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Dissertation Defense: Amy Schott


Jun 13 2019 - 9:30am

Thursday, June 13, 2019
9:30 a.m.
Haury Building, Room 215

Title: Dry-Farming and Eolian Landscape History in Petrified Forest National Park: A Geoarchaeological Approach to Understanding the Long-Term Use of a Marginal Landscape

Abstract: This study uses geoarchaeological analysis of sediments, soils, and geomorphology to understand human-environment interaction in the eolian landscape in Petrified Forest National Park, Northeast Arizona. The Petrified Forest National Park is located along the southern end of an extensive sand sheet. This landscape was inhabited by Ancestral Pueblo people, and contains evidence of widespread dune farming for agriculture. Soil geomorphic analysis and optically stimulated luminescence dating document discrete periods of eolian activity in the study area. The oldest eolian activity dates to ca. 16,000-11,000 B.C., and is expressed as stable dunes on upland landscape positions. Extensive remobilization of eolian sediments occurred in at least two to three periods in the late Holocene, at ca. 300 B.C., from A.D. 300-600, and from A.D. 1000-1600. The sequence of deposition of eolian sediments is similar across nearby regions on the Colorado Plateau, suggesting a climatic control on deposition. Soil physical and chemical analyses indicate that soil nutrients across the study area are generally low, making farming possible but not highly productive. Water retention in dunes is improved by stratification of sand over clay deposits, and by high amounts of clay within sand dunes. Micromorphological and clay mineralogical studies show that clay within the dunes is distributed as aggregates, likely incorporated into the dunes from locally eroded Triassic clay deposits. The prehispanic landscape surface includes soils that demonstrate some landscape stability, even though eolian deposition was ongoing during use of the landscape for agriculture. Archaeological sites that likely reflect agricultural use of the eolian landscape are preferentially located in areas with soils with higher amounts of clay, which would have improved water holding capacity. High variation in soils and sediments at the site-scale relates to geomorphic position, and suggests that prehispanic farmers may have carefully selected field locations to take advantage of micro-environments with better hydrological characteristics, such as contrasting soil textures. Prehispanic farmers may have used similar farming techniques to those of modern Indigenous groups to improve conditions to cultivate sandy eolian soils. Though we have limited archaeological remains of field systems, variation in soils across the landscape of Petrified Forest show soil qualities that affected how agriculture people directly used and interacted with a dynamic landscape. Soil quality is relatively poor, but the improved water holding capacity of the sandy soils due to the local geologic environment may have made this area more attractive for dune farming, and may be one reason this landscape was inhabited for such a long time.

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