by Philip S. Prince, Virginia Division of Geology and Mineral Resources
Hillshade imagery from a new LiDAR dataset provides an incredibly detailed look at landslides of unknown age within the Valley and Ridge province. This 1-meter dataset reveals slides unlikely to be identified by any other means, and it shows details of the slide masses that reflect aspects of their movement. Nearly all of the features have developed on Siluro-Devonian sandstone ridges, where strong sandstone layers have detached in interbedded shales or calcareous sands and slid downslope. The large, thin slide pictured below is a good example. I have included it in posts before, but seeing it at the new resolution takes it to another level:
A video showing a model version of this style of slide, along with what the landscape looks like without a LiDAR hillshade overlay, can be seen here:
My favorite part of this slide (and others shown in this post) is the tensional cracking on the toe ramp anticline. I doubt these cracks would be discernible in the field, even if you knew they were there, due to vegetation and leaf cover, and they certainly are not visible in coarser hillshades. With regard to age of the features, I have to wonder how long the subtle cracks would remain visible after slide emplacement in the humid, forested Appalachians.
Folding at the compressional toe is a normal feature of intact landslide masses, and distinct anticlines can be observed when the slides are thin, translational features like many of those shown in this post.
As a toe anticline grows with slide displacement, its upper surface is forced to stretch as curvature increases. This stretching results in tensional cracking. In the analog model example shown at left above, the toe anticline is very narrow because the sliding mass is very thin. At this early snapshot moment in the model’s evolution, the anticline is developing like a fault propagation fold at the leading edge of the moving slide mass (which is like a thrust sheet). As the fault breaks through to the surface at the base of the toe fold, the slide mass will begin to travel over the former land surface, rotating and stretching the anticline crest to create tensional cracks (cross section at right). In the real LiDAR example at the top of the post, the cracking is occurring on a broader ramp anticline, formed when the sliding mass “draped” across its toe ramp after significant displacement. In the ramp anticline example, the anticline forms farther back within the sliding mass, not at its leading edge. Analog models can faithfully produce these cracks in association with their toe anticlines:
Broad, thin slides with cracked toe anticlines are ABUNDANT in the central portion of the Virginia Valley and Ridge. Cracking toe anticlines have formed in both minimally-displaced fault-propagation and more displaced, buried ramp scenarios. The slides appear to be extremely thin–maybe just a few meters thick–and obviously slid with minimal friction to remain largely undeformed behind the toe fold. A big question is how old these features are. They show very crisp lateral scarps, and toe anticlines are not cut by small channels. In some cases, minor channels appear to be uplifted by growth of the toe folds. It also seems unlikely that the cracks on the toe anticlines would remain visible over very long periods (10^4 yrs or more, maybe?) in the humid-temperate, highly vegetated Appalachian landscape where soil production is efficient and tremendous amounts of leafy and woody debris are produced. In any case, it’s always exciting to identify these features using a newly-available tool and then go out and see what they look like in person.
Here are some interesting examples of these thin, slabby slides and their diagnostic toe folds.
The “slab-slides” aren’t the only mass wasting features in this area. There are plenty of flow-type features and more deep-seated, rotational failures to be seen as well. Some examples: