20 July 2018
Via Twitter, a Friday fold from Maddy Rushing:
— Maddy Rushing (@komaddyite) May 25, 2018
This is in the Alps of Switzerland; I don’t know more about it than that. If you recognize the site or the geology, educate us in the comments!
16 July 2018
Religion Explained is an interesting book. It examines the phenomenon of religion in human beings from the perspective of our best understanding (as of 2001) of neurology, psychology, anthropology, and evolution. It takes the modern scientific understanding of how brains evolved, and looks there for the origins of religious thought and its tenacity despite countervailing forces. Pascal Boyer’s contention is that we can come to understand religion better by thinking of it not as socially adaptive (a frequently invoked hypothesis), but as a “parasitic” in the sense of taking advantage of pre-existing structures (algorithms) in our brains. Boyer contends that when several different mental pathways in our minds (each themselves adaptive in the sense of natural selection) become co-opted by religion, a new mental phenomenon emerges and is maintained. It’s a ‘chewy’ book, with lots of anthropological field work observations in diverse human cultures coupled with novel ways of articulating the thought processes and rituals of modern Western religions. It explores the evolution of key mental algorithms (such as the perception of agency, and the ability to mentally assign it to non-agents) in detail, sometimes perhaps excessively so. But it’s compelling: Many of Boyer’s detailed explorations hit home with me as surprising and insightful. The overall thesis makes more sense to me than alternate explanations. I found it a moderately recommendable read – interesting stuff, and possibly a correct vision for explaining religion, but also pretty weighty in construction; Each sub-topic is explored in such detail that it ends up being hard to pick up the main thread of the argument when Boyer eventually gets back to it. I think a good editor could have trimmed it by a third and it would have made for a cleaner, more efficient reading experience. Recommended for those who wish to understand the origins of this otherwise quixotic (or obvious, depending on whether you’re a believer or an atheist) phenomenon.
15 July 2018
I’ve mentioned that there is an anticline at Chickie’s Rock, where Chickie’s Ridge is denied projecting any further south by the Susquehanna River. It’s a site I visited last month on a field trip associated with the National Association of Geoscience Teachers’ Eastern Section annual conference. Today, let’s take a closer look at the rocks exposed at the rest of the site.
Let’s begin our explorations with primary sedimentary features, since they are oldest…
The site is composed principally of quartz arenite (quartz sandstone to quartzite, depending on metamorphic grade), which is a tan white when fresh, but obscured in most places by weathering, lichen, railroad soot, rockclimber chalk, and graffiti.
Here’s a clean, fresh bit, just to show that this really is a light-colored rock, not a jet black one:
Also, there are Skolithos tubular trace fossils:
Here’s a set of 3D ripple marks:
Next let’s consider the lovely tectonic cleavage imparted on these rocks by Appalachian mountain-building:
Sorry there’s no sense of scale in that image – it was too high up to safely reach. I’d say it’s about 2 meters wide. Here it is annotated:
It really seems to refract when crossing more pelitic (mud-rich) layers:
Another example, with my field notebook for scale:
My favorite example of the phenomenon was this dismembered syncline hinge with a cleavage fan:
NAGT Eastern Section past president Dave Ludwikoski for scale. Annotated:
We went to another site, too, to look at a coarser facies of the Chickies Formation. The site was Sam Lewis State Park, and it showed off outcrops of the Hellam Conglomerate member of the Chickies Formation. In places, it looks like this:
And in other places it shows a more pronounced tectonic fabric, with flush grain boundaries parallel to a weak foliation, implying some pressure solution has taken place:
Unfortunately, some of the outcrops are covered with graffiti:
The best outcrop there was this one, with a clear bedding surface (sand below, gravel above) and a tectonic cleavage cutting through both layers:
And what’s up with this round patch of clean (relatively lichen free) rock, showing significantly less relief than the surrounding pebbles, which weather out in 3D? I have no idea.
All told, I found Chickie’s Rock a satisfying field trip stop: it had some primary features and some secondary structure, and both were fun to explore.
13 July 2018
Chickie’s Rock is a prominent cliff of Cambrian quartz arenite (sandstone) in Lancaster County, Pennsylvania. I visited it last month with field trippers at the eastern section meeting of the National Association of Geoscience Teachers. One of the aspects of the site is this gentle anticline with axial planar cleavage:
The yellow rectangle is my field notebook for scale. Another shot:
Happy Friday to all!
6 July 2018
Last week, I spent two perfect days camping with family at Usal Beach, in Mendocino County, California. Along the beachside cliffs there, I spotted plenty of lovely turbidites: graywacke and shale and a little bit of conglomerate that had been scraped off the subducted Farrallon Plate to help contribute to the bulk of the Franciscan complex. That accretionary process imparted some stresses on these deep-sea deposits, and in many places they show significant deformation. Here are a dozen images to highlight some of the folds I saw:
25 June 2018
As mentioned the other day, I’ve been in the Sierra Nevada this past week, which is full of geological delights. Today I’d like to show you a pretty profound unconformity in the region of Sonora Pass, with ~11 Ma volcanic rocks overlying ~89 Ma plutonic rocks.
I’d like to first examine the two major rock units, exploring their varieties, and then look at the contact between them. So our plan is:
- The granite (granodiorite) of Topaz Lake
- The overlying volcanics (lava flows and lahars)
- The unconformity that separates them
1) The granite
The granodiorite of Topaz Lake is ~89 million years old, and is distinctive on several levels.
It includes microgranular mafic enclaves (MMEs) which tend to be, as their name implies, finer-grained and darker in color than the granitoid in which they are hosted. These are interpreted to be small blobs of immiscible mafic magma within a larger felsic-composition magma body. They tend to be round, though they can be elongated by magma flow (prior to crystallization) or by post-crystallization deformation. The first would be a primary magmatic structure; the second would be a secondary tectonic structure. A few examples:
Here is one that shows internal spheroidal weathering, something I had not previously seen:
I also found a single example of a xenolith, a chunk of pre-magma host rock that was broken off and inforporated into the magma chamber prior to cooling and crystallization:
However, the most striking thing about the granodiorite of Topaz Lake are its eye-catching huge potassium feldspar megacrysts:
In places, these make up a significant volume of the rock:
In places, these huge feldspars weather out and can be collected from among the grus-like soil:
My son shows off his find, along with my friends Jason Westfall of Sonora High School, and Laura Hollister:
2.) The volcanics
The next of our two units is a sequence of mafic lava flows and lahar deposits that erupted from the Little Walker Caldera sometime around 10 or 11 million years ago. This is the caldera itself (on the east side of Sonora Pass):
The rocks which emerged as either lava or mudflows from this (today) scenic caldera look like this:
The lahars include logs of petrified wood, representing the ~10 Ma forest that used to exist here. It’s a distinctive orange color:
Lavas with flow-aligned feldspar phenocrysts:
Higher up in the sequence, I found beautiful almond-sized and -shaped amygdules:
On the banks of the stream, we found plenty of charismatic volcanic breccia cobbles – bits of lahar subsequently tumbled by the headwaters of the Stanislaus River:
3) The unconformity:
So now we can discuss the relationship between these two units:
At the right (western) edge of the Little Walker Caldera, the same situation applies: You can make out the layered volcanics directly overlying the spheroidally-weathered granite beneath:
The rocks below this surface (traced out as a black line on the annotated photos above) is an ancient erosional surface, an unconformity. The rocks below that surface cooled deep underground around 89 million years ago. The rocks above it formed on the surface of the planet as layered deposits, around 10 or 11 million years ago. This contact is a nonconformity, a type of unconformity where the rocks below the erosional surface are igneous or metamorphic (as opposed to sedimentary).
When you’re just above the unconformity surface in the lowermost volcanic strata, you can find granite cobbles included within the lahars:
As you can see, the shape of those granite cobbles varies between a bit more angular and quite well-rounded. These are interpreted to be cobbles tumbled in the 11 Ma ancestor to the Stanislaus River, the river that carved the paleo-canyon into which the Little Walker Caldera vomited its lava and lahars. Downstream (to the west), you can see some lovely well-rounded boulder gravels, some of which show clear imbrication, as Ryan Hollister demonstrates here at a roadcut on Highway 108 west of Cold Springs:
A wee bit to the west, at the gas station that marks Cold Springs, you can see these river gravels in contact with overlying lahar deposits: Here, the gravels remain in between the other two units; they have not been removed.
Overall, the thought is that with such significant (paleo-)topographic relief on the nonconformity surface in the Sonora Pass region, we’re looking here at a scene ~11 million years ago with relief not too different than today: an ancient river had cut into uplifted Sierra Nevada plutons, tumbling boulders it derived from those granites downstream. Then the Little Walker Caldera roared to life and poured all kinds of volcanic slurry down the ancient valley gradients. In the upper reaches of the (paleo-)drainage, the volcanic deposits scoured out most of the fluvial deposits, but lower down in the drainage, relief may have been gentler, or the volcanic flows had lost a lot of their scouring power, and the lahars were laid down atop the fluvial gravels. The paleo-canyon was filled up with volcanic debris. Later, erosion began anew, and cut into both rock types to carve a new canyon that partially coincided with the old one. This newer version of the canyon was enhanced by Pleistocene glaciation.
Amazingly, the story doesn’t stop there, but includes a surprising denouement: the eruption of another batch of lava into the newer version of the canyon. It was this most recent volcanism that built the Columns of the Giants, a basaltic feature about mid-way down the drainage, and the subject of Ryan Hollister’s acclaimed virtual field trip for Science Friday. The volcanism there occurred after glaciation had happened at least once, and was succeeded by at least one more episode of glaciation.
All told, I’d say the Sonora Pass region is an extraordinary example of a landscape with a beautifully repetitive series of igneous and erosional events through the past 90 million years.
23 June 2018
I’ve been lucky to spend the past week+ in California with friends, including geoscience outreach wonder duo Ryan and Laura Hollister. We spent an enjoyable 4 days on the east side of the Sierra Nevada, attending field trips through the Mono Basin Bird Chautauqua alternating with excursions to entertain our collective posse of four kids. One day, we took the kids up to Virginia Lakes to go fishing, and Laura and I were able to get in some geologizing. We hiked up to the Frog Lakes, where there is a beautiful deformed conglomerate exposed.
These rocks were very exciting to me.
Here’s a boulder: You can see the elongation of its component clasts:
Some of the clasts are themselves made of smaller clasts, and they too show internal development of the regional foliation:
Deformation here appears to have been accomplished by pressure solution, causing the clasts to warp and wrap around each other:
Here’s an example of what Chuck Bailey might call a “double-duckbill”:
Douglas, et al. (2011) used this unit (which they call a ‘breccia’) as a sedimentary record of the flavor of early arc magmatism affecting coastal paleo-California about 224 million years ago. But I was interested in it from a structural perspective: how the big, charismatic clasts serve as records of the strain accumulated by the wall rocks (or “roof pendants” in this case) during transpressional squeezing prior to (or coincident with) the intrusion of the magmas that would cool to become the Sierra Nevada batholith. This was, after all, the subject of my master’s thesis at the University of Maryland. I mapped very similar rocks south-southwest along strike from Frog Lakes. It was a delight to clamber around and check out these beautiful rocks, simultaneously showing sedimentological and structural features. It made me wish I had mapped this area back in 2003.
Here’s an example of the sort of thing that gets me excited: a boulder showing both graded bedding (fining upward, from the lower left to the upper right) and a pronounced tectonic fabric (running subhorizontal from left to right):
Here is another example, with bedding running from left to right, and foliation running horizontally from upper left to lower right, straight across the contact between a conlomerate and a mudrock:
The great thing about making the hike up to Frog Lakes was to get to see large outcrops showing the stratigraphy plainly, with a delicious tectonic fabric overprinted on it.
Some examples of what I’m talking about:
Here is Laura next to the contact between a meta-mudrock (now slate) and an overlying metaconglomerate:
Here’s another spectacular outcrop, showing a gradational upper contact with the lower conglomerate, and a crisp lower contact with the upper conglomerate, suggesting these beds are right-side-up. Additional evidence for this interpretation comes from the structural domain: the bedding in the muddy unit and the cleavage both dip to the left (~west) but the cleavage is steeper, and the bedding dips more shallowly. This is the basic relationship we would expect if the beds were upright.
Zooming in on the most informative central portion of this outcrop, where the cleavage shows a prominent deflection as it crosses from conglomerate to mudrock to conglomerate again:
Mostly the rocks here were either (meta-) conglomerates or mudrocks, without any intervening sandstone layers. I did spot one isolated “outsized” clast in the mudrock subunits:
And here is a relatively thin layer of cobbles in a rare sandy deposit adjacent to one of the conglomerates:
A hodge-podge of other images of these photogenic rocks:
Here’s an apparent flame structure (just above and to the right of the lens cap):
Zooming in on the “flame”:
A few more:
Did you notice the bonus glacial striations in that last shot (on the left)?
We’ll close it out there: you can count those striations as a small geo-lagniappe. I hope you enjoyed visiting Frog Lakes with me! I look forward to revisiting to this exciting location someday soon – hopefully it won’t take me another 15 years to return.
22 June 2018
Another guest Friday fold (keep ’em coming, folks!) – This time from Eric Pyle of James Madison University:
This is a weathered outcrop of the Connemara Marble in western Ireland, about a meter wide.
Thanks for sharing, Eric!
15 June 2018
My friend Karen Aucker from NAGT’s Eastern Section shared these images (and a video!) of the folds she glimpsed from the cable-car on her way up the Schilthorn in Switzerland. I reckon they will do for our Friday fold:
Thanks for sharing, Karen! These are great.
Happy Friday, all!
9 June 2018
Today, in Millersville, Pennsylvania, on the campus on Millersville University, I saw these contorted carbonates. They are of the Cambrian Conestoga Formation, and I saw them on a NAGT Eastern Section field trip led by Lynn Marquez of Millersville University.
This deformation is purported to be Taconian, but it looks very much like Alleghanian deformation in similar aged and composition rocks in Virginia. Interesting!