21 June 2019
I’m at a workshop in Madison, Wisconsin, this week.
I took the lunch hour today and walked over to the geology department to check out their rock garden and geology museum.
I was pleased to find a Friday fold in the rock garden: a limestone with cherty nodules/layering that has been folded….
Bonus: some nice bookshelfing/boudinage of the chert:
…And here’s another boulder of the same lovely stuff.
And here is another little train of cherty parallelograms:
There was also a chunk of what appears to be the same thing in the museum inside…
14 June 2019
Last weekend was the annual meeting of the eastern section of the National Association of Geoscience Teachers. On Friday afternoon, we visited Harpers Ferry, West Virginia, and my colleague Beth Doyle led a great field trip to examine the rocks exposed there. This was my favorite outcrop we saw:
Here is a close up of this outcrop, which is framed by (anthropogenic) rock wall:
Dipping shallowly from upper left to lower right is a tectonic foliation: that is to say, slaty cleavage. This foliation has been folded into a well-defined kink band by late Paleozoic Alleghanian deformation. The rock itself was a mudrock (~shale) when it was first deposited: the Cambrian-aged Harpers Formation. Bedding is also visible in places at Harpers Ferry, but I can’t see any in this outcrop.
Here is my sketch, from my field notebook:
…And lastly, here’s an annotated copy of the second photo:
…Happy Friday to you!
31 May 2019
It’s Friday, and I’m grateful that Bret Leslie of the NRC has stepped up with a Friday fold contribution from coastal California:
Bret says this outcrop is:
from my trip last month to the Sonoma and Mendocino coast. The first is the cliff below the Pt. Arena lighthouse. The marine deposits (now a marine terrace) are unconformably on top of the folded Miocene sedimentary rocks in the second photo.
Here is that second photo, showing the larger context and the angular unconformity:
Whimsically annotated: bedding traced out in white, with the unconformity trace shown in black:
Shaw (2007) writes that:
Subunit 11: At Point Arena, Miocene marine deposits dip gently seaward and are overlain by thin Quaternary terrace deposits. … Point Arena is three miles west of where the San Andreas Fault passes northwestward into the Pacific Ocean. Rocks to the east of the fault belong to the North American Plate and those to the west, including Point Arena, belong to the Pacific Plate. … Lower Miocene strata are exposed in cliffs that line the coast from Point Arena southward. The rocks consist of light tan colored … foraminiferal clay shales, some bituminous sandstone and cherty shales. All have a high percentage of microscopic porosity, causing the bulk rock to have a very low density. … In places, flame structures are found, indicating soft sediment deformation in which the weight of overlying beds pressurize water in the fine layers and mobilize them to move as fluids and intrude upward into stronger shales. … The Miocene strata at Point Arena dip gently in some areas and are nearly vertical in others. Tight folding is indicated.
One thing I’ll note is that the oval shapes (that are easiest to see in the first image) are not the traces of folded bedding, but just the outcrop pattern of erosion through upper bedding layers into deeper bedding layers – like the pattern of concentric rings you might see if you took an ice-cream scoop to the center of a stack of flapjacks. It’s the pattern you get when you impose a 3D outcrop surface on a more or less planar set of dipping 2D beds.
Pancakes and ice cream? Hmmmm: Sounds like weekend food. Enjoy your weekend!
Charles E. Shaw, July 2007. California Coastal National Monument Geologic Characterization. U. S. Department of the Interior, Bureau of Land Management. Downloaded 5/31/2019 from https://www.mendocino.edu/sites/default/files/ca_coastal_national_monument_geology.pdf
24 May 2019
Naomi says she found this fold:
near the Riffelhorn, Gornergrat, above Zermatt, Switzerland.
The sample has befriended my other show-off sample of a mantle xenolith from San Quintin, Baja California.
Thanks for sharing, Naomi! The xenolith is a nice bonus!
20 May 2019
I have a question for you. It’s about teaching geology.
Consider this photograph, by Tim Johnson, of an outcrop in the Buchan block of Scotland, near MacDuff:
Tim took the photo because he was impressed by the graded bedding and the “baby load casts.” And rightly so!
But there’s much more in that photo than just those features. It also shows percussion marks, and veins and joints (fractures), and it’s got a bit of colorful lichen on it too. And in addition to the coin (for scale), there’s modern sand in the lower right.
In short, there’s a lot to look at.
If a teacher were to be focused on teaching primary sedimentary structures such as graded bedding, and they wanted their students to see the “graded bedding” part of this image without risking getting distracted by the other details, would it be ethical to remove some of those other details using digital manipulation of the photo? (i.e., specifically by using Photoshop’s “cloning” tool)
Tim’s outcrop inspired me to try!
…For instance, what if we took away the percussion marks?
…What if we got rid of the veins?
…What if we got rid of the fractures?
…What if we did a “virtual pressure washing” and stripped away the lichens?
So the final result would be rather significantly less “real” than the original, but it may be more useful for teaching purposes, allowing students to focus on the graded bedding, and not be distracted by the other types of information/data/noise in the actual outcrop. Images modified in this way are not truly authentic, but they may make for easier learning, especially among novices who don’t know what to focus on. (I’m reminded of Werner Herzog’s “ecstatic truth“…)
Is it ethical to modify images in this way? In textbooks? In classes? (…on blog posts?)
I’d be curious to hear your thoughts about where the limits of this sort of digital manipulation lie.
Here are a few more examples I worked up, with the goal of stimulating discussion:
Relict cross-bedding in Weverton Formation quartzite, Thoroughfare Gap, Virginia:
Layered gabbro in the Stillwater Complex, Montana:
Fold in metagraywacke, Billy Goat Trail, Maryland:
Okay – let’s hear it: How do we most ethically utilize this powerful technique?
17 May 2019
This week, for Friday folds, I offer up some random folds that have passed my perceptual transom this week.
First up: In the new Netflix series Our Planet, in episode 7 (Fresh Water), an anticline/syncline pair makes a brief appearance as David Attenborough discusses glaciers as a reservoir for fresh water. Here is a screenshot:
I’m not sure where this is in the world: Greenland? Antarctica? Let me know in the comments for this blog post if you recognize this scene.
Cropped to zoom in on the fold, and minimize those glacial distractions:
Here, I’ve traced the bedding out, using red for the nearer of the two mountains, and yellow for the mountain that’s further away:
I made an animated GIF of the flyby, which imparts a better 3D sense of the outcrops:
The second fold I’d like to share comes from Highlights, a magazine for children that we subscribe to for the sake of my son: In this month’s issue’s science spread, they feature a fold from an Australian island under the title “How to Bend Rock,” though they don’t say which island.
Zooming in on the (annotated!!) fold:
I asked around on Twitter for other suggestions for “found folds,” and Jefferson Chang suggested the brake light pattern in the Chevy Bolt:
Sure, that works: It looks like an overturned asymmetric fold pair, perhaps a fault-bend fold.
Where have you found folds lately?
Have a terrific Friday and a rejuvenating weekend.
8 May 2019
I’ve just finishing reading Rising, by Elizabeth Rush. The book is about sea level rise (subtitle: Dispatches from the New American Shore) and it was a finalist for the 2019 Pulitzer Prize for general nonfiction. It’s an interesting book – lyrical and artful, with evocative phrases like “osprey’s creosote shadow.” Occasionally, these can go beyond clever/poetic turns of phrase, and can invoke other corners of literature, as when she describes a flood survivor “floating on a couch in her wine-dark living room,” – a shout out to Homer’s frequent use of the phrase “wine-dark sea” (whatever that means).
The book explores the biology and sociology of living in Pensacola, Miami, Jacobs Point, Rhode Island, Staten Island (New York), and Isle de Jean Charles, Louisiana, San Francisco Bay, as well as an interlude in an inland forest preserve in Oregon.
There are sections of the book that are essentially extended quotes from other people: testimonials that tell the person’s story without narrative filter. These feel kind of like “readings from the primary literature,” personal anecdotes told by those who were impacted by sea level rise. They are full of passion and fear, and very impactful emotionally. You feel the speaker’s pain, their sorrow. However, their parlance is at odds with Rush’s own lyricism, and alternating back and forth between them makes the reader compare the varied voices. It made me think of a scrapbook – a newspaper clipping here, and handwritten remembrance on the next page. The effect is to make Rising feel more like a collection than a cohesive through-going work of reportage.
The book features an extended discussion of sexual harassment from one of her sources, framed as part of a larger consideration of vulnerability. Rush is vulnerable as a woman reporter, conducting interviews in the field, and she draws parallels between that and the risk faced by people of limited means leaving in coastal wetlands. This is also invoked in a more extended analogy with the most vulnerable species leaving in increasingly flooded coastal wetlands. Our infrastructural footprint blocks them from migrating inland with the shore, so they feel the squeeze acutely and painfully. It’s not fair, but they have no good options.
Rising is a really personal book: In many places, Rush goes beyond mere reportage to describe her own emotional reaction to sea level rise. She reports on her own thoughts in the same way she reports on her interviews with coastal residents or scientists. She describes her dreams at length in several sections of the book. This is novel; the author’s midnight imaginings are outside the realm of typical nonfiction reporting. It’s simultaneously powerful and a bit uncomfortable in its intimacy. Is it legitimate to report on one’s own dreams in a book about sea level rise? I guess it is – not traditional source material, but powerful in the imagery it describes, the emotional state it draws the reader into. In sum, I’d say this is an interesting book from the perspective of considering the limits of traditional reporting and narration, and also a useful summary of where we are now with our submerging national coastlines.
3 May 2019
Darrel Cowan of the University of Washington steps up today with another contributed Friday fold:
I made this photo when I took my class deep into Monarch Canyon in Death Valley NP this March. From work by Stanford and others, the rock is probably medium-to high-grade metamorphosed Pahrump Group, now a well-foliated gneiss. If we consider the foliation, here vertical [I’m looking directly down], the XZ-plane of bulk strain, the little dikelet or leucosome is folded where normal to the foliation, and stretched where oblique–just as predicted.
Perfect structural fodder to start your Friday morning off right! Thanks, Darrel – as always!
1 May 2019
Peter Wohlleben is a forester, managing a forest in Germany. Over decades among the trees, he has had major insights into the “inner lives” of the trees, and uses this book to collate them and share them with a wider audience. The book opens with an anecdote: he walks by some moss-covered lumps in the forest, and peels up the moss to see what he expects will be “stones” underneath. He is surprised to find wood instead, and realizes that this ring of woody protuberances is actually a set of remnants of the outer diameter of an ancient tree, the middle having long since rotted away. On this observation, he estimates that the tree itself was felled centuries ago — but here’s the kicker: the living tissue of the tree survived, bearing the green signature of chlorophyll. Though this ancient giant had been felled generations ago, the stump had been kept alive ever since. It turns out that trees communicate and share food underground, via their root systems. This “dead” tree’s community of neighbors had sustained it for centuries. That’s amazing, and it’s just the start. Wohlleben leads his readers on a fascinating journey into the forest, viewing trees as thoughtful beings who live on an utterly different timescale from our own frenzied lives. The subtitle of the book is: “What they Feel, How they Communicate: Discoveries from a Secret World,” which gives you a sense of the content as well as the author’s attitude. What; they “feel?” Yes! Trees feel, and they can count, and they hurt, and they wait patiently. Never have I read something that successfully anthropomorphizes trees so well. I live in a forest, and I’m going to think about it more deeply and with more satisfaction and curiosity after assimilating Wohlleben’s ideas. Very interesting stuff; very well written. Recommended.
26 April 2019
This Friday, let’s hearken back seven weeks, when a similar Friday saw me and the ‘Streetcar to Subduction’ team in Marin Headlands, north of San Francisco. Black Sands Beach there is a great place for pillow basalts. You can see the knobby texture of the pillows on the far side of the beach, but today, let’s look at the big boulders in the foreground…
Let’s scoot over to the left a bit to that big boulder with the vertical stripes…
Close-up, these stripes reveal themselves to be upright and folded chert layers, with gazillions of tiny fractures perpendicular to the bedding planes:
The cherts here are gray, almost bluish-green, in stark contrast to the brick-red variety further to the east, near Kirby Cove and the Golden Gate Bridge.
On the underside of a cliff overhang at the west end of Black Sands Beach, there are more folded cherts of this gray color:
Happy Friday, all!