19 June 2020

Friday fold: Scaglia Rossa chevrons at Lago di Fiastra

My friend Alan Pitts is orchestrating a virtual field camp for George Mason University this summer, utilizing outcrops in central Italy’s Apennine Mountains. Here’s a 3D model he just posted of one of the most impressive outcrops there: the chevron folds in the Scaglia Rossa limestones at Lago di Fiastra. I featured the site as a Friday fold 3 years ago when Alan took me there in person, but this gives a much fuller picture of the site:

by alanpitts on Sketchfab

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5 June 2020

Friday fold: Harbledown Island

Reader Christian Gronau writes with this Friday fold contribution:

Greetings from Cortes Island, BC – at the opposite end of the Strait vis-a-vis Lopez Island.

Your Mountain Beltway blog is always of interest, and I have been following it for several years by now. Thank you for putting the effort into this worthwhile website.

Quite regularly your posts elicit “echoes” and make me go back to some of my own geological notes and photographs. The Lopez Island tension gashes are exquisite in their elegance and classic simplicity.

A little further north again from Cortes Island lies Harbledown Island, eponymous of the Harbledown Formation (Early Jurassic sediments, often fossiliferous). The west of the island has a bay called Parson Bay, eponymous of the Parson Bay Formation (Late Triassic, also fossiliferous) – making this geographical area one of rare distinction as well as historical complexity (early 20th century stratigraphic work).

The attached photograph shows part of an extensive stress zone in Parson Bay limy shales, looking remarkably similar to the image of your last “Friday Fold,” though a bit more complex and, perhaps, confusing. I thought you might be interested in seeing it:

That’s a lovely outcrop photo, Christian! It has many of my favorite features: a multi-chapter story (i.e., it’s a palimpsest rock), good structural geology, a well-placed sense of scale, and a lack of vegetation, barnacles, and/or anthropogenic detritus. A very pleasing sight to contemplate at the end of a rough week.

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29 May 2020

Friday fold: Lopez Island tension gashes

Darrel Cowan returns with a Friday fold for you! He writes with these

awesome fractures. They formed in slightly metamorphosed sandstone on Lopez Island in the San Juan Islands, Washington. The rocks are probably Late Jurassic-Early Cretaceous in age and part of the Lopez Structural Complex [I can give a reference for the setting: GSA SP 221]. The sandstone exhibits a weak foliation imparted by dissolution and reprecipitation of quartz; the quartz in the fractures is evidence of the role of fluids.

You can interpret what we see. I tell my students that these formed in a localized shear zone, even if the boundaries are not expressed in the outcrop, which is glacially polished and striated, by the way. I challenge my students to infer the sense of shear and to explain why the fractures are sigmoidal and thicker in the middle [they kept opening while rotating and extending their wings outward!]. The youngest tips are oriented at 45° to the boundaries of the shear zone, as predicted.

I’ve wanted to go back and visit these outcrops, but access on Lopez is extremely difficult. Someday, after covid-19.

Best wishes to all. On several levels, the Friday fold feels like a frivolous indulgence in such dark times for the nation, but perhaps its a reminder of the sort of content that acts as a beacon of normalcy. It’s the content I would prefer to focus on, were pandemic disease, racial injustice, culture war, environmental degradation, and looming facist oppression not occupying my thinking instead.

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22 May 2020

Friday fold: Anticline in Glacier National Park

Working on a virtual field experience for Glacier National Park this weekend, I came across this amazing GigaPan of a portion of central Glacier by Joe Giersch of the USGS:

What really struck me about it is that big arc of white rock (quartzite, no doubt) within the lower green argillites of the Appekunny Formation… Just left of center. Zoom in and see if you can find it!

I saw that and thought, “Ooh! A fold!” But then I thought that perhaps it was just a trick of the outcrop: i.e. a curved landscape intersection a planar bed, as in the false syncline I noted in Titus Canyon. To corroborate, I looked to adjoining ridges. And…sure enough, it can also be seen in the next ridge over to the right, highlighted by vegetation.

I found the place in Google Maps, and shifted the perspective so I was looking along strike of the anticline’s axis:

Because the fold trends north-northwest, and the valleys run east-west, you can see it appearing in cross section on at least three ridges there! Here’s a link on Google Maps to get this same view, should you care to poke around there yourself.

Sure enough, on the Whipple (1992) geologic map of the park, there’s an anticline mapped at that location:

A little virtual geological discovery for me… What an amazing place; I miss it pangfully.

Whipple, J.W., 1992, Geologic map of Glacier National Park, Montana: U.S. Geological Survey, Miscellaneous Investigations Series Map I-1508-F, scale 1:100,000

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15 May 2020

Friday fold: Argentinian turbidites

Zoltán Sylvester brings us this Friday’s fold:

And there’s more where that came from:

These are deepwater strata of the Lower Cretaceous Rio Mayer Formation, exposed near Lago Argentino, Argentina, south of the lake’s northwest “arm,” about here. They were deformed so exquisitely during Andean mountain-building.

Zoltán is a talented photographer, and you should check out more of his work here.

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8 May 2020

Friday fold: Wills Mountain Anticline

Eric Fulmer (who pitched in with last week’s Friday fold) returns this week with another treasure. He writes,

I was in Hopeville, WV a couple of years ago. The entire area between Cabins and Hopeville is a real joy (geologically and recreationally) as some of the most resistant rocks of the Mid-Atlantic Appalachians are folded and exposed in quick succession and with great relief. I am particularly fond of seeing the western-most exposure of Tuscarora Sandstone in the Wills Mountain Anticline along the North Fork Gap.

The Wills Mountain Anticline is a much larger structure than the Hopeville Anticline and the most obvious member of the antiform is the resistant Tuscarora Sandstone. North Fork Mountain (south of North Fork Gap) is essentially the eastern limb of the anticline and resistant Tuscarora forms white cliffs along the crest of the mountain for miles. The western limb of the anticline includes nearly vertical beds of the same unit plunging directly into the earth; Seneca Rocks are equivalent rocks further to the south.

Click to enlarge

Click to enlarge

The best view that I have seen of the Wills Mountain Anticline is from the first ridge of North Fork Mountain Trail (northern trailhead). The path winds up the mountain and then takes you to a fabulous viewpoint across the North Fork Gap towards New Creek Mountain, where the Tuscarora Sandstone can be followed from east to west across the entire anticline. The drop into the gap is also quite dramatic for the East Coast.

Highly recommended for a long day trip or overnighter.

Once again, thanks to Eric for contributing a fold. Other readers are welcome to do the same. Just drop me an email.

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6 May 2020

The Broken Land, by Frank L. DeCourten

You might think that the last two months would have been a good time for reading, given the social isolation and stay-at-home orders. But that hasn’t worked out to be the case for me. The stresses of the pandemic, new and different work responsibilities, new homeschooling responsibilities, ongoing textbook writing and an impending move for my family have all conspired to gobble up my time, and there’s been very little time left over for reading books. I managed perhaps 2 or 3 pages per day. I started Frank DeCourten’s The Broken Land before I went to Death Valley in early March. But I didn’t finish it until this morning. It was good. It’s a comprehensive summary of Great Basin (Basin & Range) geology: outcrops, locations, events, and interpretations, accompanied by anecdotes and DeCourten’s own hand-sketched illustrations. I found it an excellent compilation, and I’ll keep it on my shelf for the rest of my career, for the next time (hopefully) that I’m lucky enough to return to the Basin & Range. One distinctive aspect to the book is that each chapter opens with a personal story about some experience DeCourten has had on a geological field trip. These are an attempt to draw the reader in toward the experience of geological explorations in the area. I found them familiar enough and therefore engaging, but I’m increasingly conscious that they may not appeal to everyone – they are the experiences of a able-bodied white male professor, and I wonder how they would land with any other audience. Unfortunately, I’m the wrong person to make that call. My gut tells me that the book would have been stronger without them, though. Overall, the geological content is solid and comprehensive and useful. It’s a valuable resource for anyone seeking a better understanding of how the Basin & Range came to be.

And now: Onward to the next book! Hopefully I’ll have more reading on which to report before another two months elapse.

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1 May 2020

Friday fold: Hopeville Anticline

Reader Eric Fulmer has contributed a fold as a balm for the end of another week of COVID-19 self-quarantine. Check it out:

Eric writes,

I was in Hopeville, WV a couple of years ago. The entire area between Cabins and Hopeville is a real joy (geologically and recreationally) as some of the most resistant rocks of the Mid-Atlantic Appalachians are folded and exposed in quick succession and with great relief. I am particularly fond of seeing the western-most exposure of Tuscarora Sandstone in the Wills Mountain Anticline along the North Fork Gap. Immediately next to Hopeville (Harman’s Cabins, just off of WV-55), a textbook anticline of Oriskany Sandstone is exposed where the “North Fork of the South Branch of the Potomac” cuts through the Hopeville Anticline. The hinge line is high up the hill side and the western limb plunges into the earth at a ~35 degree angle. The competent sandstone forms a nice cliff face and exposes easy to identify bedding layers across the outcrop. Decent views of the entire anticline can be found from the Cabins Assembly of God upper parking lot.


Thanks Eric!
Happy Friday to all.


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24 April 2020

Friday fold: The geologic map of Pennsylvania

This is such a gorgeous map:

Link 0.43 Gpx GigaPan of a geologic map by Berg, T.M., Edmunds, W.E., Geyer, A.R., Glover, A.D., Hoskins, D.M., MacLachlan, D.B., Root, S.I., Sevon, W.D., and Socolow, A.A., (1980)

I’ve been organizing a bunch of geologic maps this week for my Historical Geology students, and this is one of the most beautiful. I merged the two halves of the map that were available online and posted the result on GigaPan, which allows students to zoom in and out to various interesting parts of the map. To me, the contrast between the tightly-folded Valley & Ridge and the broader folds of the Allegheny Plateau is striking – and gorgeous!

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17 April 2020

Friday fold: Episode 14 of “Rock du jour”

I’ve been making a little daily video series over the past couple of weeks about rocks I have at my home – a quarantine-driven focus on the samples at hand around the house (as well as a few local outcrops). In many of the videos, I’m joined by my son, who know goes by the title “Mini-Professor,” though in this one from a few days ago (14th in the series), I fly solo:

I offer that one here now, since it features a folded rock, and today is Friday. I owe you that!

The videos are intended to be informal, semi-goofy, and unpolished. I’m learning about video production as I go, which is interesting to me, but the resulting videos definitely are certainly not especially polished! Don’t get your expectations up too high. If you want to explore the whole “Rock du jour” series, here’s a link to my YouTube channel.

I hope you are healthy in these unhealthy times.

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