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August 26, 2022  

The Forest and the Trees: Glacial Topography of the Central German Plain

Figure 1. Typical scene driving between Usedom and Goseck (See Fig. 2 for approximate location). The hills are glacial moraines as we saw at Usedom in the last post.

The wind blows pretty steady over the North German Plain and Central Uplands, so there are wind turbines everywhere, scattered among the hay fields (Fig. 1).

Figure 2. Map of central Germany, showing stops discussed in this and the next post. The red line is the (very) approximate path taken for this post.

The topography of the Central Germany Plain is very similar to Nebraska, Kansas, and the Dakotas, because they were all created by the advance and retreat of multiple glaciers during the Pleistocene Ice Age. As we saw in the last post, advancing ice sheets as thick as a mile push rock and soil in front of them, before melting back for a few millennia, leaving piles of soil and boulders behind. They are like gigantic bull dozers.

Figure 3. Gentle slope north of Berlin. This is a ground moraine, pulverized rock forming a thin layer of rich soil just waiting for the plows of modern farmers.
Figure 4. The hills in the background are either terminal moraines or eskers (another German word). See text for explanation.

As ice slides over the landscape, scraping off whatever gets in its way, the ice at its base can melt from the friction, creating streams that transport already ground-up rocks. The usual rules of sedimentology apply to these ice-encased streams. They can deposit their sediment load as eskers, which identify these sub-glacier streams, or as piles of scraped-off soil (terminal moraines). I don’t know which I’m looking at in Fig. 4, but this image gives a good impression of their impact on the landscape. Keep in mind that the ice was approximately ONE-MILE thick above the landscape shown in Fig. 4…

Figure 5. Entering the Saale River valley near Goseck.

As I alluded to in the previous post, geology isn’t a sequence of static processes; there is always more than one cause of what we see today, and none of them are stationary. Thus, the landscape produced by the continental glaciers that advanced over the Central German Plain during the Pleistocene were constantly in competition with alpine glaciers created in the valleys and peaks of the Alps. The huge ice sheets had the power to overcome any obstacle…but they couldn’t surmount the steep slopes of the Alps.

The glaciers that originated in the Alps waited until the last retreat of the great ice sheets that originated in Scandinavia, before they could make their play in the Holocene. Vast quantities of easily weathered feldspar were washed down their steep slopes into a panoply of rivers, which cut through the moraines left behind during the retreat of the continental ice sheets, creating broad river valleys like that of the Saale River (Fig. 5). Germany’s central plain and uplands were cut to ribbons by these growing streams, resulting in one of the most water-navigable regions in the world.

You always have to watch your back…

August 26, 2022  

An Erratic Path: Glacial Geology in Usedom, Germany

This post finds me on the Baltic Sea, although I never actually saw it first hand. But this report isn’t about coastal geology; instead, I will be talking about an unusual feature of glacial terrains.

Figure 1. View of the lagoon that separates the island of Usedom from the mainland. An elongate lagoon is surrounded by marsh grass and is connected to the Baltic by narrow channels.
Figure 2. Map of northern Germany. Usedom is the island indicated by the pin. Figure 1 was taken on the west side, where the island is accessed via a short drawbridge.

I haven’t explicitly described glacial terrains in previous posts, and this is not going to be a summary. As always, I’m only going to discuss what I saw with my own eyes. The flat, poorly drained topography of glacial areas (Fig. 1) is often interrupted by linear mounds of loose gravel, sand and silt. These features are called moraines and they are ever-present in northern Germany, especially in Usedom (Fig. 3).

Figure 3. A glacial moraine is seen in this image, in the background, expressed as a low hill, but it is entirely composed of unconsolidated soil.

The primary glacial feature I encountered on this trip is the titular Erratic–large, rounded boulders scattered around a featureless landscape (Fig. 4).

Figure 4. Several glacial erratics have been collected and used as curb markers in this rest stop in Usedom. These boulders have been transported hundreds of miles and have no local source, thus the name erratic.

Let’s look at a couple of examples and see what they tell us about their source.

Figure 5. Photos of a boulder from Fig. 4. The left image shows the appearance of the stone in the field, and the right photo is a close-up (~4X magnification). See text for discussion.

The first erratic boulder I found (Fig. 5) contains no more than 5% quartz (left photo caption), and is dominated by K-feldspar, which is unusual. The magma from which this rock formed (deep beneath the surface where it cooled for millions of years) didn’t contain very much water, which is indicated by the small amount of quartz. The chemistry of the magma is quasi-frozen in the minerals, the second-most-abundant of which is Na Feldspar. The feldspars form a continuum that depends on the relative abundance of potassium (K), sodium (Na), and calcium (Ca); K and Na both form lighter-colored minerals whereas Ca forms dark feldspar minerals. Based on the mineralogical composition of this rock (inset in left image of Fig. 5), this would be classified as a syenite (middle left side of Fig. 6).

Figure 6. Classification of granites based on Na-K feldspar (A point), Ca feldspar (P corner), and quartz (labelled Q). We don’t need to worry about the bottom half of the plot because those are very rare minerals.

Syenites are formed in thick, continental crust. An example today would be the Alps (far beneath them) or the Himalayas, where subduction of denser oceanic crust is not occurring. In other words, the rock shown in Fig. 5 was created deep beneath the surface (~30 miles) when continents collided.

Figure 7. Photos of another erratic from the same location as Fig. 4. Note that this rock (left photo) has a less-reddish hue, an impression supported by the mineralogy (right image). K-feldspar comprises ~15% of the minerals, as opposed to 60% in Fig. 5. The increased quartz content (inset in right image) would classify this as a quartz-syenite; more water was contained in the original magma, but not enough to form significant quantities of quartz.

The boulders seen in Figs. 5 and 7 could have come from the same magma chamber because, as you would expect, there would be variations in local chemistry in a magma chamber tens of miles in diameter, and slow rates of convection wouldn’t mix the magma to a uniform consistency, even over millions of years. Magma, even when heated to 2000 F and buried tens of miles beneath the surface, is still thicker than molasses; it doesn’t mix well.

Figure 8. Photos of another erratic found along the road (Fig. 4), revealing a very different texture from Figs. 5 and 7. This sample contains large (~1/2 inch) irregular clumps of what looks like K-feldspar (note the reddish hue). The overall mineralogical composition (inset of left image) suggests that this rock came from the same magma chamber as the quartz-syenite in Fig. 7.

Phenocrysts like those seen in Fig. 8 are created in intrusive igneous rocks when they go through a multistep cooling process; for example, magma near the edge of the magma chamber loses heat to the surrounding rock and forms crystals like those seen in Fig. 8. These phenocrysts are then captured by the still-molten components of the magma and dragged along for probably hundreds-of-thousands of years (at a very slow speed, like inches per thousand years).

When the magma finally cools enough to become solid rock, it is uplifted as overlying rocks (of all kinds) are eroded by wind and water, not to mention ice. They are finally exposed in great mountain ranges like the Himalayas, where the rock breaks into smaller-and-smaller pieces along joints. When these pieces become small enough to be transported at the base of glaciers (you’ve heard the phrase glacially slow), they are dragged along, scraping over more rocks, sand, and gravel, which leaves evidence of their precarious journey (Fig. 9).

Figure 9. Photos of a glacial erratic from Usedom (not Fig. 4), showing striations that indicate it was dragged across a (rock) hard substrate.
Figure 10. A glacial erratic similar to that shown in Fig.9, but with striations (formed by movement at the base of glaciers) that flow into a joint (circled), suggesting that it was sand and not bedrock, over which this boulder traveled during the most arduous part of its journey. (Sand is mostly quartz, which is much harder than steel or window glass.)

This post has been erratic, starting out looking at a glacial terrain (Figs. 1-4), then taking a detour into igneous petrology, the chemistry of magmas, and mineralogy, with a little plate tectonics thrown in. That’s how geology is; everything is an ongoing process that never quite reaches equilibrium (e.g. the phenocrysts in Fig. 8), and the journey is unending.

I didn’t investigate the origin of the syenite boulders examined in this post, but (if memory serves) they match the mineralogy of intrusive rocks from Sweden, which is a long way from Usedom.

Stockholm is about 500 miles north of Usedom…

August 19, 2022  

Eidersperrwerk: Keeping Out the North Sea

My last post explored the mud flats bordering the North Sea in northern Germany, where we found conflicting methods applied to control and protect the levee system. This post investigates more aggressive measures implemented at the mouth of the Eider River. We will briefly look at the Eidersperrwerk, a gate system designed to control both storm surge incursion up the Eider, and river outflow

Figure 1. Aerial view of the Eidersperrwerk gate system, looking southward. The North Sea is to the right. We will examine the mud flats to the right of the roadway in this post. Note that more than half of the original mouth of the Eider River has been blocked by the levee.(Image from Wikipedia.)

We will focus on the seaward mud flats in this post. Let’s take a look at the south side of the river first (upper-right of Fig. 1).

Figure 2. Sediment retention fences on south side of Eider River. Note the erosion at the base of the fence running across the image from left to right (perpendicular to shore). They are intended to trap sediment, but that doesn’t appear to be happening.
Figure 3. Detail of gate on river side. The gates were closed when we visited at low tide, possibly to keep water depths navigable in the estuary.
Figure 4. Lock approach from seaward. This small enclosure was constructed with steel plates, but the lock gates were closed except to allow the passage of a tour boat, during our study.
Figure 5. Looking north along the seaward side of levee (lower part of Fig. 1). Note the grass that has filled in where only mud was present before (presumably, Fig. 1 is an older image).
Figure 6. Close-up near the junction of levee and lock enclosure (see Fig. 1 for location), showing clumps of grass (center of image) surrounded by pieces of stone used to armor the levee. This looks like recent erosion to me, because the grass grass was probably contiguous with the thick growth near the levee toe.
Figure 7. View looking north in the natural embayment (see Fig. 1 for location). Note the drier sediment near the top of the image. This is a berm that is semipermanent, formed of silt and minor sand by tidal and wave action. It is cut by multiple rivulets, formed as the tidewater drains from the nearshore area (to the right). The scattered boulders are evidence of intense erosion during storms.
Figure 8. View seaward from tip of lock embayment, showing eroded riprap, vegetation clinging to the toe of levee, runnel at low tide (strip of water running north-south in Fig. 1), and berm from Fig. 7 turning seaward.

Comparing Fig. 1 to Figs. 2 and 5-8, we can see the effects of years (probably decades), during which interval the northern margin of the river mouth filled with sediment and grass was established (Fig. 5). Subsequently, it seems that erosion removed some of this soil and grass (Fig. 6). Meanwhile, storms have been slowly wearing away the boulders armoring the base of the levee (Fig. 8) and a semipermanent fair-weather berm was constructed (compare Figs. 1 and 7).

In summary, something appears to have changed in the dynamic environment around the mouth of the Eider. It should come as no surprise that constructing a gate system and cutting off a major sediment supply for at least half the time had dramatic effects on the nearshore. Mud flats are very sensitive to sediment supply, and it could have been either reduced alongshore transport from the north, or the almost-complete denial of rive-borne mud that led to the current situation.

Some scientists propose that storminess varies on many scales, from decadal to millennial as climate fluctuates…

August 18, 2022  

Coastal Restoration on the North Sea

Figure 1. Sign introducing the coastal area and the restoration project. The mud flat here is a mile wide (estimated) because of about 20 feet of tidal range, twice a day.

Today’s post takes me to the North Sea coast of Germany, the city of Husum, and to one of the famous mud flats from the region. Rivers running from the Alps drain Germany, transporting mud (silt and clay) to the north coast, where it is transported along the coast and stirred around by strong tidal flows. We are going to look at efforts to stop dramatic erosion caused by a reduction of sediment input, because of dams and coastal construction, leading to a serious threat to the levee protecting Husum from the North Sea (Fig. 2).

Figure 2. Photo of levee that protects the city of Husum from the North Sea. (Right side of inset map of Fig. 1) The building to the right is an abandoned hotel inside the levee. The building to the left is a restaurant on pilings where people swim during high tide. The asphalt road is the path to the seashore.

The mud flats schematically shown in Fig. 1 are covered with fence-like structures designed to catch mud brought in the the high tide (Fig. 3).

Figure 3. Image of nearshore area (covered by grass), sediment retention fences, and reinforcing riprap where erosion occurs. Note that in this image, the fences do not appear to be collecting sediment on the landward side (to the right).

A quick look at the past. This area was covered by glaciers that filled the North Sea and transported rocks from Sweden to the north. These glacial erratics are rounded and scattered around the land in a random manner (thus the name). We found one used as street decoration in Husum (Fig. 4).

Figure 4. Close up of glacial erratic left along the coast. The boulder was about 3 feet in diameter. This close-up shows muscovite (shiny minerals), orthoclase feldspar (pink), amphibole and/or biotite (dark), and quartz (gray). This granitic rock was transported as much as hundreds of miles by ice, from Scandinavia.

In addition to boulders transported during the ice ages (less than a million years old), there are remnants of sandy sediment from the Quaternary, before the area was overwhelmed by mud (Fig. 5).

Figure 5. Image to the north in Fig.1, showing trees and a village on top of a low pile of quaternary sand, probably the erosional debris of a stream or coastal beach from the last ten-thousand years. This photo was taken to the east side of Fig. 1. Note the sporadic filling by grass, especially the sheep. This is interesting because sheep eat grass, so why they are loose in an area supposedly being reclaimed is confusing.

The result of the sediment retention project can be seen in Fig. 6.

Figure 6. The landward limit of the fencing project, less than 100 yards from the levee. Note erosion along the fence, leaving it standing 2 feet above the exposed mud. This could have been the result of long-term erosion, or a single storm.

This are represents an attempt to reconcile the problem of coastal development (the port of Husum ships out grain) and the protection from storm waves provided by a wide mud flat (which dissipates wave energy). Another issue is the encroachment of sheep grazing, which appears to be legal (there are fences and gates, etc). And then there is entertainment; this is a popular swimming location during high tide. Not to mention environmental degradation and fish hatcheries. Several attempts at mixing these applications can be seen in the hardened and dredged channel leading to the port (Fig.7), and buried groins which were apparently intended to keep the shipping channel open (Fig. 8).

Figure 7. Shipping channel to the port of Husum hardened by mortared rock.
Figure 8. Groin in mud flat. These coastal engineering structures are designed to prevent sediment being carried along the coast and blocking channels, as well as retaining sediment between adjacent groins. This is probably contributing to the erosion seen in Fig. 6.

It is difficult to reconcile the many uses the coastline is required to fulfill. This trip revealed that it is unreasonable to mix methods designed to preserve the status quo (Figs. 7 and 8), and those intended to change it (eg. Fig. 6), especially when these techniques are mixed (Fig. 3). A difficult decision will have to be made soon, or the levee protecting the bustling cit of Husum will be in danger of breach during a severe storm, which is becoming more common in the North Sea.

August 8, 2022  

The Last Few Miles

Figure 1. View looking uphill, along a small ravine in Rock Creek Park. This is a small tributary that shows what this post is about: the stream bed is interrupted by layers of rock every few yards.

This is going to be a brief post, mostly because it is very difficult to convey what I want to communicate in photographs; the camera lens (on my iPhone) simply doesn’t capture image depth well. For example, Fig. 1 was actually pretty steep, but it looks as unintimidating as my driveway.

Figure 2. Topographic map of Rock Creek Park. Note the steep gullies leading to Rock Creek from the west (indicated by dark shading). Figure 1 was taken in the deeply incised terrain east of the Nature Center (top-left of image).

I’ve been talking about the bedrock exposed along the bed of the Potomac in several posts (e.g., Geological Bottleneck and Great Falls), but those are specific locations. Those significant drops in river elevation are part of a larger pattern, one that is displayed even at the scale of Fig. 1. It doesn’t take much of a drop to generate enough potential energy to spin a waterwheel (Fig. 3), which can do a wide variety of work–from grinding corn, to operating a machine shop.

Figure 3. Waterwheel at Peirce Mill used to grind grains like wheat and corn into meal, constructed in the early 1800s, at the lower part of Rock Creek Park (bottom-center of Fig. 2). The stream’s flow was subdivided by channels like that seen in the left of the image to supply water to several mills in the area.

The staircase structure of streams along the transition from crystalline rocks to coastal plains (aka the Fall Line) is so important to the ecosystem that artificial barriers were constructed within the park to ameliorate the impacts of road and bridge construction (Fig. 4).

Figure 4. View looking downstream from a bridge near the top of the map in Fig. 2, showing blocks arranged to replicate the natural steps as seen in Fig. 1. This construction was completed to reintroduce the herring migration. They spawn in the upper reaches of Rock Creek.

Rock Creek National Park deserves its name, not just because of its rock bed. Cambrian sedimentary rocks exposure along the steep tributaries leading to the creek (river?) suggest that bedrock lies not very far beneath our feet (Fig. 5).

Figure 5. Large exposure of Cambrian sedimentary rock formation (image height is about 20 feet), consisting of interlayered sand, siltstone, and shale. Where sand is the predominant component, blocky outcrops like this occur. Siltstone and shale produce more fissile outcrops.

Water has been struggling with rocks for the last 200 million years, always trying to reach the sea. It exploits every nook and cranny in the bedrock until it forms a stream, then a river, and it cannot be stopped. Thanks to the perseverance of water, driven by the steady pull of gravity, the first European immigrants to North America were able to establish a toe hold on what was (to them) a new land…

July 5, 2022  

Recap…

This is a quick post to summarize what I said about modern Japan being an analogue to the Taconic orogeny. For example, here’s a photo of Mt. Fuji, seen from the ocean (Fig. 1). (Imagine being in the back-arc basin during the Cambrian period.)

Figure 1. Mt. Fuji from the sea.

The Sea of Japan is more than 500 miles across at its widest point, so sediment eroding from the mountain chain that forms the backbone of Honshu is collecting along the western coast of Honshu as well as in deeper water offshore.

Here’s a schematic cross-section of the most-likely geography during the Taconic orogeny (Fig. 2). Imagine Honshu as the island arc shown offshore of the ancient North American continent (to the left in the cartoons).

Figure 2. Schematic cross-sections of North America and a hypothesized island chain during the Taconic orogeny.

Modern Honshu and the Sea of Japan are most representative of the Taconic orogeny earlier than 543 my, before subduction began on the western margin in the top panel. There is no subduction in the Sea of Japan today; in fact, spreading stopped about 20 million-years ago; details are hard to find because there are no easily accessible seismic sections of the Sea of Japan. Thus, to apply the cartoon from Fig. 2, ignore the subducting back-arc ocean crust (black layers) and focus on the deformed gray areas in the middle panel.

The lower panel is probably what will happen to Honshu in the distant future. For example, the Pacific plate is being subducted at ~10 cm/year (4 inches). We can use an average width of 1000 km (625 miles) to estimate that it will take 10 million years [1000 km/(10 cm/y)] for the lower panel of Fig. 2 to become reality.

With respect to the scale of the analogous processes occurring in the Japanese Islands and N. America (during the Taconic orogeny), we can do a simple comparison (Fig. 3).

Figure 3. The left panel shows the extent of rocks associated with the Grenville orogeny, associated with closing of a precursor to the Atlantic Ocean (Iapetus Ocean). The right panel shows the island of Honshu. The arrows show a hypothesized, similar-sized island arc (outlined in black) during the early stages of the Taconic orogeny, when sediments eroded from the earlier (Grenville) mountain belt (orange) were buried deeply beneath the back-arc basin between the Grenville Belt and the offshore island. They would have been subjected to intense heat and pressure. Note the immense scale of this geologic province, based on the similar size of Honshu and the island arc that became the rocks we’ve seen throughout eastern N. America.

This has been a very simple, hypothetical reconstruction, but I hope it helps you envision what the proto-north American continent was experiencing. The key point is that a massive mountain-building event, something like the Taconic-Acadian–Alleghanian orogenies, which lasted throughout the Paleozoic era, wouldn’t have been an earth-shattering event…

July 4, 2022  

Deja Vu

As we entered the Taconic Mountains on US 4 in Vermont, something didn’t look right, or it looked too familiar to be correct. It took a while to realized what was wrong with Fig. 1.

Figure 1. Road cut along US highway 4, near Rutland, Vermont. This is Precambrian (1000 to 500 Ma) schist with strong foliation dipping to the west.

These are the mountains for which the Taconic Orogeny (550-440 Ma) was named. They were deposited as long ago as a billion years in a shallow sea (e.g. Sea of Japan) and then buried, before being compressed and heated, finally being pushed onto the porto-north America continent by 440 Ma. During this long period of metamorphism, the clay minerals comprising the bulk of the sediments recrystallized into mica (mostly muscovite), a platy mineral that creates both a sheen and a fissile texture, the tendency to flake apart (Fig. 2).

Figure 2. Close-up of Fig. 1, showing the glistening caused by alignment of platy muscovite in the sun (upper center), and fissile texture caused by the same alignment and weathering as water works its way between mineral grains. Image width is about 6 feet.
Figure 3. Close-up of the foliation surface of the rocks seen in Figs. 1 and 2. Note the linear ripple-like texture, which may be remnant from the original sediment (note the surfaces seen in the last post), or a coincidence. The brightness is caused by aligned muscovite crystals. (Image is 2 feet across.)

The Taconic Mountains are the remnants of a mass of metamorphic rock that was pushed over younger, less-altered rocks in this region. This occurs along low-angle thrust faults when the rocks are buried less deeply, so that they break rather than fold like putty. Speaking of ductile deformation, we saw plenty of evidence of that in the White River‘s exposed bed (Figs. 4-6).

Figure 4. Photo of White River near the village of South Royalton, VT, showing exposures of Precambrian schist and gravel bars. The following photos were taken on this outcrop.
Figure 5. Detail of orthoclase (pinkish area) and albite (whitish) feldspar minerals squeezed out of the original muddy sediment during metamorphism. These minerals may have originally been present as lenses of sandy sediments or be the product of remineralization, which includes a component of concentrating incompatible elements. Very little quartz was present.
Figure 6. Image of nearly vertical foliation (i.e. layering) of schist in White River bed. This broad area of irregular feldspar and quartz may have been a large sand lens (e.g. a flood deposit) in the original sediments.

This post is titled “Deja Vu” because we saw schist with a similar composition and orientation in the Potomac River, more than 500 miles to the south, in a band tens of miles across, centered on Great Falls, Virginia. Such a broad distribution tells us that a vast mountain belt eroded about one billion years ago, and then its erosional remains were buried so deep that they nearly melted. The subsequent collision was no laughing matter. I have been using Japan as an analogue for the Taconic Orogeny for two reasons: (1) Honshu, the largest Japanese Island is about 800 miles long and it is depositing vast quantities of mud into the Sea of Japan; (2) using a modern analogue demonstrates that mountain building is a slow process, barely noticed by the inhabitants of island arcs destined to be smashed onto the continents facing them.

Rocks like those seen in this post are already buried beneath the Japan Sea and deformation has no-doubt begun. We just have to wait 400 million years for them to come out of the oven…

July 4, 2022  

The Outer Limits

This is the first of several posts, reporting the roadside geology of western New York and central Vermont. Today, we will visit Binghamton, New York. This small city (urban population less than 50 thousand) sits at the confluence of two perennial, gravel-bedded rivers (Fig. 1).

Figure 1. View from Confluence Park in Binghamton, NY, showing the Susquehanna River (left side) and Chenango River (right). Note the weir on the Susquehanna, which maintained sufficient depth for a lock (Fig. 2), which permitted access further upriver.
Figure 2. Photo of submerged lock that operated in the 1800s along the Susquehanna River. Similar structures are present along the Potomac in VA and MD. The outline seen in this image may be only the foundation. The stonework was probably removed to create a clear channel for the modern bridge (Fig. 3).
Figure 3. Photo from the confluence of the Susquehanna and Chenango rivers, showing the bridge that has replaced the lock structure shown in Fig. 2. Note the gravel bar to the center right of the image, where a small tributary has been channelized but still is depositing large gravel and small boulders in the Susquehanna channel.

Enough of Holocene and Anthropocene geology. The fascinating thing about this region is that it preserves a huge volume of sediment eroded from mountains that were growing during the Devonian Period, about 350 million-years ago (Fig. 4).

Figure 4. Geologic map from Rock D, with Binghamton near the center. Note the tan-shaded area that expands from Albany westward. These rocks were originally sediments, deposited from a high mountain range located somewhere east of Albany, carried by ancient rivers as far as 500 miles to the west . (The scale is in the lower left of the image: 100 km is about 63 miles.) This humongous basin, collecting boulders, sand, silt, and mud about 350 my ago, was preserved because it was a precursor of what was to come 100 million years later, when Africa and N. America collided, burying these rocks deep enough to save them from erosion, but not deep enough for them to lose their sedimentary character. This perfectly preserved basin–frozen in time as if in a museum–is called the Catskill Delta.

We didn’t have the time or resources to go on a quest for rocks that would reveal what was happening during the Devonian Period, so we took some photos of charismatic blocks that had been removed from their original location and “deposited” along the path that followed the Chenango River through downtown Binghamton (Figs. 5 and 6).

Figure 5. Photo of slab of mudstone (not in original orientation), showing irregular ripples accentuated by silt against a matrix of mud. This is a very common environment in river flood plains during high-flow events, when gentle currents separate silt from mud. Flow during these intervals is insufficient to form unidirectional ripples, and the result is seen in this image. The sample is about two feet across.
Figure 6. Photo of silt surface in shallow water from unreferenced rocks from the Catskill Delta. Note that this bedding plane has a more criss-crossing pattern of “ripples” and contained more silt (light-colored). This sediment was probably deposited in a similar environment to Fig. 5. Both were quickly buried during a flood that occurred not long after these fragile sedimentary structures were created. If you were to step on these sediments in a modern stream, they would be what we term “mud” and avoided if possible.

The title of this post refers to the outer limits of a broad plain that was receiving gravel, sand, silt, and mud from a rapidly rising mountain belt–probably like western North America today (e.g. the Sierra Nevada mountains). It wasn’t a continental collision, but it was pretty massive, with elongate swaths of sediment subsequently buried by what came later.

I’m talking about a Clash of the Titans...

June 27, 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.

June 5, 2022  

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?

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