Saturday, 15 July 2006

Soils and Landforms as Indicators of Prehistoric Human Occupation at Big Bend National Park, Texas.

Lynn E. Loomis, USDA Natural Resources Conservation Service, PO Box 1557, Marfa, TX 79843 and Thomas C. Alex, Big Bend National Park, PO Box 129, Big Bend National Park, TX 79834.

Terminal Pleistocene and Holocene age soils are of archeological interest at Big Bend National Park because of their contemporaneity with human occupation. Humans have used this landscape for at least the last 12,000 years; certain soils are associated with remnants of stable geomorphic surfaces which ancient people might have occupied. Identifying these soils enables archeologists to target areas for investigation, where erosion has exposed important cultural sites.

The Hurds-Sanmoss-Mitre complex, 1 to 8 % slopes, soil map unit exists as two delineations within Big Bend National Park; one in Pine Canyon, the other in Green Gulch. The map unit corresponds to a mountain-valley fan major landform. Along a transect in Pine Canyon, soils were described at 10 locations spaced at 160 m intervals.

Local relief was about 5 m. Four geomorphic components were recognized; from highest to lowest they were erosional fan remnant summit, fan remnant footslope, inset terrace, and channel floor. Soils were not described on the channel floor, which was not extensive. Overall, the mountain-valley fan sloped eastward at a 4% gradient. Individual geomorphic components had slope gradients of less than 12% and variable aspect. All soils formed in very gravelly alluvium and had a thermic, aridic ustic soil climate regime.

On erosional fan remnant summits, Petrocalcic Paleustolls of the Mitre series formed in mid-Pleistocene age conglomerate. These were the most highly developed soils, situated on the highest, oldest landforms within the map unit. Diagnostic horizons were a mollic epipedon, a clayey-skeletal argillic horizon, and a petrocalcic horizon. Slope gradients on this landform ranged from 15 %.

Aridic Argiustolls of the Hurds series formed in late-Pleistocene age pedisediment on footslopes of fan remnants. The footslopes occurred 12 m below the fan remnant summits, and 12 m above the inset terraces. Diagnostic horizons included a mollic epipedon and a loamy-skeletal argillic horizon. Slope gradients on footslopes were mainly 18 %.

Pachic Haplustolls of the Sanmoss series formed in Holocene age alluvium. Sanmoss soils occurred on terraces 12 m above channel floors, inset 12 m below fan remnant footslopes, and 23 m below fan remnant summits. The soils had no diagnostic horizons other than a mollic epipedon 200 cm thick. Though sediment stratification was very evident, soil color did not vary irregularly with depth. Slope gradients of inset terraces ranged from 13 %.

The Mitre soils have been subjected to extensive geological erosion, but the resistant petrocalcic horizon preserved the associated landform, providing a stable surface that survived throughout the Holocene period. This surface was available for use by prehistoric inhabitants throughout the Holocene, and archeological sites may contain artifact assemblages from any part of that period, or may contain a mixture of temporally diagnostic materials representing repeated episodes of reoccupation at one location on that surface. Artifacts from the Middle Archaic Period (25005000 yr BP) were observed near one sample location in Pine Canyon. The younger Sanmoss soils would logically yield cultural materials from the later prehistoric periods. Mesic periods, typified by the dominance of deposition over erosion, resulted in burial of cultural sites within these earlier deposits. Late Holocene erosion is now eroding these buried soils, exposing prehistoric human occupation sites. In Green Gulch, terraces associated with Sanmoss soils have yielded Late Archaic and Late Prehistoric materials with a conspicuous lack of earlier period evidence.

Palynological and macrobotanical analyses of Holocene age deposits yield data that adds to the interpretation and understanding of climatic changes and the associated changes in vegetative cover. The general trend toward desert conditions is generally represented by the loss of trees and the increase of shrubs, desert succulents, and grasses. The archeological record throughout the prehistoric period reflects how prehistoric people adapted to changes in food subsistence base and modified their toolkit according to the available food sources. The links between soils, geomorphic processes, and archeological evidence ultimately aid in interpreting how humans used these ancient landscapes and adapted to a changing environment.

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