic and political systems that are at work to create such large modifications of the landscape. This research may also be useful in studying regional agricultural changes as part of macroscale political changes. 

References cited
Hirth, K., 2000. Archaeological research at Xochicalco. Salt Lake City: University of Utah Press.

Parsons, J., 2008. Prehispanic Settlement Patterns in the Northwestern Valley of Mexico: The Zumpango Region. Museum of Anthropology Memoirs, No. 45. University of Michigan, Ann Arbor.

Sanders, W., 1976. Agricultural History. In The Valley of Mexico: Studies in Pre-Hispanic Ecology and Society. Albuquerque: University of New Mexico Press

Villasenor Iribe, E., and Morehart, C. (2017). Proyecto de ecología histórica del norte de la cuenca de México, informe de 2018. Informe enviado al Instituto de Antropología e Historia d México, CDMX.

Early Agricultural Practices at La Playa, Sonora, Mexico: A Multi-Scalar Geoarchaeological Study of Prehistoric Earthen Irrigation Canals

Rachel Cajigas

Earthen irrigation canals were an important technological development in arid lands farming. The earliest farmers on the floodplains of the Sonoran Desert used careful planning, group cooperation, and experimentation to create productive agricultural landforms. The diversity in size and complexity of Early Agricultural period (2100 B.C.–A.D. 50) farming communities requires examination of the specific factors that favored this significant investment in landscape modification in order to understand the development of these practices. The examination of local environmental conditions and their relationship to the timing of agricultural practices is key in understanding the regional development and demise of early earthen irrigation canals.

La Playa is located in Sonora, Mexico (Fig. 1). This area has important implications on the understanding of the development of agricultural techniques because it is the single largest Early Agricultural period site in the Southwest U.S./Northwest Mexico region. This geoarchaeological research examines when and under what environmental conditions this technology was developed and abandoned. 

Figure 1. Map showing the area of La Playa

A multi-scalar, multi-technique geoarchaeological methodology was employed to investigate the canals at La Playa and resolve the timing of use and abandonment of the canals and determine how they articulate with floodplain environmental conditions. These methods are organized into three phases to accomplish these research goals.

Phase 1: Reconstruct spatial extent of canal system - Pedestrian Survey and GPS Documentation, Satellite Imagery and GIS Analysis, and Magnetic Gradiometry

Phase 2: Characterize depositional conditions - Particle Size and Thin Section Analysis

Phase 3. Constrain the timing of canal network - Accelerator Mass Spectrometry (AMS) ¹⁴C and Optically Stimulated Luminescence Dating (OSL)

Modern erosion is actively destroying the La Playa site (Fig. 2), highlighting the urgent need for documentation of archaeological features. In areas that have been highly eroded, much of the archaeology and irrigation canals have been destroyed. As much as 50% of the site is estimated to have been eroded. 

Figure 2. Map of La Playa showing the areas where samples were taken.

Some areas of the site have been affected by low-energy sheet wash erosion. In these areas, wind or water have winnowed away fine, silty sediments, leaving behind larger clasts, such as fire-cracked rock that lined the irrigation canals. Fire-cracked rock was likely used in canals to slow water velocity and prevent erosion during use. In some areas of the site, concentrations of fire cracked rock from eroded canal remains are so dense they are visible in the satellite imagery. 

Using high resolution satellite imagery, the remains of approximately 10 km of irrigation canals were documented in eroded areas of the site. An additional 2 km of canals were documented during pedestrian survey using a GPS (Fig. 3).

Figure 3. Map showing the irrigation canals.

In order to examine intact canals, magnetic gradiometry was used to detect canals buried below the floodplain surface in areas that had not yet been eroded. 3 km of buried canals were detected, as well as agricultural fields and several circular structures. Magnetic data were georectified using ArcGIS and UTM coordinates were generated to locate canals. Thirteen trenches were excavated to examine canal stratigraphy and collect samples for particle size analysis, thin section analysis, AMS ¹⁴C and OSL dating.

Stratigraphic information suggests that low-energy depositional conditions made the floodplain environment suitable for farming. Particle size analysis showed that the canal sediments and associated soils were fine in texture with very little variation. Thin section analysis on soils surrounding canals indicate these soils were saturated with water and composed of fine sediments with organic material.

AMS ¹⁴C and OSL dates on canals corresponding to the early and late portions of the Cienega phase (800 B.C.–A.D. 200) of the Early Agricultural period, which is the period of peak occupation at the site. These dates also corresponds to the earliest direct dates on maize at La Playa (A.D. 20–240) (Carpenter et al. 2015). Deep erosion of the floodplain occurred after A.D. 350, and there is no evidence of subsequent canal use. 

In sum, this research documented 15 km of irrigation canals that are at risk of erosion. These canals were in operation during a period of low-energy floodplain deposition that created a wetland environment conducive to agriculture. Earthen irrigation canals were used until A.D. 350 when severe floodplain erosion occurred. Although the site was not abandoned at this time, there was a sharp drop in site use (Copeland et al. 2012). 

The precise chronology of the spread of agricultural technology across the Southwest U.S./Northwest Mexico is disputed. The earliest known maize in the Southwest dates ~2100 B.C., but the earliest known canals were constructed much later, by ~1500 B.C. in the Santa Cruz river valley (Herr 2009). Prior to this geoarchaeological research, canal systems in northern Sonora were not closely examined. This research helps to address this gap in the regional approach to documenting the development of irrigation technology. La Playa shows agricultural intensification occurring later than similar Early Agricultural period sites in the Tucson Basin to the north. This is compelling evidence that the development of irrigation technology in the southwest US/northwest Mexico is more complex than simple northward diffusion models.

References cited

Cajigas, R., 2017. An Integrated Approach to Surveying an Early Agricultural Period Landscape: Magnetic Gradiometry and Satellite Imagery at La Playa, Sonora, Mexico. Journal of Archaeological Science: Reports 15, 381–392.

Carpenter, J.P., Sánchez, S., Watson, J. and Villalpando, E., 2015. The La Playa Archaeological Project: Binational Interdisciplinary Research on Long-Term Human Adaptation in the Sonoran Desert. Journal of the Southwest 57(2), 213–264.

Copeland, A., Quade, J., Watson, J., McLaurin, B., and Villalpando, E., 2012. Stratigraphy and geochronology of La Playa archaeological site, Sonora, Mexico. Journal of Archaeological Science 39, 2934–2944.

Herr, S.A., 2009. The Latest Research on the Earliest Farmers. Archaeology Southwest 23(1), 1–3.

Mabry, J.B., 2008. What's so Archaic about the Late Archaic? Recent Discoveries in Southwestern North America. The SAA Archaeological Record 8(5), 36–40.