Overturned bedding at Maryland Heights

The Lilster & I drove out to Harpers Ferry, West Virginia, today, and crossed the Potomac River to hike up to the overlook at “Maryland Heights,” which is what they call the Blue Ridge north of the river. On the way uphill, I noticed this nice example of Harpers Formation bedding and cleavage dipping in the same direction (~east):

Note that the cleavage is dipping more gently than the bedding: this suggests that the bedding is overturned. No big shocker here: that’s the standard interpretation for the western edge of the Blue Ridge province; but it’s nice to see some meso-scale evidence of the regional structure.

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Uniformitarian

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Heat-stressed map of the Chesapeake Bay / Washington, DC region, as seen at Kenilworth Aquatic Gardens. Looks like mudcracks, eh?

Similar stresses; similar strains.

Crystal ghosts

The first time I went to the Billy Goat Trail (Potomac, Maryland) with geology as the goal (as opposed to mere recreation), it was 2002. The trip was led by a professor at the University of Maryland. I was a graduate T.A. then, and didn’t know anything about the local geology. I remember at the end of the trip, the professor sent us out to search for “kyanite ghosts” (pseudomorphs of sericite after kyanite, produced during retrograde metamorphism). We didn’t find them on that trip, but the evocative phrase “kyanite ghost” stuck in my head.

Several years later, after I had cultivated a deeper understanding of the story told by Billy Goat Trail rocks, I was poking around in the area near the trail’s “emergency exit,” and found something that fit the “kyanite ghost” bill. I took a photograph of it:

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My next step was to confirm what I found with my mentor and local rock guru, the geologist E-an Zen. E-an had been training me to take over leading geology hikes as a volunteer for C&O Canal National Historical Park. I e-mailed him the photo above. E-an wrote back to congratulate me on finding and photographing the exact same outcrop that was used in Cliff Hopson’s 1964 book The Crystalline Rocks of Howard and Montgomery Counties to illustrate the pseudomorphs. Hopson used a pencil for scale, and I used a Swiss Army knife, but otherwise the photos are identically composed:

hopsonImage: Plate 20, Figure 2; Hopson (1964)

That’s pretty uncanny, eh? Two photos taken just over half a century apart, of the exact same square foot of clue-bearing rock.

So, we have here large, bladed crystals that formed as porphyroblasts of metamorphic minerals during prograde (↑P,↑T)  metamorphism, then those same porphyroblasts found themselves unstable as temperatures and pressures dropped (retrograde metamorphism; ↓P,↓T). Their elemental constituents found themselves in disequilibrium, re-reacted, and formed new minerals which occupied the same space and shape as the large, bladed porphyroblasts. Today, you’ll finded that these “large, bladed crystals” are really aggregates of sericite (super-fine-grained muscovite).

So the question is, what were the metamorphic porphyroblasts that formed at peak P/T (and were subsequently replaced)? I mentioned kyanite as one possibility, right? However, Hopson noted these ‘ghostly’ shapes as “sillimanite (?).” Kyanite and sillimanite have a lot in common, but they aren’t the same thing. Like their polymorph andalusite, both kyanite and sillimanite have the chemical formula Al2SiO5. Both also grow in long bladed crystals. Check out these examples to prove this to yourself: kyanite | sillimanite

But in spite of these similarities, there’s a big difference between kyanite and sillimanite: they are stable at different combinations of temperature and pressure. Consider this classic P/T diagram:

Al2SiO5 triple point

If the sericitized pseudomorphs on the Billy Goat Trail were once sillimanite, then it implies higher temperatures. If they were once kyanite instead, then the temperatures were potentially lower. These rocks have plenty of un-retrograded sillimanite, but George Fisher (1971) was the one to invoke kyanite as the peak-P/T-porphyroblasts. He uses petrologic evidence to make the case that they were once close to ky/and/sil triple point. He says:

…the pelitic rocks contain many stubby crystals of andalusite, partially altered to sillimanite, and now largely pseudomorphed by fine aggregates of sericite. Andalusite partially altered to sillimanite is common at this end [south] of the island*, while at the north end of the island only bladed crystals of kyanite altered to sillimanite have been found. It appears as if the rocks at this end of the island must have entered the sillimanite field from the andalusite field, while the rocks farther north entered the sillimanite field from the kyanite field. If so, the rocks in the center of the island must have passed close to the triple point in the system Al2SiO5., about 5000 bars pressure [0.5 Gpa], and 650° C. The presence of muscovite and quartz in the sillimanite-bearing rocks reinforce this conclusion…

I assume he’s basing those statements on detailed petrologic evidence, but I haven’t seen his thin sections myself.

Tangentially, we’ve only been discussing the metasedimentary rocks so far, but E-an Zen and Phillip Candela point out in a 1998 guide to the area (for the University of Maryland geology department’s 25th anniversary hike) that the amphibolite units (meta-igneous, presumably) also contain kyanite or sillimanite but have not melted, which suggests temperatures in the range of 540° to 680° C, and pressures between 4.2 and 7 kbar (0.7 GPa).

So which is it? Kyanite or sillimanite? I can’t claim to know the answer: perhaps someone with more metamorphic petrology experience than me can shed some light on which mineral they they think they see in these ghostly pseudomorphs.

When I was out on the Billy Goat Trail last Friday with my GMU Structural Geology students, we ended up in that same general area. I challenged them to find the pseudomorphs, and it wasn’t five minutes before several of the students found excellent (though small) outcrops. Not the one that Cliff Hopson and I found, but other ones! Here are some shots to show their discoveries:

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I have two questions for you: (1) What’s your favorite example of retrograde metamorphism? and (2) Have you had a similar photographing-the-same-spot-someone-else-did-many-years-before-you experience?

______________________________________________

Bierman, Paul, Zen, E-an, Pavich, Milan, and Reusser, Luke (2004). The Incision History of a Passive Margin River, the Potomac Near Great Falls, in USGS Circular 1264: Geology of the National Capital Region. Field trip guidebook.

Fisher, George  W. (1971). The Piedmont crystalline rocks at Bear Island, Potomac River, Maryland. Maryland Geological Survey Guidebook No. 4, prepared for the 1971 annual meeting of the Geological Society of America, Field Trip No. 4.

Hopson, Clifford A. (1964). The Crystalline Rocks of Howard and Montgomery Counties. Maryland Geological Survey, Baltimore.

Zen, E-an, and Candela, Philip (1998). Department of Geology, University of Maryland: 25th anniversary geology hike to Great Falls, and the Chesapeake and Ohio Canal National Historical Park. Field trip guidebook: September 19, 1998.

* The “island” in question is Bear Island, which is not really an island (except during times of highest flooding). It’s just the land between the C&O Canal and the Potomac River in the vicinity of the Billy Goat Trail.

The working life

It’s a rough life, working in the places I have to work… here are a few photos from yesterday’s field trip on the Billy Goat Trail with my NOVA Physical Geology students. Photos are courtesy Dr. Meg Coleman, who joined us for the hike.

A post-lunch lecture on river incision (note the two prominent bedrock terraces, a.k.a. “straths” in the background):
strath

The crew climbing the dreaded “Traverse” section of the trail:traverse

We had a nice hot day yesterday: almost 90° F! Tragically, the snack bar was closed when we got back to the visitor’s center, so we were denied our salutatory Italian ices. Back to the trail tomorrow, for the 4th of 5 trips this week…

Here, ptyggie ptyggie ptyggie!

Yesterday, I took my GMU structural geology class to the Billy Goat Trail, my favorite local spot for intriguing geology. Unlike last year, we managed our time well enough that we got to clamber around on the rocks downstream of the amphibolite contact. Here’s Sarah, Lara, Kristen, and Alan, negotiating a steep section:

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Justin, Joe, Nik, Aaron, Jeremy, and Danny find a chunky amphibolite boudin in metagraywacke. Notice how Jeremy is gesturing about the orientation of the metagraywacke foliation wrapping around the boudin.

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The thing that we found that really made me happy were these ptygmatic folds. Most of my readers will doubtless already be familiar with ptygmatic folding, but in case you’re new to this, check out this photo (ballpoint pen for scale):

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Ptygmatic folding is “intestine-like” in appearance. It results where there is a particularly high viscosity contrast (viscosity is resistance to flow) between the folded layer and the surrounding matrix. The higher viscosity material makes broad lobes, while the lower viscosity material may be found in the pointy cusps between those lobes. If ptygmatic folding is well developed, the limbs become parallel to one another (isoclinal), and the visual similarity to guts is disconcerting. Here’s a smaller version, a few feet away from the first one:

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I’m headed back to the Billy Goat Trail today to discuss the trail’s geology with a crew from Sigma Xi‘s D.C. chapter. I wonder what we will discover today?

Sugarloaf

Sunday morning, NOVA adjunct geology instructor Chris Khourey and I went out to Sugarloaf Mountain, near Comus, Maryland, to poke around and assess the geology. Sugarloaf is so named because it’s “held up” by erosion-resistant quartzite. It’s often dubbed “the only mountain in the Piedmont,” which refers to the Piedmont physiographic province. Here’s a map, made with GeoMapApp and annotated by me, showing the general area:

A larger version of the map can be viewed by clicking here.

On the far west, you can see the Valley & Ridge province, which ends at the Blue Ridge Thrust Fault. Then the Blue Ridge province runs east from the Blue Ridge itself to Catoctin Mountain. From there, you enter the Piedmont, including both the “crystalline” Piedmont (Paleozoic metamorphism of various ocean basin protoliths, plus infusions of granite) and the Culpeper Basin, a Triassic/Jurassic rift valley. The Potomac River cuts a series of three spectacular water gaps across the Blue Ridge province just west of Sugarloaf. Harpers Ferry, West Virginia, is located at the confluence of the Potomac and the Shenandoah Rivers by the westernmost of these water gaps, and the name for the easternmost one is “Point of Rocks.”

Here’s a look at a detail from the southeastern corner of the geologic map of the Buckeystown, MD quadrangle, by Scott Southworth and David Brezinski:
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The map pattern shows a that the area around Sugarloaf Mountain is a doubly-plunging anticlinorium of Sugarloaf Mountain Quartzite [SMQ] and overlying (younger) Urbana Formation. Overall, it’s got that typical “Appalachian” northeast-southwest trend. Notice the thrust fault on the west side: a typical hanging wall anticline? The ridges, including the summit of Sugarloaf Mountain itself, are held up by the toughest quartzite. This overall “squashed donut” shape shows up pretty well in the physiographic map up at the top of this post.

Sugarloaf is quartzite (metamorphic), but you can clearly see the sand grains that composed its protolith (sedimentary). There’s also reports of cross-bedding, and so Chris asked me to take a look at a few structures to assess them with my point of view. I found a pervasive cleavage in the rock, far more than I would have suspected would be there. We did find bedding exposed as compositional/grain size layers in several locations, including on the summit. I also paid a lot of attention to the many quartz veins which cut the metasedimentary quartzite. These veins of “milky quartz” are often arranged in lovely en echelon series, like these tension gashes:

tension_gash_array_sugarloaf_web

I took the above photo several years ago on a visit there, but it’s typical of the sorts of stuff we saw Sunday. The kinematic sense of this outcrop would be “top to the right.” Interestingly, none of the Sugarloaf outcrops show really deformed tension gashes (i.e., they’re not folded into Z or S shapes like those I showed you a few days ago).

What we really wanted to get a sense of, though, was which way was up in these rocks. We were in search of geopetal structures: primary sedimentary structures that indicate the “younging direction” of the beds. Graded beds can do this, though I didn’t see any unambiguous graded beds in the SMQ on Sunday’s trip. We wanted some cross-beds. We found some hummocky / swaley examples, looking approximately like this USGS photograph (black & white; hammer for scale) of an outcrop somewhere “north of the summit”:

crossbedding_USGS_sugarloafImage source: USGS

Ours wasn’t as beautiful as the one pictured above, but it was clearly hummocky cross-bedding, and it was right-side-up (in beds tilted at ~30°). Interestingly, the SMQ has been correlated by Southworth and Brezinski (2003) with the Weverton Formation of the Chilhowee Group, a rock unit exposed in the Blue Ridge. Just as the Weverton is overlain by the finer-grained Harpers Formation, so too is the SMQ overlain by a finer-grained unit, the Urbana Formation. Both are interpreted as metamorphosed continental margin deposits. The Urbana is mostly phyllite in the areas I’ve seen it (including phyllite that’s full of quartz grains, a first for me). The Urbana is well exposed in a creek-side outcrop north of Sugarloaf Mountain, and I took Chris there to show him the lovely intersection of bedding and cleavage.

Here is a weathered piece of the Urbana Formation that Chris collected there, looking at the plane of cleavage (ruler in background for scale):

urbana Image source: Christopher Khourey

You can see the bedding running ~horizontally across it, though the photo cannot convey the lovely phyllitic sheen that results from waggling these samples back and forth in good light. It’s pretty cool. In places, the transition from sandy to phyllitic is gradational, probably relict graded bedding.

So, what does it mean if Southworth and Brezinski (2003) are correct in their correlation, and the Weverton and the SMQ are really the same rock layer, but in different provinces and at different metamorphic grades? Recall that the Blue Ridge province to the west is also a thrust-faulted anticlinorium, launched up and to the west by the Alleghanian Orogeny from an original position deeper in the crust and further towards the east. It’s a shard of the craton, snapped off and shoved bodily up and to the northwest. (In class, I often liken it to Joe Theismann’s leg: a compound fracture of the continental crust.) Might the Sugarloaf Mountain Anticlinorium [SMA] be a smaller version of the Blue Ridge pulling the same trick? It too is arched up and snapped off …but it would be a “Mini-Me” that’s only just surfacing, like a baby whale swimming above momma whale’s back…

whales_analogy

We know that deeper down in the Blue Ridge stratigraphy, we find the Catoctin Formation, the Swift Run Formation, and the basement complex. If we drilled down through the crest of the SMA, would we find the same units (or more metamorphosed equivalents thereof)? It’s an intriguing thought…

Update

Just got back from three days of geology conferencing at the Northeastern & Southeastern Joint Section Meeting of the Geological Society of America in Baltimore. No time to blog whilst there, though I shot a dozen or so tweets up to my Twitter feed: small beer compared to a nice meaty blog post. Apologies if it was insufficient for your needs. Great to meet up with everyone there… I’m off tonight for a two-day field trip in the Appalachians, and I’ll get back to blogging this weekend when I return.