DELINEATING GEOLOGIC AND GEOMORPHIC FEATURES FROM SHADED RELIEF MAPS DERIVED FROM HIGH-RESOLUTION DIGITAL TERRAIN MODELS

Shaded relief maps generated from elevation control data associated with high-resolution orthoimagery have been used to improve 1:24,000-scale geologic mapping in two geologic provinces in Virginia. In the Elkton West 7.5-minute quadrangle, located in the Valley and Ridge Province near the southern end of Massanutten Mountain, areas underlain by residuum, mountain slope colluvium, and older debris-flow deposits are not evident on a standard 1:24,000-scale topographic map with a 40 foot contour interval. These map units can, however, be identified on the shaded relief map due to subtle differences in slope pattern and dissection. The map also allows for accurate delineation of modern flood plain and terrace deposits along the South Fork of the Shenandoah River. In the Providence Forge 7.5-minute quadrangle, located in the Coastal Plain Province east of Richmond, the shaded relief map enables correlation of Pleistocene terraces and underlying marine and nearshore facies of older stratigraphic units, previously difficult to resolve. Scarp and terrace morphology, which generally follows consistent elevations, can be further refined by extending the use of shaded relief maps across the Coastal Plain.

Shaded relief maps derived from high-resolution digital terrain models (DTMs) have been demonstrated to be useful in delineating geologic, geomorphic, and geologic hazard features, especially in highly vegetated areas. Features such as terraces, sinkholes, fault scarps and landslide deposits are often difficult to detect on the ground, on topographic maps or on aerial photos, and may only be visible on shaded relief maps derived from high-resolution DTMs. Many states now use bare earth light detection and ranging (LiDAR) elevation data, but, at present, Virginia lacks comprehensive LiDAR coverage. However, the DTMs generated from high-resolution orthoimagery have proven to be a useful alternative in delineating geologic features when LiDAR data is unavailable, and they are substantially better than standard 7.5-minute topographic maps.