USGS B 2123 -- Surficial Geology

Three surficial units were mapped: undifferentiated alluvium and fine colluvial debris (Qal), colluvium (Qc), and alluvial terrace deposits (Qt). Topographic relief and surface-water runoff play an important role in the depositional processes and resultant landforms of these units. Depositional processes include fluvial transport (alluvium), debris flow (debris), as well as gravity and freeze-thaw processes (colluvium). These units are spatially distributed from upper to lower slopes, and each has a characteristic landform (Jacobson and others, 1990). The alluvium and fine colluvial debris underlie sinuous flood plains along creeks and rivers. Alluvial terraces and fine colluvial debris from side slopes are transitional with alluvium and are included with it on the map. Large areas of coarse cobbles, boulders, and blocks of quartzite and metabasalt are shown (by overprint pattern) as colluvium on Blue Ridge-Elk Ridge and Short Hill-South Mountain. The colluvium is concentrated in hillslope depressions as boulder streams, boulder fields (fig. 26), and talus, but virtually all of the mountainous areas are mantled by thin colluvium. Coarse cobbles, boulders, and blocks of quartzite that were transported by colluvial processes are found as debris fans and debris terraces along the Potomac River at Blue Ridge and Short Hill Mountain. The tops of these debris deposits are terraced, which suggests modification by fluvial reworking.