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The 1847 debris flow event in Clay County, North Carolina shows crazy storms aren’t a new thing for the region

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Seeing the results of past extreme storms in a region is an important part of understanding potential landscape behavior. Thomas L. Clingman (yes, that Clingman) provides an invaluable record of a couple of past storm events in his extensive writings about western North Carolina. His discussion of the results of a storm of July 7th, 1847, in Clay County is particularly interesting. The document is linked here, but the excerpt below contains the important parts found on page 76 of the document. Anyone that saw the effects of a Helene debris flow will quickly recognize exactly what Silas McDowell described to Clingman.

McDowell was very clearly describing debris flows to Clingman (not “waterspouts” as we use the term today), and Fires Creek Mountain was obviously hit with an impressive cluster of them during this storm. Because debris flows visibly scar the landscape, could a geologist still see the effects of the storm on Fires Creek Mountain nearly 180 years later? I took a stab at this a few months ago, and I was impressed at how well the combination of Clingman’s notes and 21st century lidar imagery worked together.

If you want to go looking for 180 year old debris flows, you need to know what sort of scars you might be looking for on mountainsides. An effective way to do this is to examine the lidar-visible scars of known debris flow events. The November 1977 storm produced quite a few debris flows southwest of Asheville in the Bent Creek area. These were captured in 1982 aerial photography when their scars were still visible in the forested landscape. These air photos can be matched to lidar imagery to directly confirm what debris flow scars look like in the landscape. The GIFs below show this process, with yellow arrows indicating the upslope starting points (initiation zones) of the debris flows.

Debris flows like these begin as an initial landslide, producing a distinct scar in the landscape with steep, sharp edges where the slide began. This scar transitions into a visibly scoured track where the debris flow rakes saturated soil from the slope in its path, adding to the flow’s mass. The GIF below gives a general idea of this starting process and the scar that it makes, from the initial slide to the early phases of scour.

Within the grayscale world of lidar-derived hillshade imagery, debris flow scars can be distinguished from small, water-carved channels due to the effects of the initial sliding and subsequent scouring processes. Fluidized landslides similar to debris flows, but without the long, scoured track (referred to as “blowouts”), also produce distinct scars. The sketch below offers a basic summary of what you’re after as you peruse lidar imagery.

So, with this knowledge in mind, it’s time to take a look at Fires Creek Mountain. Clingman’s geography and distance estimates are always excellent, so I drew a line four miles due north from the Fort Hembree historical marker’s general area. The line met the top of Fires Creek Mountain at about 3.9 miles in a debris flow-susceptible area…not bad. The lidar imagery does the rest.

There are indeed an impressive number of debris flow scars on Fires Creek Mountain at the top end of the black line, exactly where they should be. Their appearance is unmistakable, and they distinctly cluster within a limited area as Clingman notes in his report. Yellow arrows point them out below; many more are visible in the zoomed-out view that follows. They aren’t labeled in the large view–see how many you can find.

The scars are well-preserved and look like the 1977 scars, though a bit worse for wear after 180 years.

This 1847 storm was likely an isolated storm “stuck” on the mountain ridges, given the limited size of the most intense debris flow activity. Clingman references observers in the valley below watching the storm on the mountain but not experiencing the intense precipitation themselves. Had this storm happened today (or in the era of easy photography, particularly aerial photography), it would have been intensely documented and firmly cemented in local lore. Today, it might make international headlines, particularly if someone caught a video of one of the debris flows reaching the valleys. How readily visible the debris flows and blowouts were from the valley below in 1847 is hard to know, but it was obviously eye-catching enough to have attracted attention and investigation from locals at the time. The images below show brown outlines over the visible features.

Extreme storms are likely to become more common in a warming atmosphere, but they’ve always been a part of the southern Appalachians. Helene’s impacts were exceedingly widespread, but more localized–and more intense–storms have happened in the past and will continue to happen in the future. It’s worthwhile to look back on these events and their impacts as a reminder that all sorts of things can happen in our region when conditions are right.

What happened to mountain slopes during Helene? Geologists are gathering information to prepare for the next storm

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The Toodies Creek debris flow in Yancey County tragically claimed the lives of residents living along the small stream which the debris flow followed.

In the nine months since Helene’s arrival in western North Carolina, geologists have worked steadily to better understand how to reduce future landslide-related impacts on life and property. While landslide themselves cannot be prevented from happening under extreme precipitation conditions, decision making during, and particularly before, a storm can save lives and reduce damage to infrastructure and personal property. Understanding what made certain landslides more damaging than others requires both extensive fieldwork and study of remote sensing data, like lidar imagery. ALC Principal Geologist Jennifer Bauer and Project Geologist Philip Prince recently presented some of the findings of their post-Helene work in US Geological Survey seminars. Video recordings of the talks are linked below. If you have ever wondered what a geologist sees in one of Helene’s thousands of landslide scars, these videos will give a glance of how we do our work day-to-day.

Jennifer Bauer

Jennifer’s talk focuses on the use of landslide mapping and modeling to understand and (more importantly) communicate landslide hazard before storms hit. Understanding landslide potential in a given landscape requires that geologists understand the landslide history of a landscape. Landslide inventories involving both lidar imagery analysis and lots of boots-on-the-ground fieldwork help geologists learn what has happened in the past.

Helene certainly was not the first landslide-producing storm in the western North Carolina Blue Ridge. Geologists study lidar imagery (left) and field-verify observations to determine where past slope failures occurred. This information illustrates the type of topography that may produce future slide events.

Once the geologic details that can produce landslides are understood (slope shape, slope steepness, soil type, etc.), models of potential landslide hazard zones can be developed. As Jennifer’s talk shows, the overwhelming majority of Helene’s landslides came from mapped hazard areas, but not every hazard area produced a landslide…this time. Hazard mapping can show mountain residents areas that are potentially dangerous is storms (don’t worry; it’s not everywhere-not even close) and help with decision making regarding where to live and how to prepare for the next big event.

Improving understanding of where debris flows will go as they move downslope–and how wide they may be–will be a focal point of landslide research in coming years. The teal color shows existing modeling of a debris flow path; the yellow outline shows the actual extent of the affected area.

Philip Prince

Philip’s talk is centered around the geologic details of Helene’s debris flow landslides. Debris flows are fast-moving, fluidized landslides that can travel long distances very quickly. Often called “mudslides,” debris flows actually carry huge amounts of rock and boulder debris and tremendous numbers of trees, so a debris flow impact is much more damaging than what might result from mud alone.

A debris flow conceptual model (top) attempts to illustrate the thin soil “birthplace” of many western North Carolina debris flows. Starting as small landslides which fluidize due to soil saturation, debris flows pick up more and more material moving downslope. The lower photography shows what a debris flow starting point (initiation zone) looks like in the field. Note the smooth bedrock surface exposed by the initial slide.

Philip illustrates where debris flows start in the landscape and how they accumulate so much material on their path downslope. A large debris flow could cover a football field with a few feet of mud, rocks and trees, but even small debris flows are surprisingly destructive. By understanding what type of geologic materials and slope settings produced debris flows during Helene, we can better understand what areas may be hazardous in the next event. Planners can also what parts of the landscape may be more susceptible to landslides when disturbed for building, as well as what areas at the foot of the mountains might be reached by debris flows.

This soil consisting of stacked, shingled rock fragments doesn’t look like much, but it is actually the material which slid to initiate one of the deadly Craigtown debris flows in Fairview, North Carolina. Understanding the distribution and behavior of soils like this is critical to improving understanding of potential debris flow behavior.

Is this the biggest boulder in the Appalachians?

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by Philip S. Prince

In Pisgah National Forest a bit northwest of John’s Rock, there’s a really big boulder in the woods. Like so many Appalachian geologic features, it looks really nice when viewed with lidar imagery. This boulder is particularly satisfying to look at because it sits alone on the floor of a small valley, and its point of origin on a cliff above is quite obvious. The boulder traveled about 800 feet to its resting place, and appears to have moved alone and not as part of a larger rockslide.

The boulder is about 130 feet long, 70 feet wide, and at least 20-30 feet thick, possibly more. It is composed of a granite-like rock, but it shows a different mineral alignment (a foliation) associated with metamorphosed rocks in the region. Despite its size, this boulder is not particularly dramatic when viewed from the ground. I tried to reproduce the ground-perspective lidar shot below during a field visit in December 2022. “A” and “B” label corresponding features of the boulder.

This boulder is actually so big that it’s hard to get a sense of its size from the ground. Enough soil has developed on parts of its surface to allow trees to grow, and the surrounding forest breaks up its outline. Adding a geologist stepping across a large crack in the boulder offers a bit of scale (lidar shot shows the crack’s location), but, ultimately, this thing is just too big to fully appreciate without a bird’s eye view.

This boulder is definitely a monster, but is it really something special within the Appalachians as a whole? I thought this was an interesting question, so I prowled a bunch of lidar in boulder-prone areas to see what I could find. Big boulders need a tough, resistant rock mass as a source, weaker rock downslope to allow them to be undercut and detached, widely-spaced fractures to permit detachment of big blocks, and enough steepness to allow them to move away from the source outcrop. These combined parameters narrow down big boulder areas, making portions of the Appalachian topographic Blue Ridge and the sandstone-capped Appalachian Plateau the best places to look. I think the southern parts of the Appalachian Plateau are better, as aggressive freeze-thaw processes during the Pleistocene likely increased fracture density to the north and reduced maximum free boulder size. The few examples below are my top contenders for biggest boulder after cruising a whole lot of lidar. I did not, of course, look everywhere, but the search produced interesting trends summed up at the end of the post.

A definite contender for Appalachia’s biggest boulder is Split Rock on the Blue Ridge Escarpment in Rutherford County, North Carolina (it’s on private property and can’t be freely accessed). Split Rock is about 150 feet long in its longest dimension, though splitting into three pieces has allowed it to spread a bit. Its proportions are actually quite comparable to the Pisgah boulder and are probably just about identical if Split Rock were “un-split” and reassembled.

Like the Pisgah boulder, Split Rock is too big to really appreciate from the ground and could be easily confused for in-place bedrock outcrop. The photo below shows the edge of Split Rock at one of the namesake splits; the lidar shot shows the photo location.

Split Rock is composed of metamorphic gneiss-like bedrock that is distinct from the granite-like rock of the Pisgah Boulder. Split Rock’s most impressive detail is that it did not completely fall apart on its trip downslope, as the rock is full of weaker mica-rich horizons along which it might break apart. Its variable compositional layering (which is a metamorphic foliation) gives it the ragged edges visible in the field photo above.

Several boulders in the 100 foot size range occur within Split Rock’s part of the Blue Ridge Escarpment, but the most prolific giant boulder province in southern Appalachia–and likely all of Appalachia–is the western edge of the Cumberland Plateau in Tennessee and Kentucky. Here, thick sandstone layers undamaged by thrust faulting and folding are well-suited to forming giant boulders. Folded and faulted layers in the Valley and Ridge contain too many fractures to make bigger boulders than the Plateau sandstones, and mica content and general weatherability in the Blue Ridge limit huge boulder potential outside of isolated extreme examples.

The slope above the Obey River shown below is a good example. The boulders looks small, but they are actually just shy of the size of the Pisgah boulder. Boulders this size are actually very common in this area, where soluble limestone beneath the sandstone caprock and a history of river incision set the stage for moving huge blocks downslope. The boulders shown below traveled as part of a larger landslide, but give the appearance of having moved independent of one another after the initial failure of the cliff line.

To the northeast, on the Big South Fork River near the Kentucky border, two large sandstone boulders detached from a cliff outcrop and plowed a clearly visible path downslope toward the river. the largest boulder, closest to the river, is about 115 feet long in its longest dimension.

A friend provide me with a photo of the 115 ft boulder, taken right at the end of the yellow leader line in the image above. Notably, these big sandstone boulders are not deeply buried in soil like the Pisgah boulder and Split Rock.

Further north in Kentucky, the Red River Gorge area is also full of ~120 foot sandstone boulders. The lidar shot below (taken from the Kentucky Geological Survey’s site) shows two 120-footers, indicated by red arrows. Boulders between 50 and 100 feet along their longest dimension are extremely common in this area. The joint-controlled, right-angle patterns in the cliff lines in this area are also interesting to check out.

The zoomed-in shot below shows the lower right boulder, which has traveled about 400 feet from the cliffline.

If you’ve paid attention to the measurements, 120 feet or so seems to be a common measurement for the biggest boulders to be seen in boulder-prone Appalachian landscapes. The Pisgah boulder and Split Rock aren’t alone in the southern Blue Ridge area; there are more 100-120 ft boulders to be found there, but they aren’t as numerous as boulders of that size in the Appalachian Plateau. In both areas, though, maximum size is intriguingly consistent, though only among boulders that have moved several boulder lengths from their source outcrops. Bigger chunks of rock can be found right along cliff lines, but these detached blocks have not actually moved or slid and experienced the associated physical forces. I imagine that the ~120 ft number is some reflection of the physical properties of the boulder-forming rock unit, both in terms of how it resists falling apart during sliding and how it controls topography to make cliffs and slopes to source and move big boulders. The image below compares sizes of the some of the boulders, with zoom adjusted so that it’s possible to directly compare them.

So, is the Pisgah boulder unique in any way? I think so. It is definitely at the top end of boulder size within Appalachia, though you can’t justifiably conclude it’s bigger than a reassembled Split Rock or something lurking in the Cumberland Plateau. It also traveled a significant distance from its source outcrop. This is notable because it contains aligned mica-rich layers that present plains of weakness along which it could break apart. The fact that the Pisgah boulder (and Split Rock) traveled hundreds of feet at their significant sizes and ended up in (mostly) one piece is impressive and probably the result of some amount of coincidence. Thick-bedded, very quartz-rich sandstone boulders lack aligned mica layers, so the potential to move a single huge piece of rock without breakup might be greater.

I don’t know how mechanical properties of the respective rock types would compare. Tensile strength of all Earth rocks is quite low compared to compressive strength, so rock doesn’t do well when forces try to pull it apart. Mica-rich zones or zones of extreme mineral weathering might lower tensile strength even more, so non-sandstone rocks might be less likely to hold together in single chunks than a physically hard and chemically tough quartz-rich sandstone. I also don’t know how these boulders move. I have always assumed that they slide, as tumbling would subject them to forces that would break them down to smaller pieces (that whole tensile strength thing, again). Check out what happens to the big block of rock in Switzerland shown below at this link.

Ultimately, geology superlatives (biggest, oldest, etc.) aren’t really worth much, but trends and patterns are useful in understanding how landscapes work. I looked at hundreds of big boulders, and not too much over 100 feet in the longest dimension is as big as you’ll see for a boulder that has moved significantly. This size is well-represented in certain sandstone-rich areas. Notably, sandstone-capped areas in Kentucky and Tennessee have, on average, bigger boulders than sandstone-capped portions of West Virginia. This may reflect local geologic details, climate and latitude, or both. The Blue Ridge serves up arguably the biggest boulders, though by an insignificant margin, and they are much less numerous due (presumably) to rock type details. Perhaps most interesting is that no one has ever seen a boulder the size of these biggies actually moving in the Appalachians, and none of them appear to be freshly emplaced. What makes them move and whether or not they do much moving under modern-day climate conditions is a big question in and of itself.