March 20, 2020

Cracked mountaintops of the Virginia Valley and Ridge

Posted by larryohanlon

By Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab

A newly-released LiDAR data set reveals impressive ridge-top cracks associated with large rock slides in the Virginia Valley and Ridge. While the cracks are easily visible with LiDAR hillshade imagery, they appear to be covered by normal forest vegetation and would probably look like elongated depressions in the forest. Distinct styles of cracking and associated rock sliding are associated with different sedimentary rock sequences. I have not visited any of these locations in person, and I would bet that they might go unnoticed to a field observer that was not actively seeking them. Their age and movement history, along with that of the slides, is unknown. Some interesting examples are shown below, along with three physical models that produce similar features.

Cracking is very clear on mountains developed on Siluro-Devonian sandstones like this one, as they tend to develop smooth-looking slopes. As mentioned below, this slide is actually very large despite appearing to be made of pebbles…

The example shown above has developed in Siluro-Devonian sandstones overlying weak shales, within the same general interval of rock that hosts very well known giant rockslides southwest of this example. The lengthy system of cracks with surface depressions suggests the sandstone sequence has detached along most of the ridge shown here, with a portion of it later failing completely to produce the obvious slide mass at center. The model below may present a similar failure sequence.

While only a small part of the slope fully slides, the entirety of the slope detaches and moves. In some parts of the model, there is not obvious slide toe despite obvious disconnection at the ridge crest.

While the boulders on the slide make it look like a collection of pebbles, it’s still very large. The obvious slide mass at center is about 2,000 ft (600 m) across, and width of the cracks projecting away is very impressive. Extracting and dating organic matter buried in the cracks might offer useful insight into their age.

The yellow line is a Google Earth measurement across one of the cracks, which may actually contain a downthrown block of sandstone. The yellow line is 144 ft in length, which gives some sense of the size of this slide feature and the “pebbles” on its surface.

A few miles to the southwest, the Nutter Mountain Anticline shows a similar crack feature extending away from an obvious slide scar.

Slide “scar” is probably not the best term here, as I think the slide mass is only slightly displaced. The crack projects away from the obvious headscarp where the slide did fully break away.

The Nutter Mountain crack system has an interesting jagged experience and may have developed small “mechanical karst” sinkholes where surface drainage is captured by the crack.

The GIF below shows another scenario in which cracks spread away from a fully displaced slide mass. Here, everything starts with the slide at the center.


The underlying geology of the Nutter Mountain Anticline is exactly the same as the first example, with a sandstone sequence detaching and sliding on underlying shales. Nutter Mountain hosts another giant rock slide just to the southwest, but it lacks obvious cracking extending away from the slide zone.

The large crack remains visible on the otherwise smooth intact portion of the Nutter Mountain. The big slide on the left side of the image is upslope of the Johns Creek Gorge. Whether stream incision pre- or post-dates the slide is unclear.

I think the most interesting examples of large-scale ridge cracking occur in the uppermost Devonian-aged sandstones and shale of the Foreknobs (Chemung) Formation.

Without LiDAR hillshades, would you ever stumble across these cracks?

Many of these crack features do not associate with an obvious slide mass and reflect only very slight downslope motion. They are entirely invisible in aerial photos, and I don’t think anyone would ever know to look for them because nothing in the topography suggests any kind of slope failure here.

The cracks obviously reflect downslope movement on bedding planes (check out layering visible near the bottom of the image). That being said, there is no visible toe at the base of the slope.

These cracks are impressively large, and their crisp edges make them readily visible in the hillshade imagery. Despite their impressive size compared to a human observer, they easily get lost within the large zones of rugged topography developed on Foreknobs outcrop zones.

The yellow arrow points to the cracks shown above. The red tabs are projections of layering in the broad syncline, which preserves a large thickness of Foreknobs sandstones and shales. The entire unit is over 2,000 ft (~600 m thick) in the area.

In this area, the Foreknobs Formation tends to be exposed in broad synclines (red areas above are projection of the layers), and the thickness of the unit and its typical structural context creates a unique topography that is different from that produced by the Siluro-Devonian sandstones. As a result, slope failures here occur into the cores of synclines, which may ultimately arrest motion of slide masses.

Just up and right of “Axis of syncline,” the dip reversal of the fold can actually be seen in the bedding. Many variables are obviously at play on this large slide, but it seems to have ground to a halt where the bedding plane on which it was sliding changes orientation instead of progressing to the neighboring valley floor.

Some of the crack examples in Foreknobs topography are associated with slides further downslope, but the cracks themselves are still surprisingly far uphill towards the ridge crest.
cracks themselves are still surprisingly far uphill towards the ridge crest.

The cracks pointed out by the leader line are 1,050 ft long and about 70 ft wide. It might be reasonable to assume that movement of the slide further down the slope destabilized upslope areas, but who knows. This is clearly a thick translational slide complex, with failure on weak shaly bedding planes between sandstones.

This model produces cracks and sags well upslope of an obvious, shallow slide zone. The uppermost cracks and sags do not develop until failures further down the slope reduce slope stability.

The mountaintop shown below hosts another example of cracking occurring well upslope of any obvious slide mass. This crack looks as if it may contain an intact, downthrown block of sandstone.

This crack system cuts right across the summit of the mountain. Again, obvious failures further downslope are almost certainly associated with the development of the cracks, but the exact movement history is totally unknown.

The depth of Foreknobs sliding that produces the cracking is not immediately clear and may be quite variable. There is some evidence in the area of very deep-seated detachment and sliding (upcoming post). The entire Foreknobs unit is composed of interbedded sandstones and shales, so many sliding surfaces are present. The slope shown below provides a cross section through a small portion of the unit, indicating the presence of a glide plane beneath a sandstone package. Whether or not this particular glide plane is active throughout the area is unclear.

The yellow arrow indicates slide motion. Rupture can be seen to cut down from the surface through the sandstone layers, presumably flatting into a shale sequence.

Collectively, these slope features further highlight the use of LiDAR hillshades in identifying potentially significant geologic features that are otherwise nearly invisible to a ground observer. The age or movement history of these features is completely unknown, but many appear very crisp and sharp in the landscape, clearly contrasting with older and more deteriorated features.

It’s risky to try to associate age of emplacement with physical appearance, but there are obvious differences in this area.

Large-scale rocksliding is obviously widespread throughout a variety of rock types in the Virginia Valley and Ridge. Even if these features are not recent or prone to reactivation, it would be worthwhile to know more about what caused their emplacement.

This post was originally published on The Geo Models blog.