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

Thanksgiving Day, Lily and I took a hike in Hickory Nut Gorge State Park, North Carolina, just south of the much-better-known Chimney Rock, which was closed for the holiday.



Just outside the park, on the public right-of-way, I collected this lovely chunk of granite gneiss which shows both foliation and lineation:


This is classic Blue Ridge province basement rock; it formed ~1.1 or 1.2 billion years ago during the episode of mountain building known as the Grenvillian Orogeny. We've got many of the same sorts of rocks (though slightly younger) up in Shenandoah National Park in Virginia. However, the thing that caught my eye about this one is the fact that it has such well-developed lineation. You're probably already familiar with foliation, the planar alignment of mineral grains in many metamorphic rocks. Lineation, a linear alignment of mineral grains, is somewhat less common, as it requires a different sort of stress field to form. In the scanned image above, you're looking straight at the plane of foliation. Within that plane of foliation is the pronounced lineation, which indicates that the maximum principle stress was directed perpendicular to foliation (plane of the screen), an intermediate principle stress was directed left to right, and the minimum principle stress was directed top to bottom, which is why the gneiss squootched out in that direction*. Hemmed in from the sides, smushed from the front and back, it had nowhere to go but "up." The strain ellipsoid here would be shaped something like a flatworm, or a baguette that had been run over by a steamroller.

* I'm assuming a monoclinic stress field.

Labels: blue ridge, granite, igneous, metamorphism, north carolina, structure

The Duke Quarry in 1928, three years after its purchase. Image: Duke University Archives.

Duke University is a private university with a strong reputation. It is located in Durham, North Carolina. The campus has a distinctive "Old World" look with its gothic architecture, and many movies and television series have filmed there. Duke gets all its exterior building stone from a quarry near Hillsborough in Orange County, NC, that they purchased 85 years ago. Here's a Duke Magazine feature on the quarry: "By the numbers." This seems like a pretty clever move to me: they end up owning the source of their own stone, and that allows them to "brand" the rock used as unique to Duke. The university has found brick to be six times as cost-effective for modern campus additions, though they're still finding ways to integrate "Duke Stone" in those buildings too, as this nice little video explains (RealPlayer video).

After checking out other metavolcanics in the North Carolina Piedmont, Rob took us to see the Duke Quarry. It's pretty cool: a big operation, with lots of interesting rock about. Here's a view from one of the active walls, looking out over the semi-vegetated quarry (with Rob in the middle distance):


Piles of the rock await Duke architects and stonemasons:


The rock itself is a meta-volcanic assemblage, metamorphosed to phyllite. Original volcanic clasts can be seen, stretched out parallel to the foliation:

So the deal here is that these are ~andesitic island arc volcanics, originally erupted out in the Iapetus Ocean somewhere. Later, they were accreted to the edge of the Laurentian continent (ancestral North America). There, they were squished and squeezed during Appalachian mountain-building during the Paleozoic. I don't know any ages for this particular unit: if anyone can supply either a crystalization age or a metamorphic age, I'd be interested to hear it. Please leave it in the comments section below, along with any published references I should read.

The color of the clasts and the matrix varies quite a bit from one area of the quarry to another.


In a couple of places, I was delighted to find bedding/foliation relationships exposed. Here, you can see bedding as ~horizontal layers in the photo (look for the grain size change):


And here it is again, a bit more explicitly (you're looking at the plane of foliation):


A lovely rock. I want to learn more about it...

Labels: igneous, metamorphism, north carolina, piedmont, volcano

For me, Thanksgiving meant an opportunity for geologizing. The little lady and I headed down to western North Carolina to spend the holiday with her family, stopping along the way to visit with our friend Rob Greenberg in Chapel Hill. Rob took us out on a morning of geologizing, visiting various metavolcanic rocks in the Piedmont province. It was a nice sampling of what North Carolina has to offer geological visitors.

Pyrophyllite-rich metarhyolite at Eno River State Park:


Crystal-lithic meta-tuff alongside the road:


Greenstone (meta-basalt) with amygdules rich in chlorite and epidote:


More information on each of these rock units here. Many thanks to Rob for taking the three hours to show us these outcrops (and one more, which I will discuss in a different post).

Labels: igneous, metamorphism, north carolina, piedmont, volcano

(Parts 1, 2, 3, 4, & 5 of this series...)

As we were climbing up a steep snowfield, we saw something that made us rush up to the top:


Interpretive sketch:

At first, we thought this was a big isoclinal synform that was cross-cut by a ptygmatically*-folded granite dike, but closer inspection at the "axis" of the "fold" revealed that it was instead just the trailing edge of a big boudin. It pinched down and then swelled again in the downward direction, hidden in this photo by the snowpack. Not quite as cool... but still pretty cool. And I can never say no to ptygmatic* folding, regardless of the setting.

This is also kind of cool:

What you're looking at here is a gneiss, with alternating layers of coarse-grained mafic and felsic minerals. The view of the photo is orthogonal to the plane of foliation, but the boulder has been weathered so that in some places the uppermost mafic layers has been worn away. There's one spot where you can "see through" the mafic layer into the underlying felsic layer (upper right) and another spot where there's a little isolated scrap of the mafic layer where the surrounding material has been weathered away. This reminded me of a larger-scale phenomenon where the same thing happens to thrust sheets: an erosional hole through a thrust sheet into the rock beneath is a tectonic "window" or "fenster" (German for window). An erosional remnant of a thrust sheet is a "klippe." The Grandfather Mountain Window in North Carolina is an example of a fenster. Chief Mountain in Glacier National Park, Montana, is an example of a klippe. So this little boulder gives us a nice physical analogue for regional-scale tectonic/erosional features.

Ahh... what cool stuff to see and think about. But the sun was setting, and we had to head back to camp and the rest of our team... Tomorrow: the story of the long hike home.

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* Really, more of a "cuspate-lobate" fold, without the parallel limbs that make for a truely ptygmatic fold.

Labels: analogies, art, montana, national parks, north carolina, nova, structure, travel, wyoming

A reader of NOVA Geoblog forwarded me an announcement for a geology/education position at UNC-Chapel Hill, which led me to check out the rest of the UNC Geological Sciences website. (No, I'm not applying for the job -- quite happy where I am!)

I cited an important paper* by Allen Glazner in my geology master's thesis, which led me to poke around the author's website a bit. He has a nice collection of photos, including field work in the Sierra Nevada (and elsewhere).

One of my favorites is this awesome (and funny) shot of a shear zone. Check out the kinematics on that sucker! It's "textbook"!

Another is this mouthwatering fold.

There are also some great aerial shots featured. This series of the Deep Creek playa reminded me of a very cold night I spent camping in the Deep Springs Basin, then hiking out on the playa and finding a dead bat that had been mummified in the salt. Nice memories...

Anyhow, enjoy the whole series -- a pleasant way to while away fifteen minutes!

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* The article I cited was a really interesting one:

Glazner AF, Bartley JM, Coleman DS, Gray W, Taylor RZ (2004) "Are plutons assembled over millions of years by amalgamation from small magma chambers?" GSA Today: Vol. 14, No. 4 pp. 4-11.

It posits that igneous pluton emplacement is really drawn out, for instance consider the case of the Half Dome Granodiorite, which took ~4 million years to crystallize:

Figure 5 from the paper. The caption reads: "Summary of geochronologic data for the Tuolumne Intrusive Suite, modified from Coleman et al. (2004). Ages are from concordant U-Pb zircon data. Bar height is equal to +/- 2-sigma error and bar color is keyed to rock unit color on inset map. Ages for units are arranged in sequence from outermost to innermost (Kse-Sentinel Granodiorite; Kga-Kkc-tonalite of Glen Aulin-Kuna Crest Granodiorite; Khd-Half Dome Granodiorite; Kcp-Cathedral Peak Granodiorite; Kjg-Johnson Granite porphyry). Horizontal scale is not linear distance, but places samples according to the fractional distance from outer to inner contact of individual units (see Coleman et al. [2004] for a complete discussion)."

I recommend reading the whole paper, especially if the details of pluton emplacement interest you.

Labels: california, field trips, granite, north carolina, plutons

Sorry for the late notice... this is for today at lunchtime.



Abstract: The Silver Hill Mine in Davidson County, North Carolina was the first important underground silver mine in America. Discovered in 1838, it produced significant quantities of silver and lead into the mid-1840's. As the oxidized ores were depleted, abundant, rich, lead-zinc sulfide ores were encountered. These complex primary ores presented the mine operators with difficult metallurgical problems. Mine development and production slowed. Nearly a decade passed as the owners experimented with new processing and smelting technologies. These efforts were largely unsuccessful and the mine closed in the early 1850's. The Civil War created an urgent need for lead to supply Southern troops. The Confederate government operated the Silver Hill Mine to provide an alternate source of lead in case the mines at Austinville, Virginia should fall into Northern hands. Lead concentrates with high silver values were shipped from Silver Hill to the newly constructed Confederate smelter in Petersburg, Virginia. After the War, the mine continued to operate for several years but the problems of the refractory sulfide ores were not solved and the mine closed again. For more than a century after production stopped, the Silver Hill Mine was the repeated target of both mining companies and stock promoters.

Where: Pier 7 Restaurant, 650 Water Street, SW, Washington, DC (within walking distance of the Waterfront Metro on the Green Line) Free parking with validation from Pier 7 Restaurant.

11:30 - Social 12:00 - Lunch 12:30 - Speaker

Meeting cost: $20.00 for Washington, DC Section SME members $25.00 for non-members

Contact Steve Stokowski with questions

Labels: dc, history, meetings, minerals, mining, north carolina

I'd like to congratulate my friend and fellow MSSE candidate Rob Greenberg for being awarded this year's Outstanding Earth Science Teacher award for the state of North Carolina. (Link goes to GSA website where winners are listed; I read about it yesterday in this month's issue of GSA Today.)

Rob's one of the most enthusiastic people I know, and a gifted educator. He loves geology, astronomy, climate, and is a strong environmental advocate to boot! If you have ten years to spare, you can check out the wealth of materials he has online at his instructional website.

Congratulations, Rob!

Labels: environmental, msse, north carolina, teaching

Would you like a little geoblogosphere with your coffee this morning?

There's some great stuff out there today...

Andrew Alden (Geology.About.com) showcases the Fransiscan melange on a trip to Shell Beach.

Watch Perito Moreno glacier do some AWESOME calving at En Morrenas (Spanish-language geoblog). Watch the whole thing for perspective (3 minutes), but the really spectacular collapse occurs at ~2 minutes into the video. Watch the splash and watch the huge chunks of ice go zinging off into the surrounding air. Wild!

Dave Petley (Dave's Landslide Blog) reviews the dangers of a collapse of a volcanic flank in the Canary Islands, and what it means for Atlantic Ocean tsunami risk.

And for the geobloggers in the house, Chris proposes getting together in January at a science blogging conference in North Carolina. I think this could be cool. I just signed up.

Time for another cup of coffee... Good morning!

Labels: blogs, california, geology, glaciation, landslide, meetings, north carolina, south america