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11 September 2018

Drawing geological structures, by Jörn Kruhl

After blogging about geovisualization, reader James Safranek alerted me to this new book about two of my favorite things: drawing and structural geology! I requested a review copy from the publisher, who kindly provided one. It’s great!

This is “a whole book” about drawing and geology and specifically structural geology. As such, it’s not going to be as pertinent to every reader as it was to me. But I found it a joy to read, with thoughtful advice about the drawing process (as opposed to the final outcome), our motivations for generating useful geological illustrations, and the specific process to follow when generating certain kinds of images.

To the first point, consider this quote, which is now part of my performance repertoire when I give my geo-visualization talk:

A special merit of drawing is that it requires us to look closely. The click of the camera cannot do this. While we are drawing, we must already geologically assess what we are drawing. Therefore, not just the drawing, but also the path to it, is relevant. Graphical representation — manual drawing — is nothing old-fashioned and superfluous, or just a nice pastime. In the digital age, it is urgently needed, because it teaches us to observe and reflect and it leads to concentration and mindfulness.

Another thing that resonated with me is Kruhl’s comments on “tools,” specifically paper and pen. He doesn’t like lined paper or graph paper, and I wholeheartedly agree. My field notebooks are blank. I even number the pages myself! And I prefer pen to pencil, and so does Kruhl. He lists the importance of consistent line width as one motivation (for as a pencil’s sharp tip is worn down, it creates wider lines), but I also hate the smudginess of ordinary pencils, but I’m also turned off by the dimness of the lines that “hard” pencils leave behind. They don’t smudge, but neither do they really write. I like pen, and Kruhl does too.

Kruhl addresses several kinds of image-making. In particular, I found his comments on drawing thin sections and drawing what he calls “stereograms” (regional structural summaries) to be illuminating. The examples he shows from his own work are nothing short of inspirational. It’s a fun read, and I’m going to have this one within easy reach on my bookshelf for the rest of my career. Recommended!

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7 September 2018

Friday folds: Continuing contorted Conestoga carbonates

Happy Friday to you all! I’ve previously showcased folded Conestoga Formation on the campus of Millersville University in Pennsylvania for the Friday fold. Today we’ll go a bit further afield, to the Burle Business Park on New Holland Ave in Lancaster. There are plenty of nice outcrops of Conestoga Formation limestone exposed there, and they have a few cool things to show us.

I visited this site on a field trip during June’s Eastern Section meeting of the National Association of Geoscience Teachers. Here’s Martin Schmidt, David Brink-Roby, and Don Duggan-Haas traipsing across one of the exposures:

Sedimentologically, the site is noteworthy because it includes some submarine landslide breccias, as seen here:

But for me, the best part was the plethora of sweet folds:

Happy Friday!

As a friendly reminder, I welcome submissions of “Guest Friday folds” for the blog. Just shoot me an email if you’ve got some cool folds to share!

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31 August 2018

Friday folds: Rifugio Fontana in the Dolomites

Another guest contribution for the Friday fold. Today, my friend Maitland Sharpe chips in with this mountainside he saw in Italy, specifically the

Dolomites, near Rifugio Fontana, Longarone

…And here is the same image, annotated in red to show the trace of bedding:

The strata here appear to be carbonates. I’d have guessed limestone, but given the fact that this is the Dolomites, I guess the chances are good that they are dolostone layers instead. The folding style is somewhere on the spectrum between kink banding and chevrons. It’s loooooooovely!

Two close-ups follow, one annotated:

I’m not even going to attempt tracing out bedding in that last one – I fear it may drive me insane!

Thanks for sharing, Maitland!
Readers, please know that I value your contributions to the Friday fold series – I love seeing the folds you see on your travels.

A happy Friday to all!

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27 August 2018

Beautiful Swimmers, by William Warner

The subtitle of this wonderful book is “Watermen, Crabs, and the Chesapeake Bay.” It’s an excellent account of crab ecology in the Chesapeake Bay as it stood in the mid-1970s, and simultaneously a sympathetic portrait of the lives of the locals who capture those crabs for sale to the seafood market. The writing is thoughtful and calm, paced very similarly to John McPhee’s writing, rich in quotes from the watermen speaking their local vernacular. It’s no surprise to me that it won the Pulitzer Prize for general nonfiction in 1977 (two years after Annie Dillard’s Pilgrim at Tinker Creek and a year prior to Carl Sagan’s The Dragons of Eden). I found it utterly engrossing, and would recommend it to anyone with an interest in the Bay’s economy or culture, a yen to learn more about the seasonal rhythms of an extraordinary animal, or just a taste for a soft-shelled crab sandwich, one of life’s finest pleasures (in my humble opinion). The title, I should note, is a translation of the scientific name of the Chesapeake Bay blue crab, Callinectes sapidus, which I’ve always appreciated, given that the first two syllables are pronounced identically to my own first name. Recommended.

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24 August 2018

Friday fold: Gneiss from Arapaho Pass, Colorado

It’s Friday, and I have another guest Friday fold to share:

This one is from my Denver friend Greg Willis, who tells me it’s

from near Arapaho Pass, near where we rain-hiked.

Ahhhh, yes – a singularly soggy hike up in the Colorado Rockies. I remember it well, and it looks like Greg had better weather on this jaunt!

Happy Friday to all!

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22 August 2018

Q&A, episode 5

After a bit of a respite, it’s now time for a fresh edition of “you ask the questions” here on Mountain Beltway. Anyone can ask a question, serious or spurious, and I’ll do my best to answer it here. Use the handy Google Form to to submit your questions anonymously.

Here’s this week’s question:

7. What is foliation and what makes it so important to the structure of rock?

Foliation is a planar alignment of mineral grains in a rock. It’s dependent on those mineral grains not being spherical, but having some non-equant shape, like a flake or a line. If you get all those flakes (or all those lines) in a common orientation, it imparts a “fabric” to the rock, in the same way that the alignment of plant cells in a log give it a “grain.” Just as it’s easier to split a log along the grain than across it, it’s often easier to split a rock along the foliation than across it. In this case (easier to split in one direction), we’d say the rock has not just a foliation, but a cleavage. So a cleavage is a foliation that is also a plane of weakness.

Foliation can have several causes: it could be a primary feature, say due to flow in a molten rock mass, but more commonly it refers to a tectonic feature, imposed on a pre-existing rock by differential stress (stress that’s greater in one direction than in other directions). As such, foliation is a secondary structure, and example of strain, a deformational feature that develops in rocks that have been squeezed by geological processes. Because rocks in these settings are often recrystallized under high temperatures and pressures, rocks that are foliated are often also metamorphic rocks. But you can have the one without the other.

A common way to get foliation to develop is through pressure solution. Pressure solution is a phenomenon where certain minerals dissolve under high pressure, and re-precipitate under lower pressures. Calcite is particularly susceptible to this, but quartz does it too.

Here’s an example of evidence for this phenomenon: “bearded” porphyroclasts in metamorphosed turbidites exposed on the north shore of the Isle of Arran, Scotland, just west of Lochranza:

Now an annotated version , to show the shape of the stiff, strong grain (porphyroclast) that resists dissolution, and the “beard” or “fringe” of precipitated quartz that grows in the “pressure shadow” adjacent to the grain:

All that quartz in the fringe adjacent to the porphyroclast had to come from somewhere. And where it came from is likely the direction 90° to the long axis of the fringe. It dissolves in the highest-stress direction (up and down in these photos) and re-precipitates in the least-stress direction (left and right in these photos). Here’s another example, also from Lochranza:

This one shows an additional feature, a dilational crack that has partially split the porphyroclast and opened up a little vein (also in the low-pressure zone). I’ve highlighted it with a blue arrow:

A rock body full of grains like these, shortened in one direction and enhanced in another, attains a fabric. It gets foliated. These Scottish rocks I’ve shown so far are interesting because the bedding (defined by grain size) and the foliation (defined by alignment of the mineral grains) are parallel:

That implies the maximum stress direction was perpendicular to bedding, which is to say, vertical (all else being equal).

In many circumstances, this is not the case. Next, take a look at an outcrop showing ~horizontal bedding and ~vertical foliation (Chancellor Group limy slates in Yoho National Park, British Columbia). Here, they are perpendicular to one another:

All else being equal, this might be the default expectation for metasedimentary rocks: the bedding is initially horizontal, and so is the tectonic shortening direction (thanks to plate tectonics), so we would expect a more or less vertical foliation.

Here is an outcrop showing bedding and foliation in metasedimentary rocks (Frog Lakes Conglomerate in the high Sierras of California), showing a non-parallel, non-perpendicular relationship:

Here is bedding and cleavage expressed in limestone and shale/slate of the Edinburg Formation, near Lexington, Virginia:

There’s clearly a lithological control on foliation’s development, right? It’s only weakly developed in the pure limestone, but where there’s a bit of clay in the rock layers, foliation/cleavage is pronounced and obvious. [Link to a GigaPan showing a similar outcrop]

Again there is an angular relationship between the two planar features. But this one illustrates nicely how bedding and cleavage both dip to the right, however bedding is steeper (closer to vertical) and cleavage is shallower (more moderately dipping), the opposite of the California example. This tells us that these beds have been tectonically inverted (flipped over).

Where seams of pressure solution are localized with big gaps between them, we dub them stylolites, and to believe the testimony of marker units such as calcite veins, they are apparently capable of dissolving away huge volumes of squeezed rock:

Where the pressure dissolution is more evenly distributed through the body of rock, and in particular where it changes the average grain shape within the rock or aligns non-soluble platy minerals (such as clays or micas), you get a proper foliation (± cleavage), like this example from the Swift Run Formation in the Virginia Blue Ridge:

And here is an example from a slab of building stone in Baltimore, Maryland:
IMG_1814

Now for some interactive media: embedded GigaPans and GIGAmacros and also 3D models. We’ll start with the Chancellor Slate (shown earlier in this post), because it’s such a beautiful example of bedding and cleavage relationships:

Here’s an example of mylonite, a highly foliated rock formed along hot fault zones:

Here’s an example of cleavage refracting across sedimentary layers with varying amounts of clay vs. silt (Neoproterozoic Konnarock Formation of Virginia):

 

Here is an example of gneiss (Archean basement of Montana), which shows a much coarser flavor of foliation: it’s a 3D model, so you can spin it around and examine how the foliation looks from different angles.

Here is another 3D model, an example of an “L-S tectonite” (phyllitic metavolcanics from the Phoenix Mountains of Arizona), which shows a well-developed lineation on the plane of the foliation:

 

As for the second part of the question, about why foliation is important, I’d say that the single biggest reason is that it is one of the principal signatures of tectonic shortening (in one direction); implying tectonic elongation in another direction. Mountain belts form when rocks are shortened horizontally and thickened vertically. Though mountains are ephemeral features on the face of our planet (because erosion reduces their topographic expression over geologic time), the rocks at their roots will show foliation, and that tells us about the ancient episodes of tectonic squeezing they have enjoyed.

If you have other questions about science, the Earth system, or anything else, please ask them.

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20 August 2018

Under a Green Sky, by Peter Ward

I’ve got a few books to catch up on from my summer reading. The first is Under a Green Sky (2007), by University of Washington geoscientist Peter Ward. I picked this up because it was referenced in another book I’d read recently: Peter Brannen’s The Ends of the World (2017). (I’ve got a review of that one coming out in a forthcoming issue of EARTH magazine, by the way!) Brannen described Ward’s book as highly accessible, though I’d say that Brannen’s book is the best one I’ve ever read on the subject of mass extinctions. Under a Green Sky covers the same topic from an insider’s perspective: While Brannen is a journalist, Ward is a scientist who works on mass extinctions. His writing style here is a little clunky – a little overwrought, excessively wordy, floridly descriptive. The sentences have a lot of clauses. But I’d say the charm of the obsessive narrator is congruent with this style of writing, though, and it comes off as intense and impassioned. And there’s no substitute for hearing it “straight from the horse’s mouth.” Ward recounts quirky and interesting aspects of his field work around the planet, grappling with nature and logistics to come to a better understanding of how mass extinctions take place. He covers the theoretical basis for his field expeditions, and then the implications that result from finding something critical (or failing to find it) in the field. Ultimately, the “point” of the book is to outline the biogeochemical thinking behind the current paradigm for mass extinctions. That is: large igneous provinces putting out lots of CO2 and shutting down ocean circulation, which leads to ocean anoxia, and if that gets shallow enough (into the photic zone), you get the proliferation of certain bacteria that produce hydrogen sulfide. That kills off adjacent marine animals, and can leak into the atmosphere to poison the land fauna as well. Of course, this has implications for the current non-volcanic carbon loading of the atmosphere – a prime example of how geological research about the past can serve as critical insights for the present day. Between this book and Brannen’s, I’d say Brannen’s is the better one, but that’s a sort of “on the shoulders of giants” situation, with a decade of insights between the two volumes.

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17 August 2018

Friday folds: Moomaw Reservoir outcrops

My friends Josh Benton and Kristi Leigh are the sources of this week’s Friday folds.

Kristi reports that on a recent kayaking trip, they visited:

Lake Moomaw, a reservoir that’s just about 3 hours away in western Virginia.

The Gathright dam blocks the Jackson River and the lake first filled to capacity in 1982 – relatively recent for US reservoirs! Ostensibly Lake Moomaw was created for flood control and to dilute pollution from a downstream paper mill in Covington.

They saw a bunch of folds in the Devonian-aged sedimentary rocks exposed on the sides of the reservoir. Josh’s orange kayak serves as a sense of scale in these photos:

A subtle antiform:

And a complimentary subtle synform:

Thanks for sharing, Josh and Kristi!

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16 August 2018

A suite of deformational features in Lancaster limestones

In the Landisville Quarry, Lancaster, Pennsylvania, there is a quarry that cuts into Cambrian limestones. (The exact identity of these limestones is apparently a matter of some dispute, but that’s not going to stop us!) I visited the quarry in June on a field trip offered through the NAGT’s Eastern Section annual meeting. We witnessed multiple varieties of deformation there.

First off, there was straight-up brittle extension, resulting in bedding-perpendicular veins:

In other places, there was an element of boudinage (brittle/ductile behavior) with veins located at the boudin necks, as here:

(Note the lobate shape of the fractured bed’s contacts with its neighbors.)

The structural geologists who work on this site have interpreted that there is a thrust fault which runs through the quarry, with folding in the footwall, and mylonitization along the fault zone. Here’s David Brink-Roby showing us what they mean (red = mylonite):

Here’s some of the carbonate mylonite, a novel rock sighting for me:

Another slab of it:

And this example had some nice S-C fabric developed, with chlorite highlighting the tectonic fabric:

Here is an example of (right lateral) slickensides, a signature of brittle faulting:

Finally, there was folding, and accompanying development of axial-planar cleavage, as here:

Another view of this same site:

All told, it was a nice display of different deformational styles in a relatively small area!

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10 August 2018

Friday fold: Gnarly chert from Spy Rock

Happy Friday!

Today for for the Friday fold, I have another guest fold: These are Franciscan mélange cherts from Iron Mountain, California, near Laytonville in Mendocino County, on Spy Rock Road. It’s the same stuff you see at Marin Headlands, with outcrops of serpentinite nearby to boot!

These photos were shared by my sister-in-law Gabriella and her husband Jesse; the outcrops are just up the road from their farm on Spy Rock.

Sense of scale in these photos is Jesse’s hand, a lip balm tube, and Gabriella herself!

Thanks for sharing, Jesse and Gaby!

Happy Friday to all.

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