Coincidence and spatial variability of geology, soils, and vegetation, Mill Run watershed, Virginia
The Mill Run watershed is a structurally‐controlled synclinal basin on the eastern limb of the Massanutten Mountain complex of northwestern Virginia. Bedrock contacts are obscured by coarse sandstone debris from exposures near basin divides. Colluvium blankets more than half the basin, masking geomorphic surfaces, affecting vegetation patterns, and contributing to the convexity of the alluvial, terrace, pediment and erosion surfaces. Examination of the bedrock geology, geomorphology, soils and vegetation shows distinct distributional correspondences. Vegetation is strongly interdependent with geomorphology, bedrock geology, and soils. On convex colluvial slopes, mixed hardwood forests are most common. In concave coves and deep gorges, mixed hardwoods are replaced by conifers. In thin colluvium, in poorly developed soils, and on blockfields, chestnut oak is singularly prevalent. Conifers dominate shaley bedrock areas. Soils and surficial sediments have a major effect on near‐surface hydrology. During wet seasons, cemented horizons in the subsurface cause temporary saturation in the superjacent horizons; lateral movement of soil‐water effectively eliminates a vertical component of ground‐water recharge. Vegetation is strongly dependent on water availability and thus reflects the distribution of subsurface barriers and sediment‐soil fades changes.