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:

“Slab-slide” is a made-up term, but it seems appropriate for broad, thin, sheet-type features like this one. Friction on the sliding surface must have been very low to keep the mass intact.

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.

Downslope movement of the slide mass generates a toe thrust, which produces associated folding.

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:

The cracked toe anticline above has displaced only slightly from the slide’s toe ramp. Cracks are parallel to the axis of the anticline. Thin, parallel cracks can be seen in the real-world example at right, produced by draping a hillshade over Google Earth.

This broad ramp anticline also produces cracking, but some of the cracks are set well back from the slide toe. This style is consistent with the real-world example below.

This cracking anticline, developed atop the slide ramp, is set back ~150 meters from the actual slide toe. The geometry seen here indicates that the sliding surface climbed upwards through the layering in a ramp-flat-ramp style before reaching the surface. The slide body at the toe is thus thinner than the main body.

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.

This slide complex shows three cracking toe folds on separate displaced masses. The fold at upper right appears to have uplifted a minor channel/gully (look just right of the red leader line).

Detail of the three fold image. Cracks on fold crests are apparent, even though the folds are quite small (~50 meters across).

This larger slide feature also shows internal complexity, with different displaced masses. A thin, rectangular slab at the top of the image has produced folding on its margins. It appears to have undergone minor clockwise rotation, leading to more developed folding at its lower right edge. A thicker mass shows two well-developed crack systems (“micro-grabens,” in this case). Hummocky “galloping ground” at the slide toe probably reflects disaggregation of the outermost toe. Presumably, bedding that can be traced along the top of the small slab should be restored to the ridge crest (arrow and question mark at upper left).

All shapes and sizes. This long, thin slab developed an obvious toe fold that reflects its odd shape. Minor folding along its left margin is also visible. The curved head of the slab can be fit to the matching (but subtle) head scarp at the top of the image.

More of the same. A faint crestal crack is visible along the toe anticline.

This feature is small and minimally displaced, but still shows diagnostic features.

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:

Green line outlines the source area of the debris lobe outlined in red. The slab slide shown above can be seen at top left…it’s pretty small! This fluidized flow dammed an adjacent stream, leading to ponding of sediment, outlined in blue.

The segmented blocks at the head of this slide are well-defined and are definitely visible on foot. This slide mass, however, is completely undetectable in all digital topography pre-dating this 1-meter dataset. This thicker slide detached in a weak shale layer, indicated by a faint red dashed line. The road grade at the toe of the slide is the trail to The Cascades, a popular waterfall hike near Pembroke, Virginia.

A big, ugly, and complex feature. This one appears to reflect more rotational movement along a deeper failure surface. Two or three subsequent failures/flow can be seen on the overall slide mass. Good road access!

This post was originally published on The Geo Models blog.