19 July 2019
A final Friday fold from Madison, Wisconsin: this one a slab of cut and polished banded iron formation from Australia:
What exquisitely beautiful rock!
12 July 2019
Here is an outcrop of folded limestone along route 430 in Newfoundland, inside Gros Morne National Park, just west of the crossroads called Wiltondale:
A detailed look at the left antiformal portion of the outcrop:
A zoomed-in examination of the rightmost part, where a goopy looking synform resides:
Just down the way, a second outcrop shows another fold with the same sense of asymmetry, on a smaller scale:
Happy Friday to you. Enjoy your weekends.
11 July 2019
I’ve been traveling on the island of Newfoundland for the past few weeks, seeing world-class geology, but not having a lot of time to delve into blogging about it. I have been posting a few pictures each day on Twitter, though, including this one:
OK, geoTwitter super sleuths, what do you see here? pic.twitter.com/SSIqljzj99
— Callan Bentley (@callanbentley) July 10, 2019
There’s a lot to see there. It’s from a place called Sandy Cove, specifically a headland called “Greenings Point:”
This is a site (#63) from an excellent map + guide for the geology of Newfoundland, called Newfoundland & Labrador Traveller’s Guide to the Geology: Guidebook to Stops of Interest, by Stephen Colman-Sadd and Susan Scott (first published in 1994, reprinted in 2003). I was sent a copy of this invaluable guide by Tim Sherry, and I’m very grateful for its guidance, for I doubt I would have dropped $100 for it on Amazon on my own. Thanks Tim!
Okay, back to the rocks! Here’s a blog-hosted version of that same image:
To me, the key elements of the outcrop are bedding and cleavage. There is a variable amount of clay in the beds, resulting in variable development of cleavage. There is a prominent contorted bed in the middle of the outcrop. And both cleavage and bedding are highlighted by differntial weathering of the outcrop in its sea-spray-blasted coastal location.
Overall, contorted bedding is what drew me to this site: the Proterozoic sedimentary rocks here are interpreted as a massive submarine slump deposit, mixing mud and sand and gravel and beds of each of those sediments into a big chaotic jumble. Later, Appalachian mountain-building overprinted the whole mess with a penetrative cleavage.
Let’s start our examination with the slumpy aspects of the site…
There is a diamictite of big bits surrounded by finer stuff:
A boulder of “mixite”
Mudchip-rip up clasts surrounded by coarse sand and grit:
A seam of conglomerate stretches down through this next photo, surrounded on either side by sandstone, with a huge block of (white-weathering) mudrock :
Note that there are no clasts of igneous or metamorphic rock here. This is not a glacial tillite, bringing in material from all over. Instead, it’s just different varieties of clastic sediment. That, and the folded bedding suggest soft sediment deformation and an overall submarine slumping interpretation for the genesis of these rocks:
One curious aspect of the site is that the mudrock weathers white, though on a fresh surface it’s almost jet black. You can see that in this next photo (in the lower right), which shows coherent beds of mudrock on the flanks of a central blob of diamictite (“mixtite”) which contains a few folded slabs of mudrock (presumably ripped off one side or the other – I’m not at all confident which way is “up” in this mess):
On the scale of beds, consider this outcrop:
Relatively undeformed beds there overlie relatively deformed ones. This deformation is not tectonic in nature — it’s ‘soft sediment deformation,’ which is to say the folding and sqloorping* was post-deposition, but pre-lithification.
* not a real word, but it’s got onomatopoeia going for it.
Another fine example of folding:
A thinner example of a convoluted bed, so thin that the convolutions (± subsequent erosion) have separated the bed into distinct segments:
So we’ve got mud and silt and sand and gravel all being deposited somewhere in proximity to one another, and later slumping of these sediments underwater shuffles them up on both the grain-by-grain and the bedding scale, depending on where you look.
Here’s a coherent bed, folded up and surrounded by chaoticly sorted diamictite:
Here’s a similar spot where you can see big slabs of sandstone folded up and mixed in with grit and gravel:
The most spectacular example of contorted bedding I saw at the site was this one:
Note the flat-lying bedding at the bottom (easy to miss it, given the glory of the jellyroll above!).
Here are a set of coherent beds shuffled into a few different orientations, with an upper curved package capping a more steeply dipping thick lower package:
A big overturned antiform:
Here’s a nice view showing three packages of sediment: a lower, darker one that’s more or less horizontal and in its original form, a ~20cm (~1 foot) thick contorted bed that weathers white and crops out in a blocky pattern, and a thick brownish upper package showing serious internal deformation:
Okay, so that’s the sedimentary scene. But another prominent aspect of the site was the cleavage, a tectonic structure superimposed upon pre-existing sedimentary features during later Appalachian mountain-building.
Here’s a Z-fold with cleavage running cleanly through the whole thing:
The trace of the cleavage and the bedding could both be really enhanced by a bit of seaside weathering:
Here’s a neat spot that shows clasts of mudrock surrounded by sandstone, but only the mudrock is discernibly cleaved:
I should also mention: there was also a substantial igneous dike at the site:
Another perspective (from the opposite side), though lacking the same lovely sense of scale:
The rock making up this dike was a greenish sort of basalt, hosting a few amygdules:
Finally, there was a chasm that kept us from exploring further along the outcrop, but its deep declivity hosted waves that impacted at the back end and echoed/reverberated in the deepest tones through the rock and air – a sound felt as much as heard. Here is a look into the abyss —
…Stare not too long into it, lest it stare into thee…
I think that about covers it! Thanks for joining me in exploring this super cool outcrop.
5 July 2019
Today’s Friday fold is rendered in stained glass, along with a bunch of other geological details, as seen at the entrance to the geology museum at the University of Wisconsin (site of our previous Friday fold):
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.