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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.

Gullies cut in alluvial‐fan piedmont deposits at the southern end of the Jornada del Muerto basin, New Mexico, reveal evidence of periodic sedimentation and soil formation. The gullies show a succession of four major sediments, each of which has a distinct soil, indicating that each period of sediment deposition was followed by a period of stability and soil formation. The two youngest sediments are restricted primarily to a broad drainageway position and have soils with relatively weak Bt and carbonate horizons. The next older sediment, buried in the broad drainageway, has a soil that has prominent textural B and carbonate horizons and that emerges at the land surface on either side of the drainageway. Towards the basin axis the sediments thin and there is mergence of the land‐surface soil with the oldest soil exposed in the gullies. The land‐surface soils overlie much older soils and in places are so thin that the buried soils must be considered in classification of the land‐surface soils. Gradual mergence zones of major sediments can conceal complexities greater than might be suggested by the generally smooth relief. Filling of channels in old sediments by younger sediments can cause prominent and abrupt soil changes not suggested by the smooth relief and uniform slope which crosses the soil boundaries. These phenomena are especially prevalent in and adjacent to drainageways and in areas of thinning of major sediments.