23 April 2013
Return to the Outdoor Lab
Two years ago, I took a trip to the Phoebe Hall Knipling Outdoor Lab, which is Arlington, Virginia’s outdoor education facility in the Pond Mountains (southern continuation of the Bull Run Mountains), on the eastern edge of the Blue Ridge geologic province. I was invited back last week to look at some new exposures. I brought the GigaPan along.
There had been additional erosion on the saprolitic exposure of Harpers Formation in the creekbed, so we set about documenting that. Here, Outdoor Lab nature guru Anthony helps me clear the scene for an unobstructed view of the rocks:
Here’s what we saw – lovely kinky little crenulated folds in the slate/schist.
Explore the whole outcrop for yourself here:
But that wasn’t the main attraction. Rather, we were mainly interested in heading to the top of Biscuit Mountain, to a new parcel of land that the Outdoor Lab has recently purchased. It includes stellar outcrops of Weverton Formation quartzite (meta-quarz-arenite) atop the ridge, just south of Thoroughfare Gap.
It’s a beautiful scene up there: tafoni-bedecked quartzite and spring blossoms. Plenty of black bear scat, too!
The exposures of rock were not only scenic, but also instructive. Here’s a lovely small-scale fault we saw, for instance:
Previously, on the north side of Throughfare Gap, I was flummoxed by some enigmatic structures which looked like cross bedding that had been sheared with the upper side moving to the west (presumably during Alleghanian mountain building). One of the limitations of the outcrops on the north side is that there were only a handful of examples – it was hard for me to reach a conclusion I felt confident in, given such a limited set of data.
And that’s why I’m so glad the Outdoor Lab bought the new property on Biscuit Mountain. There are dozens of additional examples of this same phenomenon.
I’m convinced that these are scours, concave-up, some of them internally adorned with cross-bedding. These beds are right-side-up. I think there is some top-to-the-left shearing that’s taken place, too, which accounts for the consistent asymmetry (west side shallowly dipping to the east; east side steeply dipping to the west or else overturned and steeply dipping back to the east).
Here’s another site, a meter or two away:
Because the exposure here wasn’t as crystal clear, I thought it warranted a comparative view, with my annotations visible in addition to the unannotated GigaPan.
Explore them all, and tell me if I’m missing anything!
22 April 2013
Monday macrobug: Carpenter bee
These guys are the bane of my existence lately. Now that the ladybugs are gone, we’ve got a dozen carpenter bees orbiting the house, seeking an opportunity to drill holes in it. Here’s one after an encounter with a tennis racket I keep on the porch expressly for the purpose of controlling their population:
Look at those amazing eyes!
Ventral view, rotated to make it look like he’s flying above us:
21 April 2013
Sleeping Inn with the Martinsburg Formation
Three new GigaPans I shot last Friday east of Staunton, Virginia, at a semi-legendary exposure behind the Sleep Inn at the 250 / 81 intersection.
Students: Which way is up? Which criteria did you use to make that determination?
19 April 2013
Friday fold: Another from the depths of Mosaic Canyon
More folds from the charismatic Noonday Dolostone in Mosaic Canyon, Death Valley National Park, California:
Annotated:
Happy Friday!
17 April 2013
Four new GigaPans from an intriguing contact
Jay Kaufman of the University of Maryland and I met in the field this weekend to do some outcrop preparation and structural documentation with the GigaPan.
This is a tricky bit of business, as it turns out – or, at least, it’s trickier than the typical GigaPan shoot.
To start with, we had to clean the exposure, a block that shows the contact between the Neoproterozoic Fauquier Formation and the overlying Catoctin Formation.To clean it, we needed a pressure washer. But there was a good sized stream between the car/road and the rock. So here’s Jay hauling the pressure washer across Goose Creek with the help of his graduate student Huan Cui.
All in the name of science.
Next, we had to haul water from a flooded quarry pond up a steep, muddy slope to the site where the intriguing rocks live. There were several slips and spills, and my cell phone took a swim in the pond. All in the name of science. Jay’s daughter Colleen was helping, too, and she hauled tools like a saw and branch loppers to the site.
We power-washed the rock, a block of float with some intriguing structure. Pond water misted surrounded us: we breathed in its microdroplets and their microbial cargo. All in the name of science.
Then it was time to GigaPan: Jay hauled a ladder over, which was key for getting the GigaPan up to a proper height for photography orthogonal to the plane of the exposure.
Photo by Huan Cui, courtesy of Jay Kaufman
Photo by Huan Cui, courtesy of Jay Kaufman
As I supervised the GigaPan (it was on auto-focus, and needs supervision in that condition; otherwise it sometimes skips a shot if it can’t decide what to focus on), Jay cast shadow on the block, so as to have a consistent level of lighting across the final image. Braving muscle fatigue as he perched on a branch for 15 minutes, between the Sun and the rock, Jay persevered – all in the name of science!
Photo by Huan Cui, courtesy of Jay Kaufman
Photo by Huan Cui, courtesy of Jay Kaufman
Here are the four GigaPans that resulted from our labors:
The happy team at the end of the expedition:
Photo by Jay Kaufman
So what do we see in these images? I think our background strongly colors our interpretation.
Jay is a sedimentologist and geochemist, one of the four authors of the bombshell 1998 “Snowball Earth” paper that ignited the modern explosion in examining and re-interpreting Neoproterozoic sedimentary rocks. He interprets this contact as showing pillow basalts, and carbonate diapirs between the pillows. This is very important, because if indeed the block records primary soft-sediment deformation (hot submarine lava flowing onto submerged carbonate mud), it allows us to use the radiometric date for the Catoctin Formation (meta-basalt / greenstone) as a constraint on the age of the underlying Fauquier Formation (meta-limestone / marble). It would mean that the relationship is conformable (and the metamorphism later overprinted the primary configuration with a light enough touch that it wasn’t fundamentally reorganized), and that helps us say when the “Ediacaran” phase of Snowball Earth glaciation took place, since the lower Fauquier shows features that are consistent with glacial outwash. Here’s my rough interpretation of this reading of the rocks:
Photo by Jay Kaufman; Click to enlarge
Caveat: Jay and his colleagues may draw those lines differently; I refer you to their paper for a more complete discussion of their interpretation. Jay has also added snapshots to one of the GigaPans to help tell the story.
However, I’m trained as a structural geologist, and so I see something different when I look at this block. I see boudinage, tension gashes, and metamorphic reaction rims (chlorite + garnet most prominent minerals in these “skarn”-like veins), with enormous crystals of vein calcite filling most of the volume of the boudin necks. The fact is that we see the reaction rims and the multi-cm-scale calcite crystals only in the spaces between the blocks of greenstone (and not along the bottom/top contacts). The inverse is seen with the dark-colored, fine-grained “chill zone” that one might say is where the quenched obsidian formerly was located: you don’t see that in the spaces between the greenstone pods. This suggests to me that interpreting them as Alleghanian-age boudins and boudin necks is the more parsimonious interpretation. A fine way to test my interpretation would be to compare the orientation of these structures to Alleghanian-aged structures from elsewhere in the region. Unfortunately, this is a block that is displaced from its original position (float, not outcrop), so the geometry is divorced from its in situ orientation. At any rate, the pervasive Alleghanian deformation of Blue Ridge rocks gives me reason to suspect the that this contact isn’t pristine enough that I’d feel comfortable deeming it as definitely conformable.
Photo by Jay Kaufman; Click to enlarge
Jay sees primary structures, and I see tectonic structures. Maybe there’s a bit of both?
I love this site, and the rest of the Fauquier Formation, for this reason – it’s not 100% clear what it means. That means it’s a great place to train students to think for themselves about the paths we follow from the observations we make on site (or virtually “on-site”) to the stories we tell about the past based on those observations.
Regardless of the final interpretation, these GigaPans offer plenty of information to stimulate discussion and inquiry. And the great thing about our efforts last weekend is that now everyone can “visit” these rocks and use their observations to power their own independently-formulated hypotheses.
Enjoy exploring them, and viewing them through the lens of your own training and background. Let us know what you see!
16 April 2013
Squashed pillows
Today, let’s take a look at another site from the pre-GSA-Minneapolis field trip to examine the structural geology of the sub-province boundaries in the Superior Craton:
Do you recognize what you’re looking at there? Here’s another example, this time with scale:
Those are pillow basalts, metamorphosed into greenstone! Annotated:
Take a fresh look, and hopefully their round, pillowy shapes will pop out at you:
These pillow basalts crop out near Ely, Minnesota, and are part of the same broad package of rocks that includes the banded iron formation near Soudan, and the turbidites exposed at Pike River Dam.
The circled “4-X” numbers refer to field trip stops on the fourth day of the trip. These Archean pillow basalts were seen at stops 4-2 and 4-3.
Where primary structures survived, the pillows were defined by a differently-colored rind and a train of vesicles (amygdules) interior of that.
Here’s another vesicle train…
But these pillows have been squashed – and a penetrative cleavage has overprinted all the rock in the outcrop. Can you spot it?
Let’s zoom in.
Thank goodness for the occasional 3D exposure! Here’s that same scene, garnished with annotations:
Cleavage strikes northeast and dips steeply to the south. It’s a nice example of flattening strain (X=Y>Z, in terms of the axes of the strain ellipsoid).
Here’s an animated GIF to give you a sense of this “biscuit-like” geometry:
Plenty to talk about here:
Time to head on out to another outcrop…
15 April 2013
Boston Marathon: 17 years later, and a couple minutes
On Tuesday in 1996, I showed up very tired for my 8am paleontology class at the College of William & Mary. I had driven all night back to Williamsburg from Boston, where the day before I’d run the 100th Boston Marathon. It was my first marathon (I ran it unofficially – I was what runners call a “bandit”). It was an amazing experience to take on a run of that length, and finish it. Though I went on to run two other marathons – Big Sur and Adirondack – with better times, I see the 100th Boston Marathon as the pinnacle of my running life. Along the entire route, spectators cheered me on in the most affirmative way. It was joyous and amazing. I had written “CALLAN” in Sharpie across my yellow Chiquita banana shirt, and that was the catalyst for thousands of shouts of personal affirmation. “Go Colin,” they yelled. “You can do it, Callahan!” And I needed the encouragement; though I had trained with a maximum of a 19-mile run before the race, going 26.2 miles was really, really hard.
My spirits flagged around mile 24, and I stopped running and started to walk. I walked for about half a mile or so, until an Irish fellow came along and clapped me on the back, saying “Come on now, let’s finish this thing!” He gave me a smile and nodded his head toward the finish line. With his joyful entreaty, I began running again, ploddingly at first, and then sprinting faster and faster. I felt elated, giddy – In retrospect, I think that was the one and only time I’ve ever felt the fabled “runner’s high.” I crossed the finish line 4 hours and 12 minutes after I started, and the mix of emotions I felt at that moment were at profound levels. I was simultaneously laughing and crying, drooling and dizzy. It was amazing.
I’m so thankful for that man’s encouragement. He made me push myself, and by finishing the race running, he made my day.
Murder is awful, and every innocent life that’s lost is worthy of breaking my heart, but there are so many lives ended every day in awful, preventable ways, that I find I cannot dwell on them to the extent they deserve. Only the most horrific tragedy, like the murder of schoolchildren at an elementary school, can break through this wall of emotional distance that modern culture has encouraged me to build. But today’s bombing of the Boston Marathon, at the finish line, at the bombs went off at 4:09:44 race time, 2.5 minutes before my own (unofficial) finish time, strikes home in a very personal way. It pierces that wall because of the intensely positive experience of being at that exact place myself. That intoxicating cocktail of emotions I felt – I can’t imagine that ebullience suddenly turned completely on its head, and ripped apart, showing the starkest, darkest side of my awful species. It’s so completely wrong.
Athletic achievement, the perseverance, the struggle and success – so much positive energy put into preparing for a marathon, for running it, for helping your fellow humans – and then for someone to take that special moment, and turn it into an opportunity for the most depraved act, … I just don’t know what to say. It turns my stomach.
Of course, my thoughts are with the people who were put in that position today. Just as they were about to experience one of the best moments of their lives, some murderer turned it into the worst; perhaps even the final moment. It is the most incomprehensible barbarity – How can anyone see this action as justifiable? I wonder if the perpetrators will invoke politics or religion as their motivation?
Monday macrobug: Twilight of the ladybug infestation
Spring has arrived in the Fort Valley. We went from snow and sub-freezing nights to two 80°F+ days in a row, and then throttled back to normal springtime temperatures.
The pulse of heat brought about a big release of ladybugs from our walls, and at times it was as apocalyptic a scene as it was back in October and November when they swarmed all over the house, looking for a way in. Here is a look at the interior of our screened-in porch:
But – great news – over the weekend, the numbers dropped down to approximately zero! It appears that they have all flushed out, and the survivors have dispersed into the great outdoors. If I were to go out again and re-photograph these same corners, they would be blissfully devoid of ladybugs.
My plan is to seal the exterior of the house this spring with expanding foam and silicone caulk, in hopes of limiting our overwintering population of ladybugs next year.
12 April 2013
Friday fold: Mosaic Canyon
For the Friday fold today, let’s return to the warmth of Mosaic Canyon in Death Valley National Park, California:
Can you see it there? Let’s zoom in on the dark area in the middle of that first photograph….
Ahh! Now that’s a fold to end the week on! Enjoy the weekend, everyone.
10 April 2013
Primary structures in the Lake Vermillion Formation
This is the final outcrop that we visited on the fall 2011 “Structural Geology of the subprovince boundaries in the Archean Superior Province” field trip that I took just before the fall 2011 annual meeting of the Geological Society of America in Minneapolis. It was a great trip, full of great outcrops, but this one was one of the best. I’m going to split my discussion of the place into two posts – one for the primary structures (today, here), and another later in the week for the secondary (tectonically-induced; deformational) structures.
The Lake Vermillion Formation is well exposed at Pike River Dam in northern Minnesota. It’s a gorgeous succession of turbidite strata. Graded bedding is very common there, and exquisitely photogenic.
According to the field trip guide, 64% of the 201 graywacke beds in this outcrop exhibit grading, as do 9% of the inter-lying silt beds.
There are also lovely examples of soft-sediment load-induced deformation:
This means that a fresh load of turbidity-current-deposited sand was dumped atop some squishy mud, and the sand sagged downward in broad lobe-like forms, while the less viscous mud squished upward into pointy cusps called “flame structures.”
Callan Bentley is an assistant professor of geology at Northern Virginia Community College in Annandale, Virginia. He is particularly interested in structural geology and the evolution of the Appalachian mountain belt. Callan draws cartoons and writes for EARTH magazine. He lives in the Fort Valley of Virginia.
Recent Comments