Archive | June 2022

Difficult Run: Exploring Potomac Tributaries

Figure 1. View downstream in Difficult Run, towards the Potomac River. (See Fig. 2 for location.)

This week we went exploring south of Great Falls, along a tributary that cuts through Precambrian metamorphic rocks, before joining the Potomac River.

Figure 2. Topographic relief map of the Potomac River area southeast of Great Falls (circle at upper left). The irregular black line traces out path along several trails within the Difficult Run trail system. The photos were taken at the numbered points.

Difficult Run twists its way through a mass of hard rocks that we have met before, Precambrian Schist and gneiss, forming a series of quiet pools (Fig. 3) separated by resistant, rocky sections (Fig. 4).

Figure 3. Images taken at Site 1 (see Fig. 2 for location). (A) Gravel point bar deposited. (B) Flood plain about 10 feet above low-water level, incised with secondary channels, forming an anastomosing stream at high water. The floodplain sediment comprises mostly silt and mud, with minor sand, primarily in point bars. (C) Quiet pool formed between rocky sections.

It was a shady walk beneath a tall canopy of mature hickory, ash, and other temperate forest trees. There was plenty of evidence of the recent spring floods. Large trees were jammed up on rocky outcrops and among the trees covering the floodplain. After a short hike, we came to what looked like an abandoned quarry (Fig. 4), which can be identified by the bright spot in the relief map of Fig. 2, just above the label for Site “2”.

Figure 4. Photos of quarry located at Site 2 (see Fig. 2 for location. (A) The rear wall of the cutout was hidden between trees and vines, but the top of the ridge was irregular (blue line in Panel A). The white circle highlights some of the weathered rock from the area. Access was not possible. (B) Exposure in the east wall of the quarry, showing unconformity of contact between the reddish rock on the right, and the gray rock on the left of the yellow highlighter. Incongruous gray sediment is circled in white. Note the reddish sediment to the right of the hypothesized contact. This could be a fault or simply a change in lithology but apparent foliation across the contact was not uniform. The image is approximately 12 feet high.

I was able to examine several slabs of the rock exposed in the quarry and along the river bed (see Fig. 1 for appearance) at Site 2, revealing foliation and inclusions similar to other exposures of this rock (Fig. 5).

Figure 5. Close-up images of typical exposures of rocks along Difficult Run. (A) Loose boulders near their origin in the cliff. Note the juxtaposition of nearly original bedding (central part of lower rock), folded beds (upper part), and completely destroyed bedding accompanied by recrystallization. The white blebs in the lower part of the photo are feldspar minerals that formed from the original composition of the shales and siltstones before metamorphism. The boulder is approximately 2 feet across. (B) Close-up (4x magnification) of quartz squeezed out from a quartz-poor shale during metamorphism. (Image size is 2 inches). (C) Angular inclusions of feldspar (light-colored minerals) in a fine-grained matrix of Fe-rich minerals that weather to form a rust-colored surface. This area is analogous to the disrupted bedding in Plate A. The image is about 12 inches high.

We’ve seen the structures displayed in Fig. 5 before, in this same rock, at Great Falls and other locations along the Potomac River. I’m presenting these examples to give the reader some idea of the scale of these processes. For example, the juxtaposition of foliation in Fig. 5A suggests a frenzy of activity, like in a pan of boiling water; that analogy is reasonable if we adjust the viscosity, temperature, pressure, and time scale from water on the stove to rocks buried deep beneath the surface, but heated from below–just like the pan of water. I’m speculating here but, just to get an idea of what I’m talking about, the crazy structures in Fig. 5A probably took on the order of ten-million years to form.

It might help to see the problem from a more god-like perspective.

Figure 6. This is a hypothesized cross-section across Loudon County from west to east. It is based on thousands of geologic measurements. Note the pink/gray area with brown lenses pointing upward to the left. This represents the Neoproterozoic metamorphic rocks we’ve encountered all along the Potomac River. There are several points to note from this drawing: (1) the dominant foliation (brown lenses) is dipping to the east; (2) the contact with surrounding, older rock is folded (squiggly lines to the left); (3) it forms a massive bedrock structure that cuts the Potomac River almost at a right angle. (The Potomac would run approximately along the cross-section.) (4) It is filled with igneous intrusions from multiple orogenies, represented by the lenticular blebs and orange, white, and gray dikes trending up and to the left in the image (on the left side). All of that igneous activity supplied quartz and feldspar to fill voids. (The cross-section is approximately 25 miles across.)

As you might expect, the exposure of such a deformed and mineralogically diverse set of lithologies along the Potomac’s course produces features like Great Falls, as well as what we’re examining today.

Figure 7. Example, from Site 3, of a resistant ledge of metamorphic rock forming a cataract backed by a pool as in Fig. 3. The orientation of the ledge is approximately 30 degrees east, the same as the strike of the rocks we have examined throughout Virginia (so far). This is the regional structural pattern, which I discussed in previous posts.

There is more to these rocks than metamorphic structures, including folding, foliation, and inclusions. All of those were formed between 1000 and 500 million-years ago. After deep burial (maybe 15 miles) beneath an enormous mountain range, these rocks hardened and were exhumed by erosion of the overlying rocks. They were brittle and, as isostatic pressure relaxed, they cracked just like a cooling pumpkin pie, forming joints.

Figure 8. Analysis of joint sets from Site 3. The images have been rotated to approximately align with north and east. (A) Note the thin, white lines of (probably) feldspar filling joints, which are represented schematically by black lines labeled “X” and “Y” (east and north joints). I have no way of knowing what angle at which the joints intersected the surface of the outcrop, so this is speculative. (B) The same convention is used for labeling the joints, again seen as thin, intersecting white lines in the outcrop. Axes have been labeled “X'” and “Y'” for this photo (B). The northward joint is in good agreement between Plates (A) and (B), but there is a significant difference in the east joint. It was difficult to tell in the field, but my overall sense was that these two outcrops (separated by less than 100 feet), were oriented differently. I indicated my uncertainty with a question mark.

Confused by what I had seen so far (i.e. Figs. 4, 5, and 8), I followed the trail to the confluence of Difficult Run and the Potomac river (Fig. 9).

Figure 9. Difficult Run joins the Potomac River at a steep debouchment, with large boulders littering the creek bed at the large, angular bend in the Potomac (Site 4 in Fig. 2), seen in the background.

Looking across Difficult Run to the south at Site 4 (see Fig. 2 for location), I was once again bewildered.

Figure 10. The south bank of Difficult Run at its confluence with the Potomac River at Site 4. Note the exposure of Precambrian rocks along the base of the steep slope, large boulders, and a shallow ledge at the creek’s mouth.
Figure 11. Close-up image from Fig. 10, showing unconformity between the rocks with foliation (below the unconformity), apparently dipping about thirty degrees away from the camera, and overlying reddish-weathering rocks with similar orientation, except for the tan block, just to the right of center in the photo, which have an apparent dip of more than 45 degrees to the left.

It is tempting to assume that the overlying rocks in Fig. 11 are sedimentary, deposited on an erosional surface in the underlying metamorphic rocks (angular unconformity); however, the geologic map (Fig. 12) reveals that these are similar in age and lithology.

Figure 12. Geologic map of the study area (from Rock D iPhone app).The location of Fig. 11 is shown by the black marker. This lithology is described the same as the green unit to the west of Difficult Run. These units were probably given (forgotten) unique names before they were found to be similar. Nevertheless, differentiation is useful considering the unconformity in Fig. 11.

I summarized the geologic history of this area in a previous post, so I’d like to wrap up by demonstrating how pervasive deformation is, in this post. Imagine the deformation seen in Fig. 5A scaled up several orders of magnitude, to the scale of a bluff (Fig. 11). We also saw evidence of rotation of porphyroblasts at another location along the Potomac and again, more than a hundred miles to the south, in Lynchburg. This deformation actually extends to the microscopic scale, but we had neither proper samples nor a microscope to demonstrate it for these rocks.

Think of metamorphosis and ductile deformation as being like a peach pie, the contents trapped between the bottom of the pan (deeper, more resistant rocks) and the pie crust (overburden); the filling is boiling in the oven, overturning, even displacing smaller pieces of fruit. That is what’s happening miles beneath the mountains, on time scales of millions of years rather than minutes.

Review of “The Stainless Steel Rat’s Revenge” by Harry Harrison

The blandest of several covers used on the print copy of this book, which was published in 1970. I read an ebook with this cover, borrowed from the Fairfax Public library.

I vaguely recall the Stainless Steel Rat series when I was in high school (Harrison wrote more over the following decades). I didn’t read it or the first one because the covers looked kind of childish to me at the time. I was reading more “mature” SF authors like Heinlein and Asimov. I think this was meant for young teens, which I’m not above reading.

As with every other book I’ve reviewed, the grammatical errors increased after the halfway point, but they still weren’t distracting. The writing style wasn’t that different from the probable target audience, i.e., long-winded sentences that sometimes changed topic unexpectedly. The witty comments reminded me of the Spiderman comics I read in middle school.

The protagonist is a spy who works for some kind of galactic overseer agency, saving planets with help from gadgets and his sidekick/wife. From Jim diGriz’s repeated escapes from the same sinister empire, I’d say his imagination exceeds the author’s although it mostly relied on smoke grenades and perfect timing. The “action” was nonstop but highly repetitive. I was hoping for a surprise ending–the hero getting killed.

It was, however, interesting to see how limited an author’s imagination can be, especially in a genre like SF. For example, Harrison couldn’t imagine wireless devices (this was in 1970), and the cars (far in the future on another planet) sounded like the big-block V-8s that were popular when he wrote this book. And, despite the existence of warp drives (this was after Star Trek), the spaceships were apparently constructed of steel, and rusted. There are no drones, but there is a driverless cab and robot house cleaners.

This would probably be okay for a very narrow audience today, mostly boys between 10 and 13, because it doesn’t contain any sex but lots of junior-high innuendo (e.g. large bosoms and his jealous wife).

I’m glad I didn’t read it when it first came out because I probably would have thrown it in the trash can, wasting a few of my scarce dollars…

Awkward Meeting

I will never forget how I met Dr. Zola Brown. My thesis advisor, Dr. Daniel Greeman, wanted us to meet in a setting she would find comfortable because, apparently, she didn’t like meeting new people outside of the classroom. From his description of her personality, I expected the soon-to-be outside member on my Ph.D. thesis committee to resemble my fourth-grade teacher, who happened to have the same surname; that’s a thing about being African-American with a lineage that extends into the slavery period of American history. We all have similar surnames, like Washington, Jefferson, Lincoln, Brown (you can guess where that one came from), and Wilson, which happens to be the appellation associated with my family’s distinguished lineage. The resemblance between Mrs. Brown and Dr. Brown didn’t end with their last names. 

Dr. Greeman had declined to tell me why he wanted me to meet Dr. Brown incognito, in disguise as an undergraduate enrolled in Physical Geology at San Francisco State University, on a field trip to Marshall’s Beach. I had done some research because I didn’t know if he was playing a prank on a colleague or making a fool of a new graduate student—some kind of initiation rite to the halls of academia. I was determined to play along because, to be honest, I was overwhelmed by the halls of academia and wanted nothing more than to have fun while joining the two-percent of Americans with a Ph.D., and if that meant pretending to be someone I wasn’t in order to meet an antisocial future collaborator, I was all in. 

“What are you doing here?” 

Oh yes, Dr. Brown and Mrs. Brown had a lot in common. She examined me from head to toe. She frowned at my makeup, shook her head at my large earrings and nose pins, scoffed at my form-fitting top and jeans; and her jaw dropped in shock at the high-heel pumps that adorned my feet. 

I smiled nervously (easy to do under the circumstances) and replied, “I’m a new student. I missed the van and drove so that I wouldn’t miss the field trip. This is really important to me.” That part turned out to be true, but I’ll get to that later.

Her response caught me off guard. 

“You look like a Klingon at a Death Ritual. What were you thinking, coming on a geology field trip dressed like that?” Her downturned nose hid what must have been nostrils flared with righteous indignation, her eyes indicating that I would probably fail her class.

I grinned at her comment, pleased that she appreciated my carefully applied makeup. I had always been enthralled by the Klingons and their culture, in which women were the equals of men, willing to kill and die for their honor. Furthermore, the Klingons are dark-skinned, not pasty like their Vulcan cousins, although my complexion isn’t as dark as Dr. Brown’s, which is probably why my surname is Wilson. 

“Take me to your leader.”

I was certain I would become accustomed to her abrupt demeanor, but I was surprised when a rock hammer was suddenly thrust at me, along with a pair of safety glasses. “Join the others and let’s see if you can tell dip from strike.”

I accepted the heavy tool and tiptoed through the tall, wet grass fronting the low bluff, inwardly grimacing that I would probably ruin my new shoes in the muddy soil. The students were confused by the twisted mass of shale and what was probably sandstone; some of them were pouring dilute hydrochloric acid on the unresponsive silicate rocks (HCL fizzes on limestone); others, mostly guys, were beating the rock to death with their cheap hammers. I joined a mixed group of men and women who were peering through tiny magnifying glasses at what I had learned from Wikipedia was a melange of sand and mud, buried deeply enough to harden into rock, and then scraped off as the Pacific Ocean’s seafloor plunged beneath North America. Feeling empowered by this knowledge, which the undergraduate students enrolled in Dr. Brown’s class apparently lacked, I nevertheless refrained from sharing my wisdom and instead acted as confused as they. 

“It’s all messed up,” a man with dark skin and close-cropped curly hair exclaimed in frustration.

A white girl with dark, straight hair, who could have lost a few pounds, added, “I don’t know if it’s sandstone or shale.”

 I pointed to a chunk of shale, embedded in a matrix of mudrock, and tossed out, “How can sediments like this, deposited on the seafloor, become so tangled?”

A black girl who had been studying one of the lightly colored sandstone blocks exposed along the bluff put her hands on her hips and declared, “I can’t tell what this is. I don’t see any bedding like in the shales, even the pieces we’ve been looking at.” She poured some more dilute HCL on the shiny, unspeaking rock and added, “It isn’t limestone.”

“I think it’s a schist,” announced a white guy who looked like he hadn’t gotten enough sleep the previous night.

I shook my head in frustration and corrected him. “Schist would have foliation that would be continuous. These rocks look like pieces that were mixed up together.” I looked towards the Pacific Ocean, hoping they would get my drift.

Another student joined our group, a black guy who had been diligently pounding on the helpless rock. “This is fucked up!”

It was time to collect their observations and come to a conclusion. “Let’s put it all together.”

They looked at me blankly. It was hard to believe I had been just like them only a few years earlier. Before I could elaborate on my statement, Dr. Brown’s commanding voice broke everyone’s train of thought. “Okay, you’ve had enough time to make your observations. Does anyone know what we’re looking at?”

A few hardy souls spoke up about it being igneous or metamorphic, but the guy who had called the rocks “fucked up” held his hand up, before quickly lowering it and, with a grimace that said a lot about his classroom experience with Dr. Brown, offered, “It’s all mixed up, some shale, a little of maybe granite or sandstone, something like that, but now it’s all confused…a mishmash, like my mom’s surprise dinner on Saturday night.”

There were a lot of laughs at his analysis.

Dr. Brown corrected him. “You are correct, Damon, but we call it melange. However, that isn’t granite, but only recrystallized sandstone that was mixed in with the shale matrix. It lost its original bedding when it was buried, every rock has its own chemical signature and responds differently to changes in pressure and temperature. But what caused it to look like this? To become a mishmash—a melange?”

The overweight girl looked at me, then at the calm Pacific Ocean, and offered, “It was caught in the subduction zone.” She pointed towards the sea and, with the others nodding enthusiastically, added, “It got all tangled up and was dragged along…” She grimaced and waved her hands as if imagining being dragged along with the subducting Pacific plate. 

I guess she was taking Geology 101 as a science requirement for a performing arts major.

Dr. Brown then strode to the outcrop and proceeded to show her students the details that revealed the complex history of what is now called San Francisco, a tumultuous interval that began more than 150 million years ago and continues to this day. I was as fascinated by her recitation of the Mesozoic past of Northern California as any of the undergrads. I was smiling in anticipation of sharing my discovery with Dr. Greeman as I handed the heavy-duty hammer back to Dr. Brown while the students filed back to the van. 

She looked at me with narrowed eyes as she said, “You aren’t on my class roster. I don’t mind an interested observer joining my field trips occasionally, but I don’t like disruptions…you fit in well with the others and seem to know strike from dip. I can see that you are serious but, still, don’t make a habit of it. You should register for the class if you want to take part in future field trips. I would welcome your participation.”

I finally found the appropriate response to her earlier comment about my uninvited appearance on her field trip. I showed her my best Klingon smile and said, “We Klingons do not believe in wasted effort or time, and we have no tolerance for foolishness or idle talk…”

She was still watching me, her jaw agape, as I climbed into my antique Toyota Corolla.

Review of “The Knowledge Machine: How Irrationality Created Modern Science,” by Michael Strevens

The author is on the faculty of the Philosophy Department at New York University and has been writing about a range of issues relevant to the scientific enterprise for more than twenty years. Some of his essays and papers sound quite technical, but this is a book written for entertainment as much as the serious ideas it presents.

First things first; the grammar was a little clumsy at times and I had to reread several paragraphs. This is a typical complaint I have with science writers; I think our thoughts jump before the pen has caught up. I noted (for the umpteenth time) that the number of (blatant) grammatical errors increased towards the end, as if he just wanted to get it finished. Still, it was well written overall.

I enjoyed the general approach used; setting out to debunk two of the more famous ideas about science is a great attention getter. He did a good job of it too. However, I’m not sure if the basic tenet (I’m not giving it away) justifies a book of this length. Some of the examples used to present his ideas weren’t very convincing although I agree with his conclusions (in general). Of course, I am a scientist, so maybe…actually, I would go further than he does in criticizing the practice of science .

The author goes out of his way not to offend anyone; all of the examples he uses are from the physical sciences, e.g., physics and chemistry, but social sciences like sociology and economics aren’t discussed. However, by neglecting disciplines that don’t have the clarity of conceptually simple measurements, his discussion can be interpreted as either chauvinistic or overly simplistic. The closest thing to a discussion of social sciences is taking aim at two famous philosophers of science, as the thesis of the book. This omission makes the author’s premise less convincing, possibly even reducing it to no more than biased speculation after an evening searching the internet for anecdotal stories.

I read this book on a Kindle, so I couldn’t easily go back and search for something forgotten. Thus, despite the repetition of several stories and ideas, I don’t recall exactly what “The Iron Rule” is.

That may be because I’m stupid, but still, shouldn’t even the densest reader get the main point to a book-length discussion of what is basically a simple idea?

Despite my criticism, I recommend this as light reading, especially for non-scientists who may mistakenly think that SCIENCE knows what it’s doing. I think the author paints a pretty accurate picture of how scientific inquiry proceeds, despite his bias.

As the author concludes, science is an unthinking golem, despite being practiced by the most intelligent animal on earth…

A Geological Mystery at Bears Den

Figure 1. View of the Shenandoah Valley, looking west from a crest of the Blue Ridge Mountains.

Beautiful landscapes and geological wonders are never far from your door here in Northern Virginia. Today, we took a hike to meet the Appalachian Trail near the border of West Virginia (Fig. 2).

Figure 2. Geologic maps of the study area, located NE of Washington DC (see small inset map), about 30 miles west of Sterling. We climbed about 1000 feet along Route 7 (purple line crossing E to W above the small inset map), crossing a Proterozoic metamorphic terrane represented in shades of purple and gray. The northern part of this zone is a light-colored (leucocratic) granite that has been metamorphosed whereas the southern part comprises a granitic gneiss. These batholithic intrusions are crosscut by a spectrum of metamorphosed granitic rocks known as Charnockite (orange lines). West of this area of Grenville intrusives, we crossed metabasalts of a somewhat younger age (green zone in map), the Catoctin Formation, which we saw in an earlier post. The leftmost inset map extends the large USGS map of Loudon county because the field area was just outside its domain. The oval indicates the area discussed in this report.

To reach the Bears Den rest area on the Appalachian Trail (see oval area in Fig. 2), we had to follow a winding path along one of the many irregular crests that define the Blue Ridge Mountains (Fig. 3).

Figure 3. Trail head for the short hike to Bears Den.

Along the way, we noted that the trail followed the top of a deeply eroded landscape, littered with boulders and weathered rocks (Figs. 4 and 5).

Figure 4. The path from the parking lot along Route 7 to meet the Appalachian Trail (see Fig. 2) was littered with boulders like these.
Figure 5. Large boulder dissected by a weathered joint. This was a local topographic high. This rock had faint lineations oriented nearly vertical that can be seen in the photo. However, as large as it is, this boulder may have rolled from an outcrop.

We finally reached the rocky point from which Fig. 1 was photographed. A large outcrop crosscut with veins greeted us (Figs. 6-8).

Figure 6. Top of the outcrop of (thus far) unidentified rock holding up the Blue Ridge Mountains. Note the white layer visible in the lower-left part of the photo.
Figure 7. Photo of the top of the outcrop in Fig. 6, showing joint sets filled with a light-colored mineral (either quartz or feldspar). The veins are standing out because the host rock has weathered, suggesting that they are quartz, which is very resistant to chemical weathering.
Figure 8. Close-up of thin, intersecting joints, but at a much lower angle than those seen in Fig. 7.

Intersecting joints like those seen in Figs. 7 and 8 occur when rocks that have been deeply buried are exhumed as overlying rocks erode. The upper mantle relaxes and lifts its overburden in a process called isostatic rebound. Rocks thus uplifted are no longer soft but respond like a solid in brittle deformation. What is intriguing about these rocks (whose age we haven’t yet determined) and the joints that permeate them, is the origin of the quartz and feldspar filling the joints.

It’s time to talk about the host rock seen in Figs. 4-8. The west side of the oval outlined in the inset map of Fig. 2 reveals that these are fluvial-to-shoreface sedimentary rocks deposited between 541 and 511 Ma (source: Rock D lists many sources for this interpretation).

Figure 9. Hand sample view of host rock, revealing a sandy texture that has been altered by thermal metamorphism, as indicated by the overall sheen (caused by quartz remineralization). The image is about 6 inches across.
Figure 10. The original sedimentary texture is evident in this photograph, although it has been modified after burial. There is no evidence of permeating ductile deformation (e.g. folding of sedimentary layers).

An intrusive magma filled joints (Figs. 7 and 8) and heated the country rock to the point of remineralization (Fig. 9), yet sedimentary textures are retained (Fig. 10). What’s going on?

Figure 11. Close-up (magnification is ~3x) of quartz vein in country rock, revealing a recrystallization aureole as seen in the gray area transitioning into mixed light and darker areas at the top of the image. This is a classic example of contact metamorphism. The host rock (as seen in Fig. 5 for example) wasn’t being deformed. It was actually fracturing (Figs. 7 and 8), when a new intrusion occurred, one not reflected in the Proterozoic granites found within the area (Fig. 2).
Figure 12. Photo (~4x magnification) taken from same exposure as Fig. 11, showing an intrusive texture, which is indicated by undeformed quartz (gray) and feldspar (white) crystals.


There are some general rules in determining relative geologic age. For example, layered rocks are younger as you ascend in a stack of them. This rule applies to both sedimentary and volcanic rocks, although the contacts aren’t as uniform in the latter. Another rule is that rocks that cut through other rocks (i.e. veins and dikes) are younger than the rocks they invade.

The sedimentary host rocks at Bears Den were deposited where rivers fed coastal deltas and a sandy beach (shore face) about 500 million years ago, long after the granites we see to the east (Fig. 2) were intruded and metamorphosed to become metagranites. In other words, the quartz veins and granitic dikes (Figs. 7-12) did not occur when the older (1600-1000 Ma) igneous rocks were emplaced; these veins and felsic intrusions must be associated with the Taconic orogeny, which started about 440 million years ago. The geologic map (Fig. 2) doesn’t show any evidence of granitic intrusions from this period, which could have filled joints created by isostatic rebound in these rocks.

These sedimentary layers were laid down during the collision that created Pangea, so they would have had to be buried deeply enough to become lithified, exhumed to a sufficiently shallow depth to form joints (a sure sign of brittle failure and thus uplift), then invaded by an undisclosed intrusive magma at a very shallow depth (probably less than 5 miles).

Alternatively, they could have remain buried until the breakup of Pangea, beginning in the Triassic period and progressing in stages. Magmatism has been associated with this event in Northern Virginia.

All of this is plausible, given the immense span of time involved, but…

Where are the rocks?