Thursday, November 26, 2009

New Shenandoah map gets place of honor

I hung up my new copy of the Geological Map of Shenandoah National Park Region, Virginia in my office, using neodymium magnets (the best!) to "pin" it to a metal shelf. Gosh, it's beautiful.

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Saturday, November 14, 2009

The Shenandoah basement complex

These days, I'm engaged in the lovely process of rediscovering the geologic record of Shenandoah National Park. This 'rediscovery' was prompted by the recent Virginia Geological Field Conference based at Big Meadows. While I wasn't able to attend in person (I was in Yosemite that weekend!), colleagues like Pete Berquist and John Weidner were there, as well as three of my Rockies students from last summer. They've all shared their perspectives on the conference with me, and John loaned me a copy of the field guide to the conference. This guide, authored by other colleagues like Chuck Bailey of William & Mary and Scott Southworth and Bill Burton of the USGS in Reston, makes for great reading. I'd link to it so you can read it too, but it's not online.

The guide led me to the revelation that there is a new geologic map of the park and the surrounding area that was published earlier this year by the survey. This map* is authored by Chuck, Scott, and Bill, along with their peers at the survey and other institutions. Why wasn't I informed? (Just kidding) It's a beautiful work of art and science. I'm having the NOVA duplicating services team print me out a copy this week.

The new insights offered by the map (and the VGFC field guide) include the fact that the oldest rocks in Shenandoah National Park are diverse and complicated. It used to be that geologists considered these rocks to be a granite gneiss called "the Pedlar Formation," which was intruded in places by younger granitoid plutons. Modern work in the park has revealed that it's more complicated than that. There are a dozen or more separate rock units comprising what the pros are now calling "the basement complex." These rocks are distinguishable based on texture, mineralogy, and age. (These newer, more precise ages are one of the key advances of recent work by John Aleinikoff of the USGS: the granitoids and their metamorphic successors have crystallization ages ranging from 1,183 Ma (+/-11 Ma) to 1,028 Ma (+/- 9 Ma).

I've updated my Shenandoah web page to reflect the new preferred terminology plus these new dates. More updates to come -- I've got many new tidbits of inspiration from reading the 100+ page write-up that accompanies the new map. The web page, like all of my web pages, is a work in progress. Nothing makes that clearer to me than a steaming helping of fresh science!

When I was out in the park last weekend, I found this new outcrop of the basement complex, which shows some of this intriguing diversity:

Annotated version:

The outcrop is on the hike over Bearfence Mountain, described (and mapped) in the new VGFC field guide. It's a granite gneiss, partially altered to unakite (the plagioclase and pyroxene in the graniotid reacted in the presence of water to generate epidote. A pronounced foliation is cut by no less than 3 separate sets of fractures, two of which are filled in with fibrous quartz, and another by something dark. The granitoid formed during the Mesoproterozoic Grenvillian Orogeny, and was deformed later in that same episode of mountain building. The fractures formed at some point after that: just when, I can't say. Vein sets 1 and 2 are infilled with apparently identical compositions, which would be consistent with them being contemporaries. Vein set 3 has something else lining its fractures. At first I thought it was just mildew, but Elli suggested some mineralogical possibilities. Vein set 3 does not show the same amount of dilation as the other two sets. Cross-cutting relationships show vein sets 1 and 2 cross-cutting vein set 3, which suggests I was too hasty in labelling them in my photo. "3" is the oldest; "1" and "2," despite their names, are younger. Maybe they're related to Neoproterozoic breakup of Rodinia, or Alleghanian mountain-building, or uplift? So many mysteries...

More to come on this topic, surely, as I get re-introduced to my local national park.
* Southworth, Scott, Aleinikoff, J.N., Bailey, C.M., Burton, W.C., Crider, E.A., Hackley, P.C., Smoot, J.P., and Tollo, R.P., 2009, Geologic map of the Shenandoah National Park region Virginia: U.S. Geological Survey Open-File Report 2009–1153, 96 p., 1 plate, scale 1:100,000.

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Sunday, November 8, 2009

Shenandoah, with UPJ

Yesterday, we had a joint NOVA-University of Pittsburgh at Johnstown field trip to Shenandoah National Park. It was a great day of examining new rock outcrops and old treasured favorites. UPJ is responsible for one of the only departmental student-centered geology blogs that I am aware of, Mountain Cat Geology. A couple of weeks ago, igneous and metamorphic petrology professor Elli Goecke contacted me about local rock options, and I invited her crew to team up with the NOVA GOL 135 field course to check out Shenandoah. [Geoblogger small world: Elli studyed under Kim Hannula in Vermont!]

Together, the sixteen of us checked out evidence for the two Wilson Cycles recorded in the rocks of Virginia's Blue Ridge province, and had a pleasant time hiking around and enjoying unparalleled fine weather. Unfortunately, November means the days are short, and we had the sun set on us before we got to the final stop (at Signal Knob Overlook). We took a group photo there: see if you can spot who's a NOVA person and who's a UPJ person...

The annotated version, to show who's who:

Thanks for a great day in the field, everyone!

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Sunday, August 30, 2009

A few insects

Yesterday, I took a hike with some friends in Shenandoah National Park, and we encountered a bunch of interesting insects. I took a couple of photos, the best of which I'll share here.

Monarch butterfly caterpilar:
Monarch caterpillar

Moth Butterfly and hover-flies enjoying thistle nectar:

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Monday, August 17, 2009

Upcoming events

First Geological Society of Washington meeting of the new academic year
September 23
Dupont Circle, Washington, DC

Virginia Region of the National Speleological Society caving convention (but without the caving)
September 25-27
Battle of Cedar Creek Campground (Route 11, between Strasburg and Middletown, Virginia)

New York State Geological Association meeting
September 25-27
New Paltz, NY

Virginia Geological Field Conference
October 2-4
Big Meadows, Shenandoah Nat. Park, VA

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Tuesday, June 23, 2009

Amygdules!! (two exclamation points)

A funny coincidence transpired a couple of weeks ago. I posted about "Amygdules!" and so did Andrew. We were both so excited by these cool primary igneous structures that we added an exclamation point to our post titles. Over the weekend, I found some more. These are in Dark Hollow, in Shenandoah National Park, above the falls. Pretty sweet, eh?


I hereby give them two exclamation points. Let's see if anyone else can come up with two-exclamation-point-worthy examples of amygdules...

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Monday, June 15, 2009

White Oak Canyon

Our third Rockies training hike took place Saturday. Six of us hiked White Oak Canyon, in Shenandoah National Park. It was about an eight mile loop, with six big waterfalls on it. There were a lot of plungepools where other hikers were swimming.

There wasn't an astounding amount of geology on the trail: it was mostly Catoctin Formation, with a few outcrops of underlying Grenvillian granitoids. A few nice amygdules; no columns.

The waterfalls sure were purty, though. Here's Jason at the uppermost falls (86 feet tall):

Me departing from one of the lower falls:
white oak
Photo by Chris McMahon

I got home tired and sore from this hike -- it was a good time, but I slept well last night as a result!

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Friday, June 5, 2009

Little Devil's Stairs


So, it's a month until my Rockies class starts. I've been encouraging all the students to get in shape, because the high elevations, rough terrain and multimile distances we'll be hiking in Montana and Wyoming could really kick an east-coast flatlander's arse. So we've scheduled a few training hikes to help everyone physically prepare for the Rockies experience. Last weekend, we did a 5.5-mile circuit up the steep Little Devil's Stairs trail in Shenandoah National Park. I was joined by five Rockies students + one of their kids. Here's a map of the loop we did:

Here's a few photos of the hike, and the geology we encountered along the way:


John poses next to some jointed columns in the Catoctin Formation, a Neoproterozoic rift-related series of flood basalts (subsequently metamorphosed during Alleghenian mountain building).

End-on view of one of the columns:

Overhanging cliff showing columns weathering out along jointed surfaces:

Bob poses next to a cliff, helping me demonstrate how difficult it is to take a well-exposed photo in the jungle of the Virginia hardwood forest:

A wiggle in some columns:

Jared thought these columns were better than the first ones he saw, at Old Rag Mountain.

Here's me with a fifteen-foot-long section of columns, indicating that the flow from which this boulder was derived must have been at least fifteen feet thick, maybe more:


But it wasn't all columns. There was also a lot of column-less massive Catoctin Formation, and some nice inter-flow conglomerates which are interpreted as stream deposits that developed atop a cooled flow before the next flow erupted. These conglomerates imply a reasonable amount of time passed between successive eruptions of the Catoctin flood basalts. The lichens obscure the rock, but note for instance the fingernail-sized chunk of greenstone an inch above my hand:

More chunks in the conglomerate:

And more:

Jared guards the way forward:

The view from the top:

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Thursday, May 28, 2009

Shenandoah class

Last Saturday was my Field Studies in Geology trip to Shenandoah National Park. Here's a few shots from the day's geologizing...

Garnets in the Pedlar Formation granite gneiss, oldest rock in the park at ~1.1 Ga.

Meta-basalt columns of the Catoctin Formation (photo by Mathina Calliope):

At the end of the trip, I have the students order a series of strips of paper with different geologic events in the park's long geologic history. They have to figure out the proper order based on what they learned that day:

Lastly, a group photo overlooking the Browntown Valley:

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Wednesday, April 22, 2009

Structure trip 2: Limberlost columns

After the Garth Run high-strain zone and a night hanging out by the campfire at Heavenly Acres with the William and Mary Structural Geology class, the second stop on our Structural Geology trip was in Shenandoah National Park, looking at the deformed meta-basalt columns on the Limberlost Trail. Longtime readers of the blog have seen these unique (in my experience) columns before, in a post from last May.

This is an outcrop of the Catoctin Formation, a series of (mainly) basaltic lava flows that erupted sometime older than 565 Ma (only the youngest, rhyolitic layers have been dated, and evidence suggested that significant amounts of time may have passed between the eruption of each stratum of basalt deeper down in the stratigraphic stack). As the lava cooled, it developed cooling fractures that formed perpendicular to the isotherms. These fractures likely initiated at the top and the bottom of the flow, and propagated towards the middle over time.

Later, during Alleghenian mountain-building (~300 Ma to ~250 Ma, roughly), the rocks were subjected to greenschist-facies metamorphism, and were deformed. The basalt's consituent minerals re-equilibrated and reacted to become other minerals, most notably chlorite and epidote (both of which are green).

Here's John and Joe checking out the columns:

Exquisite! Even arrest lines on the side of each column are preserved. In an undeformed basalt column, these arrest lines would be perpendicular to the column edge. Here, they have a pronounced angular relationship, indicating the shearing of the overall column:

Bobby measures the angular shear along the length of the column:

Goofball professor poses with column:

Jay plays the column like an electric guitar:

We found some nice plumose structure too:

Finally, we evaluated the concentric rings of minerals filling amygdules (vesicles that had been infilled with mineral deposits after lithification) in an attempt to determine whether they could be used as strain markers, or whether they may have attained their ellipsoidal shapes due to stretching of the bubbles in the originial lava (i.e. like this) and then been infilled with minerals:

...and then we were off to Field Study Area #3...

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Saturday, April 11, 2009

Old Rag II: Catoctin feeder dikes

Almost a week after the field trip to Old Rag Mountain, and the Facebook-hosted pictures keep trickling in. Here's some shots by NOVA student Eileen Lodovichetti, and an ensuing discussion of feeder dikes and supercontinent breakup.

Here's a shot of the upper reaches of Old Rag, showing the characteristic spheroidal weathering of the Old Rag Granite and the relative lack of trees on top:

photo by Eileen Lodovichetti

...And here's a shot that Eileen took which shows the interior of one of the weathered-out feeder dikes we had to hike through on our way to the summit. You can actually see the classic geoprofessorial arm-waving caught in blurry motion!

photo by Eileen Lodovichetti

This is one of the coolest things about hiking Old Rag: after scrambling up on top of spheroidally-weathered granite domes, you drop into these tabular "hallways." The astute observer will note that the floor is made of a fine-grained, dark-green-colored rock, quite distinct from the light-colored, coarse-grained granite that makes up most of the mountain. These are dikes of metamorphosed basalt that intruded the granite during the breakup of the supercontinent Rodinia in the Neoproterozoic era of geologic time.

Here's one of my former Field Studies in Geology students, Mike Nelson, pointing out a similar dike along Skyline Drive, in the main part of the park:

Basically, the story goes like this: Around 1.2 to 1.0 Ga, continental fragments amalgamated into a supercontinent called Rodinia. In Virginia, this is recorded in the rocks of the Blue Ridge province, where the basement consists of granitoids (granites and related rocks) and metamorphosed granitoids (gneisses, mylonites). Among the youngest of these is the Old Rag Granite, which intruded the Pedlar Formation granite gness around 1.0 Ga.

Later, Rodinia broke apart, resulting in an extensional tectonic regime and mafic volcanism. Fractures opened up in the Old Rag Granite and funneled mafic magma towards the surface. Massive eruptions of basalt blanketed the landscape. The resulting layers of basaltic lava are known as the Catoctin Formation. At Old Rag Mountain, we can see some of the plumbing that led to these flood basalt eruptions: these are feeder dikes, because they "fed" the eruption above them.

Because the dikes (which were metamorphosed to greenstone during ~300 Ma Appalachian mountain-building) weather more rapidly than the Old Rag Granite, they are typically recessed into the landscape. That's what makes the "hallways" in the photograph above. Here's two more images, showing these weathered-out feeder dikes:

Check out how there's moderately-developed columnar jointing extending across the dike. These columns form perpendicular to the cooling front, and the dikes would have lost their heat out the sides. In horizontal lava flows, the heat is lost from the top and bottom surfaces, so you get vertical columns. Here, a vertical dike produces horizontally-oriented columns. Hikers appreciate these "steps" as they squeeze through the dikes on their way up the mountain.

Here's a map of part of Shenandoah National Park:

Please ignore the "hover" instructions at the lower right. I've reproduced the "hoverable" image below. Key: the orange is the Pedlar Formation. The pink is the Old Rag Granite, and the green is the Catoctin Formation. Feeder dikes of the Catoctin are shown as green lines.

Now, let's take away the map, and just preserve the orientation of the feeder dikes. This will tell us the overall tectonic stretching direction:
Various plate reconstructions show either Amazonia or the Congo craton offboard of Virginia at the time Rodinia broke apart and the Iapetus Ocean began seafloor spreading. I've illustrated it here as the Congo, but that might be wrong.

So: the hike up Old Rag is great exercise, and offers scenic views, but for those willing to consider the rocks and how they got there, it's an insightful view into the tectonic past.

Lastly, here's a lovely, well-developed weathering rind on the Catoctin meta-basalt (greenstone). When the dark green rock adjusts to the conditions at the Earth's surface, it breaks down, resulting in the tan/"buff" color on the outside. You're watching the rock "rot" from the outside surface, working its way inward:

More on the geology of Shenandoah National Park can be seen at this page on my website.

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Tuesday, May 27, 2008

Shenandoah geology class

On Saturday, I took a group of NOVA summer school students to Shenandoah National Park to look at some rocks. We had great weather, and saw evidence of Grenvillian mountain building, the breakup of Rodinia, and the transgression of the Sauk Sea. A real crowd-pleaser was an outcrop of what was once columnar basalt (the Catoctin Formation). I say "was once" because the basalt has been metamorphosed to greenstone, and the columns have been squashed into more lathe-like shapes.

Here's a few photos of the columns:

Columns like these form as the cooling mafic lava contracts a bit in volume as it loses heat. This causes a series of fractures to form. The intersection of the fractures defines the hexagonal columns. We've seen the same phenomenon at the Giant's Causeway, Northern Ireland. When the lava of the Catoctin Formation was extruded, Rodinia was breaking up and the Iapetus Ocean was being 'born.' One of the things I really like about the Shenandoah columns is that, even though they're metamorphosed, they show clear 'ribs' on the side -- arrest lines as the propoagating fracture worked its way down into the flow. An inch at a time, the columns grew.
Finally, here's me doing some "arm waving" amongst rapt students (ha!):

All four photos by Nicole LaDue (NSF). Thanks Nicole!

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Monday, May 5, 2008

Shenandoah NP: Corbin Cabin area

This weekend, I took a backpacking trip in Shenandoah National Park. Thought I would share a few photos today: scenery first, geology second...

Here's the view looking east from Skyline Drive:
Looking East

The temperature difference due to elevation was striking. It was still early spring up on the top of the mountains, on Skyline Drive:
Brown above

...But down below, it was green and lush (and sodden with pollen!):
Green below

I camped out for two nights near Corbin Cabin, and did a day-hike around Thorofare Mountain on Saturday, visiting this waterfall at lunchtime:

The geology of Shenandoah National Park is interesting: it records the assembly of the early supercontinent Rodinia at about a billion years ago, and then the breakup of Rodinia about 600 million years ago. The first event recorded is the generation of granite gneisses and granites due to the Grenville Orogeny. The oldest unit in the park is the 1.1 Ga Pedlar Formation, a granite gneiss. There's a slightly younger granite which intrudes it called the Old Rag Granite (~1.0 Ga), but I didn't see any outcrops (or float blocks) of it, so I'll not mention it further. There's a thin, patchy sedimentary cover called the Swift Run Formation deposited directly atop the granite gneiss and granite, providing a nonconformity surface. Atop that is a series of volumnious tholeiitic basalt flows: these mafic extrusions record the breakup of Rodinia and the opening of a new ocean basin: the Iapetus. In many places in the park, you can see "feeder dikes" of the Catoctin cutting through the older plutonic and metaplutonic rocks (see image below). There are also some sedimentary rocks layered atop the Catoctin (the Chilhowee Group), recording the transgression of the Sauk Sea on the North American platform. But I didn't encounter any good outcrops (or float blocks) of them on this trip, so I'll stick to the tectonic story: the Pedlar Formation shows us Rodinia getting put together, and the Catoctin Formation shows us Rodinia breaking apart. Later metamorphism due to Appalachian mountain-building resulted in changes in both of these rocks (development of "blue quartz" in the Pedlar, and the Catoctin metamorphosed to greenstone).

Here's a massive dike (possibly a "feeder dike" feeding surface lava flows) of the Catoctin basalt cutting through the Pedlar Formation granite gneiss, just north of the Marys Rock Tunnel. Note the columnar jointing extending perpendicular to the walls of the dike:

Having covered all that, I now propose to spend the rest of this blog post showing you the variety of cobbles and boulders in my campsite. I camped at the little wedge of land above the confluence of two streams. One stream's catchment basin was Catoctin, and the other drained outcrops of Pedlar. As a result, the "float" in my camp was all either Pedlar Formation or Catoctin Formation. I'll just run through them one after another so you get a sense of the range of variety in each formation.

You'll notice that the Pedlar is sometimes coarse, sometimes fine, sometimes well foliated, sometimes not so much. You'll also notice that the Catoctin varies a lot in terms of its extrusive texture: sometimes aphanitic (fine-grained), sometimes amygdular (formerly vesicular), sometimes it even runs to volcanic breccia. All of these original lithologies have been metamorphosed to various degrees in the Catoctin, which here can be seen by comparing the amount of green in the rock. This green comes from two metamorphic minerals: chlorite and epidote. Enjoy!

Pedlar Formation:










Catoctin Formation:













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