Dolly Sods

Over the long Labor Day weekend, my fiancée Lily and my friend Seth and I took a three-day backpacking trip in the Dolly Sods Wilderness area of West Virginia:dollysods_04

Dolly Sods is a unique place, a little patch of flora that is more typical of Canada. It sits atop the eastern Continental Divide, and most of the area drains to the Gulf of Mexico via the Ohio River. Parts of Dolly Sods are sparsely treed, and resemble Arctic tundra. It is the easternmost bit of the Appalachian Plateaus province. Many places reminded me of Alaska:dollysods_24

Rolling meadows and bogs occur in patches, interspersed with forest of spruce, hemlock, and aspen (yes, aspen!):dollysods_04

The area was used as a proving ground during World War II, and there are still some dangerous bits and pieces left over from that time:
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Here’s our happy trio, ready to set off on Friday afternoon:
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Very quickly, I clued into the wealth of small blueberries which were omnipresent in the “tundra” landscapes. I snacked on these continuously throughout the weekend:dollysods_05

A glimpse of two forms of power generation off to the north: Mount Storm on the left (a coal-fired electric generation plant) and a field of windmills on the right:dollysods_06

Here and there, outcrops of white rock rose up above the lichens and shrubs:dollysods_11

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This is the Pennsylvanian-aged quartz sandstone of the Conemaugh Group. Occasionally, it outcrops as bedrock, and other times, you just get these clean boulder fields, surrounded by tundra vegetation:dollysods_07

So what do we see when we zoom in on these outcrops and boulder fields? Well, mostly, we see quartz sandstone:
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…Although there is a regular smattering of quartz-pebble conglomerate, too:
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Occasionally, primary structures jump out at the eye, like some graded bedding…
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…or these cross-beds:
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Annotated copy:
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There were even some fossils, like these plant scraps:
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Plant scraps compressed en masse make coal, and there are coal interbeds to be found in places in Dolly Sods, and bituminous coal can also be found as float, as with these chunks:
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There was even some structure to observe!
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Annotated version:
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A bigger outcrop, right around the bend, showed even more pervasive distortion of the sedimentary layers:
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Annotated version:
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What’s going on with these folds? After all, the Allegheny Plateau isn’t known for pervasive structural shenanigans… I’m guessing this might be soft-sediment deformation: slumping and sliding of sedimentary layers before they got lithified… Any other thoughts? (chime in via the comments section below, if so).

Here is sand weathering out of the sandstone, the grains free and loose again for the first time in ~300 million years:
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The plants were a joy:
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Here’s the view at sunset from our third campsite:dollysods_26

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Yesterday (Monday) morning, when we woke, we found that the temperature had dropped below freezing overnight, and a coarse layer of frost covered everything:
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Detail of the frost crystals on my tent’s rain fly:dollysods_31

The sun rose, and starting melting off the frost and dissipating the fog:

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Soon only the fog remained:dollysods_32

In the warmth of the new day, we hiked out, got apple dumplings at the Front Porch Restaurant across from Seneca Rocks, and drove back home along good old (new) Route 55. It was a great weekend away, just the right distance, in wild country, with great weather. I felt rejuvenated by the experience.

Harpers Foldry

Cleaning out the backlog of photos I haven’t popped up here yet… Here’s three shots from last weekend, of folds (some kinky) which deform Harpers Formation foliation, just south of Harpers Ferry, West Virginia:

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The Harpers is a Cambrian-aged lagoonal mudrock, dated via Olenellus trilobites in Pennsylvania. It is part of a transgressive sequence that followed Iapetan rifting of the mid-Atlantic, and was later deformed during Alleghanian mountain-building. That’s when the pronounced foliation was imparted, and when that foliation was folded (also overturned). There are plenty of nice exposures of kink folds in this charismatic rock throughout historic Harpers Ferry. Check it out if you’re ever there on a history field trip.

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.

Hackles, ribs, plumes

Today, you get a photo from GMU structure student Nik D. This is a small exposure in the Hampshire Formation (Devonian) on New Route 55 in West Virginia. It shows a fine example of plumose structure with the not-often-seen concentric ribs running perpendicular to the ‘plumes.’ At the edge of the joint, you can see the flaring fringe of hackles. Top edge of a Rite-In-The-Rain field notebook for scale:ribs_plumose

Where shall we zoom in? How about these two boxes?ribs_plumose4

Close-up of the concentric ribs:ribs_plumose2

Close-up of the hackle fringe on the edge of the joint:ribs_plumose3

…Even the hackles have hackles!

Good stuff: I love me some fine plumose structure!

Flames and pillows, Route 55

I took a look at some interesting blobby structures in the Swift Run Formation last week, and walked readers through my logic in tentatively concluding that they were ball & pillow structures (soft sediment deformation), though overprinted by a pervasive (Alleghanian) cleavage. As we move west in the Appalachian mountain belt, the rocks are less cleaved: the strain is instead taken up in large anticlines and synclines with a few thrust faults thrown in. Though arched and fractured, the rocks’ fabrics remain pretty close to what they were at the time of deposition.

Fortunately, in the Valley & Ridge province along New Route 55 in West Virginia, you can see some sweet examples of (undeformed) ball & pillow in the Hampshire Formation (Devonian; part of the Acadian clastic wedge). Here’s a view looking up at the bottoms of some of these sandy sags (quarter for scale):

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I like that partial weathering-out of the features into the third dimension…

To refresh your memory, ball & pillow forms when a heavy load of sand gets dumped (underwater) on top of a soft, squishy deposit of mud. The sand sags downward in broad “balls” (if you have a point locus of sinking) or “pillows” (if the sags have a linear axis to them). In between, the low-viscosity mud squirts up in cuspate “flame” structures. Check out this fine example (quarter for scale), found by GMU structure students Joe M. and Justin O. on the northern side of the road:

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…And here we have the same photo, with the sand, mud, flames, and ball & pillow all labeled for you. The white arrows represent the downward sagging of the sand; the red arrows represent the upward squirting of the mud.

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In some places, the pillows have weathered out. Here’s one (quarter for scale) that is now upside down on the grassy knoll beneath its source outcrop:

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Note the thin laminae of mud clinging to its exterior, like a coat of paint!

Here’s a second sandstone pillow that has weathered out of the cliff and popped down onto the grassy slopes below. Obviously, the surface being photographed is a cross-section through the saggy pillow:

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The “upper left” of this sample would have been the lowpoint of the sag if it were in situ on the outcrop. You can see the smooth margin on the left side, and the rougher zone on the right where it detached from the overlying part of the sandy layer. Let’s now zoom in on this box to see something really cool:

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If we go deep here, we can see that the laminations of sand within the pillow show small Z-folds (and S-folds on the other side, though they’re not as obvious in the photograph) that are “parasitic” on the main fold. Here they are, highlighted (white arrows) and traced out (black lines):

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The axes of these small-scale structures verge on the axis of the main fold, meaning that their axial planes (blue traces) “tip over” with increasing deformation towards parallelism with the main fold’s axial plane. We’ve seen similar things before. The axial planes play the same game as the cleavage plane. This is the first time I have ever observed such a structure associated with soft-sediment deformation, however.

Has anyone else ever seen S- or Z-folds associated with soft sediment deformation? It makes total sense to me that they would be there based on simple physics, but this was the first time I had ever seen it myself. I’d be curious to get a sense of how common or rare this might be.

More mud

Remember the mud I saw on Pimmit Run?

Turns out that West Virginia mud pulls many of the same tricks as Virginia mud. Here’s some mud cracks I noticed on Sunday afternoon on the shoulder of New Route 55 in eastern West Virginia:

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Notice how West Virginia spices her mudcracks with chunks of Silurian quartzite and fresh crabgrass. A tasty combination!

Transect debrief 6: folding and faulting

Okay; we are nearing the end of our Transect saga. During the late Paleozoic, mountain building began anew, and deformed all the rocks we’ve mentioned so far. This final phase of Appalachian mountain-building is the Alleghanian Orogeny. It was caused by the collision of ancestral North America with the leading edge of Gondwana. At the latitude of Virginia, that means northwestern Africa (Morocco and/or Mauritania).

Whereas the first two pulses of Appalachian mountain building were relatively provincial affairs, this Alleghanian phase was a full-on continent-on-continent smackdown. The Himalaya (India colliding with Eurasia) would be a good modern analogue for the Pennsylvanian and Mississippian Appalachians.

When I was live-blogging the trip, I posted this photo of Judy Gap:

It was a bit hard to get it all into one measly iPhone frame (hence the tilted angle: those trees are in fact vertical!), but what you’re looking at here is the erosion-resistant Tuscarora Sandstone (Silurian in age; quartz-rich beach deposits) that outcrop as a ridge. However, here at Judy Gap, there are two ridges. What gives? This is where I was introduced to a new term that is apparently becoming a common phrase in the structural geology literature: contraction fault.

The story most Physical Geology students get about fault types is that tectonic extension causes normal faults, while tectonic compression causes reverse faults. Contraction faults are faults that display an apparent “normal” sense of motion, but were caused by a compressional tectonic regime. How the heck does that work, you may ask? Consider the following diagram:

So the deal with contraction folds is that they might start out “reverse” but are then rotated and tipped over as deformation proceeds. The former footwall becomes the new “hanging wall,” and the sense of motion is obscured by this new orientation. This means that they do represent contractional strain, but a freshman geology student is unlikely to spot it at first glance.

The Germany Valley to the east of Judy Gap is a big breached plunging anticline, as I attempted to show with this iPhone photo from the Germany Valley Overlook along Route 33:

It’s a bit easier to see if you jump up in the air 10 kilometers or so. Fortunately, that’s precisely why God created Google Earth:

The valley is hemmed in by a big V-shaped fence of mountains, all held up by the Tuscarora. It’s tough stuff. During Alleghanian folding, the crest of the anticline was breached, and water was able to get inside and gut the weaker rocks. The quarry annotated in the photo is mining the same Cambrian and Ordovician carbonates seen in the Shenandoah Valley back in Virginia (Lincolnshire and Edinburg Formation equivalents). A pattern geologists have noted with eroded anticlines is that older rocks are exposed in the middle of the structure, with younger rocks flanking them along the sides.

So that’s a glimpse of the big picture of deformation in the Valley & Ridge, but we can also see cool deformation at smaller scales… Stay tuned…

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