Quartzite-cobble weathering in alluvial-fan soils of the Virginia Blue Ridge

Extremely weathered quartzite cobbles are embedded in Paleudults, Hapludults, and Fragiudults developed on alluvial fans along the western footslopes of the Blue Ridge, central Virginia. The cobbles are intriguing because their weathering patterns and physical condition imply longterm in situ weathering. This study was conducted to explore the feasibility of using the cobbles to correlate the soils by determining if there is a soil-cobble weathering relationship. Ninty-three cobbles were collected from 10 locations. Cobble bulk density (rho (cb)) was compared using the Kruskal-Wallis nonparametric procedure. Free Fe from the cobbles (Fe(cb)) was compared with a two-level nested analysis of variance for unequal sample sizes. Pearson's correlation coefficients were calculated for rho (cb) versus several soil and cobble properties. Among five sites on gently sloping landscapes, weathered pcb ranged from 2.17 to 2.32 Mg m(-3) and was not significantly different. At the other sites rho (cb) was significantly different and ranged from 2.39 to 2.52 Mg m(-3): Variation of Fe(cb) among cobbles within sites was 72.8%, thus making It difficult to suggest a within-site weathering scenario. Variation among sites of the Fe(cb) was only 13.1% suggesting similar weathering processes controlled cobble alteration across the study area. No significant correlation was found between rho (cb) and soil physical properties or cobble sand fractions. Correlation coefficients for rho (cb) versus Fe(cb) (r = -0.73**), hydroxy interlayered vermiculite (r = -0.76**), and slope (r = 0.69*) indicate that pedoenvironment is a primary factor for cobble weathering. rho (cb), Fe(cb), and hydroxy-interlayered vermiculite seem to be promising parameters for correlating Blue Ridge alluvial-fan soils when sites are within narrowly defined slope classes.