by Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab
The YouTube video linked below shows an impressive late 2015 rockfall above Evolène, Switzerland, that releases two large boulders on lengthy downhill paths. The boulders are visible from the valley viewer’s perspective beginning at 00:37.
The scars left by the tumbling boulders are visible in summer 2016 Google Earth imagery. The first two images shown below are from the boulder that travels towards the viewer’s right in the video; the third image shows scars left by the larger boulder that travels towards the viewer and comes to rest much further downhill. The right-traveling boulder can be found at 46.108308N 7.470488E on Google Earth.
My work with Appalachian Landslide Consultants along the Blue Ridge Escarpment in western North Carolina left me wondering if similar impact scars could be identified in a forested Appalachian landscape. Relief, lithology, and tectonic and climatic history along the Escarpment are VERY different from the European Alps, but many portions of the Escarpment landscape are littered with boulder debris that clearly originated significantly upslope. I was able to find what I believe to be a few examples of “boulder tracks” using outstanding LiDAR hillshade imagery from the North Carolina Geological Survey (QL1 lidar data acquired by NC Dept. of Public Safety, raw point clouds assembled and processed by Corey Scheip of the NC Geological Survey Landslide Hazards Program.) All features shown occur in generally gneissic bedrock on extensively forested slopes that have been logged within the last century.
The first potential example is interesting because the apparent scars suggest bouncing of a boulder as it accelerated downslope. At the end of the small black arrow near the center of the image below, three aligned scars can be seen. They are deeper and wider on their upslope ends, producing a faint “teardrop” shape with a thicker upslope end. Parallel to these possible scars are additional features that are also likely boulder tracks given the source area above them.
The image below shows the possible scars (indicated by yellow arrow) in the larger-scale context of the mountainside…they are very minor and hard to pick out! Relief in this area is 1300 ft (400 meters) or so. The large cliffs cover an elevation interval of about 500 ft (150 m). Today, home sites are located at the foot of the cliffs. Foundation pads can be seen at the lower left of the image.
The scars cover ~390 ft (120 m) of ground length, descending 140 ft (42 m). The three deepest apparent scars are more visible in the detail below. The scarp/cliff line source is visible at the very top of the image.
The scale of this system is MUCH smaller than the Alps example, but the shape and pattern of the scarring seems to bear some resemblance. The image below shows the scars of the tabular boulder from the Alps example as seen from my iPhone on Apple maps (I don’t know the date of the imagery).
The largest/longest example shows a swarm of impact tracks originating from a scar high on a spur projecting out from the Escarpment. The tracks extend over 1,300 ft (400 meters) ground length, descending 480 feet (147 meters). These are unequivocally impact marks, and their travel path offers an interesting contrast to water movement on the slope (thin blue line).
This feature is very subtle, and I passed over it several times when examining the LiDAR hillshade imagery for this area. A close zoom shows clear contrast between the scarred area and the smooth, un-scarred slope. The scarred area is lower than the smooth portions of the slope, indicating that the scarred area is definitely not deposited material.
The impact tracks are interesting because they cross the headwaters of a small headwater stream (blue line) without being “funneled” into it due to the momentum of the boulders that produced the scars. Once momentum was lost, the boulders deflected back onto a straight-down-the-slope course to produce the gently curving track of impact scars before continuing off of a ~200 ft (60 meter) cliff line, which distorts the Google Earth overlay shown below. The dashed line shows a generalized track path.
The GIF below gives a perspective view across the Google Earth overlay with a moving “boulder,” which tumbles along the scar path before heading off the cliff. The Google Earth elevation model is not vertically exaggerated, so this gives a reasonable idea of steepness of the area. An impressive accumulation of boulders can be seen at the base of the cliff line.
I have no idea of the age of the rockfall(s) that produced the scars, or if it was a single event or the combined effects of several events. There is some vegetation irregularity on the scarred area on Google Earth, but this may not be a legitimate indicator of young age. That said, it seems unlikely that soil scars would persist very long in the very rainy, humid, and forested Appalachian landscape.
One more example, which is even more subtle, occurs nearby. The probable impact tracks are very faint (hence the dark and hard-to-read annotation).
These scars cover 390 ft (120 meters) of ground length, descending 200 ft (60 m) before reaching the cliff line. I think some scarring may be visible on the cliff line as well. The foot of the slope below this example is littered with boulder debris, but I have yet to try to visit any of these scars in person to see if they are identifiable. The LiDAR dataset offers roughtly 1/2 meter resolution, so it will resolve very faint features that would be tough to pick out in a heavily vegetated setting.
I cruised around the Evolène area a bit more, and I think some very faint and small-scale scars may occur at 46.066222N 7.529256E. I indicate them in a screen shot below. I believe that they are deeper on their upslope sides, but the spray of soil kicked up by the impacts gives them a tear drop shape of the opposite orientation of the LiDAR features discussed above where only the physical scar can be seen.