The other day, I mentioned the lineated granite gneiss we saw when hiking in Hickory Nut Gorge State Park on Thanksgiving Day (hey, maybe it's not quite up to the standards of last Thanksgiving's hike, but I'm cool with that). The next day, we headed to Chimney Rock to check out the scene there.

Here's a Google Map of the site (satellite view):


...and a zoomed-in view where you can see the tan ellipse of Chimney Rock itself:


Chimney Rock is located right at the Blue Ridge mountain front, where the mountainous terrain underlain by Grenvillian basement complex (Mesoproterozoic) gives way to the multiple metamorphosed oceanic terranes of the Piedmont province (like the metavolcanics mentioned earlier this week). Here's a view east across this physiographic (and geologic) boundary:

This boundary is called the Brevard Zone, a fault/mylonite zone of complicated structure. I don't know much about the Brevard Zone, but those Carolinian geologists are all over it. It's something I'd like to learn more about. If you have any particular expertise to contribute, please leave a comment telling us more (and giving us outcrop recommendations!).

Here's the star attraction of Chimney Rock Park:

You can see from the bridge and the flag that it has been developed, and much of the park exhibits "improvements" from the natural state.

Before climbing up to the Rock, we decided to hike out to the waterfall upstream. Chimney Rock projects from the wall of a deeply incised canyon carved by the Broad River. A tributary of the Broad flows over the lip of the canyon, providing a lovely waterfall. This scenic location was the spot where they filmed the final scene in the movie version of Last of the Mohicans:


Here's a view across the gorge (from underneath an overhang), looking towards the north:


At the site of the waterfall, I was intrigued to note that the rocky walls were exhibiting "onion skin" weathering (exfoliation jointing) that in my experience is more typical of granites (say, like those in the Sierra Nevada):


Here's a smaller version of the same phenomenon: a flake parts with its source rock, leaving the source rock more spheroidal than it was before. Oak leaves provide a sense of scale:


The rock exposed in Chimney Rock Park is a gneiss. I didn't see any here that was noticably lineated, but it had a pronounced horizontal foliation. The rock varies quite a lot in its texture and degree of deformation. Here's some photos:


Penny for scale:


Penny for scale:


In places, the metamorphic foliation has been deformed. Mainly this is evidenced in charismatic, high-contrast folds, but there is also some small-scale faulting visible, and some boudinage. Here are some images of the folds:


Isoclinal fold. Penny for scale:


Penny for scale (bottom):








Lily points above her head at some parasitic folding:


Here, Lily appears to hold up a big ellipse with an axial ratio of 6 or 7:

This is a section through an isoclinal fold, so that the fold axis is transected once on the left and once (on a differently oriented surface) on the right.

Finally we approached Chimney Rock itself, a looming monolith whose presence was indicating by a loudly flapping flagpole. When a gust of wind came along, the sudden clatter of the flag whipping in the wind was quite disconcerting. From below, it sounded a lot like a rockfall had initiated somewhere up above us. Note how the shape of Chimney Rock appears to be a compromise between the ~horizontal fissility of the gneiss and the spheroidal weathering associated with exfoliation:


Little wooden walkways and staircases are draped all over the face of the mountain, including a catwalk out to Chimney Rock itself:


Atop Chimney Rock, we found these little holes which were filled with water. I forget the name of these things - can someone remind me in the comments section below?

Essentially what's going on here is a self-perpetuating focusing of weathering. A small initial divot in the rock face allows water to accumulate. That water facilitates additional weathering through freeze-thaw action and chemical breakdown of the minerals in the gneiss. This weathering enlarges the size of the depression, which allows more water to accumulate, which triggers more weathering. It's a nice example of a positive feedback loop: a small initial perturbation auto-catalyzes itself into a much larger final effect. I've seen similar structures atop many mountaintops.

Labels: blue ridge, metamorphism, movies, north carolina, piedmont, structure, travel, weathering