Human Modifications to Sonoran Desert Lanscapes during the Early Agricultural Period: Geoarchaeological Investigations at Tumamoc Hill, Arizona, U.S.A. and La Playa, Sonora, Mexico

Abstract

The Early Agricultural period (2100 B.C. –A.D. 200) is characterized by the introduction of cultigens and increased sedentism in the southwest United States/northwest Mexico. During this period, people modified their environment in significant ways for agricultural and other cultural purposes. La Playa and Tumamoc Hill are two archaeological sites that demonstrate the extent and diversity of landscape modifications employed by Early Agricultural period (EAP) communities; modifications that required substantial community effort and cooperation to construct and maintain. The La Playa (SON:F:10:3) site in Sonora, Mexico is the single largest EAP site. It is located on an alluvial fan along the Boquillas River and has the remains of an extensive earthen irrigation canal system. GPS survey, satellite imagery analysis, and magnetic gradiometry document over 15 km of canals, cultivated fields, and circular earthen structures that are at risk of erosion. Information from canal stratigraphy, radiocarbon dating, and optically stimulated luminescence dating indicate that the earthen irrigation canals were in use during the Cienega phase (800 B.C.–A.D. 200), which represents peak occupation at La Playa. Canal use also coincides with the earliest direct dates on Zea mays, indicating that the introduction of this cultigen corresponds to agricultural intensification at the site. There is no evidence of canal use following destructive floodplain erosion at ~A.D. 350, well after the end of the EAP. In contrast to the floodplain location of La Playa, Tumamoc Hill (AZ AA:16:6[ASM]), is a volcanic knob composed of basaltic andesite (~23–24 Ma) in Tucson, Arizona that contains a village on the summit comprised of circular domestic structures and linear terraces. Evidence of occupation dates to two distinct periods: the Cienega phase and the Tortolita phase (A.D. 550–650). Radiocarbon dates, field observations on soil profiles, and micromorphology analysis indicate that during the EAP, people modified the summit and upper hillslopes to create massive walls that encircle the summit of the hill. Weakly developed soils on the terraced backslopes of Tumamoc Hill suggest that soil development began in the late Holocene, after terrace construction began. Well-developed middle to early Holocene age soils on the summit of the hill were modified to build circular structures. This dissertation research demonstrates the utility of a multi-scalar geoarchaeological methodology in characterizing human modification to EAP landscapes.