Archive | April 2022

Geological Bottleneck

Last week’s post showed some of the effects of erosion along the banks of the Potomac River, which flows lazily along a broad floodplain while not becoming so sluggish that it meanders. We know this condition doesn’t last, however, because the broad and anastomosed channels of the Potomac are forced into a narrow throat bordered by immovable Precambrian schist, as we discovered in a previous field trip. Today I am going to approach this series of cataracts from upstream and document the changing river morphology (Fig. 1).

Figure 1. Overview topographic map of the Upper Potomac study area.

Previous posts have described the floodplain morphology from Algonkian Regional Park (circled area to the upper left of Fig. 1) to the west (upstream), a terrain defined by relict riverbed topography incised by modern stream erosion from the surrounding terraces. At the other extreme, we visited Great Falls in a previous post, where we discovered Precambrian schists that resisted the river’s erosional power.

The field trip began at the Nature Center (black circle at top of Riverbend Park inset map). We followed a trail over poorly sorted gravelly sand (Fig. 2) cut by numerous channels, through what appeared to be a mature and healthy forest. I don’t know what kinds of trees they were but they were at least 80 feet in height.

Figure 2. Poorly sorted sediment including rounded boulders up to three inches in diameter.

The assortment of gravel and boulders seen in Fig. 2 is classified as a conglomerate. The rounded boulders and poor sorting suggests that these are fluvial. The boulders became round by rolling along the bottom of the river. This conglomerate (sediment is an unconsolidated rock to a geologist) is matrix supported, which suggests that high-flow events were common, but the majority of the sediment was the product of chemical weathering of rocks like the diabase we saw in a previous post. (Minerals with complex compositions react to water easily, compared to quartz.)

I will return to this later in the post.

However, there were angular pieces of a fissile, dark rock showing up as regolith along the trail (Fig. 3), suggesting that the conglomeratic sediments were a thin veneer over resistant bedrock.

Figure 3. Photo of a loose fragment of bedrock along the trail. Sometimes, smaller pieces of this rock (probably meta sediments but not quite schists) appeared to have been intentionally laid along the trail in places. (They weren’t.)

The trail dropped to the river where outcrops of resistant bedrock appeared within the shallow river channel (Fig. 4). This was a substantial change from a few miles upstream

Figure 4. The shallow Potomac channel seen from VA, showing outcrops and the bifurcation of the river into two channels, as suggested by numerous islands upstream.

Our path follows the red line along the river bank in the Riverbend inset map of Fig. 2. We are approaching Great Falls. The trail is no longer constructed on conglomerate, but now is traversing sandy silt sediments deposited during the Holocene epoch (Fig. 5).

Figure 5. The trail is following deeply eroded fine sediments. Note the young trees leaning precariously towards the river, a sure sign of rapid erosion.

Schistose rocks with nearly vertical layering appear along the riverbank, and begin to obstruct the trail (Figs. 6 and 7).

Figure 6. Fissile rocks (probably Precambrian schists deformed during the closing of the Iapetus Ocean) are now nearly vertically oriented and, despite significant mechanical weathering, have resisted the Potomac’s onslaught.
Figure 7. Massive exposure of Precambrian schist along the trail, less than 100 yards from the river.
Figure 8. Closeup of an exposure of schist. Note the rounded block of rock bounded by flow lines, at the top of the image. More malleable rock flowed around this large clast during medium grade metamorphism. Although not seen in the photo, the rock had a distinctive sheen caused by the alignment of platy muscovite crystals. The image is about 12 inches across.

Our short hike led to the Aqueduct damn, which supplies water to Washington DC (Fig. 9), where the river transforms into a raging torrent that is challenged only by experienced kayakers.

Figure 9. The Aqueduct dam maintains a minimum water depth to supply sufficient flow for the DC area’s municipal water supply.
Figure 10. View looking upstream from the Washington Aqueduct dam. This image shows a placid river flowing around islands, eroding its banks (note the young tree falling into the channel), and collecting behind the dam (Fig. 9). It doesn’t get any better than this.
Figure 11. Downstream view from the Aqueduct dam. Outcrops of Precambrian schist, metamorphosed sediments from a previous ocean deformed by the collision of continents to form Pangea, are more numerous. The channel is now defined by the rocks and not the erosion of the river. The rocks are in charge.

The transformation of the Potomac, from the placid stream in Fig. 4, to the convoluted and dangerous channel that follows no commonsense rules of river flow seen in Fig. 11, took place in less than two miles (see Fig. 1). I know because I walked the river bank, passing from one era to another before being confronted by a past that will not die…

There is one last point I’d like to make in this post. The imposition of Holocene erosion–streams fed by the Pleistocene highlands bounding the Potomac floodplain–applies here as well as in the gentler topography we saw upstream. The transition from the conglomerate we saw at a major stream draining into the Potomac (Fig. 2) to the more typical fluvial sediment (sand/silt/mud) we found further downstream (Fig. 5) reflects the input of erosion of bedrock only a few miles from the Potomac. This was documented in a previous post, which showed the breakdown of regolith into cobbles, which were transported inexorably to the Potomac.

It is my opinion that this is what has been recorded in the rocks along the banks of the Potomac River, creating the juxtaposition of sediment types along the path of a river that is draining the roots of an ancient mountain range.

The Dynamic Potomac

Figure 1. View of the south channel of the Potomac River looking toward Tenfoot Island (see Fig. 2).

This post returns to the Potomac River. We have previously discussed several features along this stretch of the famous waterway: Precambrian metamorphic rocks at Great Falls; sediment contributed by tributaries as well as erosion; and emplacement of intrusive rocks during rifting of Pangea to form the Atlantic Ocean. This time we’ll see evidence of recent erosion, as evidenced in Fig. 1, which shows a large block of stone that has collapsed along the steeply eroded south bank. The bank consists of silt and clay just like further upstream.

Figure 2. Study area in Northern Virginia (see inset), showing locations for other figures in this report.

Site A (see Fig. 2) is where Fig. 1 was taken. The meandering stream to the east in Fig. 2 is Sugarland Run, which we examined further south, near its headwaters, in a previous post. The Rock-D geologic map suggests that the river is underlain by a Triassic (237-203 Ma) fining-upward sedimentary sequence consisting of sands to shales. A close-up image of the boulder at Site A (Fig. 3), despite a covering of mud and some biological material, reveals no apparent bedding. This is contrary to the description of the Balls Bluff (sedimentary) Member of the Bull Run Formation, contemporaneous with the Newark Supergroup although no longer considered stratigraphically equivalent.

Figure 3. Close up of boulder shown in Fig. 1, revealing a reddish, fine-grained texture. No clean surfaces were available, but neither individual crystals nor bedding are visible . Image is about 2 inches across.

Figure 3 doesn’t look sedimentary to me, but more like the diabase we saw further upstream on Sugarland Run. The streak of white material in the upper-left corner looks like quartz. If these are Triassic sedimentary rocks, they wouldn’t have been metamorphosed, so this exposed block is anomalous. It is possible that this is an outlier of sedimentary rocks that were thermally altered when intrusive rocks were emplaced during Triassic. The area consists of faulted and folded diabase, sedimentary rocks, and metasediments–intruded, deposited or altered, respectively, during the breakup of Pangea during the Triassic period.

Figure 4. Looking upstream with Tenfoot Island to the right. Note the trees falling into the river along the recently eroded shoreline. The concrete and steel is the remains of an electrical power plant that was removed.

The bank is steeply eroded with trees collapsing into the river, indicating rapid lateral erosion during the lifetime of a typical tree (less than a century).

Figure 5. Image along the south bank (Site B in Fig. 2), showing logs deposited during a previous flood. The bank is low at this location, the swale winding landward around a relict river bar, deposited before the Potomac cut downward to its current elevation.
Figure 6. Downstream end of Tenfoot Island with Maryland on the other bank (see Fig. 2 for map view).
Figure 7. Natural levee along the VA side of the Potomac River at Site C (see Fig. 2 for location).

This section of the southern flank of the Potomac River is characterized by a wide floodplain covered with hummocks that represent bars on the original river bed. Superimposed on this older morphology is a natural levee (Fig. 7) that varies in height, steepness, and distance from the modern channel along the river. This landscape has been cut by numerous streams that drain the Pleistocene highlands overlooking the incised river.

Substantial islands (Figs. 5 and 6)divide the Potomac river into two anastomosed channels (see Fig. 2) that have become unstable during the last ten thousand years, as demonstrated by rapid downcutting and lateral instability (Figs. 1 and 4).

Review of “The Garlic Ballads” by Mo Yan

My international literary adventure has moved to China. This dark novel about life in a rural town after the rise of Deng Xiaoping was published in Chinese in 1988 and translated into English in 1995. The translator deserves credit of course, so I would like to congratulate Howard Goldblatt for doing an excellent job. I know nothing about Chinese, but the translator is well known and worked with the author, so it’s probably a pretty good English translation.

I’m not sure where to start.


Hmmm…I’m going to just jump right into the deep end of the pool.

At first, I thought this was a slapstick, black comedy because there was so much head-banging, kicking, and urine drinking. (No that wasn’t a typo.) The author was hung up on physical abuse (short of death) and the central character’s multiple experiences with drinking piss. It added nothing to the story, so it must have been fun to write about. I think (as if I would have any insight into the characters’ lives) it was to demonstrate the miserable state of the peasants who are central to this novel. But it came across as a Marx Brothers’ story, with their abusive behavior towards each other pushed to the point of nausea…

However, there was no laughter in this book, which was the problem because the entire story could have been told in a fraction of the words used. The repetitive beatings, abusive treatment, wailing and moaning about the plight of the peasants, and general patriarchal corruption and familial misbehavior lost its shock factor after twenty or so pages. The ending was as predictable as I imagine a Russian tragedy (I’ll be reading Dostoevsky and Chekov next), with practically mass suicide by the characters, described in hopeless detail. The survivors were either in prison or left to starve…

I lost track of the dialogue because of a complete lack of dialogue tags. He and she was used so much, in conversations with several characters, that I had to reread way too many paragraphs to sort it out, especially since everyone had the same grim, hopeless view of life. This is a dystopian novel of the first degree, right up there with 1984 and A Brave New World, but it didn’t have a message.

The story was broken up, with very few plot markers because all of the characters were the same. There were different threads centered on several characters, but there was no consistent timeline. The scenes jumped around, covering a year or so in current action, with many sudden flashbacks. The disarrayed storyline reminded me of “Catch-Twenty Two.” The disconnected development was exacerbated by the aforementioned abuse of pronouns, and sudden leaps from a third-person, past-tense narrator to a first-person, present tense narrative. What was that about?

Chapters started repeatedly with He, and matters were made more confusing by the characters having only a few surnames because it was a rural village with only a couple of family lineages present. (The translation follows the Chinese convention of surname first.)

And then there was the garlic.

This is a literary drama, which means it uses metaphors and colorful descriptions until the reader is fed up. If I never hear another reference to the smell of garlic or jute, it will be too soon. And what was with everybody having green sparkles associated with their eyes? Even the birds and lizards. And not by one character, but several. Were they all schizophrenic? There were some great metaphors and the hint of good literary fiction, but the author failed to broaden their horizons, metaphorically speaking. Rather than revealing the breadth of the rural experience, they kept jamming the same images down my throat…again and again, until I was fed up.

I can’t recommend this book. I can’t even think of a reason someone would want to read it. The blend of juvenile behavior (e.g., bodily functions), slapstick physical humor that wasn’t funny, and complete hopelessness doesn’t work for me.