Mass Wasting in the Blue Ridge

Debris Flow Chute

The most common mass-wasting process in the Blue Ridge is probably downslope movement of colluvium and residuum induced by windthrow of trees. Slopes are steep and heavily forested. Although disturbed material may only move a few feet in an event, the process is widespread and continual, and so may accomplish more erosion than any other mass-wasting process.

Among rapid mass-wasting processes, debris flows appear to be by far the most common in the Blue Ridge. They seem to occur much more frequently there than in other Appalachian provinces. This abundance may stem in part from the greater permeability of regolith derived from crystalline rocks on steep slopes, relative to the regolith in other provinces. In contrast to the fine-grained colluvium derived from shales that is common on the Appalachian Plateau, for example, this colluvium may be very insensitive to the prolonged, slow rainfall that commonly sets off landslides on the Plateau. Blue Ridge colluvium issensitive to rainfall that is intense enough to infiltrate the soil. The large number of debris slides and flows in the Blue Ridge may reflect the intense rains thatoccur in the Province. The susceptibility of crystalline rocks to saprolitization, which provides an abundant source of hillslope debris for slides, may be another contributing factor. Recurrence intervals for flows at a given location probably range from hundreds to thousands of years. Debris flows usually start as slides that move down "chutes"into river valleys. Typical chute lengths are 50-300 m, with widths of 8-24 m. Thickness of slide masses ranges from 1 to 6 m. Slides commonly occur in troughlike depressions down hollows.here, convergence of surface and subsurface drainage produces high pore pressure that lowers shear strength and makes slope failure more likely. Commonly, only a small part of the material sliding down the hill is deposited at the base of the chute. The greater part travels down the valley as a debris flow or is washed away by subsequent flood waters. Where the chute emerges onto a broad valley, however, debris fans may result. Such fans maycontain boulders6-10 feetin diameter.

debris-flow fan

Large areas of bouldery colluvium occur along the bases of mountains in many parts of the Blue Ridge. The origin of these unsorted, unstratified deposits has been attributed to solifluction in ice-age climates. The similarity of the colluvium to the material found in the debris fans, however, suggests that a large part of the colluvium was probably placed by debris slides and flows during intense rainstorms.

Debris Flow Fans

The most studied surface deposits in the Blue Ridge Province are along mountain footslopes. Theyoccur either as a continuous apron along the base of the mountains or, more commonly, as alluvial fans in gently sloping embayments of the mountain fronts known in some places as "coves". The fans range in area from several acres to several square miles. Larger fans average 15 to 20 m in thickness, and seldom exceed 30 m. Some of the fan material is alluvium, particularly in the distal part of large fans, but much of it is unsorted and unstratified material that appears to have been emplaced by debris flows. The major exception to these generalizations occurs in the Virginia Blue Ridge, where fans up to 10 square kilometers in area deposits occur in the Great Valley at the base of the northwest flank of the Blue Ridge. These fans appear to be composed mainly of coarse alluvium, and can exceed 100 m in thickness.

The Blue Ridge fans range widely in age. The majority appear to be late Pleistocene in age, as shown by the similarity of weathering and soil development to those of late Wisconsinan glacial till in Pennsylvania. Much hillslope colluvium probably is also of this age. However, many fan surfaces have deep red soils containing decomposed stones, and are clearly much older than late Pleistocene. They may be early Pleistocene or older. A date of 1.5 million years has been reported for one old fan remnant in the North Carolina Blue Ridge (Mills and Granger 2002). The soils on these old fan surfaces are more like residual on saprolite soils than like soils on young fan deposits.