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Zoltán Sylvester

@zzsylvester.bsky.social

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Zoltán Sylvester @zzsylvester.bsky.social · 8d

A thread about how two geologists went to a beach with a plywood board and a protractor to see how variable wind directions influence bedform orientation - and published a paper in Science that blew my mind. 🧪⚒️🧵 1/23

Title and abstract of the Science paper by David Rubin and Ralph Hunter - Title: Bedroom Alignment in Directionally Varying Flows; Abstract: Many kinds of sediment bedforms are presumed to trend either normal or parallel to the direction of sediment transport. For this reason, the trend of bedforms observed by remote sensing or by field observations is commonly used as an indicator of the direction of sediment transport. Such presumptions regarding bedform trend were tested experimentally in bidirectional flows by rotating a sand-covered board in steady winds. Transverse, oblique, and longitudinal bedforms were created by changing only two parameters: the angle between the two winds and the proportions of sand transported in the two directions. Regardless of whether the experimental bedforms were transverse, oblique, or longitudinal (as defined by the bedform trend relative to the resultant transport direction), they all had trends that yielded the maximum gross transport across the bedforms. The fact that many of the experimental bedforms were neither transverse nor parallel to the resultant transport direction suggests that transport directions cannot be accurately determined by presuming such alignment.

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Zoltán Sylvester @zzsylvester.bsky.social · 6h

It's not a big deal but I find it annoying when I am told to use only one slide (or three, whatever) when giving a short talk. I am old enough to figure out how to use visuals in a limited amount of time; let me decide if I want to use fifty slides or none

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Hope Jahren @realhopejahren.bsky.social · 9h

ah the leaves are turning and the men are giving each other prizes again the first whispers of winter

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Jeff Sharlet @jeffsharlet.bsky.social · 4d

It appears that my employer, Dartmouth, one of the Trump 9, has said no to the compact. All the better given that our president is cited within it. But she’s saying no. Count the small victories when they come.

Office of the President
Dear Dartmouth community,

As many of you know, Dartmouth was one of nine universities asked by the White House to give feedback by Oct. 20 on a draft of its “Compact for Academic Excellence in Higher Education.”

I am deeply committed to Dartmouth’s academic mission and values and will always defend our fierce independence.

You have often heard me say that higher education is not perfect and that we can do better. At the same time, we will never compromise our academic freedom and our ability to govern ourselves.

Best,
Sian Leah Beilock
President

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Dr Dan Shugar 🇨🇦 @watershedlab.bsky.social · 5d

🚨FOUR tenure track positions in my dept @ucalgary.bsky.social @ucalgaryscience.bsky.social in applied & computational geophysics, subsurface geochemistry, sedimentary geology, and sustainable soil science. careers.ucalgary.ca/search/jobs?...

(please reskeet widely!) #academicsky 🧪⚒️🇨🇦

Opportunities matching 'earth'

Search 4 Careers available at University of Calgary.

careers.ucalgary.ca

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John Schwartz @jswatz.bsky.social · 6d

“It trades autonomy for subservience, academic freedom for censorship, gender science and history for ideology, and the best interests of UT students and faculty for the favor of an administration intent on destroying our university.” www.texastribune.org/2025/10/02/u...

UT-Austin considering offer to adopt Trump priorities for funding advantages

The funding deal would reportedly ask the university to adopt a stricter definition of gender and a tuition freeze, among other conditions.

www.texastribune.org

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Callan Bentley @callanbentley.bsky.social · 6d

This is so, so horrible. Who are these unaccountable government agents? What gives them the right to abuse people this way? Where is due process? I weep for these poor children, and I weep for the nation that condones this abominable treatment.
This behavior is sadistic.

Anjali Enjeti @anjalienjeti.bsky.social · 6d

My god.

“Watson said she saw agents dragging residents, including kids, out of the building without any clothes on and into U-Haul vans. Kids were separated from their mothers…’It was heartbreaking to watch…seeing kids coming out buck naked and taken from their mothers, it was horrible.’”

Massive immigration raid on Chicago apartment building leaves residents reeling: 'I feel defeated'

The Department of Homeland Security said federal agents with Border Patrol, the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives arrested 37 people in the raid.

chicago.suntimes.com

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Robert Mahon @bedform.org · 6d

its this part
"If universities sign and then violate the terms of the compact, they could be forced to return any money given to them by the federal government that year as well as any private contributions." 👀

Stay in line OR ELSE

AAUP chapter @UT Austin @utaustinaaup.bsky.social · 6d

www.wsj.com/us-news/educ... UT Austin is one of the 9 schools offered a deal: accede to White House demands and get funding priority

Exclusive | White House Asks Colleges to Sign Sweeping Agreement to Get Funding Advantage

An initial round of nine schools is being asked to sign a wide-ranging accord.

www.wsj.com

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Zoltán Sylvester @zzsylvester.bsky.social · 6d

Thanks for adding these! This review paper has been a great resource in my process of trying to catch up with the eolian dune literature: www.ipgp.fr/~narteau/pap...

www.ipgp.fr

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M. Colin Marvin @mcolinmarvin.bsky.social · 6d

I also like to use some of their examples from previous underwater experiments (Reffet et al., 2010; doi.org/10.1130/G308...) to show how acute and obtuse diverging winds result in different dune types (inspired by @agunn.bsky.social 's Sharp Lecture last year):

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Zoltán Sylvester @zzsylvester.bsky.social · 7d

Here is a zip file with the OBJ file and the textures: www.dropbox.com/scl/fi/pp8e2...

www.dropbox.com

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Clastics Lab @clasticslab.bsky.social · 8d

@txgeosciences.bsky.social QCL Clastics Lab at Tusher Canyon, north of Green River, UT. Origin & architecture of channels incising parasequences.

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Melaine Le Roy @subfossilguy.bsky.social · 8d

When the dust will clear up...💨

Preparation of Morteratsch Glacier samples (Little Ice age, 1st two), Fee Glacier (Roman Period) and Mer de Glace (Bronze Age) today! 🤩🌲💍

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Joe Mason @moreorloess.bsky.social · 7d

Great example of a flood produced by the end-of-monsoon-season rainstorm on an ephemeral desert stream with a drainage area of 1817 square miles. A bit west of Phoenix.

Hydrograph (plot of stream flow over time) for Centennial Wash in Arizona west of Phoenix. Flow rises from 0 to more than 400 cubic feet per second and then drops back to 0, in a little over 12 hours.

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Mike Tice @ticeonmars.bsky.social · 22d

I've been asked a few times over the last few days what I think the chances are that the "leopard spots and poppy seeds" on Mars will turn out to be actual evidence of life. People are naturally skeptical given the history of possible signs of life on other planets. Strap in: long thread ahead...

Image collected by WATSON on the Perseverance rover of "Cheyava Falls," a rock on Mars containing potential biosignatures

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Zoltán Sylvester @zzsylvester.bsky.social · 7d

*gross bedform-normal transport* - when will my gadgets finally learn that I am much more likely to talk about bedforms than bedrooms 🙄

Reposted by Zoltán Sylvester

Joe Mason @moreorloess.bsky.social · 7d

Great explanation (and illustration) of Rubin and Hunter's work. Dave Loope impressed the importance of this on me, and it was the basis for our paper on longitudinal dunes in the Nebraska Sandhills: doi.org/10.1126/scie.... See Figure 2 in particular. In the supplemental material, included in this

Large Wind Shift on the Great Plains During the Medieval Warm Period

Spring-summer winds from the south move moist air from the Gulf of Mexico to the Great Plains. Rainfall in the growing season sustains prairie grasses that keep large dunes in the Nebraska Sand Hills ...

doi.org

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Zoltán Sylvester @zzsylvester.bsky.social · 7d

*Courrech du Pont et al., 2014* #autocorrect

Zoltán Sylvester @zzsylvester.bsky.social · 8d

Those of you who have worked on eolian geomorphology/ sedimentology much longer than I have: comments & corrections are welcome. I had to write this stuff down so that I do not get myself into trouble again when trying to explain these ideas in front of 25 people in the field. 23/23

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Zoltán Sylvester @zzsylvester.bsky.social · 8d

... and then for wind vectors of unequal magnitude: 22/23

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Zoltán Sylvester @zzsylvester.bsky.social · 8d

However, I think that the simplicity, elegance, and explanatory power of Rubin and Hunter's 1987 paper are striking. Here are two animations that summarize what happens as we increase the divergence angle, first for wind vectors of equal size, ... 21/23

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Zoltán Sylvester @zzsylvester.bsky.social · 8d

Since 1987, several studies have been published that provide an increasingly sophisticated understanding of bedform development under variable winds. A key contribution was the insight that bedform orientation also depends on sediment availability (see Courrech du Post et al., 2014, Geology). 20/23

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Zoltán Sylvester @zzsylvester.bsky.social · 8d

The cross bedding resulting from this migration is not easy to distinguish from the migration of compound transverse dunes; the thing that we can rely is the divergence between the dip directions of cross sets of the smaller, superimposed dunes when compared to the larger bedforms. 19/23

Sketch of superimposed small bedforms on a larger longitudinal / oblique bedform, with the smaller bedform crests being roughly perpendicular to the trend of the large dune. From Rubin and Hunter (1985).

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Zoltán Sylvester @zzsylvester.bsky.social · 8d

So if oblique and longitudinal dunes are so common today, why is it that their deposits are rarely recognized in the geologic record? Rubin and Hunter also wrote a paper about that, in 1985. They point out that more often than not oblique/longitudinal dunes have a net migration. 18/23

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Zoltán Sylvester @zzsylvester.bsky.social · 8d

Hunter and Rubin argue that "transverse, oblique, and longitudinal bedforms are not necessarily different kinds of bedforms in a dynamic sense, but rather can be explained by one governing principle." 17/23

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Zoltán Sylvester @zzsylvester.bsky.social · 8d

As many wind regimes on Earth consist of wind directions that form an obtuse angle with each other, large oblique-to-longitudinal bedforms are quite common in modern deserts. This example is from the Rub al-Khali in Saudi Arabia (source: Apple Maps). 16/23

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