23 June 2017

Friday fold: Canoeing the Célé River, France

I was in southern France last week, exploring an awesome suite of caves cut into the Causses limestone plateau.

My family and I took an afternoon to paddle a canoe down 5 kilometers of the Célé River. While floating along, we spied a gentle, open fold in the limestone layers that crop out along the banks.

This low-amplitude fold is highlighted with the “horizontal” reference line of the river’s edge.

It’s a little anticline/syncline pair, a pleasing sight for a structural geologist floating through a landscape of relatively undeformed rocks…

Comments/Trackbacks (0)>>



16 June 2017

Friday fold: toothpasty Tomstown

I spent last weekend at the National Association of Geoscience Teachers’ Eastern Section meeting, based out of the Community College of Baltimore County in Catonsville, Maryland. One of the two field trips I took headed out to the western Piedmont, Blue Ridge and Valley & Ridge provinces of western Maryland. On that trip, we took a tour of Crystal Grottoes, a commercial cave south of Boonsboro. I was impressed at how deformed the carbonates were there. The alternating limestone/dolostone strata appear to have flowed like toothpaste squeezed from a tube.

These are Cambrian-aged limestones of the Tomstown Formation, which were deformed (folded and cleaved) during the Alleghanian phase of Appalachian mountain-building. The style of folding is reminiscent of another example from an equivalent structural position much further south. In this example, cleavage is parallel to the axial plane of the folds. I found it interesting that I could keep track of my ‘compass’ orientation in the cave by the angle between bedding and cleavage, and their dip direction. These rocks have attained a fundamentally pervasive fabric as a result of the overthrusting of the Blue Ridge on top of the easternmost Valley & Ridge: a tectonic “grain” that these lucky layers could never have hoped for when they were deposited in calm Cambrian seas.

Comments/Trackbacks (0)>>



9 June 2017

Friday fold – or is it a fauxld?

Check this out: is it a fold?

Annotated to show the 3D expression of the ‘bed’ (left) and cross-sectional view (right):

Here’s a 3D model of the outcrop to better convey its shape:

This is in the same sandstone unit I blogged about on Tuesday with the apparent soft-sediment deformation. This could be another example of the same general class of pre-lithification structures, or it could be an optical illusion. Again it’s hard for me to make the call since the folded “layer” is the same composition as the layers above and below, yet if folded, it shows a significant rheological contrast. I don’t get it.

Comments/Trackbacks (0)>>



7 June 2017

The epistemology of carbon atoms

I have some questions for you. You answers determine whether you’re ready to begin talking about climate policy.

  • Do you believe that carbon atoms exist?
  • Do you believe that carbon can bond to oxygen?
  • Do you believe that the bonding of carbon to oxygen is an exothermic reaction?
  • Do you believe that exothermic reactions make heat?
  • Do you believe that heat can be used to boil water?
  • Do you believe that boiling water can be used to turn a turbine?
  • Do you believe that the turning of a turbine can make electricity?
  • Do you believe that people can utilize electricity?
  • Do you believe therefore that people are motivated to find carbon atoms and react them with oxygen?
  • Do you believe the carbon atoms continue to exist after the electricity is generated?
  • Do you believe that the accepted method of disposing of the oxidized carbon atoms is to throw them into the air?
  • Do you believe that the oxidized carbon atoms in the air tend to stay there for a while?
  • Do you believe that these carbon atoms bonded to two oxygen atoms (CO2 molecules) are invisible to visible wavelengths of light, but opaque to infrared light with wavelengths 2.7, 4.3 and 15 micrometers (µM)?
  • Do you believe that CO2 having this selectively transparent property results in a finite positive quantity of energy remaining on Earth rather than being radiated to space?
  • Do you believe that heat is the agent of temperature change?
  • Do you believe that a sufficiently large finite positive quantity of extra energy remaining on Earth will raise its average temperature?
  • Do you believe that a higher global temperature could have negative effects?

Your answer to all of these questions should be “yes.” If it were not so, your beliefs would run counter to what we have verified about the physics and chemistry of the Earth system.

Finding yourself at odds with physics and chemistry is a bit unsettling. You might ask yourself why you should have such a state of mind.

You might ask yourself, “What would it take for me to change my mind on these questions?”

My list of questions is incomplete. I haven’t specified the size of the “sufficiently larger finite positive quantity of extra energy.” I haven’t indicated the level of the increase of the planet’s average temperature where negative effects become negative enough to matter in the grand scheme of things.

And there are other questions to be asked at the end, like

  • Do you believe that a higher global temperature could have positive effects?
  • Do you believe that policies exist for reducing average global temperature that might be a boon for society?
  • Do you believe that policies exist for reducing average global temperature that might be a cost for society?

Again: yes, yes, yes.

These sorts of binary queries help frame a survey of possibilities for the future. Our goal is to get to the point where we can then ask some more open-ended questions, like

  • Do the positives outweigh the negatives, balance them precisely, or do the negatives outweigh the positives?
  • Do the answers to the previous question apply equally to all people? To all ecosystems?
  • How much should we pay per year to keep some of that oxidized carbon out of the air?
  • Where are we going to put it?
  • What strategies would be wise to prevent harm to ecosystems as a result of the oxidized carbon we opt to leave in the air?
  • What strategies would be wise to prevent harm to humans as a result of the oxidized carbon we opt to leave in the air?
  • Which humans will we prioritize protecting?
  • Which negative effects demand the greatest attention to ameliorate?
  • Which positive effects should be most celebrated?
  • Would inducing global cooling have negative effects commensurate with the magnitude of negative effects induced by global warming?
  • What’s the ideal average global temperature to shoot for?
  • How comfortable should we get pulling levers and steering in the planetary “driver’s seat”?

With these sorts of qualitative considerations, we venture into the realm of policy decisions. Once social priorities and economic implications are added in, the situation’s true complexity emerges. Science can help inform the answers, but it’s no longer the exclusive arbiter of valid answers. The initial chain of “yes” facts establishes the situation in which our species finds itself, on this, the only planet it has ever inhabited. We all have to get to that point before we start talking about policy options. Science establishes our understanding of the situation; no politics need be applied. The realm of politics should be focused on the squishier third group of open-ended questions.

Somehow the initial chain of questions have also been politicized. It strikes me as bizarre that this is so. Do I really have to ask “do you believe in carbon atoms?” and then lead a skeptic by the nose through a chain of “yes”s? The more interesting stuff is contained in the messy third set of questions. That’s the realm Oliver Morton explores in The Planet Remade, which you should read if you have any curiosity about the human-influenced future of the planet Earth. That’s the realm where our society’s attention should be focused. We need to get to a point where we can discuss those questions in a mature, honest way.

Comments/Trackbacks (1)>>



6 June 2017

Loading sags in homogeneous lithologies?

I was out in the field over the weekend, and saw something new. Readers, I’d be eager to hear what you think of it.

The feature is what appears to be a series of small “ball and pillow” type loading structures (soft sediment deformation), but the thing that’s weird about them is rather than being sand sagging into mud, we have instead silt or fine sand sagging into identical sediment. Check it out; in each case I offer a plain photo followed by a version with the base of the “pillows” highlighted:

Some of these have very compelling shapes. It just seems so bizarre that they could exist in sediments where the lower layer appears to have the same grain size (and thus mechanical properties?) as the upper layer. Where’s the differential viscosity coming from? Perhaps there was a component of seismicity that induced these structures to form through preferential liquefaction of the lower layer? Does that even make sense? Could it be that there used to be a lot more mud here, but it all got squished out of the outcrop through volume loss, and so doesn’t appear in modern cross-sections? The last one I think is most compelling as a traditional case of density inversion, with a slight difference in grain size preserved through the differential weathering profile: beds crisp at the base, then grading slightly upward to the crisp base of the overlying bed. It may be that examination under a microscope would reveal a discernible grain size difference.

It may be relevant that this site is very close (just a few meters above) the mass transport deposit I showcased previously.

Thoughts? (Thanks in advance)

Comments/Trackbacks (2) >>



5 June 2017

3D models of sedimentary structures

I’ve been busy making 3D models lately. Here are three ones united by a theme of being sand that was deposited relative to mud. In one case we have scouring to make flutes, in another case we have have localized sagging to make “ball & pillow” structures, and in the third case we have an extraordinary submarine landslide deposit. For two of them, the shale has been preferentially etched away, leaving only the sandstone. For one, the shale remains (but it’s breaking apart fast!).

In each case, you should be able to spin the models, zoom in and zoom out, and read the information in the annotations.

Here’s a 3D model of flute casts, from a slab on display on the campus of Penn State University:

Question for students: Which way was the current flowing?

A 3D model of load casts in the Hampshire Formation, Corridor H, West Virginia:

Question for students: Is this the top or the bottom of the bed?

And finally a 3D model of an outcrop of uppermost Devonian Spechty Kopf Diamictite on Corridor H, showing a mass transport deposit:

Question for students: Can you find a “jelly rolled” sandstone bed within this mess? What events are implied by this feature?

Comments/Trackbacks (0)>>



1 June 2017

The geology of the Yesnaby coast, Orkney

Ahhhh, a raw Scottish coast.          …Let’s go there.

There’s sedimentology to be learned here, and coastal geomorphology to be ogled.

This is Yesnaby, on the west side of Orkney. The rocks there are part of “the Old Red Sandstone,” a neat package of Devonian strata, flavored here and there with an igneous dike, a fault, etc. But overall, I’d like to focus on the strata.

The strata I saw at Yesnaby last summer came in two varieties: flagstones and dune sandstones. The flagstones are alternating layers of sandstone and shale, deposited in an ancient lake. They are gently dipping:


Link 0.31 Gpx handheld GigaPan by Callan Bentley

Here’s another coastal outcrop:


Link 0.69 Gpx handheld GigaPan by Callan Bentley

Between the packages of “flags,” there is the “Sandwick Fish Bed,”  Fish fossils extracted from these rocks are Eifelian Age (Middle Devonian), and were therefore deposited 398 to 392 million years ago. The images here are from the package below the Sandwick Fish Bed, called the Lower Stromness Flags. This package crops out between the small car parking area and the goal of our visit last summer, Yesnaby Castle.

There’s beautiful sedimentary structures to be seen, like these fine examples of bioturbation – trace fossils etched into the mud and filled in with sand.

On the bedding plane, these burrows appear as semi-cylindrical wiggly-squigglies:

Here’s an example of soft sediment deformation within some of the Lower Stromness Flags’ strata:

Oscillation ripple marks indicate shallow water conditions:

These are (weathered out) mud-chip rip up clasts included within a sandstone:

They suggest a story of changing water energy: calm conditions to deposit the mud, then energetic conditions to rip up the chunks of self-cohesive mud, and bring in the sand to mix them in with.

Further down the coast is a sequence of aeolian strata that are probably the same age, though (because they are coastal dune deposits) they aren’t as fossiliferous and thus aren’t as easy to date. They crop out in dramatic sea stacks and cliffs:

Zooming in, you can see the cross-bedding:

And of course there are nice “storm beach” examples along the tops of these cliffs.

Does it count as a sea stack if there’s a tiny little 2 foot wide “bridge” connecting it to the headland?

This particular sea stack has a dramatic name — Yesnaby Castle — and it has a dramatic story to tell:

Yesnaby Castle in the medium of GigaPan, two views:


Link 0.92 Gpx GigaPan by Callan Bentley


Link 0.37 Gpx GigaPan by Callan Bentley

If you zoom in on it, you’ll find excellent examples of large-scale cross-bedding:

The implication of these structures is a series of coastal dunes adjacent to the lake wherein the Stromness Flags were deposited.

Here’s my field assistant and I setting up the GigaPan to capture this extraordinary feature:

…Thanks for joining us for this virtual day in the field!

Comments/Trackbacks (0)>>



30 May 2017

A Field Guide to Lies, by Daniel J. Levitin

In the library the other day, this book’s title caught my eye. I grabbed it and readily consumed it over the past week. It’s a guide to exercising our best critical thinking skills during a time when our attention is awash in claims both vital and derivative, important and erroneous. How do we tell truth from fiction? Politically, the timing could hardly be more propitious for the release of this volume, but politics is only a very small part of the Field Guide‘s purview. There’s plenty about health and medicine, conspiracy thinking, statistics of risk, and the role of expertise/authority. Levitin lays the book out in three parts: quantitative, qualitative, and the practice of science. It’s clearly written, with plenty of good examples rendered quickly in succession, providing a more robust view of the techniques / ideas under discussion. I appreciated the extended discussion of ways graphics can be manipulated in order to deceive. There’s also a bite-sized primer on formal logical statements and fallacies that result from mangling them. Bayesian probability and how to calculate it also gets a lot of attention, and comes close to being a “theme” that runs through the book. A final section examines four disparate “case studies” where Levitin examines what it makes sense to think or do. Because these case studies are explored in more depth than the examples cited elsewhere in the book, this section has a very different feel to it – it’s not as fast-paced. Overall, I found the book to be a useful read, and would recommend it to anyone who’s not already well versed in critical thinking.

Comments/Trackbacks (0)>>



26 May 2017

Friday fold: Macigno turbidites II

Samuele Papeschi contributes today’s fold photo:

These are

liquefaction-related folds in turbidites of the Macigno Formation sandstones at Cala del Leone, Livorno (Italy).

If that name sounds familiar, that’s because it’s the same formation that gave us another celebrated Friday Fold via Alan Pitts.

Great image, Samuele. Thanks for sharing.

Comments/Trackbacks (0)>>



25 May 2017

Chert-slab conglomerate from the Fig Tree Group, Barberton

A return to the Barberton Greenstone Belt in South Africa today….

Let’s examine a fascinating chert-slab flat-pebble conglomerate in the (Archean aged) Fig Tree Group’s Mapepe Foramtion, exposed at one of the stops along the R40 “GeoTrail”:

These shots show close-up images of the texture of a single 6m thick unit that cuts into (and is overlain by) deep-water ferruginous and tuffaceous shale. It was probably deposited in a single violent moment in geologic time. It may have been triggered by a tsunami, which itself may have been triggered by an earthquake or meteorite impact.

I visited it on a terrific field trip I took prior to the International Geological Congress meeting I attended in Cape Town last summer. The field trip was led by Don Lowe, Chrisoph Heubeck, and Gary Byerly.

The clasts are slab-like in shape: roughly 1:15 axial ratios as exposed on these bedding-perpendicular outcrop surfaces. The largest slab has roughly the dimensions of a family-sized pizza!

The clasts are oriented roughly (paleo-) subhorizontally, though there’s plenty of imbrication to be observed, too.

It’s intriguing to think of the story implied by these clasts: chemical deposition of chert in calm waters below the wave base, for years and years. Then, a sudden violent event (tsunami from a distant meteorite impact??) fractures the chert and triggers a turbidity current of the unsettled debris. This tumbles its bits and pieces down a submarine channel en masse, to be dumped in a huge pile. Wow: sudden enough and violent enough to qualify as downright catastrophic!

And then, it was over, and the debris rained out, and the water cleared, and a more gradual deposition of sediment resumed in its same old uniformitarian way.

Comments/Trackbacks (0)>>