Soils at Prehistoric Agricultural Terracing Sites in New Mexico: I. Site Placement, Soil Morphology, and Classification1
- J. A. Sandor,
- P. L. Gersper and
- J. W. Hawley2
Soils and landscapes used for runoff terrace agriculture sometime between 1000 to 1150 A.D. in a semiarid area of New Mexico were studied in order to infer criteria for site selection, and to determine possible long-term effects of the land use. Terracing involved construction of small rock dams across hillslopes and drainageways with subsequent sedimentation occurring upslope of each dam. Sites occur primarily within a narrow range of possible locations, implying deliberate placement with respect to climatic, topographic, and soil factors. Locational characteristics commonly include elevations of 1800 to 2000 m, 3 to 10% slopes, <1- to 8-ha drainage areas, and soils with strongly developed argillic horizons (claypans). These soils, Paleustolls and Argiustolls, are developed in Pleistocene volcanic alluvium. The claypan probably served to confine moisture within the crop rooting zone. Terracing functioned to reduce runoff velocity, increase soil moisture, and thicken a naturally thin loamy A horizon overlying the claypan. Soil morphological changes continuing to the present are mainly limited to surface horizons. Although A horizon texture was not appreciably altered by terrace sedimentation, A horizons are usually 10 to 30 cm thicker in cultivated soils. Peds in cultivated A horizons are generally coarser, blockier, and compacted relative to the dominantly granular structure of their uncultivated counterparts, and are lighter in color value. The structure and color changes are attributed to decreased organic matter contents resulting from cultivation and persisting conditions of reduced grass cover and accelerated erosion.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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