August 28, 2020
By Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab
Mountainsides in Virginia’s Valley and Ridge have a well-documented potential for naturally occurring landslides, so it should be no surprise that significant excavations on slopes can result in anthropogenic (human-induced) landsliding. LiDAR hillshade imagery reveals numerous landslides which have developed above open-pit iron ore (limonite) mines that roughly follow the contour of the hillslope. The mines, located in Botetourt and Allegheny Counties, were abandoned in the 1920s, and I have found no evidence that slope instability led to their abandonment. The landsliding is confined to areas upslope of the mine workings and is distinguished by crisp, well-defined scarps and surface features. The example shown below has developed above the former Wilton and Circle Mines southwest of Iron Gate, Virginia.
This is a large slide feature compared to the apparent depth of the mine workings along the slope. The headscarp is ~1,000 ft (~320 m) ground length from the mine cut and 400 ft (120 m) above it. The image below is rotated to give some sense of the slope and the feature’s position on the mountainside.
The “washboard ribs” (check out the first image, too) are a distinct feature of this and several other mine-related slides in the area.
I have not visited this location, but I think it is likely that the ribs result from the sandstone layers slipping past each other within the slide mass as it extends and stretches while moving downslope. This particular style of movement results from the orientation of the layers in the slide mass–they dip gently back into the mountainside, unlike the downhill-dipping layers exposed further up the slope. A generalized cross section through the slide might look something like this:
The GIF below offers a general conceptual model. This is not drawn to scale, and the “domino block” rotation between the broken sandstone layers creates an interesting volume issue beneath the slide mass. Pulverized rock might be adequate to fill this space; my sketch uses it to fill in the gaps beneath the tilted blocks. The actual size of the blocks and thickness of the slide would also have bearing on just how much space might develop beneath and between the rotating blocks.
Northeast of the Wilton and Circle Mines, the Callie Mine pits have created a larger slide complex which extends even farther up the slope. The image below shows the two mine complexes relative to each other, with orange outlines around the slide zones.
The Callie Mines slide occurs under the same geologic circumstances and shares many characteristics with the Wilton/Circle Mines slide.
It shows the same well-developed headscarp and “washboard” texture, along with evidence that the slope has begun to slide far above the main headscarp.
The “washboard” ribs are visible between the mine workings and the main headscarp; underlying bedrock structure is oriented the same way here as at Wilton/Circle. In this location, it looks like movement of the main slide has destabilized the mountainside well uphill, causing a huge slab of rock to start sliding along the bedding planes of the sandstone. Cracking is visible in the LiDAR imagery; the cracks are ~2,500 ft (762 m) ground length and 700 ft (213 m) above the mine workings.
Similar mine-related slides abound in the area, and they frequently show the same “washboard” texture when the sandstone layering has the appropriate orientation above the mining cut. Another example to the west is shown below.
A smaller and less extensive mine complex to the northeast has produced two small grabens upslope of the cuts. There has been very little actual movement of the slope here, but enough to create easily visible downthrown blocks.
These limonite iron ore mines are part of the Oriskany mining district of western Virginia. The ore is developed where limestones and iron-rich black shale are in contact. Acidic, iron-containing waters moving through the black shales were neutralized by the limestone, causing hydrous iron oxides to precipitate. This ore formation process may have occurred when the rock was very near the surface, or it may have occurred at greater depth and temperature, possibly during tectonic deformation. Ore-rich areas in this part of the Appalachians are very obvious to the field geologist, but old mine workings like the ones shown here are not nearly as clear in the field as they are in LiDAR. All of the mines shown here are completely reclaimed by forest cover.
The sharpness of these landslide features suggests they may still be slowly moving, but very little disruption to vegetation is visible in satellite imagery, so movement is probably very slow. Since their maximum age is known (the time of mining; late 1800s-1920s), they offer interesting comparison to older, natural landslides in the area, which tend to have softened, rounded features due to weathering and erosion. The yellow arrows point to several examples below (note that these slides have traveled farther, among other distinctions).
This post was originally published on The Geo Models blog. It was reproduced here with permission.