Rockfall at Big Green Mountain
Rock Cuts Loose from a “Solid” Wall,
May 24, 2021
Bill Jacobs, June 2021
At about noon on May 24, 2021, a large slab of rock fell 175′ or so from Panthertown’s Big Green Mountain into the forest at the base of the steep granite cliff known as the “Great Wall.” Before falling, the slab was, to all appearances, securely embedded as part of that cliff wall, and was on an established climbing route.
This note will explore the causes of the rockfall.
I am indebted to Corey M. Scheip, a geologist with the North Carolina Geological Survey’s Asheville Regional Office. Corey works in the NCGS Landslide Program, and visited the site a few days after the event — when he could still smell and taste the rock flour spread across the forest as the slab shattered. He provided some of the accompanying photographs (using a drone to get otherwise unobtainable perspectives), as well as thoughtful commentary about the rockfall.
Figure 1. Note that the source of the fallen slab is high above the tree tops at the base of the cliff. Also note the white abrasions resulting from falling rock impacting protrusions on the cliff face.
Drone-based image courtesy of Corey Scheip.
The fallen slab is an estimated 30 – 40 foot rectangle, with a thickness ranging from about 3 feet to an inch or so (thicker at the top and along the right side). Thankfully, no one was on the rock or at its base at the time of the fall. Nearby campers heard the fall, and initially thought it was a thunderclap. The descending slab struck lower portions of the wall at two places, and was broken into boulders that came to rest up to almost 100 feet from the wall’s base. The larger pieces toppled several mature trees. Other smaller rocks sprayed out like shrapnel, damaging and even impaling other trees. It appears that the fall was explosively sudden and occurred without warning.
Like much of Panthertown, Big Green is part of the Whiteside pluton, which intruded into the the Ashe Metamorphic country rock about 465 million years ago. It’s basically tough, resistant granite, and the exposures seem solid. Certainly rock climbers would consider it a more secure (albeit challenging) climbing surface than the layered, schisty rocks common in the surrounding country rock. To deal with the pluton’s steepness and scarcity of “features” to serve as handholds and footholds, climbers place bolts into the rock to attach their safety gear. The ability of granite to safely host those bolts, on a long-term basis, is hardly given a second thought.
But plutons can harbor a hidden weakness.
Figure 2. Setting for the rockfall. The Great Wall of Big Green Mountain is at image center-right, with the lower 100′ or so hidden by trees. The source of the rockfall is the light patch at image center.
How Does this Happen?
Because they form from largely homogenous masses of magma, plutons have fewer internal directional weaknesses than we find in the AMS country rock (which formed in an ocean as different types of sediment piled up in layers, later to be pushed out of that ocean, deeply buried, and heavily metamorphosed). But weaknesses or not, the plutons must fracture, because they expand as the pressure of miles of overlying rock is removed by erosion. Lacking multidirectional internal weaknesses to accommodate this expansion, they tend to fracture by a process known as “exfoliation” — think of it as onion-skin layers popping off the surface. The result is a rounding and smoothing of the surface (and thus fewer “features” for the climbers), and fractures that are parallel to the existing surface. The tendency of exfoliation to create and preserve rounded surfaces is reflected in the name given to mountains like Big Green (as well as nearby Little Green and Laurel Knob) — they are called “exfoliation domes.”
Now look at Figure 3. Note that the surface of the space from which the slab fell is essentially a smooth plane parallel to the outer surface of the wall. It’s where the exfoliation process generated a fracture that was hidden from view until the rock gave way.
Actually, the interesting question is not how such a slab could form and fall off the cliff, but rather why this one didn’t fall sooner.
The fracture may have been hidden from view, but it was not sealed from water, which intruded and caused the noticeable weathering and lichen/algae/soil formation shown in Figure 3, not only on the back surface but also along the top and sides. For comparison, note the bright white of freshly exposed Big Green pluton rubble shown in Figures 4 and 6. So what held the rock in place over the years (decades?) after the exfoliation fracture formed and water found a route into it?
Looking again at Figure 3, we see some clues in the white scarring of the voided area’s rear wall. These scars represent rock that has been freshly exposed, in one of two ways. One way would be if they are the remains of intact connections between the slab and the mountain, shattered when they collectively were no longer able to support the slab’s weight. Another possibility is that they formed when protrusions on one or the other of the two surfaces impacted the other surface as the slab slid out and fell. Either way, you get support (in one case direct, in the other in the form of friction between two irregular surfaces). But, whatever its source, the support is inevitably weakened by weathering, by freeze-thaw cycles, and by cycles of expansion and contraction due to exposure to the southwestern Sun. At some point, the pull of gravity exceeds the support, and the detached slab crashes down the mountain.
Rockfalls are inevitable, even on smooth, solid-looking exfoliation domes. To emphasize the point, the base of the Great Wall is littered with fallen boulders, some much larger than those let loose in the recent rockfall. The time span for these events is frequent and continuous on a geologic timescale, but infrequent on human timescales. One only can hope that they are sufficiently infrequent that future rockfalls do not catch rock climbers (or curious geologists) in their line of fire.
Figure 3. The source of the fallen slab. Note that the newly exposed surfaces are mostly weathered, indicating that the fractures (both at the rear and along the sides and top) have been present, and have been intruded by water, for the years (if not decades) required to produce the observed weathering and lichen/algae/soil formation. Dimensions of the voided area are estimated at roughly 30′ – 40′ tall, 30′ wide, and 3′ to 6″ deep.
Drone-based image courtesy of Corey Scheip.
Figure 4. Debris at the base of the rockfall
Figure 5. Looking up the Great Wall from the base of the rockfall. Note the surface scarring from impact of falling rocks. The source of the rockfall is out of sight at the top of the picture.
Figure 6. Amid the rubble at the base, the largest intact portion of the fallen slab, greater than 15′ in length. Medium-sized dog for scale.