March 20, 2020
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.
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 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.
A few miles to the southwest, the Nutter Mountain Anticline shows a similar crack feature extending away from an obvious slide scar.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.