Quantitative comparison of soil and saprolite genesis: Examples from the Virginia Blue Ridge and Piedmont

Document Type


Date of Original Version



Soil reconstruction and mass balance analysis are two techniques used to quantitatively measure the genesis of regolith materials. Although both of these techniques have been widely used, no study has attempted to compare the two methods. We used soil reconstruction and mass balance analysis to compare the genesis of residual soils and saprolites underlain by gneiss. Study sites were located on stable summit positions in the Blue Ridge and Piedmont of Virginia. Both quantitative techniques showed similar trends in quantifying soil formation. The main soil-forming processes that could be quantified were sand and silt weathering, subsequent weathering product transport and leaching, clay illuviation, and accumulation of Fe oxides and hydroxides. Saprolite formation occurs as the result of oxidation, primary mineral weathering, and weathering product transport and leaching. Strain indices for soil horizons ranged from -0.72 in the most weathered and leached E and BE horizons, to 0.41 in the Bt horizons with the largest accumulations of illuvial clay. Saprolite formation is an isovolumetric process. Therefore, even though alterations of unweathered rock to saprolite results in an average of 45% mass lost, no volume change occurs. Soil formation results in a loss of mass of the original saprolite on the order of 8 to 20%. Most of these losses occur in the upper 40-80 cm of the soil. Weathering of the saprolite results in a loss of mass of 4 to 6% Clay mineralogy of the upper meter of soil is dominated by hydroxy-interlayered-vermiculite (HIV) and kaolinite (>80% combined). Kaolinite and mica are the most prevalent clay-sized minerals in the saprolite. Halloysite is the predominant clay size mineral in the deeper saprolite of the Piedmont study site.

Publication Title, e.g., Journal

Southeastern Geology