By Philip S. Prince

I spent about 10 years studying and then teaching at Virginia Tech, and I can confidently say that it’s hard to find a better to place than Blacksburg to live and work as a geologist. The majority of my research and field teaching took place in southwest Virginia, so it’s always exciting when new lidar imagery comes along to offer a new perspective on familiar landscapes. The four large landslides shown here are not really noticeable without lidar help–I have passed close to (or across) all of them many times, and I never noticed them based on field expression alone. They are quite noticeable in 1-meter lidar hillshade imagery, and the appearance and morphology of each slide is an interesting reflection of its geologic and topographic setting. All of these features can be seen in 1-meter hillshade imagery on The National Map’s seamless viewer. In this post, they are draped over Google Earth as kmz files.

1. Huge, historically (<100 years) or currently active debris slide, New River above McCoy Falls, Pulaski County

This slide, which resembles a glacier creeping down towards the river, has developed in the debris deposit produced by a large rockslide on Walker Mountain. The rockslide laid bare white cliffs of Tuscarora Sandstone that I have always heard called “White Face.” The rockslide itself is ancient (Pleistocene?), but slide movement in the resulting debris pile is much younger. The GIF below shows the slide and its topographic context. White Face is visible at the very top center in the aerial photo image. The curving path of the slide can be appreciated from this perspective. I have looked across the river at this heavily forested area hundreds of times, so this 1-meter hillshade view is eye-opening, to say the least!

The most striking lidar feature of the slide is the crispness of its lateral scarps and headscarp. They don’t appear to be very large (in terms of height), but remain very crisp and well defined, suggesting recent or ongoing activity.

The edge of the slide can be seen cracking along the left-lateral scarp as material pushes up and out of the topographic low that holds the debris deposit.

I can’t personally speak to the movement history of the slide, but I suspect that removal of a portion of its toe to construct the railroad running along the river may have led to reactivation. A smaller slide to the north (right side of the image) was not altered as much by railroad construction, and looks very old and stable compared to its large neighbor.

2. Rock blockslide along Big Reed Island Creek, Pulaski County

This slide has experienced little displacement, but lidar reveals an impressive graben that is forming as a large mass of Hampton Shale (really a slate or phyllite here) bulges outward from the slope as it is undercut by Big Reed Island Creek. The slide and graben are at the center of the image below.

The slide and its graben are, unsurprisingly, invisible without a lidar-based image.

The graben is quite large compared to the overall scale of the slide. This indicates that the basal sliding surface slopes less steeply than the land surface, meaning the slide mass is thick and deep, particularly at its upslope end. The change in dip/steepness along the failure surface is also likely quite abrupt, forcing graben development. The block diagram below attempts to illustrate this idea.

This slide is included on the Hiwassee quadrangle geologic map (linked here), and structural data indicate the failure is occurring along the crest of an anticline that plunges northeast. The basal sliding surface could therefore be a bedding plane/foliation plane failure whose low dip is an expression of where it sits within the fold structure. The dashed form lines in the diagram generally indicate layering orientation as it might appear on this cross section cut, which is at an angle to the fold axis.

I actually went to the foot of this slide with Virginia State Geologist Matt Heller during a canoe transect of the Hiwassee map area in June 2019. The poor photo below shows what the downslope end of the slide looks like, right above Big Reed Island Creek. This huge outcrop sheds plenty of rock, and does not feel like a place you would want to stand around for too long.

3. Buck Hollow Ridge rock slope movement, Macks Mountain, Pulaski County

This slope movement is not far from the Big Reed Island Creek feature. The slide mass is Erwin Formation quartzite, just up-section from the Hampton Shale. Sub-surface failure within the Hampton Shale may have actually caused the movement, but I don’t know for sure.

This failure indicates the precarious stability of some rock slopes (particularly dip slopes) in this part of Virginia, even if they host resistant rock types like quartzite. I have hiked along the toe of this feature before, and never knew it was there. The logging road grade along the summit cuts right through numerous scarps. I think this feature is particularly interesting because the headscarps are found on the back side of the ridge crest, meaning the summit of Buck Hollow Ridge has lowered slightly due to the failure.

The slide has developed on a dip slope, with slate/phyllite beds being the likely seat of the basal failure. Dipping quartzite beds nearer the surface are visible in the lidar imagery, as are subtle flatirons within the landscape around the slide.

Progressive undercutting of the toe by Big Laurel Creek may have kept this sliding intermittently active for a lengthy period. The toe appears eroded and “shortened” next to the creek, which is actively eroding it at the tip of the arrow in the image below.

The effect of mechanical toe erosion can be reproduced in a simple model, like the one shown below. Compare the appearance of the eroded toe to the lidar shot above, and note its effect on continued movement and progressive fracturing of the slide block due to the flattening failure surface.

4. Rock Castle Gorge rockslide, Patrick County

Rock Castle Gorge is actually named for clusters of terminated quartz crystals found in the area, but its Blue Ridge Escarpment location makes it a very rough and rocky place at the larger scale. A loop hiking trail through the gorge passes through the upper portion of the rockslide shown below, which is conspicuously bouldery and rocky to an observer on the ground.

The still image below shows what I believe to be the route of the hiking trail. I have walked the trail several times, though not for a number of years.

The texture of the slide mass shows the presence of different materials, which are likely schists and amphibolites. The chunky, bouldery material appears to overlie less bouldery material, and is also partly buried by less bouldery material derived from upslope. The bouldery material is derived from the resistant horizon cut by the slide, which produces bold bedrock outcrops and a flatiron-like effect on the slope. I have tried to delineate these material-specific zones below.

A visit to this feature would quickly indicate how the stacking order of material in the slide mass reflects position of the in-place materials on the slope. Whether this slide is the result of a single failure or successive failures is unclear. It is also somewhat unique in the area. Despite its steepness and the frequent occurrence of dip slopes, the Blue Ridge Escarpment and its intensely folded metamorphic outcrops seem to host far fewer landslides than the folded/faulted sedimentary Valley and Ridge to the west.

Philip Prince is a Project Geologist with Appalachian Landslide Consultants, PLLC, in Asheville, North Carolina. He also conducts geologic mapping in the Virginia Valley and Ridge for the Virginia Department of Mines, Minerals, and Energy.  More posts related to his field experiences and remote sensing work can be found at princegeology.com.