20 April 2018
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.
18 April 2018
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.
13 April 2018
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.
10 April 2018
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…
9 April 2018
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!
6 April 2018
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!
2 April 2018
Mountain Press has released a new volume by frequent author Bert Dickas: it’s a compilation of 101 places in the United States where fossils can be viewed. Some sites are collection sites on public land; others are museums or protected areas. The book is a useful collection of information in a concise, well-illustrated form. Each of the sites gets a name, a latitude/longitude (but not directions), a short tag line, and then a page of description. Facing the text on the opposite page is a suite of three to six images illustrating the fossils or setting of the site. Two pages per site, and an average of two sites per state. A map shows the distribution of the sites across the country, but I think a stratigraphic column showing their distribution in time would have been a good addition too. The book also includes a relatively dense introductory section with an overview of key happenings in the history of life for each eon, era, and period, but its strength is the lean, efficient, “intermediate” level discussions of each site (and the accompanying photographs) that make up the bulk of the book. There’s a lot of variety in North American fossils, and this book captures that well, though I noticed that an awful lot of them feature Cenozoic shark teeth. The sites I was most familiar with were here in Virginia, and they are good choices. I was delighted to learn of a bunch of new places I’d never heard of – opalized acorns, fossil forests, the details of the Two Creeks site in Wisconsin, a glaciated stromatolite outcrop in New Jersey, and the Berlin (Nevada) ichthyosaur site. Reading this book has populated many future travels for me! Furthermore, it turns out that some of my own local go-to spots for structural geology are also good for trilobites and corals – I never knew, so now I’ll know to keep my eyes peeled next time I visit. 101 American fossil sites you’ve gotta see would make a great gift for the amateur paleontologist or rock hound in your circle of family and friends.
30 March 2018
Stop the presses!
This late-breaking Friday fold has just been submitted here at Friday Fold Headquarters. This is from Philip Prince/Virginia Division of Geology and Mineral Resources:
It’s a recumbent fold of Harpers Formation metasandstone in the James River Face area.
Pretty lovely exposure. This outcrop would make a good 3D model.
Happy Friday to all. Hope it’s a good one!
28 March 2018
After reading Mika McKinnon‘s endorsement of this series on Twitter (example), I downloaded an audiobook copy of N.K. Jemisin’s first book in her “Broken Earth” trilogy, The Fifth Season. It is a fantasy novel with a healthy seasoning (ha! no pun intended) of science fiction. The story is set in a world called “Earth,” but it’s not clear if it’s the same world as our own, in the distant past or the distant future. The people that inhabit the “Earth” of the novel are both quite familiar as humans, yet some have the ability to manifest tectonic changes in their planet. When an earthquake is about to strike, for instance, these gifted people can use their powers to sense it coming, and stop it. This takes a lot of effort, and as they draw energy from their surroundings for the task, a frozen zone manifests itself around them as they work. These “orogenes” can also break the Earth – triggering earthquakes or volcanic eruptions. Because of their power, they are feared by other humans, who seek to control them. Much of the novel’s dramatic arc is driven by the relationship between the orogenes and their “still” controllers. The training of the orogenes, their professional work, their coping with the social stigma of their talents – those are all part of the tapestry of the book. There are some odd extras that I don’t really understand: “stone eaters” which appear to be living rock sort-of-people, and strange obelisks which move through the Earth’s interior. But presumably their existence and purpose will be made clear in the latter two books of the trilogy. The Fifth Season is a fully realized imagined world, and its description and details are rich. It is worth reading for the ratio of strange and familiar it conjures. Because the version of “Earth” in this “Broken Earth” series has fundamental differences from our own world, there’s a lot of new vocabulary to master. Sometimes multiple words refer to the same thing, as how you eventually come to realize that “ragga” is a slur for “orogene.” Sometimes it’s plain that new terms are stand-ins for words in our own argot. (For instance, when orogenes curse, they yell something like “Blech! This rusting soup tastes awful!” – where “rust” has attained a new kind of “four-letter” status.) These new words can be both enticing to the geoscientist reader and also off-putting in their multitudes (at least initially). I found the book hard to get into for the first few chapters, but once I’d mastered the lingo and dipped into each of the three separate strands of the story twice apiece, I got into it properly. I would prepare readers minimally for the “Broken Earth” they are about to enter, so they can discover its strange wonders on their own, but I would emphasize to prepare themselves for three stories in one novel, one of which is told in the second-person perspective. It takes time to wrap your mind around all that, but I found that the payoff was worth it. The three are connected in ways that aren’t at first apparent, and it’s a refreshing bonus that all three feature strong women of color as main characters. I’ll be headed soon for the next book in the trilogy. If my endorsement isn’t enough, know that this novel won the 2016 Hugo award for science fiction. It’s something unique in my experience.
26 March 2018
Yesterday, I took a hike in search of anticlines.
I started off in Veach Gap, a place where I bring students and where there are four excellent 3D outcrops showing SW-plunging anticlines of ~human size in the Massanutten Sandstone. (They are presumably buffered by intervening synclines, but those don’t crop out well in Veach Gap.) Then I headed NE up the hill, hiking along the trail-less crest of Little Crease Mountain before joining up with the Massanutten Trail before dropping down off the ridge, again without a trail, into my neighborhood. The red line on the map shows my route. The yellow circle shows another water gap that cuts across Little Crease Mountain; I intend to go check that out someday soon, along with the “south face” of Veach Gap.
I chose yesterday to do this hike because it’s still winter here, and that means the trees (dominantly deciduous hardwoods) haven’t yet leafed out for spring. The way through the forest, and the views out over the surrounding landscape are thus relatively clear and relatively non-green. Also: there are no ticks out yet! And yet, the snow had melted at my house (elevation ~1200′) which led me to think that the going would be relatively easy. (It turned out that there was still a substantial amount of snow up on the ridge, and I had very cold, wet feet by the time I got home.)
All these photos are taken looking NNE (~030°) along the axis of the ridge. I’ll show each one in its raw form, and then in an annotated state, using bright green to trace the bedding out.
Here is a portrait of the profile plane of the best example my excursion revealed:
And here is the same structure, on a different outcrop, further downhill by about 40 feet:
I made a 3D model of this one:
That gives you a pretty good sense of the thing, right?
Another anticline, not quite as impressive, but still showing a well-developed arc to the bedding:
And here’s the last clear, blatant anticline I spotted, a composite image from 4 input images (hence the weird repeating branch in the upper right):
I also found a syncline:
Another perspective, about 40 feet away on the same syncline:
I wasn’t sure exactly what was going on in the middle of this one; was it like this?
…or was there a fault in there, making it impossible to trace bedding across the middle of the structure?
Those were the main outcrops of interest I discovered while ascending the “north face” of the gap.
Once I got up out of the gap, up on the ridge crest itself, the outcrops looked like this:
It was pretty consistently dipping to the NW, and I thought I was done with matters of structural interest.
But then I went through a stretch of outcrop-poor forest, and it had shifted to dipping to the SE:
Clearly, I had cross the trace of the fold axis without realizing it. I hiked on.
But then it shifted back to NW-dipping:
And back again to the SE…
So the main fold axis and the ridge were approximately but not exactly coincident.
There were also a few spots where I could almost convince myself that I was on the axis of a fold, like this one:
…and this one:
But you can see that the quality of these outcrops wasn’t great. The beds are massive and jointed, and everything’s been spheroidally weathered to some extent, not to mention covered in lichens of a half dozen varieties, and buried beneath fallen leaves and rotting logs. So I wasn’t positive that these spots were structural highs as well as topographic highs.
There were plenty of cross-beds to use as geopetal indicators:
Also, I found this lovely set of Arthrophycus bedding-parallel trace fossils:
Also, it’s probably worth mentioning the views. Thanks to the bare trees, I could actually see out over the surrounding landscape. From here on to the end of the blog post, the “default” orientation to the photos no longer applies. These next images look every which way.
Here’s a shot looking ~southeast, across the small valley cut by Mill Creek, over Massanutten Mountain, across the Page Valley, and toward the Blue Ridge on the horizon:
The Fort Valley, looking NW across strike:
The southern Fort Valley, looking south oblique to strike, Kennedy Peak prominent as a knob along the ridgeline:
The craggy Blue Ridge, looking east:
The snow highlighted things in new and enticing ways. For instance, there’s a ridge-parallel belt of resistant rock cropping out here in the western Blue Ridge, that I had never noticed before:
It appears to curve over on its southern end, suggesting it’s a very tight overturned syncline.
This is what I love about exploring – I go off and finally visit a region I’ve been seeking to visit for a while, and I see something else, on yon distant ridge, that plants the seed for another day’s adventure.
Here’s a handheld GigaPan of this view, hosted and annotated in the GIGAmacro viewer:
Finally, let’s look SSW, across Veach Gap, at the dismembered southern tip of Little Crease Mountain. This is just after I got up out of the gap and onto the ridge proper:
Now east is on the left and west is on the right.
There are outcrops up there! (and they appear to dip to the west).
I wonder if there are any anticlines to be seen?
I guess I know where I am heading on my next free day to explore!