4 September 2015
Twitter buddy Ian Stimpson posted some photos this week from his recent Arctic cruise. There are many gorgeous images in the set – you should check them all out – but this one in particular caught my eye:
This is at Prins Christian Sund (“Sound”), South Greenland. Ian suggests that the rocks are likely Palaeoproterozoic supracrustals – rocks that formed original as stratified rocks on Earth’s surface (sediments, volcanics) before being dunked down into Earth’s crustal interior and subjected to intense metamorphism and ductile deformation. Now, billions of years later, here they are back at the surface again.
Zooming in on the fold hinge:
It looks like Ian had excellent weather for much of his trip – I was surprised to see such sunny, crisp pictures from such a frigid part of the world. Check out the whole set!
2 September 2015
I just finished this excellent memoir by Mary Karr, mostly about her childhood, mostly in east Texas. It’s not explicitly geological but it does feature an oil town economy and a hurricane, as well as some consideration of the Rocky Mountain Front Range in Colorado. I didn’t read it out of any illusions it would be geological, though – I selected it from the library shelf more from a desire to broaden the diversity of the authors in the books on my nightstand. I’m so glad I picked it – it’s exquisitely written with a unique voice, snarky and deep in turns, describing experiences that run from the prosaic (but the way Karr writes about them, they seem profound and insightful) to the traumatic. There’s a lot of alcoholism described, serious family dysfunction, social friction, and two episodes of rape. It’s real, this stuff – and Karr writes so, so, so well about it all. She writes in a way that makes me envious; it’s so extraordinarily well done. She’s awesomely talented, and she has an incredible story to share. Recommended.
29 August 2015
I have a mystery for you today:
These are samples of Tonoloway Formation carbonate (not sure if it’s limestone or dolostone in retrospect), with bedding more or less horizontal in these images, and a few petite stylolites running orthogonal to that. The top sample has a gentle fold 2/5ths of the way across. All of the samples are from the same site in West Virginia, along Corridor H.
I’m wondering about the closely-spaced vertical (bedding-perpendicular) lines. Have any of my readers observed these before? What are they? How do they form?
Thanks in advance for any insight you can share.
21 August 2015
Ladies and gentlemen, I give you…
The Friday fold:
This beauty came to my attention on Monday, when I was lucky enough to go on a field trip with my friends Leigh and Mary. They are founding members of our local informal geology club, and we have been meaning to take a Cedar Creek field trip together since I moved out to the Fort Valley.
One of the sites we visited is a roadcut along the southern side of Baldwin Gap, a small water/wind gap in Little North Mountain.
While at Baldwin Gap, the question they posed to me was whether siliciclastic strata had been thrust on top of carbonates, and what unit was what.
We parked near the top of the hill (east) and walked downhill (west). We puzzled over what we were looking at – Pseudostenography of the conversation:
Was that Martinsburg turbidites?
No, can’t be Martinsburg, because here’s a quartz sandstone.
Wait, here are some redbeds – that looks like Bloomsburg. Hmmm – what unit could this be?
Sure enough, here at the bottom of the hill are limestones, including mudcracked limestones, which suggests Tonoloway Formation. Is this one unit? Two divided by a thrust fault?
…more than two…?
As far as structures go, we noted several folds (the one above was best expressed) and two small-offset reverse faults, with westward dips.
A field experience like that, for which I had not specifically prepared, raised questions in my head, and I went home eager to answer those questions.
Fortunately, the area has already been mapped.
The Geologic Map of the Middletown, Virginia quadrangle (1999) by Orndorff, Epstein, and McDowell was the resource that confirmed our field observations and put them into a regional framework. Here’s a screenshot I took, annotated with key units, and including a detail of the thrust-faulted geology of the gap itself:
The ridge into which the gap is cut hosts not one, not two, not three, but four separate blocks, stacked up along thrust faults. This explains the diversity of llithologies we saw, and –delightfully– confirmed our assessment of which units we were seeing. Sure enough, there were clastics at the top of the hill (east), thrust on top of limestones at the bottom (west). The map-scale faults dip east, so the small-scale faults we observed were antithetic to that orientation – not an uncommon thing to observe.
The fold is probably in Tonoloway (or Helderberg?) – it was pretty much the westernmost outcrop in the gap.
This outcrop is worth sharing with the world, so in the fall or winter, when the vegetation dies back a bit, I’ll revisit it with my GigaPan.
Thanks for showing me this place, Mary and Leigh!
20 August 2015
Here are a few shots of a Devonian aged reef exposed in Mustoe, Virginia – one of the sites I visited this spring with GMU’s Rick Diecchio, when he led his sedimentology and stratigraphy trip there.
At first, the outcrop made no sense to me – I kept searching for bedding, and failed to find it. Then, the reef interpretation clicked, and suddenly I didn’t “need” bedding any more…
Stromatoporoids were probably spongelike organisms, with layers like stromatolites, but in those layers they have a tiny architecture of ‘rooms’ and ‘pillars’…
A lot to see here – certainly a site I should return to some day with a GigaPan.
19 August 2015
Have you ever found a rock sample that looked something like this?
It’s got a lot of rust, but mainly around the edges. What’s up with that?
Check out my hat for a clue – this is after a long hot afternoon picking apples in the sun on Sunday:
Did you see the dirty “rind” where my sweat soaked in and moved dirt and grime along with its flow, to accumulate at the edge of the soaked zone? Flow paths depicted in the annotated version below:
In the same way, fractures (joints and faults) in rock can serve as conduits for fluid flow, and surfaces along which subterranean fluids can soak into the “fabric” of the surrounding rock.
In the case of this sandstone, the white is “unsoaked” and the brown shows the rusty signature of overlapping successive “soaking fronts”:
All the rock examples in this post are from the new Bismarck, WV, outcrop of Conemaugh Group sedimentary rocks exposed along Corridor H. Here’s an example of conglomerate overlying sandstone, with both hosting the Liesegang-like iron oxide banding:
Zooming in on the contact:
These features can be quite beautiful, making Rorschach-blot like patterns in the rock…
18 August 2015
On the recommendation of my friend Troy Holland, I just finished reading Michael Punke’s novel about Hugh Glass. The book has been optioned as a film, and because it stars Leonardo diCaprio, it will doubtless be a hit. What got Troy’s attention, though, is the director: Alejandro González Iñárritu, the guy who gave us Babel, Birdman, and Amores Perros, all of which were masterful films.
So what’s a ‘revenant?’ Wikipedia’s got it: “A revenant is a visible ghost or animated corpse that was believed to return from the grave to terrorize the living. The word “revenant” is derived from the Latin word, reveniens, “returning” (see also the related French verb “revenir”, meaning “to come back”).”
In this case, Hugh Glass is the revenant, but as an analogy. Glass, a real person, was mauled by a grizzly bear while in the employ of the Rocky Mountain Fur Company in 1823. Two men were assigned to watch over him while the main body of the party went upriver, and to bury Glass when he inevitably died. Except they didn’t – they abandoned him. Glass didn’t die. Deprived of almost every resource, he vowed to survive and visit vengeance upon the two men, one of whom was Jim Bridger (namesake of my beloved Bridger Range near Bozeman, Montana).
His odyssey is insane – the difficulties that Glass overcame, expertly described (in lightly fictionalized form) by Punke, are astonishing. Rattlesnakes, starvation, blizzards, Arikara warriors, and even federal bureaucracy all conspire to put an end to Glass’s quest for revenge. But he keeps working at it, keeps coming up with clever ways around problems, like a frontier MacGyver (though a bloodthirsty one).
The book is a great way to revisit the post-Lewis-&-Clark, pre-civlization days of the Rocky Mountain west. Punke’s narration brings to life many of the daily routines, risks, and perspectives of ‘the white man” entering a land he saw as savage. Native Americans will probably not be thrilled with the rendering of the Arikara as savages, however. They fought tenaciously to keep white pioneers from invading their homeland, but The Revenant depicts that all from the pointy end of the arrow – little sympathy is conveyed, which I think is an accurate choice, in depicting the events of the novel from Glass’s perspective. There are a few token “good Indians” in the story, but none of them have their backstories explored, as a half dozen of the non-Glass white charaters do. Instead, the more dominant treatment for the Native Americans is straight up “cowboys and Indians” (good guys and bad guys). So that’s that.
Overall, it was a compelling adventure story – although I’ll warn you that the ending didn’t really satisfy me as much as I had hoped. I’m looking forward to the movie. Here’s the trailer:
17 August 2015
I’m grateful to Mountain Press for sending me copies of all of their new books. There are some terrific volumes that have arrived in my mailbox over the past year, and I feel guilty for not reviewing more of them. But when I upwrapped this one, I was struck by two things:
1) The author is a geoblogger, and a prolific one. Dave Tucker writes Northwest Geology Field Trips, and apparently that effort led to the authorship of this book. That piqued my interest.
2) The cover image is an extraordinary image of Mount Rainier sending a lahar down to Puget Sound – beautifully rendered and as clear a blend of geoscience and art as I have ever seen.
So I read it.
The website seems like an incredible resource for western Washington geology, and the book is the same. There are some really amazing sites detailed in the book – places I would love to see. Mr. Tucker writes about them very clearly and compellingly. I read it cover to cover and learned a lot. It’s beautifully produced, with high quality graphics and color photography, and the analogies are excellent.
The one thing that drives me bonkers about the website is the ‘stamping’ of all the photos with an inelegant source ‘watermark’ (example). I understand why Mr. Tucker does that – internet plagiarism is rampant and he’s only being proactive in protecting his imagery – but it’s such an ugly way of claiming one’s photos as one’s own. To the aesthetically-inclined eye, the watermark really distracts from the geologic content of the imagery, rendering the photo less useful than it would have otherwise been. To me, it really gets in the way.
That’s not the case for the book, which allows a reader to immerse themselves as directly as possible in the geology of these diverse sites, with few reminders that Mr. Tucker is there acting as translator (meaning he does a very good job). Clastic dikes, pillow basalts, glacial erratics, lahar deposits, ghost forests, waterfalls, spits – there’s a wealth of geology in western Washington, and this book is THE resource I would use to guide me in exploring it.
I recommend this book if you’re on your way out to western Washington anytime soon.
15 August 2015
My collaborators and I are exploring the potential of big science databases, like the Paleobiology Database (see below), to enhance geoscience education and research at all types of institutions. We’re very interested in learning who is and isn’t currently using this and other databases for education and research and why. This work is supported by an NSF DUE grant #1504588.
If you could take about 10 minutes to fill out the following survey, we’d very much appreciate it:
If the link doesn’t work, please feel free to copy and paste the web address.
The Paleobiology Database (PBDB) is an online database that stores information on when and where fossils occur throughout geologic time. It includes a variety of information such as taxonomy, abundance, ecological traits, and body size, all gathered from the scientific literature. You can find out what the PBDB has to offer here: www.paleobiodb.org.
If you have questions regarding this survey, please feel free to contact Dr. Rowan Lockwood at the College of William and Mary (email@example.com).
Callan Bentley (Northern Virginia Community College)
Peter Berquist (Thomas Nelson Community College)
Christian George (High Point University)
Rowan Lockwood (College of William and Mary)
Laura Lukes (George Mason University)
Katherine Ryker (Eastern Michigan University)
Mark Uhen (George Mason University)
14 August 2015
For the past two days, I’ve been asking readers to think about some interesting rocks exposed along a newly-opened stretch of Corridor H in West Virginia.
Here’s the scene.
I visited the site twice last week – once with Dan Doctor (USGS), Mitch Blake (West Virginia survey), and Alan Pitts (University of Camerino), and then again with Jeffrey Rollins (Old Dominion University & Team GEODE) and Alan Pitts again.
We are talking about these rocks:
Today is the big reveal – a detailed look at this site. But the images I showed you above were only a taste. The real action is across the street:
Whoa! That looks like an angular unconformity, right? Check it out in these GigaPans:
link [GigaPan by Marissa Dudek]
link [GigaPan by me]
But here it is in broader context:
link [GigaPan by Jeffrey Rollins]
link [GigaPan by Alan Pitts]
A detail of the central area showing coal seams in crazy orientations (Mitch Blake christened this site “The Dragon’s Tongue”):
link [GigaPan by me]
I hope at this point you have noticed all the different angles at which bedding appears in these roadcuts, and the various truncating relationships between key marker beds. Perhaps, you are wondering what’s going on here.
These strata are Pennsylvanian in age (later Carboniferous). They are part of the Conemaugh Group.
These strata are sandstone, siltstone, shale, and coal. The coal is the black stuff:
Here are two coal seams, at a ~20º angle to each other:
As you might expect with all that coal, there are lots of plant fossils in these rocks:
Here are two samples of the distinctive diamond-shaped pattern seen in lycopsid bark:
Here’s a section of stump/branch/log/root that Jeff found:
A stem in one of the white sandstones:
And another example of big pieces of coalified wood in the coarse, white sandstone:
So, what’s going on here? Another example of a truncated coal, ending abruptly (and at an angle) relative to an overlying sandstone (here, stained orange from rust):
The bottom of this same sandstone shows features that look strongly like “ball and pillow” (soft sediment deformation due to density inversion):
So maybe these relationships are sedimentary, depositional.
But… the angles of bedding are all over the place! Look at this:
Could these layers be juxtaposted through tectonic deformation instead? Could these abrupt truncations be thrust faults? Certainly that was a common theme in the guesses offered by readers.
There is some smaller-scale structure to be observed, such as this batch of small crinkles in on of the shale units:
…or these recumbent folds in siltstone layers, a few inches away:
In one of the highly tilted sandstones, there is internal folding, too:
Here, the trace of bedding can be followed around the left and lower edges of this block weathered out from the outcrop, apparently overprinted by a weak crenulation lineation:
Look at this truncating relationship:
Jeff examines this site (same site; photographer’s perspective has shifted a few meters to the left):
Elsewhere, there are odd “lozenges” of coal in the sandstone:
Is that a coalified branch? Is it a bleb of former peat that got torqued and twisted during deformation? Was the deformation pre-lithification (and coalification), or post-?
One idea on how to interpret these rocks is that there these discontinuities represent tectonic (Alleghanian) faulting, but when we looked for slickensides, fault breccia, etc., we didn’t find it. Instead, we think the best way to interpret this outcrop is a series of paleo-slump-blocks that were then incised by rivers, removing some portion of the slump blocks and depositing coarse channel sandstones.
Here’s Alan Pitts’s annotation of the outcrop, based on a hand-held panorama (one of the GigaPans embedded above) and condensed a bit by me in order to economize on space:
This I think, is instructive on two levels: (1) Alan is a talented graphic artist, and (2) this sort of annotating a high-res image is a useful method of developing hypotheses about an outcrop. You can see, for instance, that Alan has outlined the prominent white sandstone blob in the upper left with two faults. Here’s a close up of that site from the upper bench:
It didn’t look like it was fault-bound to me; it looked to me like a channel incising into (slump-) tilted strata, and then filled in two separate episodes (earlier dark gray and medium-grained sand, later white and coarse-grained sand). In other words, in my mind, the arcuate bottom to that protrusion is primary, a depositional feature, not a later tectonic imposition.
So here’s my attempt at annotating Alan’s GigaPan (i.e. same base imagery as above, but different a geologist making the lines and interpretations, using a stylus on a Wacom Cintiq):
I did that solo, without reference to Alan’s annotations, to minimize his influence on my thinking, and see what I would come up with on my own. He may have missed some stuff I caught, and I may have missed some of the stuff he caught. Caveat emptor: Both Alan’s sketch and mine are preliminary – a method of practicing looking at the outcrop. Nothing here should be taken as conclusive. I share them here in their unpolished state to reveal the way we approached this problem, and how the imagery was a catalyst to our thinking.
I interpret these rocks as paleoslump blocks – layers of clastic sediment and peat laid down, perhaps during eustatic highstands, then incised during the regression accompanying a late Paleozoic glaciation. These incisions likely oversteepened the slopes of these soggy sediments, and they sloughed into the resultant valleys. This rotated the strata, and exposed the leading edge to erosion at the river’s cut bank. The slump blocks, as Alan pointed out, would have a “normal” sense of offset on their trailing edge, but a compressional (and potentially even “reverse”) sense of kinematics on their noses.
Later, transgression accompanied deglacation, and fluvial deposition laid down the coarse white cross-bedded sandstone (with channel lags of gravel and big branches of plants included). Then another cyclothemic cycle went into force, and the pattern repeated. That, to me, is the most economical way to explain what I see at these outcrops. But I’m open to being convinced otherwise: You now have the same data we do. Take a crack at it – and once, again, Tell Me What You See Here….