by Philip S. Prince

Pleistocene-aged sand dunes are some of the most conspicuous lidar features of the southeastern United States Coastal Plain, largely because their lumpy surfaces and non-erosional outlines contrast sharply with other “background” landforms. It’s hard to imagine the modern-day swampy Coastal Plain being much less forested and windy enough to be covered with migrating sand dunes, but high-resolution lidar imagery shows just how widespread these features once were. A particularly beautiful set of dunes is visible with lidar imagery on the east bank of the Ohoopee River west of Metter, Georgia, just north of Interstate 16.

Wind direction was from left to right (west to east), and the sand in the dunes was derived from sandy coastal plain sediments exposed in the bluff carved by the Ohoopee River. Dunes like these are interesting landforms to examine with lidar because their shape implies the material movement that formed them–it’s easy enough to imagine wind sweeping the sand across the landscape. An intersting detail of these ancient dunes is how their sandy soil controls agriculture. Note how the bright, sandy soil is visible in places where farmed trees have been harvested in the image above. I’m not exactly sure why, but tree farming closely follows the boundaries of the old dunes very closely, as seen below.

Looking for features like these dunes is a good way to train for pattern recognition in lidar evaluation, as the dunes are not as conspicuous when the observer is “zoomed out.” Once identified, however, the dunes are quite distinct from background erosional features (stream channels, etc.) and are consistent in their appearance within the region.

Interpreting landforms like the dunes with lidar imagery also requires understanding of modern-day, active features whose origin and evolution is known. There are plenty of modern dune features to see in Google Earth, and (unsurprisingly) they bear a striking resemblance to the Pleistocene Ohoopee dunes. The dunes shown below are a nice example; they have developed above a river bluff in northern Alaska.

Interestingly, modern-day northern Alaska has been suggested as a good indicator of what the Pleistocene Coastal Plain landscape was like. These Alaska dunes are located in a landscape full of elliptical, wind-sculpted lakes that are a dead ringer for Carolina Bays as well. Carolina Bays and modern-day landscape comparisons need their own post, but the similarities between these Alaskan features and many aspects of the southern Coastal Plain landscape are almost uncanny. In the image below, the yellow arrow points to the river bluff dunes shown above. The swarms of elliptical lakes are obvious enough. Lakes near the lower right edge of the image are partially covered in ice.

Dune development is a product of the laws of physics and the materials involved, not latitude or geography, and good examples along topographic bluffs can be found throughout the modern world. The coast of central Brazil has some outstanding dune fields that are useful in identifying and interpreting old, vegetated features like the Ohoopee dunes.

The island of K’gari in Queensland, Australia, is another good example.

The consistency of dune shape between all of these locations is a nice example of patterns and their recognition in the Earth system. By thinking about landforms as expressions of a given process, anyone interested in geology can work logically towards understanding why a particular location looks like it does. Pattern recognition is a good way to link the study of older, sometimes partially preserved landforms with the modern landscape to allow accurate interpretation of features that might otherwise be mystifying. After looking at the images in this post, some landforms in the lidar image below might be more recognizable.

This image is from Lee County, South Carolina, just north of Lee State Park. The Lynches River (left side of the image) carved bluffs into sandy Coastal Plain sediments, which provided a sand source for Pleistocene winds. These dune features are analogous to the Ohoopee River dunes, which are quite far away but located in a comparable geologic setting under the same Pleistocene climate regime. An observer need not be an expert in South Carolina geology to interpret this lidar image; understanding material movement and dune development in the Earth system is far more useful.

A closer look at the Lee County image reveals hundreds of small, elliptical, and very faint “pock marks,” with a few larger ones mixed in. These are Carolina Bays, which remain “officially” unexplained. Take a look back at the second Alaska image and see what you think in terms of pattern recognition…

by Philip S. Prince

Landslides occur regularly along the rugged Blue Ridge Escarpment in western North Carolina, and lidar imagery provides a way to easily view the accumulated scars of these events that are preserved in the landscape. With a high quality (0.5-meter resolution) lidar dataset, it’s hard to examine much of the Escarpment without seeing a few landslide features. Some slopes have more than others, however, and I think the McDowell County slope shown below probably has the greatest density of slides I have seen in the last 3 years of mapping. The transparent red areas cover definite (or very high confidence) slide features. The yellow arrow points to an active railroad line to provide a sense of scale…possible slides on railroad cut slopes or embankments are not marked.

The area shown above is surprisingly small to host so many slides. The area with marked slides covers about 0.3 square miles, or 0.8 square kilometers (204 acres or 85 hectares); this excludes the distant slopes seen at upper left, where no slides are marked. Like so much of Appalachia, the slides are not visible without lidar imagery. Despite their crisp outlines, many are cut by undamaged logging road grades, suggesting the slides are many decades old. The GIF below compares aerial imagery, unmarked lidar slopeshade, and marked lidar slopeshade.

Many of the slides occur in clusters and are not significantly displaced. The cluster of slides above right center and below the railroad grade is worth a look in detail.

The images shown above are captured from kmz files I made, and a zoomed-out view provides some larger scale context. The previous images are from area A1.

Area A2 shows comparable slide density, with numerous minimally displaced, largely intact slides. The slide indicated by the yellow arrow is 130 ft (40 m) across. The road grades are single-track logging roads. Notably, they are not offset by the upper two slides…this is more clear in the lower, unmarked image.

Similar landslide density is visible in area B1. The style of the slides is somewhat different–many are small “blowout”-type slides. This difference in style may result from the different aspect of the slope with respect to underlying bedrock structures.

The central portion of the image above provides a nice view of the contrast in slide style and the resulting traces in the landscape.

Both areas B1 and B2 host a very high number of slides. Again, the railroad grade at the base of the image provides some sense of scale. None of the slides marked in the image below are seen to offset any of the logging road grades, despite the crisp, well-defined appearance of the slides.

Area B3, just over the ridge (to the left) of B2 above, is also a mess. The arrow points to the railroad line for reference.

Area B3 is distinct in terms of its aspect and its relationship to bedrock structure. Dip of bedrock foliation at B3 is significantly less steep than the slopes above the railroad line, creating cliffs and ledges. Lidar imagery suggests that large amounts of colluvium and talus have accumulated below these ledges. Failure in colluvial deposits is widespread in the region, particularly as the deposits weather, so a slope like this is certainly expected to show evidence of failure, regardless of land use or construction.

An overall view looking north across area B shows slide distribution across the face of the Escarpment towards the valley at upper right. This isn’t a perfect labeling of each and every slide, but it’s pretty good. Slide density does decline significantly below the railroad grade and is much less on the distant ridge at top and right.

So, why have so many landslides occurred on these slopes? The most likely explanation is an extreme precipitation event. This area was heavily impacted in both 1916 and 1940 by extreme rainfall events, and the clustering of slides and their lack of damage to mid-20th century road grades would fit a connection to either of the storms nicely. These McDowell County slopes also resemble slopes in the headwaters of the Moorman’s River west of Charlottesville, Virginia, which experienced numerous slides due to an exceptional thunderstorm event in 1995. The two Moorman’s River slopes shown below (in lidar hillshade instead of slopeshade) reveal numerous slides that cluster in a similar fashion to the McDowell County examples. As usual, single-track logging roads provide a sense of scale.

Details of bedrock geology, colluvial or residual soil development, shallow groundwater, and land use may also control the McDowell County slide clusters. Because of the area’s history of extreme rainfall events and land use, many variables may have worked together to produce the high density of landslides. They may not have occurred together, either, although their comparable preservation and pre-logging road development suggests that they could have. This area will received detailed landslide mapping in the coming months, and field investigation may shed more light into the details of the slides’ setting. Until then, just knowing what the land surface looks like and what sort of slope behavior is possible in the area is useful and interesting.

by Philip S. Prince

Lidar-derived imagery is a remarkable tool for showing how rock, soil, and woody debris moved over the landscape during old debris flow events. Understanding how this material moves is a key aspect of understanding present-day debris flow hazard. Lidar imagery provides a way to track downslope material movement of old flows that is otherwise difficult or impossible to see in the field, which is particularly significant in forested Appalachia. This post highlights some interesting debris flow styles and paths now hidden by vegetation in Pisgah National Forest in Transylvania County, North Carolina. I have tried to label relevant features without obscuring details of the deposits, which are quite subtle but notably distinct from the rest of the landscape once you see them. The GIF below uses a transparent polygon to highlight a flow path and deposit on the north side of the Davidson River southwest of Brevard, North Carolina. This one is a good starter, as details of the initial failure and subsequent flow path are reasonably easy to pick out.

This flow appears to have resulted from the partial breakup of an intact landslide block, much of which did not flow and remains clearly visible as a detached rectangular feature on the slope. For a sense of scale, the intact rectangular block is about 200 ft (60 m) across. Due to the shape of the landscape, the debris flow resulting from breakup of part of the slide block quickly reached the smooth but sloping valley floor, where it spread out before re-channelizing toward the bottom of the image. The flow had sufficient momentum to superelevate (move at an angle to the slope instead of straight down the slope) and bend away from the faint upper channel before the material turned back towards the channel. A faint dashed yellow line highlights the curve in the levee-like deposit edge as it bends back onto a direct-downslope path.

Superelevated debris extends about 100 ft (30m) across the ground surface from the faint upper channel, which is highlighted by the blue line.

A head-on view (below) gives more context to the superelevated debris, which moved oblique to the slope of the valley before losing momentum and following the slope. The second image offers a basic illustration of how the debris “banked” its way downslope.

Lobes and sheets of deposited material, along with one large boulder, are visible further downslope. This slide-to-flow was obviously quite fluid and mobile despite the modest relief between the failure point and the sloping valley bottom. After traveling overland in the area shown above, portions of the flow re-channelized and continued downslope. The deposit terminates at a ground length distance of 1.180 ft (360 m) from the failure point, having descended about 265 ft (80 m). The transparent polygon in the image below covers the full extent of the deposited material visible in lidar.

2.2 miles (1.3 km) to the east, another subtle debris flow deposit is visible spreading across a gently sloping valley floor. The GIF below shows the extent of the deposit, which resulted from a flow originating in a small channel upslope. No clear initiation zone is visible above the head of the channel, suggesting the flow began within colluvium stored in the small channel itself. The deposit highlighted by the transparent polygon is about 280 ft (85 m) wide at its widest.

This flow also occurred in reasonably modest topography, descending about 230 ft (70 m) over a ground length of 1,000 ft (~300 m) from the apparent initiation zone to the toe of the deposit. Near the upper end of the polygon, a line of superelevated debris is visible on the otherwise smooth hillslope. This material was likely deflected onto the slope by the slight bend in the to the left of the superelevated debris as shown in the image below.

While the deposit from this flow is more subtle than the previous slide-to-flow feature, it is definitely visible once you know it’s there. The lowermost portions of the deposit stand out nicely against the comparatively smooth valley floor. Much of this deposit is draped across a faintly convex area, which is visible just below the logging road switchback that cuts into (and obviously postdates) the deposit.

The leveed channel at the tip of the deposit is particularly interesting, with the outlet for fines and flowing water clearly visible.

About 6 miles (~10 km) southwest, a comparatively complex debris flow deposit can be seen on a slightly larger and more rugged hillslope. This deposit spilled out of the narrow, shallow “trough” that steered it in two places, though the bulk of the flow reached the stream below after traveling a ground length of ~1,200 ft (~364 m). The flow descended ~380 ft (115 m).

The “spillover” zones result from the unusual topographic shape of the hillslope, where three faint channel heads are located in close proximity to one another. This failure was also quite voluminous; the failure scars are about 25 feet (just over 7 m) deep and 260 ft (80 m) wide. An oblique image (below) puts both “spillovers” into topographic context. The second image uses yellow arrows to give a sense of material movement in this failure. I have actually visited this feature in the field, and the spillover material would not be noticeable without lidar-derived imagery of this (0.5-meter) resolution.

These debris flows are particularly interesting to study because they are distinct from many other debris flows in the region, whose downslope travel is strongly controlled by topographic channels along much of their length. The flows shown here experienced less topographic control and presumably less “bulking up” by collecting soil, rock, and wood along their paths. Despite their lack of extended channelize flow paths, the spread of these flows, along with their mobility in close proximity to their initiation zones, provides an interesting glimpse into the physical dimensions of possible impact zones related to this type of slope failure. The age of these failures is unknown, but they likely occurred in 1916 during an extreme tropical precipitation event in the area. They are completely obscured by forest cover today, and the information about slope failure effects that they preserve would be completely lost without this lidar data set!

by Philip S. Prince

At first glance, the Altoona Coal Mine of Pulaski County, Virginia, seems to be in just the right place. Coal beds in its Little Walker Mountain setting occur just up-section and downhill of a prominent sandstone bed, which forms an obvious “rib” on the slopes of the mountain. The mine was opened right at the base of this rib. This must have appeared to be a great location, as it took advantage of a stream valley to ease construction of a tram railway to the mine. Prospect pits and mine adits leading up to the Altoona’s location are visible in the GIF below. The rib is marked with a pink overlay, with mine workings visible as tiny dots to its right; the image that follows the GIF is a detail of mine workings adjacent to the rib.

Despite its apparently good location, all was not well at the Altoona Mine. Coal seams in the mine were too distorted and mixed with surrounding rock to be easily extracted, leading to its ultimate failure. Early 20th century geologist Marius Campbell addresses this issue at length in the 1925 report The Valley Coal Fields of Virginia, twice calling Altoona’s location “the worst” in the general area and the obvious reason for the mine’s closure.

Campbell connected the geologic issues in the mine with its position near a complex ridge pattern that implied large-scale folding in the underlying strata. His focus was on the apparent convergence of Little Walker Mountain and Tract Mountain southwest of the mine’s location. This pattern is visible in Google Earth satellite imagery, and is also quite noticeable from ground level near the mine. Little Walker Mountain, which is typically an arrow-straight ridge, is noticeably buckled in the vicinity of the mine, and Campbell may have seen this feature as evidence of rock deformation associated with the Little Walker-Tract Mountain structural setting.

Campbell was definitely onto something, and he was certainly correct that weak rocks, like coal, focus deformation when surrounded by stronger material like sandstone or limestone. He attributed rock distortion at the mine to “cross-folding,” which appears to be his term for the structures that create the locally buckled appearance of Little Walker Mountain. As it turns out, Campbell wasn’t exactly correct, but accurate interpretation of the area is essentially impossible from field data alone due to limited outcrop. Additionally, no high-altitude imagery was available at the time, preventing Campbell from having a Google Earth-style, satellite-level view of the mine’s setting. Today, lidar-derived imagery quickly reveals the true structural issue at the mine, along with the large-scale context of the distorted coal beds.

A close look at a 1-meter lidar hillshade image reveals that the Altoona Mine is indeed positioned exactly in a fold hinge. This fold structure is more significant and affects a greater thickness of rocks than Campbell’s “cross fold” concept suggested (more on this below), but it is still subtly expressed, even in the high-resolution lidar imagery. The GIF below overlays color bands onto distinct rock units to highlight the fold, which is very tight and probably faulted. I think the fold is most visible just below and left of center, where the purple and blue bands meet in a V-shaped point. From here, the faint layer patterns can be followed towards the mine. Layers young towards the right; purple covers Devonian-aged rocks, while the mine is developed into Lower Mississippian-aged rocks.

The significant folding near and left of image center appears to die out in the vicinity of the mine, where the sandstone rib that highlights the coal location does not immediately appear offset. An even closer look suggests that the rib is deflected to the observer’s left, indicating that the rib layer and coal-bearing interval are either folded into a very tight syncline or repeated by a minor thrust fault. The mine is located nearly atop this highly distorted area, which nicely explains the messy condition of its coal beds.

Based on these lidar-derived images and my own experience in the area, the structure immediately adjacent to the mine would not be decipherable through field work. Sedimentary rock layers visible in the lidar imagery do not actually produce much surface outcrop; they only create faint bulges on the land surface that require a 1-meter resolution (or better) digital elevation model to see. The fine structural details at the mine are still not completely clear in the imagery, but I would offer the two conceptual models below as explanations of the folding and faulting that produces the observed surface pattern. They are very similar (differing only in terms of a small fault vs. a fold in the mine area), and both would produce considerable distortion of the coal-bearing horizons. The models use a wedge-type geometry instead of a single fault cutting all the way through the affected section. This is a way of reducing apparent offset of the pink sandstone marker bed, which is not obviously deflected to a ground observer or coarse-resolution map user.

Both models focus shortening into the coal zone and would result in thickened, duplexed coal seams. Thickening of coal beds is one of the details reported by Campbell, with the main coal bed in the mine reaching 125 feet (~38 meters) thick in some areas despite an average thickness of 7 feet (just over 2 meters).

In addition to locally thickening the coal, the tightness of small scale folding in the mine, along with possible faulting, would have broken surrounding sandstone layers and forced pods of sandstone into the more ductile coal. Coal beds in the mine would thus have been observed to be locally atypically thick or atypically thin, and full of fragments of surrounding non-coal rock. As a result, the mine would have been nearly impossible to develop efficiently.

As mentioned earlier, folding around the Altoona mine is not the result of the mine being between Little Walker and Tract Mountains. In actuality, the fold on which the mine is situated has developed along one of at least two thrust fault structures that climb out of the Devonian-aged rocks into the coal-bearing Mississippian aged rocks at the mine. These faults produce the bends or buckles in the Little Walker Mountain ridge and may have accommodated displacement as bedding plane faults in the Mississippian strata, either in the coals themselves or in the overlying Macrady shale. I highly doubt these larger faults would be easily field-mapped either, due to the difficulty of finding meaningful bedrock outcrop where the faults climb upward into the Mississippian layers. Where the faults are operating as detachments or flats within a single stratigraphic interval, they are not readily identifiable even when outcrop is present. The GIF below traces these faults relative to the mine’s location.

While coal extraction was not particuarly successful at the Altoona Mine (and never will be!), the mine’s coal issues highlight a structural scenario that is valuable to structural interpretations of the surrounding areas. Cove and Tract Mountains, the respective left and right elliptical ridges south of the mine, are part of a large structural mass that is difficult to relate to neighboring thrust sheets. Cove and Tract Mountain (as well as Draper Mountain to the south) reached their structural position on what should have been a high-displacement thrust fault, but evidence suggesting this high displacement is hard to find.

Activity on the detachment fault that terminates around the Altoona Mine could help explain some of the “missing” fault displacement, particularly where the additional fault to its north is considered. These otherwise subtle features would largely escape consideration were they not connected to a conspicuously bad coal mine, making the Altoona a geological success long after its closure!

by Philip S. Prince

Earlier this week, I came across a recent article from Kathy N. Ross of The Mountaineer newspaper (Waynesville, NC) describing the “mystery” of Split Mountain, a spur ridge rising above the Fines Creek Community in Haywood County, North Carolina. The article is linked here, and is worth a read in conjunction with this post!

The “mystery” specifically refers to episodes of falling rock, formation of lumpy “hillocks” on previously smooth slopes, split and tilted trees, and cracked ground that gave the mountain its name in the mid-19th century. Statesman/geologist/famous North Carolinian Thomas L. Clingman visited Split Mountain three times between 1848 and 1867, describing impressive sights that significantly altered the appearance of the mountainside and conjured up suggestions of an “upheaval” event. Interestingly, none of the features Clingman described are readily apparent today, allowing the mystery to persist.

As a geologist working with landslides in western North Carolina, I found Clingman’s accounts to be clear but unintentional descriptions of a large translational landslide on a forested slope. Given the apparent size of the feature and the extremely high resolution lidar imagery I have access to, I thought it might be possible to see the feature(s) Clingman described and give them a bit more context.

Sure enough, the south side of Split Mountain is home to a significant landslide that could indeed host many of the details that caught Clingman’s eye during his visits. The GIF below shows a slope-shade image of the feature along with aerial photography of the site. The slide is entirely re-forested, and no details of the slide-related landforms would be visible from the valley below. In fact, without the lidar dataset, this slide would go likely completely unnoticed today unless a specifically trained geologist looking for landslide topography happened to hike directly across it in just the right place!

Clingman’s observations are readily understandable in the context of landslides, and I have tried to connect them with details of the slide that are visible in lidar imagery and comparable features on a modern slide in the region. In addition to Kathy Ross’ article, a 2008 blog post (author unknown) offers a few more of Clingman’s words that contribute nicely to visualization of the Split Mountain slide. While I cannot, of course, be absolutely sure that what I am looking at is what Clingman visited, I think the chances are pretty good. The only notable disagreement between Clingman’s description and the lidar-visible slide is its size (see below), but I suppose one might expect a bit of exaggeration or at least inaccurate estimation on a forested slope.

First and foremost, the slide is located on the south side of the ridge, where Clingman said the most striking “evidence of violence” could be seen. The slide is oriented close enough to the north-south trend Clingman described, and, most significantly, it has deflected the stream at the base of the hillside.

The slide is quite elongated and mostly translational in its movement, which would have produced long, crisp lateral scarps early in its development.

A lateral scarp in its early stages of development could be the “crack not an inch wide” that “could be traced for a hundred yards.” The photograph below shows an example of this type of lateral scarp-associated crack in Rutherford County, North Carolina, with Appalachian Landslide Consultants geologist Ken Gillon standing across it.

The elongated crack Clingman described split a large poplar tree, leaving one half remaining standing. Trees can certainly be split by landslide movement, as indicated by the image below. Note that this image is on an extensional headscarp. Presumably, a similar effect could develop along a lateral scarp, particularly in the presence of extremely large trees, which are often absent from modern Haywood County’s slopes after more than a century of timber harvesting.

Despite largely translational motion, the slide appears to be thick enough to have experienced back-rotation of blocks near its head. Rotated blocks would nicely fit the description of the “hillocks” with tilted saplings that were well established prior to the tilting.

Block rotation would also create depressions on the hillside, which Clingman referenced on one of his later visits to the site. The conceptual cross section drawing below illustrates a generally translational slide whose main failure surface is roughly slope parallel, but is much steeper at its uppermost breakaway point due to the thickness of the slide. As portions of the slide mass that started above the steep failure surface move onto its less steep portions, they back-rotate and tilt the original ground surface to a flatter slope, often sufficiently to create surface depressions.

The image below, from the same Rutherford County slide that hosts the lateral crack, shows a large depression with adjacent rotated blocks, whose movement has caused tilting of a large tree at left. These landforms are typical of hummocky topography observed in the upper extensional zones of large slides. Perhaps Clingman saw a scene like this at Split Mountain?

Compressional toe features on blocks within a slide, or at its downslope toe, can also systematically tilt trees as shown in the image below. This photo was taken at the downslope, compressional end (toe) of the Rutherford County slide.

A large boulder seems to be present near the head of the Split Mountain slide, consistent with one of the features which most impressed Clingman. The source of the boulder is unclear, but its location near the head of the slide puts it in a good location to have interacted with “chasms two or three feet in width,” which were probably tension cracks in the upper, extensional portion of the slide.

Clingman described the slide area as having prolonged activity, with rockfall occurring episodically for years before the larger events. Cliffs above the slide are strongly fracture- or joint-influenced, and are certainly a good potential boulder source. I think the shape of the outcrops of the slope suggests a dip slope geometry, which would further favor rockfall (and could explain the slide itself). Early rockfall deposits may have been subsequently transported by later sliding or buried by later addition of material; Clingman references the continually changing appearance of the slide, indicating ongoing activity. The hillshade image below shows some of the boulder locations with a bit more detail than the slope-shade imagery.

The north side of Split Mountain also hosts extensive boulder deposits that are easily visible in lidar-derived imagery. Estimating the relative age of the deposits with respect to the slide on the south side of the mountain is impossible, but the extent of the deposits and the appearance of their source outcrops at the ridge crest, rockfall has been active for some time. Very large boulders are occasionally involved, as evidenced by the example below.

Perhaps the most interesting question relating to Split Mountain is whether the “shocks” referenced by Clingman and local residents describe the slide events themselves, which appear to have been surprisingly rapid for a somewhat intact slide, or small earthquakes that led to the slide events. The excerpted text I have read is not clear, but it at least suggests that earthquakes large enough to be felt by residents in the area might have preceded or actually caused the slides. Kathy Ross’ article references other accounts of tremors and “cracked mountains” southwest of Split Mountain, but these, too, may have been mistranslated over the years. Split Mountain also invites comparison to the 1874 events at Rumbling Bald in Rutherford County, where seismic events followed by or associated with slope movements appear to have occurred.

From my own perspective, I am interested by the lidar appearance of the Split Mountain slide with respect to its age, assuming it is the right one. Estimating landslide age in the field and remotely via lidar is tough, and is ultimately a somewhat subjective judgment call. A field visit to this slide would be very useful, particularly in terms of seeing how scarps and cracks degrade over a known time interval (check out the last image).

Clingman’s observations also highlight the importance of qualitative understanding of mass movement and its consequences. He probably never watched any sort of landslide happen (or slowly develop), which is certainly a hit-or-miss game in Appalachia today. Even if he had interacted with some sort of large slide event, it would have been almost impossible to have a vantage point to watch the movement and its effects on the slide mass. He also had no way to view the slide from above or without considerable obstruction by trees and vegetation (enter: lidar). As a result, he attributed the surface features he saw to a force pushing up from below, possibly in the way soil expands and cracks when lifted upward by a shovel. He ultimately settled on the expansion of subterranean gases as the cause of the Split Mountain events. Indeed, vertical offset along extensional scarps could give the impression of “upheaval” instead of rotation or relative subsidence, as evidenced by the photo below from the Rutherford County reference landslide.

Today, the perspective provided by lidar-derived imagery, an endless supply of YouTube videos, and other approaches such as physical modeling allow geologists to become familiar with the sometimes surprising types of landforms that develop due to slope movement. The small physical models shown below illustrate ground slope tilting and the complex fracture patterns that can develop in the extensional portions of largely intact landslide masses.

In the Appalachians, this basic understanding of landslide-derived landforms is particularly important, as heavy vegetative cover makes interpretation of landslide-associated topography even more difficult. Clingman’s account clearly shows he faced the vegetation challenge on his visit to Split Mountain, but he was able to use tree deformation to his advantage in describing the details of ground movement he observed. While he may have missed the mark on his interpretation of the features, his attention to detail in recording the relevant features of the landscape was right on target, and his account was immediately understandable to a geologist in 2021, over 150 years after he wrote it!

by Philip S. Prince

Lidar-derived imagery reveals many fascinating details of Appalachian landscapes, some of which are initially a bit puzzling. Fortunately, if the imagery is sufficiently detailed, it can often provide a rational explanation for the puzzling features. Landslides in the early stages of movement (or slides that have experienced very little movement before becoming dormant) often generate unusual small-scale landforms like the examples in this post. While these features would be noticeable to a geologist on the ground, they are certainly best appreciated (and explained) with the help of 1-meter lidar data! These sites are all located in Randolph County, West Virginia, and would likely be nearly unnoticeable in lower-resolution digital topography data sets.

The mountainside shown below hosts several elongated trenches or gashes that are plainly visible in lidar hillshade imagery. A matching air photo image is shown for comparison. The trenches are nearly invisible in the air photo, and while they would be noticeable on the ground, their size (up to 200 ft or 60 m long) and the forest cover would likely obscure their context.

The trenches resemble mining prospects that are common throughout Appalachia, but this zone of Appalachian stratigraphy (Foreknobs Formation; upper Devonian) is not a mineral or coal producer. The trenches also do not cut perpendicular to bedrock strike, a common prospect orientation to expose numerous bedrock units in a single excavation. The trenches don’t follow the typical prospect rules because they are actually small “pull-aparts” along the edges of a large, thin blockslide (a type of landslide defined by a moving, intact mass of stratigraphy). The blockslide has only experienced a very small amount of downslope movement. The first image below shows the entire slide covered by a transparent polygon. The second image points out the laterals (edges) of the slide as well as its toe, whose slight movement has deflected the stream at the base of the slope. The laterals are very subtle and would likely be challenging to identify in the field on the heavily forested slope.

The “pull-aparts” are open gaps between the slide mass and intact slope. They are almost certainly filled with soil and organic debris to some extent, but do not appear to be full grabens, or downthrown blocks, of the slide mass. Presumably, grabens have not formed because the sliding mass is quite thin and the cohesion of the rock composing the sliding layer prevents the gaps’ edges from collapsing. The pull-aparts have formed at stepovers on the left lateral failure surface of the slide, and are thus a result of the shape of the slide mass and its direction of motion.

The stepovers and overall angular outline of the slide results from its development within highly jointed or fractured sandstone. As strength of the underlying shale layers is exceeded by the weight and downslope force of the sliding layer(s), the slide mass preferentially disconnects along existing weaknesses in the rock. I have not visited this slide and have no idea of its movement history, but logging roads cutting across it are not disrupted. Material stored in the pull-aparts or in uplifted stream sediments at the toe could provide some insight into when the slide was active.

A few miles to the northeast on the same ridge face (and thus the same stratigraphic interval), an unusual vein-like pattern of tiny ridges is visible on the otherwise smooth slope. The “veins” extend 1,400 ft (430 m) along the face of the ridge, and are 40-60 ft (12-18 m) across.

Like the pull-aparts in the previous example, the “vein” ridges would likely be noticeable in the field, but are only faintly visible in the zoomed-in air photo image below.

The “veins” are actually compressional folds developed within a very thin blockslide. Given the width of the veins, the sliding sheet of rock is probably 20-30 feet (6-9 m) thick or so, although this estimate depends of the geometry of the folds. Lidar is again beneficial in interpretation, as it reveals subtle gaps at the upslope end of the slide mass that result from slide movement.

Compressional folding of this type is actually observed in other blockslides in the Appalachian Valley and Ridge, where sliding sandstone layers are so intensely fractured that they can fold at the surface without shattering (see this link). In this Randolph County example, the extremely narrow width of the folds and implied thinness of the slide mass is particularly interesting. The conceptual relationship between fold crest width and the thickness of the slide mass is illustrated below.

The very basic illustration above shows a simple hanging wall anticline above a thrust ramp at the toe of a blockslide. Many other possible fold and fault related geometries are possible, but almost all involve a fold width notably greater than the thickness of the sliding mass. Given that the slide is 1,400 ft (430 m) across and likely under 40 feet thick, it really is moving like a thin sheet of snow sliding on a windshield as portrayed in the link above. The diagram below is not scaled, but it offers a conceptual view of the slide geometry.

The diagram is broadly based on the three folds visible on the slide, along with the faint upslope gap.

Low-displacement blockslides like these dominate the southeast face of this ridge for about 15 miles (25 km) in Randolph County. While most are limited to the face of the dip slope, some failures are positioned in a way to affect the actual ridge crest, like the slide shown below. In the field, the effects of this slide would be apparent as an elongated depression along the ridge.

This slide has also experienced only a tiny amount of displacement, allowing the movement to be accommodated downslope without a defined toe.

Like so many older landslides in the Appalachians, the significance and cause of these features is unknown. Because they are so numerous and are only visible using lidar data acquired in 2016, they may represent an untapped resource of useful information about the recent history of Appalachian landscapes.