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December 18, 2025  

Quaternary Geology on Mt. Rainier

Figure 1. View of Mt Rainier from the west. At 14410 feet, it is the most prominent peak in the contiguous United States. It has 28 glaciers, with the largest total surface area in the lower states–35 square miles. Mt Rainier is a stratovolcano, composed of andesitic lava (rather than basalt), material ejected from the summit, and ash layers. This type of volcano is commonly found in subduction zones; they tend to have explosive eruptions (e.g. Mt Saint Helens). The oldest rocks on Mt Rainier are about 500,000 years old. It is active and listed as a decadal volcano–one of the most dangerous volcanoes in the world. Its last major eruption, accompanied by caldera collapse, was 5000 years ago, but minor activity was noted during the nineteenth century.

Figure 2. From my home in Tacoma (star) it’s a two-hour drive to Mt Rainier National Park. It is part of the Cascades Range, which comprises many well-known volcanoes like Mt Baker, Mt St. Helens, and Mt Hood.

Figure 3. This photo was taken on the south flank at an elevation of about 5400 feet, near the visitor’s center. It was a beautiful day and there were a lot of people preparing for some cross-country skiing on a couple of feet of snow. From this elevation it takes 2-3 days to reach the summit, almost 9000 feet higher. It’s hard to imagine it being so high and taking so long to reach.

Figure 4. These southern volcanic mountains are part of the Tatoosh Range, with peaks of about 6600 feet. Most of the volcanic rocks comprising these mountains are andesite, intermediate in composition between basalt and rhyolite. It is also very viscous, behaving like peanut butter and thus not flowing well. Andesite is commonly found at convergent plate boundaries where it is thought to result from mixing of basalt (from the oceanic crust), continental crust, and sediments accumulated in the accretionary prism.

Figure 5. Map of the 28 glaciers on Mt. Rainier. The glaciers fill canyons and valleys that were partly cut by ice. Figure 3 shows a smooth mountain, but in reality most of the smooth areas are the surfaces of glaciers. We’ll look at one below. The ellipse indicates the area discussed in this post.

Figure 6. (A) Narada Falls interrupts the descent of Paradise River, fed by Paradise Glacier (see Fig. 5 for location), making it drop a couple hundred feet over a thick layer of andesite. Andesite tends to form blocky flows, as shown in the right side of the photo, where the water seems to be climbing down steps. (B) Possible contact between younger volcanic and older intrusive rocks. Igneous activity within the area has been continuous for at least 50 my, during which time erosion has exposed older intrusive rocks like this granodiorite, which is part of a pluton intruded between 23 and 5 Ma. It is important to keep in mind that the volcanic rocks originated in plutons (magma chambers) emplaced miles beneath the surface. As they are exposed, new volcanoes form as more magma is injected into the shallow crust in a continuous process. (C) Differential weathering has accentuated layering in this volcanic rock, which was probably created by a series of ash layers deposited in quick succession–geologically speaking.

Figure 7. (A) View looking north towards the source of Nisqually Glacier (see Fig. 5 for location), which originates near the peak of Mt Rainier. The area delineated by the blue rectangle is the face of the glacier. (B) Closeup of the face of Nisqually Glacier. The characteristic U-shaped valley carved by glaciers is highlighted in white. Note the dark material within the glacier, probably wind-blown fine sediment. It looks like the face is a couple hundred feet high. I’ve never seen a retreating glacier before, so this is pretty spectacular to me. The Nisqually River originates right here…

Figure 8. View looking upstream along Nisqually River a mile downstream from Fig. 7. This is one of the most stunning photos I’ve ever taken because it reveals geological continuity, from the origin of a glacier 10000 feet higher, to the outwash being transported by a river. Amazing! Note the perfect U-shape where the shadow ends upriver. This area would have been covered by the glacier as recently as 10000 years ago.

Figure 9. Another mile downstream from Fig. 8 the walls of the valley have lowered, and are now rimmed by volcanic flows half-buried by detritus. Evidence of a glacier filling the valley has been erased by collapse of the valley walls. Rounded boulders fill the riverbed. The Nisqually River is overwhelmed by the huge sediment load and opens up new channels to continue flowing.

Figure 10. View looking upstream at the confluence of Nisqually River and Van Trump Creek. There are a couple of interesting features visible in this braided stream bed, less than two miles from the glaciers feeding each branch. The valley is very wide and flat-bottomed because it was carved by glaciers more than 10000 years ago. The large boulders (as large as three feet) covering the entire valley floor were transported by a glacier and became relict after its retreat because the stream flow, even during floods, is too weak to transport and erode them. The Nisqually River is cutting a channel through these relict sediments; the scarp is about eight feet in height. The white line delineates large, surface boulders from subjacent sand and silt with few boulders. Note that the surface boulders stop upstream where the white line curves sharply upward.

Figure 11. Image from 200 feet downstream of Fig. 10, showing a break eroded in the boulder-bar that crosses the stream bed at an angle. During recent heavy rain Nisqually River broke out of its current channel and created a myriad of flow structures such as the longitudinal bars seen in the lower-right of the photo. I think this bedform is actually a terminal moraine marking the maximum advance of a previous glacier–not necessarily the maximum glacial extent during the last two-million years.

Figure 12. (A) Andesite boulder (2 feet across) wet by recent rain shows fine-scale structure. The irregularity of the laminae, and phenocryst distribution, suggest to me that this sample represents ash fall rather than a flow. Magma with the viscosity of peanut butter tends to form smooth lines because it is difficult to penetrate, which would be necessary to create the mixed-up appearance between the lighter and darker shades in the center of the image. (B) Large block (~10 feet long) of intrusive rock similar to that seen at Narada Falls (Fig. 6B), but this is two-miles downstream. This relic was pushed/dragged by a glacier to this location. The white circle indicates where a close-up photo was taken. (C) Close-up (5x) image of the heavy block. It contains quartz (Q), plagioclase/albite feldspar (no orthoclase) (F), and amphibole (Am). My estimate of the composition is: 50% feldspar; 30% quartz; and 20% amphibole. Based on my estimated mineral composition, this would be granodiorite; however, I didn’t differentiate plagioclase and albite feldspar. (The former is darker than the latter.)

Figure 13. Map of potential volcanic risks associated with Mt Rainier–besides an explosion (e.g. Mt St Helens) and the eruption of ash which would cover a large area, depending on wind direction. Lahars (mud flows fed by all those glaciers) pose the greatest risk because andesite is too viscous to flow more than a few miles from its source.

Summary. I have seen evidence of continental glaciers in the Great Plains, the German Plain, and Ireland, but I never had the opportunity to observe glaciers up close. Alpine glaciers were nothing more than an abstract idea to me, something viewed from a distance.

I’ve looked out over the clouds from the summit of Haleakala crater on Maui, gazed into the cauldron of Kilauea, witnessed the boiling water rising from beneath Yellowstone’s seething caldera. I’ve seen videos of volcanic eruptions in Iceland, but I never imagined putting the glaciers and volcanoes together–right next door!

Usually, geology is observed as a series of images frozen in time, but at Mt Rainier it can be glimpsed as a real-time process that reshapes the earth’s surface–from top to bottom.

What a wild geological ride!

November 3, 2025  

Review of “The Gulag Archipelago. Volume 3” by Aleksandr Solzhenitsyn

This is the last in the series, and it continues the author’s eclectic writing style, mixing personal experiences with those of other survivors of the Gulag system. He introduced the idea of the Gulag being a nation, so this volume describes the inhabitants and extrapolates the system to the entire Russian state. His arguments are pretty convincing; you can’t create such a system of forced labor, in which the majority of the “workers” die of malnutrition, freezing temperatures, and murder by the hands of their captors.

The Gulag extended beyond the camps, forming communities of exiles after their release. Life as an exile was so bad that many former inmates returned to the work camps as “free” laborers or by committing new crimes (which was easy to do within the Soviet system). The author argues that the Archipelago so impacted Soviet society that the entire nation was swallowed by the Gulag, turning Russia into one large work camp. A society characterized by paranoia, distrust, and total submission to the state.

This photo of the author in the infamous Ekibastuz special camp (for 58s, political offenders) perfectly conveys the stoic, distrustful expression common to all survivors. That thin coat was what they wore (if they were lucky) in -50F weather working outdoors. Many froze to death at work or on the long hike back to the unheated barracks. Imagine a society with this mentality, even if they don’t show it on their faces.

These books were long, probably unnecessarily so, but they left me with an indelible impression of just how poorly people treat each other; the Gulag was filled with Russian citizens, many of whom fought during the Second World War.

Man’s inhumanity to man knows no bounds…

October 27, 2025  

Reflections of a Road Warrior

I began my journey in the overpopulated East, where the Appalachian Mountains—formed more than 250 million years ago—now lie subdued beneath layers of human settlement and urgency. The roads here are crowded, the pace performative. Drivers jockey for position, not just to arrive but to assert. In this terrain, driving is a social act, a negotiation of space and dominance. I obeyed the speed limits, but the pressure to conform was palpable. The land, ancient and eroded, seemed to whisper of restraint, but the people moved as if chased.

Crossing the Great Plains, the landscape flattened into a vast, glacially weathered expanse. Once grasslands, now farmland, the terrain offered little variation—just endless repetition. Here, the temptation to speed was not about competition but escape. The monotony of the land invited dissociation. Cruise control became a crutch, and the mind wandered. I found myself accelerating not out of urgency, but out of boredom. The road stretched like a taut string, and I felt the pull to snap forward. But I resisted. I slowed down. I began to see the land not as obstacle, but as place.

In the intermontane basins and across the Rocky Mountains, the terrain shifted again. The Rockies, surprisingly, offered no drama. I crossed them with nary a whimper. The basins between ranges were long, subdued, and emotionally neutral. Driving here felt mechanical, almost meditative. The land flattened my urgency. I became an automaton, moving through space without resistance. It was peaceful, but also forgettable. The road no longer demanded attention—it simply received it.

Then came western Montana, Idaho, and Washington. The youthful peaks struck like a cymbal crash. Steep grades, winding highways, and sudden elevation shifts pierced the monotony. I was exhausted—metaphorically speaking—by the mind-numbing landscape behind me, and now the terrain demanded vigilance. Driving became reactive again. The land had changed, and so had I. I was no longer cruising; I was contending. The road had become a teacher.

Less than a mile from my motel in Missoula, I witnessed a collision—a junker sports car and a delivery van, both likely violating traffic laws. The vehicles bounced like Tonka Toys, absurdly intact despite the violence. The driver of the wrecked car tried to restart his mangled machine, as if denial could override physics. Traffic paused, sighed, and resumed. No one panicked. No one intervened. The system absorbed the chaos and continued. It was a once-in-a-lifetime moment, and I had a front-row seat.

This scene encompassed many of the behaviors I’d observed across the country. Reckless driving wasn’t confined to high speeds—it occurred at low speeds too, often in familiar places. We rarely pause to see these events as inevitable outcomes of behavioral contagion, misaligned urgency, and systemic detachment. The stoic traveler observes without absorbing panic, recognizing the choreography of modern motion and its refusal to acknowledge consequence.

As I drove westward, I began to notice a pattern—not just in the terrain, but in how people moved through it. Flatness bred velocity and boredom. Elevation restored awareness. Geological youth correlated with behavioral tension. The land was not neutral. It shaped urgency, perception, and emotional posture. I had come to recognize a love-hate relationship with living in such a large country. The vastness invites freedom, but also fatigue. Driving is, above all else, boring—especially at highway speeds. But boredom is part of the lesson.

And then came the most important realization: Let local traffic pass; their urgency is not yours. This became my mantra. Most of the vehicles around me were not crossing states. They were running errands, commuting, performing routines. Their urgency was performative, not purposeful. I was on a different journey. I didn’t need to match their pace. I didn’t need to compete. I could let them pass. I could observe without absorbing. I could drive with intention.

This awareness led, fitfully, to acknowledging the inescapable control of the land over our minds and emotions. The terrain modulates behavior. It governs how we move, how we think, how we feel. The road is not just a conduit—it’s a medium. And to cross America solo is to engage with that medium fully. It’s to see the choreography between geology and psychology, between motion and meaning.

I did not enjoy driving fast. I found it fatiguing, disorienting, and performative. Slowing down was not just a mechanical adjustment—it was a philosophical one. It allowed me to appreciate the act of covering ground, to see the land as layered text, to learn in a hands-on way about geological and societal history that no Wikipedia article could convey. I stopped at unexpected locations. I absorbed stories sedimented in stone and soil. I saw how the land shaped settlement, movement, and memory.

I wish I’d had more time. My mind couldn’t keep up with the rapid pace. I experienced a kind of jet lag, even though I never left the ground. The body moved faster than the mind could metabolize. Reflection lagged behind experience. But that lag was instructive. It revealed the limits of perception, the need for pacing, the value of restraint.

In the end, this drive was not just a crossing—it was a reckoning. It was a slow-motion confrontation with the land, with behavior, with self. I began in the roots of the Appalachians and ended in the youthful peaks of the Northwest. I moved from assertion to observation, from urgency to awareness. I let others pass. I slowed down. I listened.

And the land spoke.

Acknowledgment

This essay was written by CoPilot after an extensive conversation, which it reduced to this piece. I accept full responsibility for the contents. The photographs are all real, taken by me along the way.

May 17, 2024  

Devonian Coastal Sediments in The Catskills Delta

Figure 1. View of Rondout Creek from the Trestle trail in NY state. Several tributaries of the Hudson River are constrained by ridges like those seen in this photo. Most of the exposed rocks are Bloomsburg Formation (Silurian, , 427-419 Ma), consisting of shale, sandstone, quartzite, conglomerate, all metamorphosed to some extent. They were deposited near a source of coarse sediment but include nearshore marine and deep water environments.
Figure 2. Billboard of the Wallkill Valley, named after the primary Hudson tributary west of the river.
Figure 3. Geologic map of study area. The location of Fig 1 is over the blue line (Rondout Creek) just below the label for State Hwy 213. The dash line is the contact between Bloomsburg rocks (427-419 Ma; gray area labeled S.S.) and undifferentiated Ordovician to Devonian (443 to 393 Ma) limestones, sandstone, shale, conglomerate (labeled L.S.). The specific dates of the Bloomsburg formation are probably due to radiometric dates from volcanic layers (probably ash) whereas the much broader interval of the other assemblage suggests that it was dated primarily through biostratigraphic methods, which are not precise. Also, the relationship suggests that the environments represented by these assemblages were spatially variable; i.e., Bloomsburg was one delta sequence among many that moved around during the 50 Ma span of deposition represented here.
Figure 4. Block of limestone located along the trail within the L.S. Area indicated in Fig. 3. The block is about eight feet across. The circled area contains several larger objects that will be examined in Fig. 4; the ridge seen in the topography of Fig. 3 couldn’t be examined, but many caves were visible at its base, suggesting chemical weathering of limestone. As a further note, the trail we followed in later images was allied Kiln trail; a kiln is a place where limestone is quarried and heated to produce lime, used as fertilizer and an ingredient for cement. It is reasonable to assume that the kiln was somewhere on the north side of Rondout Creek.
Figure 5. Close-up of block in Fig. 4, showing blue and green calcite (probably malachite) areas within a gray background. The coloration comes from high, local concentrations of Copper, Carbon, and Manganese.
Figure 6. Close-up of fossil assemblage from Fig. 4; I’m not a paleontologist but I recognize bivalves (curved) and echinoids (circular and columnar), two common groups found in Paleozoic nearshore marine environments. The living, segmented insect to the upper left is similar to a trilobite, which was probably also present on the original sea floor. This would have been the Paleozoic equivalent to a coral reef (no corals yet).
Figure 7. Exposure of Bloomsburg Formation conglomerates at the location of the blue dot in Fig. 3. The yellow lines indicate the bedding plane, which was the original river/sea bottom when the sediments were deposited. They dip about 30 degrees to the SE, very similar to rocks we found in Northern Virginia.
Figure 8. Close-up of undifferentiated rocks (labeled L.S. in Fig. 3). This photo shows that nearshore sediments are not uniform because of changes in river mouths, erosion of coastal uplands, sea level changes, etc; the laminae seen in the center of this image suggest deposition of fine particles in quiet water. Such rhythmic layering is often created by tidal flows. This exposure was only a few thousand years younger (tens of feet higher in the section) than limestone with caves; of course, this could be limestone, but it wasn’t part of a reef.
Figure 9. Close-up of Bloomsburg Formation conglomerate, showing rounded pebbles in a sandy matrix. This sample doesn’t show layering but in other locations, the pebbles were aligned in layers suggesting episodic flow events. These sandy sediments are reddish, which suggests they were deposited in fresh water (red indicates oxidized iron when deposited); marine sediments tend to be green or gray (reduced iron in ocean water). Precise dating of these events is impossible, but we can say that, within a few hundred thousand years, quiet water (e.g. Figs. 6 and 8) was replaced by high flow events in rivers transporting pebbles.
Figure 10. Image of quartzite/conglomerate exposed at the top of a ridge, showing striations formed by glaciers advancing from the NW, dragging stones with them and gouging these scratches. The arrow indicates the direction of movement. The white areas indicate where quartz has been ground into powder by the force of stones buried beneath thousands of feet of ice. Gouges can’t be dated but this was probably during the last million years, when glaciers reached their maximum thickness in this area.
Figure 11. Blocks of Bloomsburg Formation sandstone that collapsed recently, carrying ferns with them. This area was littered with such blocks carrying post-glaciation soil profiles and plants with them.
Figure 12. View looking east from a ridge near the bottom of Fig. 3 (where the trail makes a loop). This is where Fig. 10 was taken. These rocks have been exposed to the elements since this area was covered by ice.

SUMMARY. This part of central New York was very different about 400 million years ago. It wasn’t a deep ocean, but instead consisted of a spectrum of nearshore environments, from reefs to lagoons, deltas, and finally conglomerates containing rounded pebbles. The coarser sedimentation occurred later within this time interval, suggesting that mountains were rising to the east, shedding coarser sediments further west with every passing millennium.

This was the final collision of proto-North America with Eurasia in what has been called the Acadian Orogeny. We saw only a small fraction of this regional event during our walk today, but the rocks we encountered encompassed the entire event. Shallow seas filled with reefs and shallow marine environments were replaced by streams carrying pebbles less than fifty miles from their source in rapidly rising highlands.

July 26, 2023  

A question of scale: Indian Canyon Falls

Figure 1. This photo doesn’t have anything to do with this post, but this is where I parked when I explored Riverside park, along the Spokane River. I didn’t have an opportunity to get a big picture of Indian Canyon falls, which is only 15 minutes from the “Bowl and Pitcher” because it is located in an area with heavy undergrowth, hidden between basalt scarps.
Figure 2. Indian Canyon falls should be called “Hidden Falls” because it occupies a narrow crevice in a thick sequence of basalt. In this post, I will try and show how running water slowly but irresistibly forms chasms as big as the Grand Canyon. This photo shows the middle terrace of Indian Canyon; the top is about 20 feet higher and couldn’t be photographed properly to convey the correct sense of scale.
Figure 3. Photo looking into the lower “gorge” of Indian Canyon. A cave can be seen along the far wall and the bottom is somewhere “down there,” about fifty feet below my vantage point.
Video 1. This video is from the same vantage point as Fig. 3, but now you can hear the water trickling over the ledge, see the water moving in a thin flow, and see the surrounding morphology.
Figure 4. Photo from slightly further downstream. A thin, bright line shows the “water fall.”

Video 2. This short film shows the dynamics of the water running over the ledge in the context of the inner canyon. It really brings the experience to life.

Summary. I found Indian Canyon falls by looking for Lake Missoula park, which turned out to be closed to the public. However, the Park Service supplied a link to other geological attractions, with navigation instructions—GoogleMap took me to a nondescript, heavily wooded area, where I found something I hadn’t expected to see.

The rivulet of water flowing over the “fall” during the dry season is an omen of what is to come for this relatively unknown canyon (it was actually covered with biking and hiking trails). At first the trickle carries only mud, then sand, then gravel, then boulders, then …

Indian Canyon falls is how it begins. Where it ends …

Think big …

July 26, 2023  

Bowl and Pitcher: Volcanic Rocks at Riverside Park, Spokane, WA

Figure 1. Photo of a large volcanic boulder that has been named the “Pitcher” by local inhabitants for centuries. The name comes from the columnar rock with a piece protruding from its left side. I don’t seen the resemblance but I don’t see angels and dragons in the stalactites found in caves either.
Figure 2. Photo of the “Bowl” seen from a vantage point at the top of a cliff. The bowl is the dark opening to the left. These two exposures are huge boulders that fell off the surrounding cliffs when the Spokane River undercut the surrounding volcanic rocks.
Figure 3. Geologic map from RockD, showing the Riverside area and the “Bowl and Pitcher” campground. The yellow areas to the left are basaltic flows dated to between 25 and 5 my. The tan area is glacial lake outwash deposits lining the river bed.
Figure 4. Photo of the cliff where Fig. 2 was taken. Note the irregular surface of the entire cliff face; pieces of this rock are seen in the foreground, revealing strange, curved shapes.
Figure 5. Photo of the “Bowl” rock, showing the curved structure seen in Fig. 4; I have seen this kind of form in limited areas in older basalt but never comprising a cliff. It is reminiscent of ropy lava (pahoehoe), which suggests that it was very viscous and flowed slowly. There was very little sign of weathering (forming soil profiles), so much of it was extruded very quickly. Note the indent on the right side of the image, where rocks carried by the river eroded the lava; a process like this probably led to collapse of these huge boulders into the Spokane River.
Figure 6. Well-worn boulder (~2 feet long) showing lamination that could be volcanic or sedimentary in origin. If volcanic, it looks like a welded tuff (hot ash); either way, there is no rock like that contained within the basalts making up this area.
Figure 7. A recently broken fragment (not by me) of a boulder shows a composition of alkali feldspar (white, high in sodium) with some quartz (pink and gray). I think it is syenite, an intrusive igneous rock dominated by Albite feldspar. Like the sample seen in Fig. 6, it isn’t associated with basalt.
Figure 8. This badly beaten boulder remained intact and more than 2 feet long after millions of years in a river bed. It is similar to the sample in Fig. 6 but lamination isn’t apparent and, besides, I just wanted to share such an amazing specimen with you. Note that it is lying next to similarly colored rocks that haven’t been polished to such a high sheen.

Summary. Today’s excursion brings two questions to mind: 1) What is the meaning of such an immense thickness (hundreds of feet) of basalt with such an unusual form? (I’m going to call it “oyster” lava.); 2) How did rocks that are nowhere to be seen in the area (refer to Fig. 3) end up in Riverside Park?

Basalt flows are known to be highly irregular in outcrop because lava flows in tendrils, sheets, molten chunks blown out of a fissure; however, these flows (and there must be thousands of them exposed in the cliffs along Spokane River) are eerily uniform and individual flows can’t be identified. This is unusual for relatively young volcanic rocks. The problem is exacerbated by the scarcity of soil profiles; there wasn’t time for water to react with the highly reactive minerals in basalt before another layer was deposited. I don’t have an answer.

The second question is easier to answer. During the last two million years, this region was covered by thick ice sheets that periodically melted and then expanded. Dams of ice formed huge lakes like Lake Missoula, the size of some states. There is overwhelming evidence for the catastrophic collapse of such an ice dam between 20 and 10 thousand years ago. The region surrounding Spokane contains many igneous, sedimentary, and metamorphic rocks dating from Precambrian (older than 500 my) to the age of these basalts (about 10 my).

The resistant cliffs surrounding the “bowl and pitcher” channeled such massive floods many times, beating very hard rock (e.g. Fig. 8) to a pulp as the boulders bounced along and hit other equally hard rocks.

I don’t like unanswered questions but that’s how it goes because the rocks keep secrets …

August 27, 2022  

The Rest of the Story: The Harz Mountains

This post is a continuation of previous posts on northeast and central Germany, but we won’t be seeing direct evidence for glaciers in the Harz Mountains (Fig. 1).

Figure 1. Aerial view of the Harz Mountains (from Wikipedia).

Today’s post discusses some of the rocks exposed in the valleys, gorges, and road cuts that dissect the Harz Mountains (Fig. 2).

Figure 2. The large circle indicates the Harz Mountains (inset map). Locations, A through D, are approximate sites of photos and rocks discussed in this post.

We approached the Harz uplands from the east (site A in Fig. 2), where we encountered mines based on removing sedimentary rocks for use as building material (Fig. 3).

Figure 3. (A) Road and building gravel mine from the eastern end of Harz Mountains (site A in Fig. 2). (B) Close-up of tailings pile, showing uniformity of the conglomerate being removed from the open pit.

The mines in Fig. 3 were removing desired beds from the Tanner Graywacke ( age ~360-320 Ma), a poorly mixed sedimentary rock (e.g. graywacke) originally deposited in ocean trenches associated with volcanic island arcs. The Tanner graywacke ranges from mudstone to conglomerate. It forms medium beds with variable texture, and has been tilted to varying degrees (Fig. 4).

Figure 4. Photos of Tanner Graywacke near site A (see Fig. 2 inset for location). The beds in (A) are tilted about 30 degrees (unknown direction) and the outlined layer is ~6 inches thick. Panel (B) shows two nearly horizontal beds exposed within a kilometer of (A). These beds contain detailed sedimentary structures, like cross-beds, as indicated by white outlines in the lower bed.

Our route east of the Harz Mountains (red line in the inset of Fig. 2) took us to site B, where we encountered more facies of the Tanner Graywacke (Fig. 5).

Figure 5. Tanner Graywacke exposures at Site B (see inset of Fig. 2 for location). The massive layers in (A) are ~4 feet high. A thick layer of conglomerate (panel B) doesn’t form cliffs. Note the irregular cobbles exposed by weathering. (Image B is about 6 feet high.)

Our path (red line in Fig. 2 inset) led us into a valley between the small highlands where Figs. 4 and 5 were taken. This valley is filled with a lake, and probably follows a fault zone between Site B and the main Harz Mountains to the north (Fig. 6).

Figure 6. View looking north towards the Harz Mountains. This area is underlain by marine evaporites, including salt that occurs in domes (age ~250 Ma). Extensive karst development in carbonates has led to serious subsidence problems in the area as sinkholes continue to develop.

Our journey followed the southern margin of the Harz Mountains (red line in Fig. 2 inset), taking us by Site C, where we found nearly horizontal beds of Tanner Graywacke exposed along road cuts. We couldn’t stop until we found a rest area, where a large block was available for close examination (Fig. 7).

Figure 7. Images of a block of Tanner Graywacke (A) exposed at site C (see Fig. 2 for location). The boulder was not in place nor was it a glacial erratic. It was put their during highway construction. (B) Close-up of the weathered surface, showing a bright-white square in the circle; this is a grain of Na-feldspar encased in a matrix of clay minerals (weathered to black in the photo). Note the large, pinkish form in the extreme upper-right of the image. This is a block of what was probably a granitic source rock. (C) Lower magnification image of the same rock; the circled area includes dark spherules against a white matrix, which I cannot identify. If you zoom in closer by opening the image, you will see that they are actually rectangular and have smooth edges. The white weathering product is probably from feldspars whereas the dark minerals (entire crystals) may be amphibole or pyroxenes (more resistant to weathering). It is important to recall that a graywacke collects near the source, and thus includes minerals in every size and shape, and every stage of weathering. (D) The circle highlights a cavity that was occupied by a large piece of rock (instead of a mineral grain), similar in shape to the phenocryst displayed in plate (B).

We continued around the western end of the Harz Mountains and found exposures of the marine deposits (including evaporites and carbonates) that underlay the town of Kelbra (Fig. 6), including a thick sequence of either salt or anhydrite (Fig. 8).

Figure 8. Images of the marine sequence (age ~250 Ma) that is much younger than the Tanner Graywacke (age ~360-320 Ma), taken at approximately Site D (see Fig. 2 inset for location). (A) Contact between an evaporite (white rock) and overlying sediments (tan), showing several anomalous features. For example, there is some suggestion that the darker beds have been folded (zoom in on the image); several irregular blebs of evaporite (e.g. to the right of image) appear to be isolated. This may be a salt diapir (or some other ductile rock) that forced its way into younger sediments, folding them as it intruded. A fault zone is not out of the question, considering that site D is located at the margin of the Harz upland (compare to Fig. 4A). Plates B and C show medium beds (<1 foot in thickness) of resistant siltstone surrounded by mudstone/calcarenite. These beds may be tilted but not at such extreme angles as suggested by plate A.

This post reveals rocks that are widely separated in time while being found near each other, supporting the Harz uplift as they do (Fig. 2). As the title of this post suggests, geology is not a series of isolated events. Let’s get the rest of the story.

The Tanner Graywacke was deposited in an island arc, a tectonic region in which oceanic crust is being subducted beneath either a continental (or less often an oceanic) tectonic plate, about 350 million years ago. What was happening on the opposite shore of this proto-Atlantic ocean (aka Iapetus)?

I have encountered rocks of similar age in northern Virginia and discussed them in previous posts. On the west side of Iapetus, during a mountain-building event called (in America) the Acadian Orogeny, a series of island arcs were being subducted/accreted to form a series of suspect terranes. This orogeny was only a phase of the collision of Laurentia (porto-north America) and Avalon (proto-Europe), which endured for most of the Paleozoic era.

The next problem is what happened during the ensuing 100 million years, between deposition of the Tanner Graywacke and the evaporites and carbonates we encountered west of the Harz uplands (Fig. 8)? The collision was completed and Pangaea had been born of two continents…

The mountains rose and they were eroded almost as quickly by wind, rain, and ice, creating massive layers of sediment to the east (modern Europe) and the west (e.g. the Catskill Delta in New York). By 230 million-years ago, the earth’s upper mantle changed its mind and tore the newly formed supercontinent apart, creating rift valleys like that of East Africa in what is now Virginia. Splitting a continent can take as long as building one, but this was a relatively rapid event in geological time; by 200 million-years ago, diabase dikes were injected into the sedimentary and metamorphic rocks created by the closing of the porto-Atlantic Ocean (Iapetus) and the split was well under way. Alluvial and fluvial sediments were collecting in isolated basins in what is now Virginia, and evaporites were settling to the bottom of lakes and brackish coastal waters in Europe, as the ocean invaded…

Jump ahead 200 million years…

Figure 9. View looking upstream in the Elbe estuary, less than a hundred miles from the German port of Hamburg.

I love it when I can understand what the rocks are telling me…

August 6, 2022  

Ball’s Bluff Battlefield Requiem

Figure 1. A Union artillery piece facing the Balls Bluff battlefield in its approximate position during the battle.

I reported on the geology of this area in a previous post, but I didn’t have much time to explore the area on that outing, so this trip I followed trails all the way around the park. This was the site of a battle early in the American Civil War, October, 1861. The cannon (Fig. 1) is a metaphor of how geology is always in front of us; it isn’t just about really old rocks, but also rivers and beaches, even gas and lava being belched out by volcanoes. That’s all geology too. For example, this battle took place in a field (Fig. 2) .

Figure 2. View from the Union artillery position. It wouldn’t have looked that different in 1861; instead of mowed grass, the field would have been filled with stubble from the recent harvest.

There isn’t much arable land along the Potomac River here because of the rocky soil, but there are a few pockets of land suitable for farming–flood plains left as reminders of the ancient river’s meandering, while it cut its way through rock, gravel, and mud to reach its current position (Fig. 3).

Figure 3. Google Map image of the study area. Our path took us from the end of Ball’s Bluff Road to the southern edge of the map along an inland route. We then followed the bluff (indicated by dark shading) to the north, following the river to the ravine that leads to the Veterans Park trailhead. We cut back to the south, following the gully NNW of the battlefield marker.

The bottoms of the gullies were paved with tilted layers of sandstone and siltstone (Fig. 4), sediment originally deposited in intermontane basins like those that occur in western North America (Fig. 5).

Figure 4. Photo at the bottom of the southernmost valley seen in Fig. 3. Layers of sandstone and siltstone form ledges like this, spaced very hundred yards or so, along the creeks that feed the Potomac river.
Figure 5. Image from the summit of Piestewa Peak in Phoenix, Arizona. The Ball’s Bluff Formation was originally deposited in a similar setting. Sand, silt and clay would have been washed down from local peaks that were probably composed of rocks like the schists comprising the Phoenix Mountains. (Think the Precambrian schists that outcrop along the Potomac River.)

I’d like to finish this post with a thought experiment: Imagine the sediments being carried away from the camera in Fig. 5, passing into the distance to collect in the wide valley that fronts the major fault-block mountain range, seen in the distance; now, imagine everything you see in Fig. 5 being worn down by water and wind and ice, until the sand and silt filling the lowlands in front of the camera is buried beneath the erosional product of Piestewa Peak; imagine that pile of sand and silt and clay being buried many miles beneath the surface, for millions of years.

Can you imagine the rocks seen in Fig. 4?

July 4, 2022  

What Goes Up…

I’ve been talking about mountain building events that continue for hundreds of millions of years a lot in my posts, referring to the erosion of mountains into mud, silt, and sand, carried by rivers to be deposited as broad expanses of sediment. On sufficiently long time scales, this is an accurate representation of the delicate balance between uplifting mountains and the inexorable influence of rain, ice, wind, and water to eradicate all evidence of an orogeny. For example, the collision of North America with Europe and Africa required nearly all of the Paleozoic Era, beginning with the Taconic Orogeny (550-440 Ma), reaching a crescendo during the Acadian Orogeny (375-325 Ma), and culminating in the Alleghanian Orogeny (325-260 Ma). By the way, the abbreviation Ma (mega annum) is used to indicate dates that were determined by radioactive dating, rather than the more ambiguous “my” for millions of years. There is uncertainty (error bars can never be zero), but not with respect to the general timing of geologic events.

Figure 1. Ridge of Precambrian schist, metamorphosed and transported along thrust faults during the Taconic orogeny; it was subsequently uplifted during the following approximately 400 my, and is now exposed to weathering. This photo was taken on the south side, looking northward.

This post is going to examine details of how uplifted rocks can be broken down into pieces that are weathered while being transported to their final resting place, whether in a river, lake, shallow bay, or the deep ocean.

Vermont (Fig. 2) was entirely covered by ice several times during the last couple million years.

Figure 2. Topographic map of northern Vermont (See inset for location.) Smuggler’s Notch is the local name for the area shown in Fig. 1, a region covered by as much as one mile of ice during the last glacial maximum. The peaks in Fig. 1 were covered by glaciers, which carved U-shaped valleys as well as creating cliffs on the south sides of topographic highs.

The rocks at Smuggler’s Notch are the same ones we saw in the Taconic Mountains and along the White River. They are equivalent to those we encountered along the Potomac River, 500 miles to the south. What happened when rocks formed as much as 20 miles beneath the surface are exposed to low pressure and temperature?

Figure 3. Photo of a different part of the ridge seen in Fig. 1, revealing large, overhanging blocks of schist. This rock is permeated with joints, created when overburden, and thus pressure, was reduced dramatically. Brittle fracture is the result, and all of those flat surfaces implicate the effects of ice and gravity on physical weathering.

There is a lot of missing rock from the cliff shown in Fig. 3. Where did it go?

Figure 4. Huge blocks fell to the narrow valley formed by glaciers during the last couple-hundred-thousand years, littering the base of the cliff with blocks as large as houses. The largest recorded is estimated to weight 6000 tons. These blocks are piled up like dominoes, forming caves that have been used for millennia by wildlife and people as refuges. (Climber for scale.)

How did these huge blocks get where we find them today?

Figure 5. Photo of disrupted forest where a block rolled down the slope at the base of the cliff seen in Fig. 3, before coming to rest. Note the young trees and gravel slope. Falling hundreds of feet is the second stage of breaking down the mountain. The first is dislodging the block along joints, which allow water to weaken the rock by changing the chemical composition of the minerals.

Mechanical weathering doesn’t stop when the fallen block comes to rest. Then, water carries small grains and uses them as abrasives to grind the once-humongous blocks into gravel (Fig. 6).

Figure 6. Rock debris collected along a nascent stream, filled with smaller blocks of schist that originally fell from the cliffs that tower above the narrow valley. Gravel fills every nook and cranny in the jumble of rock, grinding away whenever water power is sufficient to mobilize the harder and more resistant minerals (e.g. quartz and feldspar).

All that bumping and grinding eventually produces a scene like that seen along the path of the White River (Fig. 7), with bedrock resisting the seasonal onslaught of gravel and sand carried by intermittent, torrential flows.

Figure 7. View of White River (see Fig. 2 for location), showing the eventual outcome for peaks like those seen in Fig. 1. The bedrock blocking the channel will be eroded in its turn as it is exposed to surface weathering, by isostatic uplift.

I hope this post connects the dots between the loftiest peaks (Fig. 1) and the lowest streams (Fig. 7).

January 12, 2021  

Queenstown to the Continental Divide: Proterozoic Sedimentary Rocks

The second day of our field excursion covered quite a bit of the geological history of Tasmania, so we’re going to continue discussing this very long day (from Cradle Mountain to Hobart) in this post. To summarize, we saw exposures of Proterozoic (1600-540 MY) and Cambrian (509-485 MY) sedimentary and volcanic rocks between the coast and Cradle Mountain (see Fig. 1 for final location). These were metamorphosed and indicative of hydrothermal activity in the region, as discussed in a previous post.

 Fig. 1

The last post discussed remineralization and granitic intrusion into these rocks, especially Paleozoic rocks, culminating in the extensive mining activity centered on Queenstown, the “top of the world” so to speak, because these are some of the highest elevations in Tasmania.

Today’s post is going to take us from Queenstown to Tasmania’s official continental divide. Most of the included photos were taken about 30 miles east of Queenstown, where the pin in Fig. 1 is located. We will be examining rocks primarily from the Tyennan Group, but not as strongly deformed and metamorphosed. We are moving east of the Paleozoic trough where most ore bodies were emplaced. The sediments consist of fine-grained (pelitic) schist and quartzite (sand-sized particles), and some conglomerate deposited between 1600 and 541 MY ago.

The road cuts exposed rocks that are tilted but relatively undeformed (Fig. 2), such as this sequence of fine-grained sediments, with thin sandstone layers interbedded.

 Fig. 2

Examined up close, the sandy layers have lost their original bedding but have not developed the strong visible layering (foliation) commonly associated with what are called schists (Fig. 3).

 Fig. 3

These sediments varied substantially, as seen in Fig. 4, which shows more sandy layers and a reduced volume of fine-grained sediments.

 Fig. 4

The area in Fig. 1 contains many faults associated with volcanism and intrusion during the late Proterozoic and early Paleozoic (~1600-500 MY). In some places the rock layers of this area are vertical (Fig. 5).

Fig. 5

  A  B

Figure 5B has been annotated to better show some features of deformation without strong remineralization. The irregular lines showing deformed bedding are more-or-less original variations in particle size and/composition (i.e., sedimentary layering) that has been squeezed and had the grain size of crystals increase in response to heat and pressure. The joint pattern has nothing to do with this but came later, as the rocks cracked from cooling and reduced pressure (similar to mud cracks).

A closer look reveals how far this process can go without leading to remineralization and the replacement of original minerals by new ones (e.g., pyritization or chloritization) as new elements are introduced by hydrothermal circulation.

 Fig. 6

The lighter areas in Fig. 6 are probably quartz recrystallized from sand grains whereas the darker zones are very likely quartz and muscovite that result when water is removed from clays. The heat and pressure weren’t sufficient to form new mineral crystals with larger size, however, so the mixed mud-sand assemblage remains identifiable.

These rocks were folded during compression, when some of the faults certainly occurred. We saw an outstanding example in a road cut (Fig. 7).

Fig. 7

 A B

Figure 7B has original bedding highlighted. This shows a tight fold on its side (recumbent) and juxtaposed against vertical bedding. There are certainly some faults present between these layers. The rocks were brittle enough (i.e. shallow burial) to break and slide against one another. Zooming in closer on the “C” in Fig. 7C, we see that there was no remineralization in these tightly folded rocks (Fig. 8).

 Fig. 8

But if we look at the more outward layers surrounding this structure, we see signs of substantial brittle fracture (Fig. 9) and remineralization. The former is shown by the small size of (much less than 1 foot) of individual blocks of stone (Fig. 9A) and the latter by the weathered appearance and lack of structure in some areas (Fig. 9B).

Fig. 9

 A  B

Soon after this we left the central mining district and the rocks deposited and deformed during the collision of Tasmania with Gondwana (~500-370 MY). In the case of Tasmania, the Continental Divide is between the rainy western half and the dry eastern half. Another way to look at it is that the western half was created when Gondwana was formed and the eastern half when it was pulled apart.

And finally, the King William Range, comprising peaks of fault blocks pointing to the east and a different geologic regime…

 

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