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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.

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.

Summary

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?

The Race to Ohio

Several of my posts in Northern Virginia have noted the presence of old canals which were part of the earliest transportation system to reach beyond the Atlantic Seaboard (e.g. The Potowmak Canal and Goose Creek canal). Today we crossed the Potomac River and followed the Chesapeake & Ohio Canal for several miles (between Locks 28 and 29), which took us along a scenic path through a metamorphic terrain in Maryland.

Figure 1. “Point of Rocks” tunnel at US-15 on the Maryland side of the Potomac River. This railroad was built at about the same time as the C&O canal (early 19th century) as they both negotiated this narrow right-of-way between Catoctin Mountain on the right and the Potomac. Note the cut wall of dark rock on the right. See Fig. 2 for location.
Figure 2. Map of “Point of Rocks” location on the Potomac River. The inset map shows the geology of the area indicated by the rectangle. Photos were taken at Sites A through C. Key to lithologies: L1 — Catoctin Formation (1000-541 Ma) comprises basalt metamorphosed to green-schist facies, with sedimentary rocks found as lenses, as well as flow bedding, formed from a rifting continent; L2 — Biotite granite gneiss (1600 – 1000 Ma), gray with black streaks; L3 — Pink leuocratic metagranite (1600 – 1000 Ma), gneissic foliation.

The cut for the tunnel (Fig. 1) reveals a dark rock with foliation oriented nearly vertical in the plane of the cut (Fig. 3) with blebs of pink and white material (Fig. 4).

Figure 3. Outcrop (photo is approximately 12 feet in height) of Catoctin Fm., showing irregular foliation based on original basalt bedding. Note the lighter colored area running diagonally from UL to LR, bounded on the LL by a very dark area. This is very likely a remnant of the original volcanic layering (basalt beds are often irregular).
Figure 4. Detail from outcrop in Fig. 3, showing a pinkish bleb and lens in the UR quadrant. Note how foliation flows around these zones, which are probably epidosite, a mineral produced by leaching of metals during thermal metamorphism of basalt. This image is about 2 feet across.

These basalts originally flowed out of fissures and volcanoes, probably at the seafloor, during rifting of a continent (according to Rock-D‘s summary). The origin of magma can be correlated to tectonic regime (e.g., mid-ocean ridge, rifting continent, island arc) based on the chemical signature of the whole rock (not minerals, which are altered during metamorphosis). At any rate, the age range of the Catoctin Formation (1000-541 Ma) spans the final closing of an unnamed ocean to form a hypothesized supercontinent called Rodinia during the Grenville Orogeny between 1100 and 900 Ma — and the subsequent rifting of Rodinia, which occurred between 750 and 633 Ma. Unraveling that time discrepancy is beyond the scope of this blog.

Let’s just say that these basalts (and minor sedimentary rocks) were created during the tumultuous assembly, pleasant life, and violent breakup of a hypothesized supercontinent.

Figure 5. Zoomed-in photo of irregular leucrocratic structure (estimated to be ~50 feet long) on a cliff that was more than 100 feet in height. This unusual feature may be a deformed sedimentary lens as mentioned in the Rock D description. It is too large to be analogous to the epidosite inclusion seen in Fig. 4.
Figure 6. Image of bedding in Catoctin Fm., seen from several hundred feet through thick foliage. Note the contrast between the dark and light beds. The difference is probably caused by subtle changes in the minerals that were produced during metamorphism, but they reflect original volcanic layering, squeezed and deformed during burial (maybe as deep as 15 miles). It is also noteworthy that large-scale folding is absent.

The Catoctin Formation (basalts with some sediments) were buried deeply and metamorphosed by heat and burial pressure but not compressed, at least not enough to produce folds visible at the scales of Figs. 3 through 6. This suggests that Rodinia wasn’t created by huge horizontal forces, crashing tectonic plates together like putty.

Jumping ahead in time to the present, we see these rocks still controlling the Potomac and the development of transportation during the early days of the United States.

Figure 7. View of the Potomac at Site B (see Fig. 2 for location), looking west towards Virginia. The rocky islands are the emergent parts of the basement rocks.

The outcrops seen in Fig. 7 occur very near the contact between the Catoctin Formation and a Pink leuocratic metagranite (1600 – 1000 Ma) with gneissic foliation (L3 in Fig. 2).

Figure 8. Outcrop of unidentified rock from Fig. 7, showing some sign of weak foliation but no evidence of an other texture or mineralogy, without breaking a piece off (which we don’t do in this blog). The irregular foliation was estimated to have a strike of 31 degrees (NNE) and a dip of 20 degrees ESE.

It is important to note that the outcrop in Fig. 8 has been exposed to the vagaries of the Potomac since the Pleistocene, so its irregular surface is not unexpected. Nevertheless, the estimated orientation is radically different than that of the Catoctin Fm, which is nearly vertical (see Fig. 6). One possibility is the presence of a fault between the two rock formations, as suggested by the geologic map (Fig. 2). The fault (indicated by the nearly vertical, black line south of Site B) may extend under the alluvial sediments around the Potomac and not be visible in Maryland.

Figure 9. Lock 28 of the C&O Canal operated from about 1834 to 1923. It was operated by a lock keeper, who lived with his family in the cottage to the left of the photo. The C&O canal is to the right (filled with debris and old trees) of the now-asphalt tow path, and the railroad now has two tracks to the right of the canal, out of the image.

We have one last stop today, Site C (see Fig. 2 for location), where we will try and get some closure on the fate of Rodinia (pun intended).

Figure 10. View looking SE along Catoctin Creek towards its confluence with the Potomac River.
Figure 11. The C&O Canal aqueduct at Catoctin Creek was one of many such structures, constructed to carry barges over the Potomac’s tributaries. Note the different hue of the blocks comprising this 188-year-old structure; the left abutment is original, constructed of pink granite (lithology L3 in Fig. 2), whereas the central span was rebuilt in the 1970s of a granite gneiss (lithology L2), after it collapsed.
Figure 12. View of outcrop along the B&O RR line that runs parallel to the C&O canal. The poorly exposed, and partly hidden by foliage, rocks we see are, according to the geologic map (Fig. 2), a biotite granite gneiss. About the only thing I can say about them from this distance is that they appear to have a foliation that is dipping steeply to the east (right in the photo).

Let’s take a look at some samples that were conveniently made available by the Maryland DOT, probably during construction of the park.

Figure 13. Large block (about 3 feet across) of biotite granite gneiss (lithology L2 from Fig. 2). Note that the large phenocryst in the inset (~2 inches across) has not rotated during deformation but, instead the foliation has deformed around it.
Figure14. Block of gneiss showing intrusive contacts. The red lines on the central photo indicate some kind of layering, which could be from burial of sediments or extreme burial stress on an igneous rock. There is no evidence of folding, not even at smaller scales. The right close-up shows a dark layer interacting with a pinkish intrusion. It is being drawn into the intrusive material and melting, losing its identity so to speak. Note how the thin black layer (top of right photo) has stretched right-to-left in the circled area. The left close-up shows minerals with no foliation in the intrusive (younger) rock, which was certainly in a molten form at the time. Note the presence of Cu-bearing minerals (shiny and metallic), which grew from metals drawn out of the host rock.

The rocks we saw today were of similar ages (determined by radiometric dating, not guesswork), and were deposited, deformed, and intruded during the Grenville Orogeny. In other words, based on my brief examination of the literature, these rocks represent the collision of continents to form Rodinia (1100-900 Ma), its long lifespan (900-750 Ma), and its breakup to form the precursor of the Modern Atlantic Ocean (Iapetus) between 750 and 633 Ma.

I’m going out on a limb here (again). It is significant that there aren’t a lot of sedimentary rocks (even metasediments) here, which means they were removed by erosion. Keep that in mind, when I add that even the oldest metamorphic rocks (the granite gneiss, L2) aren’t folded like the schists at Great Falls. These rocks were buried deeply, relative to the impact they survived, which apparently didn’t affect the deeper parts of the crust (at least not along the modern Potomac River). There probably was a lot of folding of the sedimentary rocks that covered these igneous and metamorphic rocks, but they were eroded away during the 150 million years that Rodinia was a (quasi) stable supercontinent. That’s a long time. The youngest of the Catoctin rocks (~540 Ma) were deposited just as the so-called Taconic Orogeny was starting up in modern-day New England…

It is extremely difficult to wrap our minds around such four-dimensional events playing out on such long time scales but, to make it even more fun, the mantle and core are dancing to their own tune and directly influencing everything we experience and can observe from our perilous seat atop the earth’s crust.

Just look at all those canals we dug to avoid the rocks…

Goose Creek

This week we didn’t have to go far to find a quiet place on the Potomac River, its confluence with Goose Creek, where low bluffs of reddish sedimentary rocks watched over the calm water of this sometimes violent tributary (Fig. 1).

Figure 1. Exposure of Balls Bluff Siltstone (237-201 Ma) on Goose Creek (photo looking north). These mixed sedimentary rocks have a reddish color (indicating deposition in rivers and lakes). They form a bluff ~20 feet high here. See Fig. 2 for location.

However, things aren’t as geologically simple as the sedate image in Fig. 1 would suggest. This photo was taken within a few hundred feet of the contact with thermally metamorphosed sedimentary rocks that were probably part of the original Balls Bluff Siltstone (Site C in Fig. 2).

Figure 2. Geologic map of the confluence of the Potomac River and Goose Creek. Figure 1 was taken at Site C, at the contact between the reddish and the purple areas. The metasedimentary rocks have been dated between 237 and 174 Ma, suggesting that they are part of the Balls Bluff Siltstone. They were altered by intense heating when several diabase intrusions (201-145 Ma) forced their way into the sedimentary rocks.

We saw diabase in an earlier field trip. Those dikes and sills were part of the same intrusive episode, when North America split away from Europe. Diabase is a fine-grained, intrusive rock with a chemical composition like basalt.

I was unable to measure the orientation of the sedimentary rocks seen in Fig. 1, but my “field estimate” is that they were dipping ~30 degrees away from the camera, with a strike of about (you guessed it) 30 degrees east of north. Such an orientation is consistent with what we saw in a previous post.

The presence of diabase crossing Goose Creek (see Site C in Fig. 2) is indicated by shallow water and a rocky bottom just west of Site C (Fig. 3).

Figure 3. Photo looking upstream along Goose Creek (west of Site C in Fig. 2), showing riffles over a rocky bottom.

Because of the shallow water over the diabase intrusion in an otherwise navigable stream, a canal was constructed to run from this point downstream to deeper water, parallel to the stream seen in Fig. 2. This was the Goose Creek Canal, finished in 1859 to reach grain mills as far as twelve miles upstream.

Figure 4. The stone lock constructed at Elizabeth Mills to allow boats to circumvent the shallow water seen in Fig. 3.
Figure 5. Gravel bed of the Goose Creek canal, west of the lock seen in Fig. 4.
Figure 6. Abutments to a bridge that spanned Goose Creek when there was an active mill at the site. The bridge was burned by the Confederate army in 1862.
Figure 7. Regolith soil profile near Site C, showing fine-grained sediment deposited by modern Potomac during flood, over boulders deposited by Goose Creek. Of course, there is also a contribution of material used to prevent erosion, including a few blocks of finished stone eroded from the Goose Creek canal’s sidewalls.

Moving back to the Potomac River, there are a few more surprises for us on this field trip (Fig. 8).

Figure 8. Summary of geology along the Potomac River downstream of its confluence with Goose Creek. The river’s flow along this stretch is determined by an outcrop of basement rock (probably Balls Bluff) at a narrow point (Plate 4). This resistant constriction (yellow area in the map) created a pattern of oscillations along the western shore for thousands of years(squiggly red lines in the map) that constructed minor headlands that collect debris during high river flow; Plate 1 is looking downstream whereas Plate 2 looks upstream. These two snags are separated by less than 300 feet. Plate 3 shows raindrop imprints in mud which will, if covered over before being disturbed (like in a sudden flood) become a feature recorded in sedimentary rocks.

This post worked forward in geologic time, in reverse to the counterclockwise path we followed (thin black line seen in Fig. 8). I was surprised to find so much geologic history on a short walk along the shore, surrounded by golf courses and million-dollar houses.

To summarize, as Pangaea began to be torn apart, the mixed sediments of the Balls Bluff Siltstone were deposited in grabens for millions of years, until oceanic crust intruded as diabase dikes and sills, heating these now-deeply buried sediments and altering them into what is called a hornfels metamorphic rock. The rocks we saw today were deposited and thermally metamorphosed between about 237 and 145 Ma (equivalent to millions of years), which is a very long time. But that 88 million-year span was brief compared to the 145 million years that have elapsed since these diabase intrusions forced their way into the picture.

As we’ve seen everywhere along the Potomac, modern fluvial processes are struggling to overcome tectonic events from a bygone era…

Exploring the Potomac: Red Rock and Balls Bluff

Observations at Red Rock Park

I have been exploring the Potomac River from Washington to Harpers Ferry in stages in recent posts. Most of the rocks we’ve seen were Precambrian schists, sedimentary rocks deformed during the closing of the precursor of the modern Atlantic Ocean (The Iapetus). The Potomac River has eroded into the roots of the ancient mountain range that was created by this event, superimposing floodplain processes on these metamorphic rocks. Today I travelled a little further upriver and found different basement rocks, which are the reason I’m excited about today’s post.

Figure 1. Photo of outcrop of layered rocks along the VA bank of the Potomac River at Red Rock regional park (see Fig. 2 for location). This photo shows debris piled against this obstacle during recent high water.

Accessing the river at Red Rock park required a short walk along a narrow ridge, left by erosion of gullies into the rocks.

Figure 2. Google map of the study area. Red Rock park is located at the bottom center of the image (indicated by green color) and Balls Bluff Battlefield park is located at the upper left (NW corner) of image. Harrison Island formed in this large bend of the river, probably caused by changes in underlying basement rocks (e.g., faults and lithology).

Figure 1 was taken at Red Rock park, after following a steep trail about 100 feet to the river bed (Fig. 3).

Figure 3. Looking upstream reveals ledges (gray horizontal objects) that create a series of steps as the water flows towards the Potomac.

A close-up of the outcrop seen in Fig. 1 reveals medium-to-thin bedded, fine-grained sedimentary rock with a reddish hue (thus the name of the park), as seen in Fig. 4.

Figure 4. Photo of intercalated blocky and fissile siltstones of the Balls Bluff member of the Bull Run Formation. These rocks were deposited in streams about 200 million years ago (Ma), and have not been metamorphosed. They are tilted, however, at an angle of about 20 degrees to the ESE (about 30 degrees south of east).
Figure 5. Photo showing irregular bedding of the fissile (contains more mud than the blocky layers above and below), typical of mud drape over silty sediments on a river floodplain. We saw modern examples of this in recent posts. We’ll see some examples in this post. This was an environment not that different than what exists today in this area.
Figure 6. Hand sample of siltstone from Red Rock park, showing the characteristic red hue of terrestrial sediments. Siltstone is formed from the tiny slivers of quartz broken off large boulders as they roll along river beds. Thus, the grains are not round and there is often a lot of mud available, from the weathering of mica, feldspars, and other minerals contained in their parent rocks. In this case, the parent rock is unknown, having weathered 200 Ma to form these rocks. It was probably granitoid rocks formed at shallow depths during the Grenville orogeny (when the schists we’ve been seeing were deformed at greater depths).

The sedimentary environment implied by the siltstones and mudstone we see in Figs. 4-6 has (coincidentally) been reproduced today, with weathering of these same rocks and others found further west.

Figure 7. Photo of narrow floodplain at Red Rock park, showing a Pleistocene river bar on the left and a swale to the right. These sediments overlie the Triassic Balls Bluff rocks and they are finer, being recycled sediments from a previous epoch.
Figure 8. Photo of stream eroding a channel around the relict bar seen in Fig. 7, following more easily eroded sediments before entering the Potomac on its muddy shore (Fig. 9).
Figure 9. Terminus of stream seen in Figs. 3 and 8, revealing a muddy bank (seen along stream crossing image) cut into sediments deposited by the Potomac on this narrow flood plain, restricted by bell-shaped bluffs less than 100 feet in height.

Summary of Red Rock Park

Fine-grained sediments were transported along rivers similar to the modern Potomac about 200 Ma and deposited on a flood plain like we see today. This was when the modern Atlantic Ocean was just beginning to form as the supercontinent Pangea was being torn apart. There would have been mountains much higher than the modern Blue Ridge to the west, and a narrow but widening (~1 inch/year) ocean to the east. Erosion of the ancestral Appalachian Mountains continued for the next 200 Ma, creating thick piles of sediment on the continental shelf of North America, depressing the earth’s crust and burying even terrestrial sediments deep enough to create the Balls Bluff siltstone from mud and silt. As erosion wore these mountains down, the crust rebounded to expose these ancient rocks to the ravages of water and ice. Now, these sedimentary rocks are being eroded as the process continues.

Requiem: Balls Bluff National Battlefield

A little further upstream (see Fig. 2 for location) is the probable type-locale of the Balls Bluff siltstone, but good exposures weren’t accessible because the bluff is higher and the bank narrower, there being no floodplain as we saw elsewhere (Fig. 7). In fact, the only rocks we saw were at the top of Balls Bluff (Fig. 10) and along a trail that led to the river, where I was able to estimate strike and dip.

Figure 10. Photo of outcrop at the top of Balls Bluff. I was prevented from getting closer by a fence, a safety feature because the bluff is more than 100 feet high and quite steep. The reddish color is due to the presence of oxygen in the pore waters when the original sediment was buried (it had no where to go), which led to oxidation (rusting) of Fe-bearing minerals (e.g. the ubiquitous clays that would have replaced original, plutonic and metamorphic minerals) in the Triassic river sediments.

These rocks contained original sedimentary layering and I was able to estimate that they were dipping to the WNW at about 20 degrees, with a strike similar to the rocks at Red Rock park (i.e. about 30 degrees east of north). This is an important finding, because this is the opposite to what we saw only a few miles downriver. I’ll try and summarize this interesting observation briefly.

Tilting of layered rocks like these siltstones can occur by either folding or faulting. Check out the links to understand these processes. Folding creates great arcs of rock, like sine waves, or ocean waves, as lithified sediments (hard rock) are compressed from both ends. This is what led to the steep folds we saw in the Precambrian rocks at Great Falls and in Lynchburg; our limited observations can’t allow us to decide what happened to these rocks on our own, but we can turn to reliable resources. The sediments that formed the Balls Bluff siltstone have never been compressed; we know this because the Atlantic Ocean is still spreading at a slow and steady rate of about 1 inch/year.

Rocks also become tilted when the earth’s crust is stretched. Even though buried deeply, if they are not in a metamorphic pressure-temperature regime, rocks break and slide around to form faults. This is a well-understood phenomenon that is occurring today in the Basin and Range province of western North America. It doesn’t take much imagination to picture new ocean crust appearing while a continent (Pangea) is being torn apart, snapping rocks buried several miles beneath the surface, like a slab of concrete being removed by a bulldozer.

Crustal stretching produces a series of opposing normal faults that create grabens. These collapsing structures occur at every scale, from outcrops to the birthplace of Humans.

Closing Thoughts

It was good to see some younger rocks, especially Triassic river sediments that are direct evidence for the splitting apart of Pangea. It was a bonus to discover evidence of block-faulting of these same sedimentary rocks after they had been buried several miles beneath the surface. Geological processes occur on time scales of millions of years, with annual displacements of inches or less. Because of the juxtaposition of fast, river-based erosion and deposition and the slow pace of plate-tectonic movements, these rocks record their entire life cycle.

And it continues to this day, as slivers of quartz and oxidized clay minerals are transported yet again towards the same ocean into which they were originally flowing, before being trapped on a primordial floodplain.

Maybe they’ll make it this time…

Figure 11. Photo of stream cut ~15 feet into fine sediments at Balls Bluff. The bank revealed overall fine sediment with occasional boulders (~2 inches in diameter). This is a very dynamic and restricted environment, where rapid downcutting is excising young sediments and reintroducing them to the Potomac River…

Update on the Potomac

We returned to Algonkian Park when the Potomac River was running bank-to-bank and about to spill onto its flood plain (Fig. 1).

Figure 1. Looking upstream at an area where there was a cut bank on our previous visit.

The water level is several feet higher than on our last visit, and is expected to crest in two days. Unfortunately, I won’t be here to document that event, but I expect the water to be covering the picnic table placed on the active floodplain in Fig. 1. Low areas were inundated (Fig. 2).

Figure 2. Water has encroached along a low area in the floodplain, possibly a relict stream bed or swale behind a sand bar.

Unfortunately, I didn’t take a photo of several large logs resting on the grassy floodplain, deposited by a previous high-water event. Nevertheless, a log can be seen lodged against the bank in Fig. 1, and many more were racing by at 3 feet/second, some more than twenty feet long.

Because of its dynamic height and flow strength, the Potomac is constantly switching from eroding its banks to depositing fine sediment and organic matter on its floodplain. Still, it is downcutting into previous river deposits and spring floods are nothing more than a temporary anomaly in the inexorable transport of sediment from the Blue Ridge Mountains to Chesapeake Bay.

What’s the Difference?

Recent posts have discussed metamorphic rocks found along the banks of the Potomac River in Northern Virginia, buried and deformed during closing of the Iapetus ocean, between 1000 and 500 Ma (million years ago). Those rocks are schist, which formed at depths of about 15 km (10 miles), and temperatures of approximately 500 C (about 1000 F). The original mudstone was ductile under these conditions and the sedimentary layers (bedding) were folded like taffy, while low-melting minerals like quartz were squeezed out and filled cracks and voids, to form veins and irregular, rounded bodies. As extreme as these conditions sound, these are intermediate-grade metamorphic rocks.

I took a trip to Lynchburg, Virginia (Fig. 1), to see some higher grade metamorphic rocks that were formed at about the same time.

Figure 1. Geologic map of central Virginia, showing the location of previous posts (square at top of image) and the area discussed in this post (circle at bottom). Note that the schists observed along the Potomac River are shown in a darker shade than the rocks we will see near Lynchburg. Note also that the ribbon of rocks discussed in this post are not evident in the northern study area. The distance (as the crow flies) between the two study areas is about 140 miles.

I looked at three exposures of Precambrian rocks on this trip. I would love to show all of the photos I took of the metamorphic and igneous structures I saw, but I will have to restrain myself. (I’ll try anyway…)

The first locale we visited was Ivy Creek Park (Fig. 2), where there was an exposure of the Lynchburg Group that revealed one of its many facies.

Figure 2. Plate G shows the location of a manmade pond on Ivy Creek, blocked by an earthen dam (Plate B). It is a fairly steep incline and there is no evidence of erosion along the silty shore (Plate A). About 100 feet above the lake level, we found an exposure of dark rock that weathered to red (indicating a high iron content), as seen in Plate C. This rock was foliated and weathered to reveal a fissile texture reminiscent of schist (Plate D). I estimate the orientation of the foliation to have a strike of about 40 degrees east of north (Plate E) with a dip of approximately 70 degrees. A close-up of this exposure (Plate F) reveals a fine-grained texture.

The rocks seen in Fig. 2 (especially Plate F) are probably either biotite schist or graphite schist. Without petrological analyses that are unavailable at this time, there is no way to tell–not that it would matter. The protoliths of these metamorphic rocks were fine-grained sediments, probably deposited in either a back-arc basin or continental subduction zone. They were buried to at least 10 miles and compressed by colliding tectonic plates; however, this small outcrop showed no evidence of ductile deformation (e.g., folds or quartz inclusions). Note that the strike of the foliation in these rocks (Fig. 2E) is consistent with the orientation of geologic trends seen in Fig. 1. So far, so good…

Our next stop took us to Candler Mountain, where a road cut exposed metamorphic rocks for more than 100 m along a narrow road, still part of the Lynchburg Group (Fig. 3).

Figure 3. Typical exposure of Lynchburg Group metamorphic rocks on Candler Mountain. Note the irregular “bedding” and delineation of grey and tan rocks due to weathering.

This exposure revealed many fascinating metamorphic textures–too many to share here. I’m going to focus on extensional features, which might not be expected since I just said that these rocks were deformed during collision of solid land masses, at least island chains like Japan of the Philippines, if not continents.

The context for this situation–stretched crust in a tectonic plate collision–is best illustrated in a cartoon (Fig. 4), which shows how the crust can actually stretch to release the stress caused by volcanism, as subducting crust melts under increasing pressure and temperature.

Figure 4. Schematic view of subduction with an island (instead of a continent). Note location (1), where the crust is being torn apart because of the immense compressional stress at location (2), which is released episodically as the subducting oceanic crust melts and creates vast quantities of magma which rise to form the extensional “back-arc basin.”

Keeping Fig. 4 in mind, let’s look at the rocks we found on Candler Mountain.

Figure 5. Photo of a loose sample from the road cut in Fig. 3. The classic “phyllite” sheen couldn’t be observed on the rocks that were in place because of their orientation. Phyllite often consists of muscovite, quartz, and chlorite, although biotite and plagioclase can also occur. All mineral grains are microcrystalline, which explains the sheen. No minerals can be seen with the naked eye.

Phyllite is a low-grade metamorphic rock that forms from mud stone at fairly low temperature and pressure (Fig. 6). It is associated with convergent tectonic plate boundaries, and is found in both subduction zones (see Fig. 4) and continental collisions.

Figure 6. Schematic of metamorphic facies in a subduction zone. Note that phyllite is formed at lower temperature and pressure than schist (~300 C or ~600 F). This figure would be representative of Location 1 in Fig. 4, although it could also be found more seaward (to the right in Fig. 4).

Focusing on extensional tectonics for this post, we can see some of the textures we’ve seen elsewhere on Candler Mountain.

Figure 7. Extensional forms like the boudins seen in the center of the photo were dominant in this road cut.

Figure 8. Large quartz/feldspar boudin.

The extensional features seen in Figs. 7 and 8 are consistent with these rocks being deformed as suggested in a back-arc basin (Site 1 in Fig. 4) but not buried too deeply (Site e in Fig. 6). However, they are not horizontal, as indicated by the steep dip seen in Fig. 8 (actually they are dipping at more than 60 degrees).

After being heated in the back-arc basin environment, they would have been crushed by the imminent collision of continental land masses, which cannot be subducted because of the low density of the granitic rocks that comprise them. During this compressional stress, they were folded and probably transported many miles along thrust faults, without being buried deeply enough to transform them into schist.

Figure 9. This image shows a quartz/feldspar boudin that has been deformed during compression, leading to partial melting of the low-temperature minerals (light colored), filling gaps in the surrounding phyllite rock. Think of this rock as being mangled between an irresistible object (N. America) and an irresistible force (Europe). They escaped deep burial to become schist and gneiss because of the presence of horizontal weak zones (photo-thrust faults), as they slid over the rocks that made up proto-North America.

The tilting of these rocks is due to large-scale folding during this later compressional period. The rocks of this area are actually part of an anticlinorium–a region filled with anticlines of every scale, from inches to miles in width. There are a couple of additional superimposed structures I would like to mention before we move on.

Figure 10. This photo shows almost vertical striations in the shiny rock, superimposed on the folded structure created during compression (fine-scale, gray rock at top of photo). Note that this structure is orthogonal to the layering seen in the tan rock to the upper-right. I don’t know what this represents, but it came later than extension or compression and was expressed only in small areas. The photo is a couple of feet across.
Figure 11. The yellow lines indicate a set of joints that resulted from decompression of these rocks during exhumation. This is the last phase of deformation. Recall that the beds are steeply dipping to the ESE (~40 degrees south of east).

There is one more piece of the puzzle that we observed on our field trip. Before we finish with a wave of our hands (and a good bit of conjecture), let’s review.

Figure 12. Summary of rocks observed in the Lynchburg area. Site A is where we saw an example of the Lynchburg Group (Fig. 2), including gneiss and schist (both high-grade metamorphic rocks). Site B is where we observed phyllite with superimposed metamorphic textures (Figs. 3-11). Site C is discussed below. The dashed line represents the approximate division between rocks of the Blue Ridge (high grade) and low-grade rocks seen in Fig. 1. Location is approximate and this is a wide transition zone.

The dashed line in Fig. 12 is oriented approximately in line with the general lithological trends in Fig. 1, as well as several faults (solid lines in Fig. 1) that have been identified. This line is meant to delineate the schist and phyllite zones (~300 C isotherm in Fig. 6) within a back-arc basin. Schist is high grade. But what do we expect to find at Site C?

The last location we visited was a slope, the edge of a bulldozed lot covered by an apartment complex. None of the rocks we saw were in place, but they hadn’t been moved far, maybe a few yards. We weren’t looking for orientation data, so that wasn’t a problem. What we found was very interesting.

Figure 13. This was a loose boulder pushed over the edge of the development. All of the other rocks were the same, including some that were in place (they were covered with grass and biological material). Individual mineral crystals can be seen as lumps from a distance (Plate A). A closer look (Plate B) reveals well-formed crystals (circled) of a dark mineral, probably hornblende. The lighter areas between the hornblende are plagioclase feldspar. This combination of minerals, and the large crystals, suggests that this is either gabbro or dolerite.

The exposure faced a ravine that had been partly filled to support a road, so we followed the slope until we found more loose boulders with a different lithology.

Figure 14. The small boulder in Plate A reveals a fine-grained, layered rock which, upon closer examination, was found to contain a distinct contact zone with the gabbro we saw in Fig. 13 (Plate B). Plate C shows this contact zone in detail (image width is ~6 inches). The circles enclose large crystals of (probably) hornblende.

I cannot say, from the available photographs, which lithology in Fig. 14 is intrusive. However, those details are not the purpose of this post; between the high-grade (schist and gneiss) and low-grade (phyllite) metamorphic rocks we’ve observed, there is a line of intrusive and possibly volcanic rocks with the same general orientation (about 40 degrees east of north). We cannot say anything about post-emplacement deformation of these magmatic rocks, but it is curious that they lie between metamorphic rocks of different grade, in an area with several identified faults (see Fig. 1) having the same orientation.

I have followed one interpretation of the rocks discussed in this post, that they were deposited as muddy sediments about a billion years ago and deformed when volcanic processes stretched the crust. The subduction zone that created this tectonic environment was subsequently caught between opposing continental plates and crushed like a beer can. At some point–probably continuously until the final cataclysm–basaltic magma collected in a subterranean chamber and periodically emerged, creating submarine basaltic flows.

I had a lot of fun with this post (and crushed a few beer cans myself)…

Seneca Regional Park: The Rocks are Awakened

The title of this post refers to the furthest upstream exposure of Precambrian metamorphic rocks (see a previous post) along the Potomac River I have encountered; Precambrian schists rise from the riverbed and surrounding hills, reflecting the plate-tectonic processes that created them, but in a human-friendly form. The perfect harmony of rocks and life is revealed in the mature forests lining the Potomac River (Fig. 1).

Figure 1. Old-growth forest in Seneca Regional Park, covering a gentle slope along the trail to the river. Note the vines (dark lines in foreground), which apparently grew with the trees for decades until they straddled the tree tops, 75 feet above the forest floor.

I have been reporting on the geology along the Upper Potomac in recent posts (e.g., this post), revealing a braided river that is cutting into flood deposits, until it reaches a bottleneck at Great Falls, where the earth slows the Potomac’s rush to the sea.

This isn’t an overview post, however, so Fig. 2 shows only shows today’s study area. Note that there are three inset maps; the largest (Seneca Park) will be referred to most often in this post.

Figure 2. Overview of the Upper Potomac study area. The red-outlined inset map indicates Seneca park with a red arrow. The path followed in this field trip is shown in black in the large inset map (Seneca Park), which indicates waypoints referred to in the text and later figures. Note that the starting point of the hike is marked by a “P” in the Seneca Park inset map.

Starting from the parking lot (P in the Seneca Park inset of Fig. 2), we proceeded towards site A, surrounded by a mature forest (see Fig. 1) established on a thick soil horizon (Fig. 3) that was incised by creeks (runs in NOVA), cut into the regolith surmounting the Precambrian basement rocks (Fig. 4).

Figure 3. Stream-cut scarp in thick soil covering hillside between Sites P and A (see inset map of Fig. 2).
Figure 4. Regolith gravel on hillside. The trail from site P to A (black line in Fig. 1) followed a stream towards the Potomac. The valley floor was incised by streams as seen in Fig. 3. This juxtaposition of basement rocks and stream fill suggests a long history of erosion and deposition in this area.

The Potomac River at Site A (Dave’s Lookout on Fig. 2) is made up of several islands (Fig. 5) and includes remnants of the Patowmack Canal (Fig. 6), which was part of George Washington’s lifelong dream to make the Potomac River navigable, to open up the frontier as far as Ohio.

Figure 5. Side channel with Patowmack Island in the background (see Fig. 2 for location).
Figure 6. Remains of Patowmnack Canal constructed by George Washington to skirt the shallows in the main channel. Another segment is preserved at Great Falls.

The ready supply of fragments of flat rock to construct the canal came from many exposures of the same schist we saw at Great Falls and River Bend Park (1000 to 500 Ma old).

Figure 7. A typical exposure of schist from the area. The top is about twenty feet high. These bedrock outcrops formed isolated hills along the river bank and a ridge that constrained the flood plain severely. The foliation (original bedding surface) is tilted about 30 degrees towards the south–a compass direction of about 150 degrees (north is zero).

The chemical alteration of the original sedimentary rock (mud deposited more than one-billion years ago) to concentrate quartz (Fig. 8) is evidence of very high temperature and pressure caused by deep burial and deformation during a geological process called metamorphism.

Figure 8. Close-up photos of quartz porphyroblasts in schist. Plate A shows a semi-round quartz nodule about 4 inches in length, as well as irregular blebs (gray material indicated by arrow) and veins (extending above circled reddish mass). The circled area is a highly weathered, iron-rich mineral such as garnet or hornblende. Plate B shows a circular, pure porphyroblast of quartz (~2 inches across) that merges with darker quartz to the upper right of image.

The formation of quartz porphyroblasts within a foliated rock like schist suggests that heat and pressure were distributed irregularly within the study area, melting the silica out of the parent rock but not destroying its original sedimentary layering. This is a fine line that is poorly understood because the extreme temperatures and pressures that produce schist can only be reproduced in the lab at scales less than a millimeter.

Figure 9. Cross-sectional photo of the schist shown in Fig. 7. Thin layers (about one inch) of quartz (highlighted in yellow) generally delineate a relatively undeformed area, where the original sedimentary bedding is undulated, from highly deformed areas above and below. Note the convoluted foliation and the quartz boudins (circled). The quartz is stronger than the surrounding minerals and was torn apart during extension while under a lot of pressure. The quartz beds (yellow highlight) may have been lenses of sand, or formed from quartz remineralized from the original sediment. The view is about 12 inches across.

Figures 8 and 9 are from the same exposure, taken less than 100 feet apart horizontally, and maybe (I’m guessing) about 30 feet separated them vertically (in their original reference frame). These schists were tilted by normal faults that occurred hundreds of millions of years after deformation, during the breakup of Pangea.

The path took us to Site B (see Fig. 2 for location), along a very shallow channel nearly blocked by a gravel bar (Fig. 10). The Potomac flood plain was wider here but erosion was just as evident as further upstream.

Figure 10. The Potomac River is choked by sediment deposited on the Precambrian basement that surfaces at Site B (see Fig. 2), forming several islands defined by poorly sorted sediment as seen in this image. There was considerably more deposition in the past because these islands are erosional rather than the result of recent deposition.

The Potomac’s floodplain is much narrower here than further upstream, as revealed in the topographic map (Fig. 2). Note the number of valleys leading to the Potomac in Seneca Park. However, because of the sudden decrease in channel size, a bottleneck is formed that causes substantial deposition. This created the islands seen in Fig.2 in the past as well as a well-developed flood plain (Fig. 11) characterized by greater foliage than at Horsepen Run. It floods frequently at this bottleneck because the river’s flow is constrained to a narrow and shallow channel as the Potomac approaches Great Falls.

Figure 11. Eroded stream crossing the Potomac’s floodplain at Site B. Note the juxtaposition of mud with boulders in this small stream. Such a mixture is common where a river channel changes morphologically like it does here at Seneca Park (see Fig. 2).

The return to the parking lot (labeled P in Fig. 2) followed a steeper valley lined with outcrops of the same Precambrian schist we saw at Site A, with foliation oriented the same (dipping to the south). The stream followed the rocks (along strike) to the southwest, finding an irregular path around bedrock that surfaced constantly. There were many ledges and dead ends, resulting in shallow pools, along the meandering path the stream had forged in its effort to join the Potomac (Fig. 12).

Figure 12. A u-shaped meander of the stream we followed, from Site B to Site C (see Fig. 2), apparently formed in response to a bulge of bedrock beneath the fallen tree. This was a cobble and gravel stream, which shouldn’t form meanders under any circumstances, especially not on such a steep incline (note the slope indicated by the trees in the background).

This was an interesting field trip. We saw how the rocks can rise up from the bowels of the earth to change the character of rivers, where they flow and what they can transport.

Maybe someday we will understand the earth well enough to explain Figs. 5, 10 and 11, using the geological clues presented in Figs. 7-9. The highest mountains and deepest canyons are the result, in large part, to the secrets hidden within the material science of geochemical processes.

Someday we may move mountains…

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).