“Geology of Skyline Drive” w/JMU

I mentioned going out in the field last Thursday with Liz Johnson‘s “Geology of Skyline Drive” lab course at James Madison University.

We started the trip south of Elkton, Virginia, at an exposure where Liz had the students collect hand samples and sketch their key features. Here’s one that I picked up:

Regular readers will recognize those little circular thingies as Skolithos trace fossils, which are soda-straw-like in the third dimension. Rotate the sample by 90°, and you can see the tubes descending through the quartz sandstone:

This is the Antietam Formation, a distinctive quartz sandstone / quartzite in the Blue Ridge geologic province. But at this location, on the floor of the Page Valley and butted up against the Blue Ridge itself, we see something else in the Antietam:

Parts of this outcrop are pervasively shattered: a variety of sized clasts of Antietam quartzite are loosely held together in porcupine-like arrays of fault breccia. Turns out that this is the structural signature of a major discontinuity in the Earth’s crust: the Blue Ridge Thrust Fault. This is the fault that divides the Valley & Ridge province on the west from the Blue Ridge province on the east. And here, thanks to a roadcut on Route 340, we can put our hand on the trace of that major fault. Here’s another piece of the fault breccia:

After grokking on the tectonic significance of this fault surface, we drove up into Shenandoah National Park, to check out some outcrops along Skyline Drive itself, but it was really foggy. Here’s a typical look at the team in the intra-cloud conditions atop the Blue Ridge:

We checked out primary sedimentary structures in the Weverton Formation at Doyles River Overlook (milepost 81.9), like these graded beds (paleo-up towards the bottom of the photo)…

…and these cross-beds. You can see that it was raining on us at this point: hence the partly-wet outcrop and glossy reflection at right:

Cutting through this outcrop was a neat little shear zone where a muddy layer had been sheared out into a wavy/lenticular phyllonite, with a distinctive S-C fabric visible in three dimensions:

Finally, we went to the Blackrock Trail, which leads up to a big boulder field of quartzite described as Hampton (Harpers) Formation. In some places, exquisite cross-bedding was visible, as here (pen for scale):

Here’s a neat outcrop, where you can see the tangential cross beds’ relationship to the main bed boundary below them:

…And then if you spin around to the right, you can see this bedform (with internal cross-bedding) in the third dimension. I’ve laid the pen down parallel to the advancing front of this big ripple:

That last photo also shows the continuing influence of the fog.

Thanks much to Liz for letting me tag along on this outing! It was a great opportunity for me to observe another instructor leading a field trip, and also to discover some new outcrops in the southernmost third of the park.

“Those aren’t pillows!”

In the 1987 comedy Planes, Trains, and Automobiles, John Candy and Steve Martin have a funny experience. It involves a cozy hotel room (one bed only) and the two travelers are huddled up for warmth. As he wakes up, John Candy thinks he is warming his hand “between two pillows.” At hearing this, Steve Martin’s eyes pop wide open, and he yells, “Those aren’t pillows!”

They jump up, totally discombobulated. An awkward moment follows.

Well, it’s not quite as awkward, but I had a similar “those aren’t pillows” moment recently. I was out in Shenandoah National Park with my GMU structural geology students, and we stopped off at the Little Stony Man parking area (milepost 39.1 on Skyline Drive). Here’s a figure showing the area in question, from Lukert & Mitra (1986):

You’ll note in the detail map at the right that it shows the nonconformable contact that separates the basement complex (here, the “Pedlar” Formation) from the overlying metabasalts of the Catoctin Formation.You’ll also note that it says “PILLOWS” with an arrow pointing at a specific spot on the trail. The word refers to basaltic pillows, which are breadloaf-shaped primary volcanic structures that form when lava erupts underwater. They are typically the size of a bedroom pillow (especially overstuffed pillows). Here’s some video of pillows erupting.

Pillows have been reported elsewhere in the Catoctin (e.g., near Lynchburg, according to Spencer, Bowring, and Bell, 1989), but this is the only location that I’m aware of where they have been reported in northern Virginia. The implications are not all that tremendous: just that a portion of the Catoctin erupted subaqueously, but it would be a neat thing to show students, especially seeing how close the outcrop is to safe parking.

Well, I’ve been to this area a half-dozen times, and I’ve never been able to find those damn pillows. It’s frustrated me, but I had an additional impetus this time around: I ran into Jodie Hayob, the petrology professor from Mary Washington University, who was out there with her students for the day. First thing we said to one another? You guessed it: “Did you find the pillows?”

While the students ate their lunches, I went off downhill (to the west), exploring and looking for these confounded pillows. Pretty soon, I found something that looked vaguely pillowy, at least in terms of have a well-defined “crust” with a dark interior (click through that link for a fine Canadian pillow, courtesy of Ron Schott). Prepare yourself for a lot of photos today… Here’s what I saw:

A few meters further downhill, I found another outcrop of the same stuff, this one veiled in a thin layer of algae (ahh, the joys of east coast geology!):

Little double-ridges which varied in parallel, defining small chunks of rock. Could these be the fabled pillows? But they’re …so small! They’re almost pincushions! I know they say size doesn’t matter, but it’s hard for me to picture a volume of lava this small hitting water and “inflating” to such a puny volume with a nice quenched glassy rind, but then having the interior to stay hot enough to crystallize into basalt. Hmmm. Starting to think something’s fishy with this subaqueous tale…

I then found a nice big cliff, 10 meters high and 20 meters wide, which was made of almost nothing but these structures. Here’s some of them highlighted by the sun (the boundary ridges weather out in high relief), despite being obscured beneath several layers of lichen:

A relatively clean, but relatively unweathered sample:

Aha, now that’s better:

The next two show more of a “classic” Catoctin coloring: chlorite green when fresh, with buff weathered surfaces on the outside:

Zooming in on one small, skinny purported “pillow”:

I climbed back up and coerced some students into joining me to check these weird things out, and they clambered down. Danny W. found a nice chunk of float which showed one of the “pillows” in three dimensions. Check it out at the top of this sample:

Three-dimensional extension courtesy of Photoshop; red line shows the long axis of this oblate ~ellipsoid plunging towards the camera. (Lara laughs in the background…)

Okay; two more… Check out how angular the boundaries of these “pillows” are:

Seeing this one really made me think: No way; “those aren’t pillows!“…

…Seeing that angular “break” on the left led me to realize that not only are these things too small* to be pillows, they also don’t have the right shape. Instead of being “pillowy,” (i.e., round) they are very angular, defined by edges that are aligned in a common direction and continue from one to the next.

* Where “too small” is defined as “smaller than anything Callan has seen before.”

I sketched in some of these planar edges:

To me, it looks like what’s happening here is that original homogeneous rock of the Catoctin Formation fractured, and then fluids flowed along those fractures, altering the rock that the fluids came into direct contact with. This produced the “double ridge” of buff-colored rock (on either side of the fracture), with the less-altered greenstone interiors being beyond the reach of these altering fluids. The intersection of the various joints and their subsequent boundary-defining alteration would look something like this example (from the online structure photo collection of Ben van der Pluijm): definitely click through to check it out.

In other words, I interpret these structures to be secondary, not primary. The end result is something that looks a lot like “boxwork” (again, please click through to get a sense of what I’m suggesting here): a phenomenon that occurs when limestone fractures, more resistant mineral deposits are precipitated in those fractures, and then the limestone blocks are dissolved away, leaving behind the “fractures” as planar ridges separating little “boxes” from one another.

Here’s two photos of boxwork, one whole-sample, one zoomed-in. This sample is in the USGS library in Reston, Virginia, and both photos were taken at my request by Bill Burton of the Survey. (Thanks Bill!)

At Little Stony Man, of course, the greenstone hasn’t “dissolved” away, but it does appear to be weathering more rapidly than the resistant buff-colored edges to these blocks, producing a distinctly boxwork-like effect.

Let’s look back at some of my field photos again, this time with the pillow boundaries highlighted in red…

(…I definitely could have hit a few more boundaries on that last one; forgive me for being haphazard and slapdash…)

This exercise convinced me that these things are not pillows, but some sort of fluid-rock interaction effect that took place on a complex fracture network. There’s no reason for the sharp edges of two adjacent pillows to be perfectly parallel and aligned.And it strains credulity to imagine ultra-tiny pillows in the first place (the size of my fingernail? Come on!).

I’ve e-mailed one of the authors of the original paper claiming pillows in this area with a link to my photos asking if these things are what he and his co-author were referring to, but I haven’t heard back anything. (I’ll update this post if he responds.) I might be totally off base here, but I can see how someone could make the claim that these were pillows. It’s just not a claim that convinces me, based on these outcrops.

What do you think? Do these look like any pillows you’ve ever seen?

__________________________________________

References:

M.L. Lukert and G. Mitra (1986). “Extrusional environments of part of the Catoctin Formation.” Trip #45 in Geological Society of America Centennial Field Guide – Southeastern Section, pp.207-208.

E.W. Spencer, C. Bowring, and J.D. Bell (1989). “Pillow lavas in the Catoctin Formation of Central Virginia.” in Contributions to Virginia geology, volume VI. Virginia Division of Mineral Resources publication 88, pp. 83-91.

Flames and pillows, Route 55

I took a look at some interesting blobby structures in the Swift Run Formation last week, and walked readers through my logic in tentatively concluding that they were ball & pillow structures (soft sediment deformation), though overprinted by a pervasive (Alleghanian) cleavage. As we move west in the Appalachian mountain belt, the rocks are less cleaved: the strain is instead taken up in large anticlines and synclines with a few thrust faults thrown in. Though arched and fractured, the rocks’ fabrics remain pretty close to what they were at the time of deposition.

Fortunately, in the Valley & Ridge province along New Route 55 in West Virginia, you can see some sweet examples of (undeformed) ball & pillow in the Hampshire Formation (Devonian; part of the Acadian clastic wedge). Here’s a view looking up at the bottoms of some of these sandy sags (quarter for scale):

I like that partial weathering-out of the features into the third dimension…

To refresh your memory, ball & pillow forms when a heavy load of sand gets dumped (underwater) on top of a soft, squishy deposit of mud. The sand sags downward in broad “balls” (if you have a point locus of sinking) or “pillows” (if the sags have a linear axis to them). In between, the low-viscosity mud squirts up in cuspate “flame” structures. Check out this fine example (quarter for scale), found by GMU structure students Joe M. and Justin O. on the northern side of the road:

…And here we have the same photo, with the sand, mud, flames, and ball & pillow all labeled for you. The white arrows represent the downward sagging of the sand; the red arrows represent the upward squirting of the mud.

In some places, the pillows have weathered out. Here’s one (quarter for scale) that is now upside down on the grassy knoll beneath its source outcrop:

Note the thin laminae of mud clinging to its exterior, like a coat of paint!

Here’s a second sandstone pillow that has weathered out of the cliff and popped down onto the grassy slopes below. Obviously, the surface being photographed is a cross-section through the saggy pillow:

The “upper left” of this sample would have been the lowpoint of the sag if it were in situ on the outcrop. You can see the smooth margin on the left side, and the rougher zone on the right where it detached from the overlying part of the sandy layer. Let’s now zoom in on this box to see something really cool:

If we go deep here, we can see that the laminations of sand within the pillow show small Z-folds (and S-folds on the other side, though they’re not as obvious in the photograph) that are “parasitic” on the main fold. Here they are, highlighted (white arrows) and traced out (black lines):

The axes of these small-scale structures verge on the axis of the main fold, meaning that their axial planes (blue traces) “tip over” with increasing deformation towards parallelism with the main fold’s axial plane. We’ve seen similar things before. The axial planes play the same game as the cleavage plane. This is the first time I have ever observed such a structure associated with soft-sediment deformation, however.

Has anyone else ever seen S- or Z-folds associated with soft sediment deformation? It makes total sense to me that they would be there based on simple physics, but this was the first time I had ever seen it myself. I’d be curious to get a sense of how common or rare this might be.

Ball & pillow in cleaved Swift Run Formation?

On my structural geology field trip this past weekend, I made one major modification compared to last year’s iteration. I added a fifth detailed “field study area” at an outcrop of the Swift Run Formation, a Neoproterozoic sedimentary unit that is discontinuous in extent between the underlying Blue Ridge basement complex and overlying Catoctin Formation meta-basalts. A month ago, I didn’t know about this location, but I was introduced to it by Chuck Bailey on the Transect Trip last month. It’s a perfect complement to my structure students’ detailed examinations of the units above and below it, and the outcrop offers an embarrassment of rich structures to measure, both primary and secondary.

Here’s something I spent some time pondering: are there ball-&-pillow structures preserved in the Swift Run? We definitely see the ‘coarse-sand-dumped-on-mud’ set up that will lead to soft sediment deformation (sagging of heavy sand downward into squishy mud) under the right circumstances.

Okay, so with that in mind, take a look at this:

You’ll notice a clear contact here between dark, fine-grained mud (below) and lighter-colored arkose sand (above). The contact is irregular, so a sedimentologist unschooled in structure might start thinking about soft sediment deformation. But it’s not so simple as that: this rock is cleaved! The photo above is looking down parallel to the cleavage plane. Annotations:

The boundary between the two strata tends to get a bit scrambled at this interface, with an increasingly zig-zaggy trace as deformation proceeds. Now, let’s rotate the same sample by 90°, and check out the trace of the bedding on the cleavage plane itself:

Pretty smooth line, eh? Not nearly so “EKG-ish” as what we observed in the first photo. This suggests that the wigglyness we saw in the first photo is a structural overprint, and not a primary sedimentary feature. We can then breath a sigh of relief, and just use this cleavage plane outcrop to point out what a nice graded bedding contact looks like:

Here’s a second sample, as viewed on the edge where cleavage and bedding intersect. The little white dot is some kind of arthropod egg case; ignore it.

Whoa! significantly more up and down wiggles here to the contact between the two beds. Again, this could be a primary feature (soft sediment deformation), or it could be a structural overprint caused by the development of crenulation cleavage. I note how the bottom of the tan bed shows the large-amplitude wiggles, but the top does not. Now let’s turn this one 90° so we’re facing the cleavage face, and see what we see there:

Some annotations:

I would not expect a structural re-organization of the bedding trace as viewed on the plane of foliation, only 90° to it. So the fact that the second sample shows the wiggles continuing around all exposed faces suggests to me that it is indeed a primary feature, but the alignment of the most-vertical parts of the sags was accentuated by the development of cleavage, as seen on the first face. This interpretation is backed up by the observation that the basal part of the sandy unit (the coarsest part) varies not only in position, but also in thickness as you trace it out around all sides of the sample.

I’m not entirely satisfied with this interpretation though, because of the asymmetry and small-scale “parasitic” wiggles on each of the potential sand “pillows.” Furthermore, when I’ve seen true ball-&-pillow soft-sediment deformation structures in the field, the mud that squishes up is typically in a cuspate form, in stark contrast to the lobate blobs of sand that sink down. Here in this Swift Run sample’s foliation plane, the shape character of the ups appears to match the shape character of the downs. On the first view (looking parallel to the bedding/cleavage intersection), however, I suppose one could argue that the mud approximates a flame structure (cuspate) while the sand pillows look more lobate… but with the cleavage overprint, it sure isn’t super obvious.

Anyone else want to chime in on these two samples? Observations? Interpretations?

More mud

Remember the mud I saw on Pimmit Run?

Turns out that West Virginia mud pulls many of the same tricks as Virginia mud. Here’s some mud cracks I noticed on Sunday afternoon on the shoulder of New Route 55 in eastern West Virginia:

Notice how West Virginia spices her mudcracks with chunks of Silurian quartzite and fresh crabgrass. A tasty combination!

Mud

A few more photos from Pimmit Run … of mud.

This mud has lots of interesting features, including dessication cracks showing lovely 120° triple junctions connecting up with their neighbors, raindrop impressions, and animal tracks.

Tracks of at least three species here:

(Clicking on this one will make it bigger)

Mud: not as fascinating as some things, but it can share some modest insights.

Hol(e)y basalt, Batman!

Today, our theme is vesicles. Here are some images of vesicles in basaltic lava flows in the Owens Valley of California, the same spot where we saw the baked fanglomerate that I showcased a few days back.

In this photo (and the zoomed-in detail shot below), you can see a couple of things. One is the size difference of the vesicles as you go up in the flow. Bigger bubbles represent larger loci of low density, and hence will be more likely to rise in a fluid batch of lava. This is the inverse of the phenomenon that causes graded bedding (heaviest grains sinking first). The result is a “graded vesicular lava flow.”

Also visible are several cooling joints that intersect to form columns. At the lower part of these columns, you can see arrest lines perpendicular to the column. Each of these subhorizontal lines represents a single instance of fracture propagation as the column separated from the rest of the flow. In composite, they form a “crack panel” like others showcased here in the past.

Let’s take a closer look at these distinctive features:

…And here’s some big vesicles, big enough to host a Swiss Army knife for scale:

They aren’t as big as some I’ve shown here in the past, but they were the largest vesicles I saw on the Owens Valley Field Forum last September. One thing I find interesting about this batch of vesicles is how they deform one another. The big one in the upper right has several smaller ones above it that “wrap around” its left edge. I envision this as the small bubbles hanging out with ~neutral buoyancy (ascendancy power), when up from below comes this massive bubble. As it pushes up (with its greater buoyancy), they smear out to the side, out of the way.

Likewise with the pair of large vesicles at lower right: it looks like the big flat one was there first, with the smaller “egg-shaped” one rising up from below and impinging on its larger upstairs neighbor. If the lava has been less viscous, the two may have merged into one, as blobs in lava lamps may be seen to do: a minimizing of surface tension, a lowering of the surface-area-to-volume ratio. Why would the smaller impinge on the larger? As I’m envisioning it, there would be a viscosity gradient in the cooling flow, with cooler temperatures towards the top (and hence higher resistance to flow). Deeper in the lava, temperatures would remain warmer, and hence the lava would be less viscous. I’m thinking that the big flat bubble had essentially risen as far as it could, but its top side was cooler than its more ductile bottom side, and so the bottom side was less resistant to the nosy intrusions of upstart bubbles from below.

Do you see anything else worth discussing in these photos?

Easter egg

Searching through my photo archives this morning for something suitably “Eastery”… something in pastel colors, perhaps? … a petrified lagomorph? … how about an egg, or something egg-shaped?

This is as close as I got:

This is in the Owens Valley of eastern California, showing a boulder of the Mesozoic Sierra Nevada Batholith bearing a faulted xenolith. I love outcrops like this, with a combination of primary structures (like the xenolith) and secondary structures (like the fault). And the fault surface appeared to have hosted some fluid flow, encouraging epidotization (hydrous metamorphism) along its surface. How appropriate, considering both the “cracked egg” implication of the round xenolith and the pastel tones of the green epidote.

I’ll annotate it up for you, because I know you love it when I do that:

Happy Easter, folks. Focus on the bunnies and candy, and not the zombies.

Sugarloaf

Sunday morning, NOVA adjunct geology instructor Chris Khourey and I went out to Sugarloaf Mountain, near Comus, Maryland, to poke around and assess the geology. Sugarloaf is so named because it’s “held up” by erosion-resistant quartzite. It’s often dubbed “the only mountain in the Piedmont,” which refers to the Piedmont physiographic province. Here’s a map, made with GeoMapApp and annotated by me, showing the general area:

A larger version of the map can be viewed by clicking here.

On the far west, you can see the Valley & Ridge province, which ends at the Blue Ridge Thrust Fault. Then the Blue Ridge province runs east from the Blue Ridge itself to Catoctin Mountain. From there, you enter the Piedmont, including both the “crystalline” Piedmont (Paleozoic metamorphism of various ocean basin protoliths, plus infusions of granite) and the Culpeper Basin, a Triassic/Jurassic rift valley. The Potomac River cuts a series of three spectacular water gaps across the Blue Ridge province just west of Sugarloaf. Harpers Ferry, West Virginia, is located at the confluence of the Potomac and the Shenandoah Rivers by the westernmost of these water gaps, and the name for the easternmost one is “Point of Rocks.”

Here’s a look at a detail from the southeastern corner of the geologic map of the Buckeystown, MD quadrangle, by Scott Southworth and David Brezinski:

The map pattern shows a that the area around Sugarloaf Mountain is a doubly-plunging anticlinorium of Sugarloaf Mountain Quartzite [SMQ] and overlying (younger) Urbana Formation. Overall, it’s got that typical “Appalachian” northeast-southwest trend. Notice the thrust fault on the west side: a typical hanging wall anticline? The ridges, including the summit of Sugarloaf Mountain itself, are held up by the toughest quartzite. This overall “squashed donut” shape shows up pretty well in the physiographic map up at the top of this post.

Sugarloaf is quartzite (metamorphic), but you can clearly see the sand grains that composed its protolith (sedimentary). There’s also reports of cross-bedding, and so Chris asked me to take a look at a few structures to assess them with my point of view. I found a pervasive cleavage in the rock, far more than I would have suspected would be there. We did find bedding exposed as compositional/grain size layers in several locations, including on the summit. I also paid a lot of attention to the many quartz veins which cut the metasedimentary quartzite. These veins of “milky quartz” are often arranged in lovely en echelon series, like these tension gashes:

I took the above photo several years ago on a visit there, but it’s typical of the sorts of stuff we saw Sunday. The kinematic sense of this outcrop would be “top to the right.” Interestingly, none of the Sugarloaf outcrops show really deformed tension gashes (i.e., they’re not folded into Z or S shapes like those I showed you a few days ago).

What we really wanted to get a sense of, though, was which way was up in these rocks. We were in search of geopetal structures: primary sedimentary structures that indicate the “younging direction” of the beds. Graded beds can do this, though I didn’t see any unambiguous graded beds in the SMQ on Sunday’s trip. We wanted some cross-beds. We found some hummocky / swaley examples, looking approximately like this USGS photograph (black & white; hammer for scale) of an outcrop somewhere “north of the summit”:

Image source: USGS

Ours wasn’t as beautiful as the one pictured above, but it was clearly hummocky cross-bedding, and it was right-side-up (in beds tilted at ~30°). Interestingly, the SMQ has been correlated by Southworth and Brezinski (2003) with the Weverton Formation of the Chilhowee Group, a rock unit exposed in the Blue Ridge. Just as the Weverton is overlain by the finer-grained Harpers Formation, so too is the SMQ overlain by a finer-grained unit, the Urbana Formation. Both are interpreted as metamorphosed continental margin deposits. The Urbana is mostly phyllite in the areas I’ve seen it (including phyllite that’s full of quartz grains, a first for me). The Urbana is well exposed in a creek-side outcrop north of Sugarloaf Mountain, and I took Chris there to show him the lovely intersection of bedding and cleavage.

Here is a weathered piece of the Urbana Formation that Chris collected there, looking at the plane of cleavage (ruler in background for scale):

Image source: Christopher Khourey

You can see the bedding running ~horizontally across it, though the photo cannot convey the lovely phyllitic sheen that results from waggling these samples back and forth in good light. It’s pretty cool. In places, the transition from sandy to phyllitic is gradational, probably relict graded bedding.

So, what does it mean if Southworth and Brezinski (2003) are correct in their correlation, and the Weverton and the SMQ are really the same rock layer, but in different provinces and at different metamorphic grades? Recall that the Blue Ridge province to the west is also a thrust-faulted anticlinorium, launched up and to the west by the Alleghanian Orogeny from an original position deeper in the crust and further towards the east. It’s a shard of the craton, snapped off and shoved bodily up and to the northwest. (In class, I often liken it to Joe Theismann’s leg: a compound fracture of the continental crust.) Might the Sugarloaf Mountain Anticlinorium [SMA] be a smaller version of the Blue Ridge pulling the same trick? It too is arched up and snapped off …but it would be a “Mini-Me” that’s only just surfacing, like a baby whale swimming above momma whale’s back…

We know that deeper down in the Blue Ridge stratigraphy, we find the Catoctin Formation, the Swift Run Formation, and the basement complex. If we drilled down through the crest of the SMA, would we find the same units (or more metamorphosed equivalents thereof)? It’s an intriguing thought…

Transect debrief 7: Brittle-ductile deformation

On the transect trip, I also saw some nice meso-scale “minor” structures that probably formed during Alleghanian deformation. Prominent among the ones that really impressed me were these en echelon tension gash arrays, deforming the Antietam Formation quartz sandstone and well exposed in blocks used to construct the wall along Skyline Drive and the Sandy Bottom Overlook in Shenandoah National Park:

Good Lord! Ain’t those things beautiful? They also give us a lovely sense of the kinematics (relative motions) of the blocks of Antietam sandstone on either side of this sheared zone. In the case of the image above, the left side of the photo has moved “down” relative to the right side. The rock in between has torn and stretched, with the gashes opening up at right angles to the maximum stretching direction. As deformation proceeds, of course, the gashes rotate and deform, folding into “S” shapes.

Here’s one that’s more subtle:

What you’re looking at in the image immediately above is a tension gash array that was a zone of weakness, exploited by later brittle deformation. The fracture which defines the edge of this block cracked through those old brittle-ductile tension gashes and split them clean in half.

Neat, eh? …Now check this out:

Remember the Skolithos trace fossils? Here, you’re looking at a sideways cross section through some cylindrical Skolithos as they are disrupted by this zone of shearing. Note that the burrows tend to be highlighted by rust (hematite) staining: the brown lines that run roughly from the top left of the photo towards the bottom right. But look what happens to the orientation of those tubes where they are cut by the tension gash arrays: they are deflected into a new orientation, rotated from their original orientation!

If that’s a bunch of gobbledygook to you, consider this annotation:

I’ve drawn white lines to show the orientation of the Skolithos tubes in their undeformed and deformed states, colored the tension gashes yellow, and drawn on a set of blue arrows to show my kinematic interpretation (top to the left).

Here’s another block, showing the same phenomenon:

Go ahead. Tell me you’re not impressed with that. I dare you. That is frakking AWESOME.

You are now dismissed.