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Pedogenesis and Origin of Deeply Weathered Soils Formed in Alluvial Fans of the Virginia Blue Ridge

USDA-NRCS, Bishopville, SC 29010

J. C. Baker^*

Dep. of Crop and Soil Environmental Sciences, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061-0404

^* Corresponding author (suebrown{at}vt.edu).

ABSTRACT

Sediment disparities strongly influence soil formation and classification on alluvial fans along the western footslopes of the Blue Ridge, central Virginia. We conducted this study (i) to show the distribution of the soils, (ii) to characterize selected soils developed on the fans, and (iii) to describe their genesis. Soil properties are indicative of long-term weathering. Two deposits occur—a younger, upper one (Unit A) and an older, lower one (Unit B). We examined soil morphology and physical, chemical, and mineralogical properties to document pedon similarities across a distance of 200 km. Transitional horizons (BE), fragipans, and weakly to moderately developed argillic horizons are common to Unit A. Thickness and soil development of Unit A influence classification at the family level. Clay content increases abruptly by 20 to 36% below the discontinuity. Free Fe increases from as little as 6 g kg^-1 in Unit A to as much as 66 g kg^-1 in Unit B. Clay maxima at 2 m range from 33 to 60%. Hydroxy-interlayered vermiculite (HIV) dominates Unit A clay mineralogy, whereas kaolinite prevails in Unit B. In some pedons gibbsite notably increases below the discontinuity, suggesting an intense weathering environment prior to burial. Clay content and the low-activity clays imply Unit B developed from sediments or lithologies additional to the weathered quartzite found in the sola. Time and climate are the most influential factors controlling soil development on the alluvial fans. The soils are part of a surficial continuum from the mountain footslopes to the rivers draining into and flowing through the Valley and Ridge Province.

Received for publication July 3, 1997.