A couple hundred million years ago, the Blue Ridge Mountains may have been as tall as the Himalayas are now. Over the course of millions of years, they have been weathered and whittled into their familiar profile: slightly rocky, but mostly forest-green. Looking at the mountain range across our valley in September, 2004, I had an epiphany. Until that moment, I had thought of erosion as a gradual process that occurs due to the slowly accumulating effects of wind and rain over millions of years. But after the rain clouds of TS Frances cleared that fall day, revealing fresh scars on the mountains, I suddenly realized that much geologic change must occur on a timeline of punctuated equilibrium. Punctuated equilibrium is a theory, first introduced by Stephen Jay Gould and Niles Eldredge, positing that biological evolution is not a gradual process (as Darwin had originally theorized). Gould and Eldredge argued that species are fairly static and stable for thousands of years, until an environmental impact or a genetic shift forces an evolutionary shock upon a population. The radical and relatively rapid speciation (creation of new species) that happens in the aftermath of a mass extinction is the most extreme example of this, but it can happen on smaller, more local scales as well.
Until 2004, I thought of geologic change as a gradual process — that mountains slowly erode, raindrop by raindrop. But after witnessing the scale of disruption after Frances scraped the landscape, I suddenly realized that mountains are actually very stable for decades or centuries at a time, until a big storm comes, and causes ten thousand years worth of erosion in a matter of hours. This, of course, happened to an even greater extent last month when Helene pummeled the mountains.
The best views from our valley are of the eastern flank of the Black Mountains, the highest ridge east of the Mississippi. A number of bare rock faces are visible, especially between the 5000’ and 6000’ contours of the mountainside. Most of these “slides” date back to previous events. Folks who witnessed the 1977 flood say that several notable slides originated or were freshly scoured at that time. I’ve spent a few adventurous days with friends exploring the ones created by Frances (2004) and Fred (2021).
Because the bare rock faces are scars in an otherwise densely forested landscape, these high elevation slides are noticeable from miles away. Up close, they are equally impressive. These slabs of bedrock once had a thin covering – anywhere from a few inches to several feet – of soil and moss and trees sheathing them. When several acres of forest peel off a mountain, it can either pile up in an enormous logjam at the bottom of the steep slope, as mostly happened in 2004 and 2021, or it can continue flowing down a watershed, sometimes for miles, as some did during Helene. These slides begin when enough rain saturates the soil to the point of becoming a non-newtonian fluid. I imagine many of the slides are initially triggered by the toppling of a tree at the top of a steep slope, loosening enough soil mass to start a cascade of debris.
After Helene, at least one large new rock face appeared in the headwaters of Ayles Creek on the northeast flank of Celo Knob. (Ayles is the creek that flowed through Micaville School and destroyed most of downtown Micaville when it collided with Little Crabtree Creek right behind the post office). Helene moved a lot of earth in this valley, and the creek and river beds were all scoured, but most of the slides seem to have started at lower elevations (between 3000’ and 5000’). This means they are not easily visible from far away – you may not notice them until you are right on top of (or below) one. Some that I’ve observed, like the one that crossed Hwy 80 and knocked Jennie Boyd Bull’s house off its foundation, started on a steep escarpment below a natural ridge – in this case, Little Celo Mountain. Many other slides started where a roadbed cuts across the face of a hillside. When roads are cut into a mountain, it creates an artificially steep bank both above and below the roadbed. Many of the slides along Hwy 80, the Parkway, and other current roads and old logging roads, began on the steep bank under the roadbed, often tearing away a chunk of road in the process. It’s hard to say how many of these slides are anthropogenic and wouldn’t have occurred without the roadcut.
You may have heard people calling Helene a geological event. It was. The USGS has documented 2000 slides so far caused by Helene, and most of the ones that I’ve noticed are not yet on the database, so clearly that number will grow. Philip Prince, aka @TheGeomodels on Youtube, has done a number of videos about Helene’s debris flows. This video is a good overview of various landslides in Yancey and Buncombe; here is one about the Buck Creek slide that in my Day 18 post; and here is another about the deadly Garren Creek slides. Another youtuber, Mark Honeycutt, hikes up and shows drone footage of one of the Garren Creek slides in this video. These videos are long, but you can speed them up to 1.5-2x speed, and fast forward through some parts, to get through them quickly.
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