26 June 2014
Okay, I photoshopped that one up. This one too…
Here are the originals…
And, if you’re a scale-off-to-the-side-of-your-main-subject purist, here’s a different shot of this quintessential boudin:
And, while we’re at it, here are some other fine boudins (of granite pegmatite) exposed along the coast of West Boothbay Harbor, Maine:
Some nice coastal ecology to be seen in that last shot, eh?
25 June 2014
Pemaquid Point, Maine, is a locally-owned and -managed park near an old lighthouse.
I went there yesterday with my family. We’re on vacation in coastal Maine for a week. At Pemaquid Point, the action of waves have cleaned the rocks, and they offer a delightful three-dimensional look at Acadian-aged metamorphics and granite pegmatite dikes, with a fair amount of structural geology superimposed on the whole lot.
Learn more about the site from this “Site of the Month” (July 2002) from the Maine Geological Survey. According to it, the sedimentary protolith of these rocks was Silurian in age, with Devonian metamorphism and igneous intrusion on top of that. (The Maine Geological Survey has a great collection of site reports like this, searchable in map format, here.)
The folds range from open…
Here are a few more:
There is also a lovely suite of examples of boudinage, on many scales:
As you can see, I brought the GigaPan with me, and took a couple of new images with it:
And here are two that are not mine (These next two from GigaPanner “eos101” at Bowdoin College):
20 June 2014
One of the real treats of the spring break field course in west Texas was visiting a road cut of the Castile Formation, a Permian evaporite formation south of the Guadalupe Mountains. It’s on the border between Texas and New Mexico, and I’d been wanting to see it for years after seeing a photo of the folds there in a structural geology paper about folding.
The strata of the Castile Formation are gypsum/anhydrite (the white layers) and organic-rich limestone (the dark layers). They are thought to represent changes in the salinity of the basin in which the strata were deposited (the Delaware Basin). The regular periodicity of the alternation of sediment type suggests some sort of rhythmic influence on the salinity of the basin, and many workers interpreted theses black and white layers as varves – seasonal deposits.
But it’s probably a more complicated story than just “seasons only” – as there are anomalously thick layers interspersed throughout:
But we didn’t come here for the layers themselves. We came to see their high-contrast outcrop pattern when folded. Though this post runs the risk of overwhelming you with gorgeous fold imagery, I’m going to just let loose the avalanche:
And, why not, let’s finish this feast with a single fault example:
19 June 2014
That’s the State Line outcrop south of the Guadalupe Mountains, along the Texas / New Mexico border. Know what you can find there? Tune in tomorrow to find out…
17 June 2014
Yesterday, I pointed out an example of differential weathering on Old Rag Mountain, in Shenandoah National Park, in Virginia. Today, I’d like to shine the spotlight on another example of weathering to be seen along the trail there: little weathering pits that occur on the top of the granite outcrops. These are opferkessel.
Some people call these “potholes,” a term I do not approve of in this context. To me, “pothole” implies focused abrasion (in a fluvial setting) – which is not how opferkessel form. Others opt to dub them “solution pans,” but I don’t love that one either, since the granite isn’t really “going into solution” at these sites so much as it is being subjected to increased levels of chemical (and, to a lesser extent, physical) weathering. The actual genesis of opferkessel is more prosaic and basic: the chaotic business of weathering a rock means some areas will be higher than others, and some will be lower. The lower spots tend to hang on to rainwater for longer, which means the minerals lining those low spots will be subjected to more chemical weathering (oxidation and hydrolysis mainly, in the context of granite). Consequently, those minerals “rot” faster than their neighbors in higher, drier positions. When the wet minerals weather away, the little low spot becomes bigger, and so can hold more water, and that can encourage more chemical weathering, and so on. It’s a self-amplifying process. Physical weathering may play a role, too – I can envision additional breakup of the granite due to the splashing action of impacting raindrops, the jostling of small waves, and even the grinding action of ice that forms in the opferkessel during winter months. Many of the opferkessel have living things in them, too, and the humic acids, hyphae, and even scrabbling of little legs may focus additional weathering power on the walls and floor of the opferkessel.
The actual summit (highest point) of the mountain (3,291 feet above sea level) has a very well-developed opferkessel, too:
(Note the quarter for scale on the “peninsula” at bottom center)
If you’re into weathering, Old Rag’s an excellent destination.
If you can’t get to Old Rag in person, here’s a GigaPan of an opferkessel there for you to explore:
16 June 2014
Old Rag Mountain is a distinctive mountain in the eastern Blue Ridge of Virginia, contained in a little lobe of Shenandoah National Park. It’s a great hike on several levels: (1) it’s got no trees on the summit, so you can actually get a decent view from on top, (2) it’s got a great section of full-body rock scrambling on the Ridge Trail, and (3) it’s long (9.2 miles round trip), which is why I brought a group of four of my Rockies students there last Friday for a training hike. Today and tomorrow, I’d like to show you some of the geology we saw there. Here’s one of the most striking geological features to be seen at Old Rag:
The sharp-eyed among you will note that most of the rock in the picture is light-colored and coarse-grained – on both the left and right sides of the photograph. This is the Old Rag Granite, a Mesoproterozoic (~1.04 Ga) granite that intruded in the final phases of the Grenvillian Orogeny, during the assembly of the ancient supercontinent Rodinia.
Running through the middle of the photo, however, is something different: a rock that is dark in color, and fine-grained. This vertical seam is a feeder dike of the Catoctin Formation, a Neoproterozoic (~565 Ma) basaltic dike. Flowing through this ancient fracture was basaltic magma that fed surface eruptions of lava that flooded the landscape. Tectonically, this was during the breakup of Rodinia, and the opening of the Iapetus Ocean. The dike was metamorphosed to low-grade greenstone facies much later, during the Alleghanian Orogeny (~300 Ma), during the assembly of the more recent supercontinent Pangaea.
The mafic (iron and magnesium rich) minerals that make up the basalt are less chemically stable at Earth’s surface than the felsic minerals (potassium, sodium, and silica rich) that make up the granite. Consequently, they fall apart (weather) more quickly, and the dike is soon etched into an empty slot in the landscape. Many of these differentially-eroded Catoctin feeder dikes can be seen on the Old Rag hike, mainly on the Ridge Trail. The trail itself actually goes through three of them, which is a very cool perspective – to be walking within the actual slot that once hosted pulsing, gurgling flows of 1100° C magma, bled out during the death throes of Rodinia.
13 June 2014
Marek Cichanski (of De Anza College near the south end of San Francisco Bay) contributed this week’s Friday fold:
The locality is a place near San Francisco called Devils Slide. It is a piece of the coastal highway built along a steep mountainside above the ocean. This unstable stretch of road was recently (finally!) replaced by some tunnels. And, amazingly, the old roadbed finally got converted to a nice paved hiking and biking trail! I never thought I’d live to see the day.
Most of the rocks here are Paleocene turbidites, IIRC. There is also Mesozoic granitic basement rock of the Salinian terrane at one end of the trail.
In the past, I couldn’t take my class here, because there was no safe place to park vehicles nor to stand by the side of the road. Now it’s all safe! I haven’t actually taken any students here yet, since the parking areas are small, and I didn’t even know the trail was finished until I was done leading my trip yesterday. But I sure hope I can take classes here, even if all we do is gawk at the cool geology and views!
This is also a great example of cool geology that people can visit when they go to AGU. A car would be pretty handy, although I think a bus line serves one of the trailheads – shades of `Streetcar to Subduction’. I think that for the more geologically-minded AGU attendees, it’s well worth skipping the least interesting half-day of talks to see this, if the weather isn’t rainy/windy. That blue-sky weather that seems to prevail so often during AGU week would be ideal, if one could get here before dark.
This image is one of a nice set that Marek shot at the trail. You should check them all out.
12 June 2014
While at the Vulcan Quarry in Manassas, Virginia, a few weeks ago, the principle interesting feature I saw on the individual blocks of rock we sorted through was slickensides. I saw dozens and dozens of examples, in both the hornfels and the diabase, but here are five nice examples to share:
No sense of scale on this last one, because I didn’t feel safe edging up to the quarry wall (for fear of something falling off and popping me in the noggin), but this slickensided face is about 4 meters tall:
This last one is the only in situ shot of the slicks – and I hope you’ll notice what EPCC geology faculty Rob Rohrbaugh noticed immediately: all the slicks that were in place on the quarry walls showed horizontal lineations, which (a) must post-date the rocks’ formation, making the faulting Triassic or later, and (b) implies horizontal (right lateral or left lateral) motion on the faults. Also, it’s worth noting that (c) there were bazillions of exposures of the subhorizontal slicks on the quarry walls – this was a pervasive rather than isolated phenomenon.
This kinematic interpretation is hard to square with a neat and simple “extension causes normal faulting” scenario for the opening of the Culpeper Basin. Clearly, there must have been some syn- or post-rifting wrench component to the basin’s extension.
…It would be a great project for an Honors student to look into!
10 June 2014
When I was flying back from Phase I of “Border to Beltway” in Texas this past March, I was delighted to photograph a bunch of local geology from the air, including this prominent diabase quarry in Manassas:
I had never been to this particular quarry before, but as it turned out, it was one of our final destinations on Border to Beltway’s second phase, last month.
Here’s a closer view into the quarry, from its edge:
There’s a big dike of diabase (mafic hypabyssal intrusive igneous rock) in the middle, cutting across layers of the Balls Bluff Siltstone, a Triassic sedimentary deposit that was baked to hornfels by the heat released by the intrusion:
Up close, the diabase weathers out along joint surfaces, with its corners and edges reacting more rapidly (due to their greater surface area) with the water in the air:
This produces some nice examples of spheroidal weathering:
Here, Chris (our guide) and Josh (one of the B2B students) view a moderately-dipping contact between vertically-jointed diabase and shallowly-dipping Balls Bluff Siltstone (hornfels):
We got to pick through a recent blast pile for souvenirs…
…And being down in the pit gave us some additional perspectives on the contact between the two units:
Later on, I’ll show you some other sights from within the quarry. Thanks so much to Vulcan and to Chris for admitting us and showing us around.
9 June 2014
Bill Bryson is a very fun writer. Like many people, I first dipped into his oeuvre when he published his book about hiking the Appalachian Trail. Though I bruised my ribs laughing as hard as I did, I went back and read many of his other books – about traveling in Australia, or the UK. The one that totally knocked my socks off, though, was his superb book about science, A Short History of Nearly Everything. If you haven’t read that one yet, it really should go to the top of your list. But this little book review isn’t about that book; it’s about a more recent offering from Bryson: a memoir of his childhood in the 1950s in Des Moines, Iowa, possibly the most middle-America location possible. I read it over the past week or so, and found it quick and enjoyable, and while it’s not as profound as A Short History of Nearly Everything nor as hilarious as A Walk in the Woods, it’s a solid memoir with fun, authentic rememberings of childhood and teenagerhood. Though I didn’t grow up in Iowa nor in the same decade as Bryson, some childhood experiences are ubiquitous among American children, and these rang with authenticity for me. The cultural reflections on that decade are the most arresting matter in the book (including some horrific documentation of racism), but most of his jottings don’t meet a very high intellectual bar, nor are they particularly serious. It’s an enjoyable book on that count – some small amount of “making you think,” but it’s dominated by “making you laugh.” An excellent choice for summer reading.