27 April 2018

Friday fold: Cape Liptrap

Sandra McLaren of the University of Melbourne is the source of today’s guest Friday Fold. Let’s join her on a journey with her students to Cape Liptrap (southeast of Melbourne in Victoria, Australia) on a lovely day:

What a spectacular place. (We have featured folds from this site previously.)

Happy Friday all!

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25 April 2018

Natural Bridge State Resort Park, Kentucky

A couple of weeks ago, when I was visiting Kentucky, I was fortunate to visit Natural Bridge State Resort Park in the Red River Gorge area in the eastern part of the state.

Zooming in on the “bridge” (which really to me seems to be an arch, given that it is fin-like astride a ridge, and not spanning any waterway):

On the trail approaching the bridge from below:

From underneath:

Joining me that day were my friends and colleagues Kent Ratajeski (my host; also at @KentRatajeski), Ryan Thigpen, Frank Tamakloe (a fellow alumnus of William & Mary), Stephanie Sparks (a former student of mine, also at @stsparks1). Here they all are, posing like the powerful posse they are, beneath the bridge:

The rock here is sandstone of the Pennsylvanian aged Corbin Sandstone Member of the Lee Formation (Breathitt Group). It has nice cross-beds, and some decent woody plant fossils, like these:

There’s also some terrific examples of tafoni, as Stephanie here demonstrates:

Here is a well-developed outcrop of tafoni on the underside of an overhanging ledge about 20 feet above me:

Frank and the tafoni wall (and the William & Mary t-shirt):

And then there was the Liesegang banding, which occurred in well-developed cylindrical shapes, and weathered out in three dimensions. It was lovely in its hues and varied forms. Take a gander:

Here, flow of fluids through the permeable sandstone encouraged the transport of reactive consituents which deposited iron oxide minerals in concentric, waving bands. It’s a common feature to be seen in rocks like these, as I’ve noted previously on this blog.

The iron-oxide cementation accentuated the development of the tafoni, as you can see here:

Zooming in on the “diapir-like” form on the left side of the previous image:

Visiting the park was a terrific experience, and I’d recommend it to anyone who finds themselves in the area. Overall, the Kentucky park reminded me a lot of Giant City, a park in neighboring Illinois, which is made of similar Pennsylvanian sandstone weathering out in near-identical conditions.

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24 April 2018

An astonishing rise in the number of humans

One interesting thing about reading T. rex and the Crater of Doom, by Walter Alvarez in 2018 is the change in world population since it was first published in 1997. In explaining to his readers how to think about measurements in “parts per billion,” Alvarez explains his mental shortcut to appreciating those numbers. He says something along the lines of “since there are 5 billion people in the world, a measurement of 3 parts per billion is a proportion equivalent to 15 people out of the world’s population.”

The global population just two decades later has increased by 50%. It’s almost 7.5 billion today. It’s astonishing how quickly this useful mental calculation has become outdated, and more to the point, how rapidly the human population on this planet is swelling. I was a single year out of college when Alvarez’s book first came out. Twenty years later, my life has trundled along its fairly predictable arc, with family and career and whatnot. And in that short time, there have been so many births, and so many more people staying alive who might otherwise have died, that when I finally get the motivation to pick up the book and read it, and I find that this particular statistic has gone stale. There are now 3 people kicking around for every 2 who I shared the planet with then. That’s really kind of amazing.

There are more people on Earth today than ever before. And tomorrow there will be even more.

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T. rex and the Crater of Doom, by Walter Alvarez

Walter Alvarez has a new book out, and its publication reminded me that though I read and appreciated The Mountains of St. Francis, I had never read his most famous work — the account of how he and his father and a team of other researchers zeroed in on an extraterrestrial impact explanation for the end-Cretaceous extinction. So last week I read T. rex and the Crater of Doom (1997). It’s excellent in that it shows in an appropriate level of detail the sequence of insights, speculations, observations, searches, arguments, and collaborations that led to a revolutionary new way of envisioning Earth history. Alvarez writes for a general audience, and somehow manages to convey a sense of the science without ever getting bogged down in jargon. His analogies are excellent, his writing is kind, and each chapter is tidy and economical with its language. As a result, thought the story being told is BIG, the book is fairly brief. It would be an ideal entry point into historical geology for anyone, and I wish I’d read it long ago. Recommended.

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20 April 2018

Friday fold: Isoclinal in Damaraland

A guest Friday fold from Graham Andrews of West Virginia University:

Graham describes this as an

almost along axis view of a huge isocline in the Damaraland belt, Namibia

Thanks for sharing! And a happy Friday to all.

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18 April 2018

Visiting St. Francis’s lovely limestone

Walter Alvarez wrote a great book about the geology of central Italy, but he chose to title it The Mountains of St. Francis, a moniker that didn’t exactly lure me in. Being a bit of a religious outsider, I knew that St. Francis of Assisi (1182 – 1226) was famous for being kind to animals and eschewing luxury in favor of simplicity. But that’s about it. I read Alvarez’s book in spite of its title, and found it a marvelous mix of geological insight and memoir. I recommend it to anyone traveling to that region of the world.

So when I was in central Italy last summer, I made a point to travel to Assisi, the town where the eponymous St. Francis once dwelled. There is a big basilica there, constructed of local limestone, and made marvelous in tribute to St. Francis. Alvarez opens his book here, talking about the delicate pink tone of this limestone, the Scaglia Rossa.

Here’s a closer look at this lovely rock, in a wall on the other side of Assisi:

Here are some charismatic stylolites that cut across bedding and calcite veins:

The Scaglia Rossa was seriously deformed during the orogenic events that built the Apennines. Based on their cross-cutting, non-parallel relationship with bedding, I’d venture that these stylolites probably formed at the same time the Scaglia Rossa was folded and thrust-faulted and tilted and smeared out in the Eocene.

A nearby poster shows an ammonite of a shade of pink that seems to match this limestone:

Ammonites died out at the end of the Cretaceous, and of course the Scaglia Rossa spans the end-Cretaceous extinction event. That catastrophe was preserved in this very rock unit, exposed in Bottaccione Gorge near Gubbio. I regret not being able to visit the paleontology exhibit advertised in this flyer; the only fossils I saw in the Scaglia were sand-grain-sized forams.

But I did walk through St. Francis’s basilica, and found it to be remarkably lovely. Check out this scene, rendered in limestones of various hues, all of them exuding a soft, enticing glow.

Alvarez makes the point in his book that “San Francesco” probably wouldn’t have approved of this grand basilica erected in his (posthumous) honor. But I found it a delight for the eye, and enjoyed a moment of silence in the lower level, contemplating St. Francis’s tomb and the legacy of a human being on future generations. The limestone of the Scaglia Rossa too shares this property of forming in one time, and acting in subsequent times: forming ridges in the Apennines, forming walls in a church, inspiring people with raw natural beauty, profound scientific insight, and the special beauty that comes from combining natural hues with anthropogenic forms.

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13 April 2018

Friday fold: Blue Ridge gneiss in the University of Kentucky rock garden

I went to Lexington, Kentucky, last week, to give a talk the University of Kentucky’s Department of Earth and Environmental Sciences. I was delighted to see their many robust geology displays, including an outdoor rock garden. The most charismatic fold in the garden was this one, apparently collected in the Blue Ridge.

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10 April 2018

An outcrop showcasing a strand of the Kentucky River Fault System

Here’s the view as my colleague Kent Ratajeski drove me on Route 27, crossing the Kentucky River heading south from Lexington, headed toward a nice roadcut:

The cliffs at the site show a fault deforming the otherwise horizontally-oriented strata of the Camp Nelson Formation (oldest rocks in Kentucky) and the Lexington Limestone. Both are Ordovician in age, but the Lexington Limestone lies at the higher stratigraphic position. The Camp Nelson formation tends to be finer-grained and thicker-bedded than the Lexington Limestone.

There were some fossils in these rocks. Here’s one block I found with a few prominent brachiopods:

Close-ups of the brachs:

There were also plenty of trace fossils to be observed. In many cases, the burrows were dolomitized, turning them tan:

As noted yesterday, the Lexington limestone has some spectacular fossils, too.

If you’re more keen on primary sedimentary structures, then here is a mud-cracked bedding plane in the Camp Nelson Formation:

But we were here for some structural geology, to sniff out of a strand of the Kentucky River Fault Zone. Here, there’s one big fault and a lot of small-offset, parallel minor faults.

There was a prominent pair of “ball & pillow” layers in the Lexington Limestone that could be used as marker beds for offset:

Here’s Kent taking a look at a prominent pillow, half obscured by vines:

Close-up pillow portrait:

Here’s a small fault in the Camp Nelson Formation:

It shows a “normal” (hanging-wall-down) sense of offset.


Note the fluid flow along this one, and the attendant increase in potential for vegetation.

Eventually, we found weathered out gullies with Lexington Limestone on the south and Fort Nelson Formation on the north. In between was fault breccia, crumbled up rock produced through the grinding action of the fault’s movement.

Here’s an example of fault breccia in outcrop…

…and in hand sample:

There’s a folding story to be told here, too, but I’ll save that for another day.

…A Friday, perhaps…

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9 April 2018

Orthocone nautiloids of the Lexington Limestone

I took a trip last week to Kentucky. My colleague Kent Ratajeski from the University of Kentucky took me out on a nice all-day field trip to examine some of the local geology. I was particularly impressed with the large straight nautiloid fossils that abounded in the Ordovician-aged Lexington Limestone. Here are a series of photos I took of these orthocones, all on pavement exposures (horizontal bedding planes) with my DSLR lens cap for scale:

I also found a loose one that had weathered out in three dimensions, like a section of stalactite:

Very cool to have so many big, detailed fossils around!

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6 April 2018

Friday fold: “wrecumbent in Wrangellia”

Darrel Cowan of the University of Washington has pitched in a Friday fold. Check this lovely scene out:

Darrel says this is

a recumbent fold in the Upper Triassic Nizina and Chitistone limestones. I took the photo when I worked for Shell in 1973 or 1974. I’m pretty sure it is on the west wall of the canyon occupied by the Root Glacier, several kilometers NW of McCarthy, Alaska. The view is west. These limestones are part of the stratigraphy that defines the Wrangellia terrane: the type locality, matter of fact.

Thanks Darrel! This is inspirational!

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