In previous posts from northern Virginia I have discussed Precambrian orogenies, the rifting of Gondwana to form the Atlantic Ocean, erosion of the deep-seated roots of mountains, and the deposition of sediments along the modern Potomac River. With so much of earth’s history displayed here in my new home, I always keep my eyes open for the next geological adventure. I wasn’t disappointed when I walked along the banks of the famous Tidal Basin in Washington D.C. (Fig. 1), surrounded by monuments to Jefferson, Franklin Roosevelt, Martin Luther King Jr., Lincoln, George Washington…
The Tidal Basin itself (Fig. 2) is a monument, cherished by D.C.’s residents, because the cherry trees that line its banks, which were in bloom during my visit, were originally given to the United States by Japan in 1912. However, it has another symbolic meaning, as a reminder that the earth doesn’t stand still, not even long enough for our civilizations to flourish and decline naturally.
The western side of the Potomac River (Fig. 3) comprises rolling hills and many creeks draining the deeply eroded topography. Capitol Hill (see Fig. 2 for location) is also high ground, but between these two topographic highs, the river’s channel has filled with fine-grained sediment (i.e. mud).
The northern end of the Tidal Basin (see Fig. 2 for location) is high and dry (Fig. 4).
The southern end of the Tidal Basin is subsiding. This isn’t surprising, but the rate is alarming. For example, a back-of-the-envelope calculation, comparing Figs. 4 and 6, suggests subsidence of about three feet in less than a hundred years (since construction of the seawall). That works out to a burial rate on the order of 3/8 inches per year. That is substantial and explains why the National Park Service hasn’t kept up with the problem.
There is nothing surprising about the settling of muddy sediments at the apparent rate I observed in the Tidal Basin. The only confusing aspect of this predictable problem is why anyone would have thought it was a good idea to build massive stone edifices (e.g. Jefferson and FDR memorials) on muddy soil only a few feet above the high-tide line.
How long until Washington Monument becomes the leaning tower of America?
This post is going to talk about fluvial processes during the last few millennia, with the Potomac River as an example. A previous post discussed the geology of the Potomac’s fall line, where it drops out of the foothills of the ancestral Appalachian Mountains to the coastal plain before entering Chesapeake Bay. I’m going to keep this simple because, to be honest, fluvial geomorphology is not a straightforward topic. Rivers are constantly changing at time scales from years to millions of years. We won’t be walking back billions of years today, only a few hundred thousand, maybe a couple of million.
The lower Potomac River is braided, with multiple channels defining wooded islands (e.g. Van Deventer Island in Fig. 1). I won’t be talking about them but instead focus on what I saw, what the rocks (river sediment is unlithified rock to a geologist) tell me. The river flood plain extends to the Pleistocene terrace (yellow line in Fig. 1), which is about 80 feet higher in elevation than the river surface. No permanent structures have been constructed on the flood plain.
Some of the features we will examine are shown schematically in Figure 2. Note however, that the image shows a meandering stream whereas the Potomac is braided, which means that its channel doesn’t take those big loops shown in Fig. 2. That’s because the lower Potomac drops rapidly from Great Falls just upstream of the study area, to Washington D.C. in this area.
We started out on the area labeled “Bluffs” in Fig. 2 and traversed the flood plain, following a tributary called Horsepen Run (see Fig. 1 for location). Note that Horsepen Run is a meandering stream, so we’ll see several features that scale downward from Fig. 2 as we cross the Potomac flood plain.
Horsepen Run (aka creek) drops quickly from the Pleistocene terrace (Fig. 1) but then crosses the Potomac flood plain and begins to meander. The photo in Fig. 3 is from a location just before this change in stream topography occurred.
The changes in stream morphology seen between Figs. 3 and 4 occur in larger streams (like the Potomac) but on much longer spatial scales.
Figure 1 indicates the presence of natural levees (lower center of Fig. 1) near the main river channel. There is no “Yazoo Tributary” (see Fig. 2) at this location, so Horsepen Run cut across the Potomac’s natural levee. This can be seen beautifully in Fig. 8.
It was a beautiful February day to hike to across the Potomac River flood plain. I hadn’t expected to find so much dynamic geology so close to my new home, but there it was. The historic Potomac River transitions from its rocky confluence with the Shenandoah River at Harper’s Ferry, to the tidal river that defines Washington D.C., right here and, like America, it is not in equilibrium. The cut banks of the Potomac and its tributary, Horsepen Run, portend of rapid changes in the relative elevation of the land and the sea.
We are in for a wild ride…
My last post examined some structures and petrology of a Metamorphic Core Complex (MCC), whereas the previous one discussed the geology of the Central Highlands of Arizona. I mentioned several times that these geologic provinces were but two manifestations of the profound tectonic change associated with uplift of the the Colorado Plateau.
Today’s post is from the Phoenix Mountains, a municipal park inside the city limits (Fig. 2). My geological interpretations and dates, etc, come from a report by the Arizona State Geological Survey.
To the west (left of Squaw Peak in Fig. 3), a deep fault has been identified, which thrust the metasedimentary rocks comprising the eastern part of the range into juxtaposition with the metavolcanic rocks of Stoney Mtn and other outcrops to the west (Fig. 4).
Today’s post is focused on the circled area in Fig. 4 which, if we look back to Fig. 3, is the highest peak within the Phoenix Mountains. I was intrigued by the view from the parking lot (Fig. 1), and compelled to explore this fault-block in person. Note that the area discussed in this post in contained within the circle in Fig. 4.
The geology of Piestewa Peak is relatively simple. Schist. In this case, the metamorphic grade isn’t too high and the rocks preserve much of their original thin-bedded layering. However, they are standing on end (Fig. 5).
There isn’t much to say about this post after my road trip to Prescott, and then a hike in the White Tank mountains. The first thing I can say with confidence, however, is that Squaw Peak (aka Piestewa Peak) is squarely located within the Basin and Range, defined by faults that have brought disparate rocks into juxtaposition, but only in a small area. The Phoenix Mountains are nothing like the vast, overlapping fault-bounded mountains of the Central Highlands, but instead they are isolated in a sea of sand and gravel, sediment eroded from the long-gone rocks that encased them for eons. There was no superimposed shear evident in these rocks as in the White Tank mountains. They just rose from the earth’s bowels along nearly vertical faults.
These rocks aren’t as old as those we encountered in the Central Highlands — by about a billion years. Nevertheless, they suggest that plate tectonics determined the history of central Arizona, even so long ago. Because of the lack of suitable rocks, no plate reconstruction can be attempted for the Precambrian (neither a geologic period, era, or eon); thus, we can only assume that things were the same but different — upper mantle processes dragging crustal plates around, but without plants, oxygen, or animals to intervene in surface erosion.
We don’t know what happened that long ago, not to mention the billion years between the creation of these sedimentary/metamorphic rocks and the emergence of multicellular life. We can only view the rocks we’ve seen in Arizona through a glass darkly…
My last post set the stage for this report, but this time I did a lot of walking to get the facts. A short drive took me to White Tank Mountain Regional Park, about 30 miles west of Phoenix. I studied geology at Arizona State University in Tempe…it must have been 40 years ago, and Metamorphic Core Complexes (MCC) were a big thing then. Let’s start with a map (actually several maps that focus ever closer on the field area).
Now for a view from the ground.
The White Tank MCC is located within the Basin and Range Province. In the middle of a low-lying flat desert, MCCs appeared within the last 60 my, in close proximity to a region defined by faulting and the uplift of Precambrian rocks on a huge scale. Time to look at some rocks.
Figure 3 reveals a complex pattern of deformation and magmatism. The most striking feature of this outcrop is the brilliant white veins that cut across the dark rocks, and folded in a crazy pattern on the right side. What is going on here?
The background is that the dark rocks are Precambrian metamorphic rocks and granites. As I discussed before, these rocks were deformed several times during the billion years spanning the resetting of their radiometric clocks and the tectonics associated with the uplift of the Colorado Plateau. The lighter-colored rocks are of Cretaceous age, injected as veins into preexisting weak fracture zones.
Figure 10 reveals some of the field textures seen along the trail, starting at the uphill end, where medium and small blocks litter the landscape as the granite weathers in place (Fig. 11). Panel A (Fig. 10) shows a sharp contact between the main granite and a whiter material with microcrystalline structure similar to what we saw at Site 1 (Fig. 3). As the magma was intruded, the melt was fractionating into a component with a lower melting temperature and so it filled fractures, which indicates there was tectonic movement at that time. Panel B is evidence of syntectonic intrusion because it shows a lineation that was present in many of the rocks. Panel C is a close up (5x magnification) showing larger crystals in a finer matrix. All of it is feldspar (dominated by Na also known as albite). Panel D is a good exposure of the relationship between the finer grained material that forms veins in the main rock. Panel E shows a salt-and-pepper texture that dominates the rocks along the trail.
A close up of a fresh surface near Sample D (see Fig. 10 for location) shows simple mineralogy of the main rock body (Fig. 12). I would estimate 70-80% albite, >15% quartz, and minor biotite (a platy mineral, a variety of mica).
The mineralogical composition can be used to classify this granite as tonalite. Tonalite is a granite that contains no potassium feldspar (no pink color), very little quartz, and mostly feldspar containing sodium and calcium. These rocks originate deep in the earth where ocean crust (basalt) melts and rises, losing much of the iron and magnesium it originally contained. The tectonic setting is an island arc, like Japan.
Just as in the Central Highlands, sediments were deposited here in ocean basins as long ago as 2.8 by and subsequently buried and deformed, changing through heat and pressure into gniess, and injected with veins of quartz and feldspar during metamorphism. Several orogenic events followed, deforming the assemblage further. It was exhumed slowly over the ensuing billion years, and eventually injected with a tonalite granite created deep beneath an island arc. This was the time when the White Tank granite was emplaced. This magma was intruded when the region was undergoing extensional stress (pulling apart) as part of the adjustment to complex tectonic process associated with uplift of the Colorado Plateau.
Rather than breaking into irregular blocks as in the Central Highlands, the White Tank mountains (and other MCCs in the western Cordillera), were formed by the uplift of Precambrian basement along deep sub-horizontal surfaces called detachment faults. In other words, the crust stretched in the Basin and Range rather than breaking into fragments.
I haven’t attempted to describe the complex tectonics of central Arizona, a task that is well beyond my experience. This is a topic that is hotly debated in the geological community to this day. This has only been a brief effort to relate the rocks I saw with my own eyes to what is known about the history of the earth.
Always listen to the rocks…
To keep this post from going off the rails and becoming a treatise on metamorphic textures and their relationship to regional tectonic trends, I’m going to address the bullet list I ended the last post with. I hoped to find the following rocks, from youngest to oldest:
- Quaternary gravel
- Quaternary and Tertiary lava flows
- Tertiary stream deposits of sand, silt and gravel with rounded pebbles
- Tertiary lake sediments of horizontal, whitish, fine-grained rock with layers of volcanic ash
- Precambrian granite
- Precambrian gneiss and schist
The descriptions in The Roadside Geology of Arizona include unique features, from outcrop to hand sample scales. By the way, the “Roadside Geology” series books (available for many U.S. states) include introductory sections for the layman, and a level of detail that will increase anyone’s appreciation of the natural world, whether driving cross-country or on a daily excursion.
I’m going to dispense with my usual lengthy introduction. For the first figure, which becomes the de facto icon for the post, I’m going to skip maps and cut to the chase. I was driving north from Phoenix on Interstate 17, scoping for interesting exposures, checking the mileposts, and trying to remember several outstanding stops from the previous night’s reading. Frustrated at having passed the exit for Crown King, the ramp and interchange (beneath the I-17 roadbed) magnificently adorned by a long road cut that displayed interlayered dark and lighter colored rocks (Fig. 1), berating myself for not taking the path less traveled, I searched for the on-ramp and, when it came into view, I hit the brakes hard and cut onto the last fragment of the slip road. Traffic on the ramp was nonexistent and, with no unmarked Arizona Highway Patrol vehicles within sight, I backed several hundred yards while avoiding the drainage ditch and concrete barricade.
All I’m going to say about Fig. 1 right now is that this was the exposure that prompted me to drive — shall I say “recklessly?” But it is only the tip of the iceberg in the Central Highlands.
This post isn’t a typical report. Instead of a trip log, I’m going to show examples of the six rock-types from my list. I took a lot of photos but I will try (really hard) to limit the interpretation to a concluding paragraph.
The block faulted mountain ranges of the Basin and Range province, accompanied by Metamorphic Core Complexes like South Mountain or the White Tank Mountains, bounding Phoenix, Arizona, on the south and west, respectively, suggest extensional forces at play. The Colorado Plateau, on the other hand, reveals no significant evidence of either crustal extension or shortening, instead comprising relatively undeformed Paleozoic sedimentary rocks that appear to have been exhumed vertically. The earth’s crust had to accommodate not only 6000 feet of differential uplift, but a change in tectonic regime, within the distance from Phoenix to Prescott in the last 80 million years, creating the Central Highlands.
The Quaternary Period spans the last 2.58 my. It is identified with the most recent occurrence of extensive continental ice sheets.
Dating sediments deposited during the Quaternary is extremely difficult because of a scarcity of suitable material for radiometric dating of the time of deposition. Thus, stratigraphic relationships are used, along with sedimentary textures, to determine the relative age of sedimentary rocks. For example, Lithology (1), Quaternary gravel, contains very little organic debris for carbon-14 dating; Lithology (3), on the other hand, may occasionally contain some plant fragments in the finer-grained components; but carbon-14 dating can’t be used for rocks older than 500 ky (thousand years).
Gravel was present everywhere, filling topographic lows and even forming cliffs. Often, these sediments were found in thick intercalated beds of silt and gravel, but sometimes cross-cutting relationships reveal erosion of older sediments before infilling (Fig. 2).
The darker gravel in Fig 2 fills a channel in the finer, buff-colored silt. The difference in lithology, and presence of an erosion surface, suggest that the upper unit is representative of the Quaternary gravels of Lithology (1). The underlying rock is a siltstone representing the Tertiary (66-2.58 my) stream deposits of Lithology (3).
Quaternary and Tertiary Lava Flows
Several volcanic complexes were active during the last 66 my in the Central Highlands. The lava sometimes flowed from fissures and often from central cones. Their remnants can be seen in the arid landscape.
The lava flows are not very thick and individual flows can be identified. I’ll say more about the lava in the following discussions because of its relationship with the other lithologies.
Tertiary Stream Deposits
Figure 2 shows a silty bed that I have attributed to older Tertiary sediments, which could have been deposited over a 60 my time span. However, specific thickness of stream deposits can be dated by their relationship to the rocks of Lithology (2), because volcanic rocks can be easily dated. When such data are unavailable (as in this situation), we can’t be more specific.
An interesting thing about Fig. 4 is that ancient stream sediments are eroding to form new stream sediments in an endless cycle of deposition, erosion, etcetera.
Tertiary Lake Sediments
Lithology (4) was broadly distributed near Black Canyon City but it wasn’t safe to stop and examine them. Figure 6 reveals some evidence of soft-sediment deformation in the underlying chalky sediment. The implication that the lava flowed into a lake is further supported by pillow structures (Fig. 7).
The oldest rocks I’ve discussed so far were less than 70 my, but now we’re going to jump back to between 2.5 and 1.5 billion-years ago. There are no rocks from the Paleozoic or Mesozoic Eras in this part of the Central Highlands.
Granite outcrops weren’t ubiquitous along the highway until Prescott, although they are everywhere within the area — just not next to Interstate 17. I had an opportunity to see them up close on the return, which took me through Peeples Valley to Congress (Fig. 8).
I drove through overlapping plutons, with variations in the granite composition indicated by differences in their color, which varied from orange (orthoclase feldspar) to light gray (plagioclase feldspar). Most were weathered to a reddish color by the oxidation of iron-containing minerals, and they were all rounded as seen in Fig. 9. The one-billion year duration of magmatism suggests that this was a time of intense continent building.
Precambrian Metamorphic Rocks
Arizona (as well as most of the earth’s surface) is underlain by a crumbled, jumbled, layer of metamorphic rocks — reflections of long-forgotten oceans that teemed with life, collecting fine particles eroded from the slowly emerging continents, as well as the bodies of earth’s first inhabitants. In other words, for metamorphic rocks to have formed about two-billion years ago, sedimentary particles must have collected in ocean basins a couple hundred million years earlier. The earth is only 3.8 billion-years old, so there could have been only so many cycles of metamorphism in any given location. I’m guessing…maybe two or three?
The title of this post refers to Precambrian rocks being torn apart during the separation of the Colorado Plateau from the lowlands represented by the Basin and Range province. Figure 12 exemplifies this dynamic process. To the north (left side of Fig. 12), it is difficult to ignore the nearly vertical lineation of rocks that are more like schist, with a sheen caused by the alignment of muscovite minerals. The tan color suggests that the original sediment contained a lot of quartz. The rocks exposed on the south (right in Fig. 12) end of the road cut are dark in color and include lenses of quartz (white flecks beneath the yellow arrow), suggesting a very different post-depositional history. They were probably mudstones before alteration.
The dark rocks (below the white arrow…) that conjoin these discordant blocks exhibited fine-scale jointing (scales of inches) and conchoidal fractures, an indicator of microcrystalline structure. I haven’t shown the photographic evidence of my description, to shorten the report. (I really wanted to.) These metamorphic textures indicate stress regimes with different orientations. Figuring it out would require substantial, detailed field work to measure stress indicators, and geochemical analyses. Not being in a position to do any of that, and unaware of any reports on this area, I’m going to treat this as a fault that cuts the plane of the road-cut at an angle similar to the steep dip of the lighter beds on the left of Fig. 12.
It is time to address the geological questions raised in Fig. 1 which, it turns out was a cause of confusion in deciphering the geological history of the Central Highlands.
The black, metamorphic rock in Fig. 14 gave the name Black Canyon to the region because it outcrops throughout the area, underlying both volcanic and sedimentary rocks.
Almost three-billion years ago, the western half of what is today North America was an ocean margin, possibly like the East Coast. Mud and sand was accumulating in environments much as we find today except there were no plants so erosion was probably more intense. By about 2.5 Ga (billion years ago) these sediments were buried to depths as great as 35 km (~20 miles) and were compressed as tectonic plates collided. This process continued, no doubt in pulses, for another billion years, producing the complex textures we see in Figs. 12-15. This area was on the trailing edge of proto-North America during the formation of the supercontinent Gondwana, between ~500 and 200 Ma. When the modern Atlantic Ocean began to open, subduction of oceanic crust began and new continental crust was created, forming modern California.
Streams crisscrossed the area much as we see today, but in a different climate. The volcanism associated with subduction of the Pacific Ocean crust didn’t reach Arizona until ~30 Ma, when volcanoes spewed out numerous lava flows such as those that cap the mesas seen in Fig. 3, and the Colorado Plateau began to rise by thousands of feet. This is when the Central Highlands (aka Transition Zone) formed, a buffer between the block-faulted Basin and Range of central Arizona and much of the western deserts of Nevada, and the uniform and mostly undisturbed Paleozoic sedimentary rocks of the Colorado Plateau.
I haven’t posted much about the geology of Arizona, where I grew up and wandered the back ways while studying Geology at Arizona State University because, unfortunately, most of these excursions occurred before the internet and are nothing more than dim memories, supported by fuzzy photographs enclosed in aging plastic as part of my photo album collection. Many of those albums were damaged by water during storage. Recent trips to the Grand Canyon State were in a motor home, which isn’t conducive to unplanned photo stops on busy highways. The good news is that I am back for a few days and ready to explore one of the state’s most interesting provinces, the Central Highlands.
Figure 1 reveals how Arizona can be divided based on the most-recent geological event in its history, the problematic uplift of the Colorado Plateau during the last 80 million years by approximately 2.5 km (about 8000 feet). Today, Flagstaff is located at an elevation of 7000 feet while Phoenix, located in the Basin and Range province sits at a mere 1000 feet above sea level. The Transition Zone (Fig. 1) reflects how the earth’s crust adjusted to this 6000 foot difference in elevation. (Note that I am using the older terminology — Central Highlands — mostly as a nod to this area being identified as a distinct geographic province long before its tectonic underpinning was understood.) It’s also a nod to the motivation for this blog, in this case The Roadside Geology of Arizona, by Halka Chronic, published in 1983 by Mountain Press Publishing.
This post is an introduction to the Central Highlands. I will be driving (in a car and not a motor home) from Phoenix, in the Basin and Range province (see Fig. 2) on Interstate 17 to the middle of the volcanic complex shown in a burnt orange in Fig. 1, and then follow AZ-69 NW to Prescott, before returning to Phoenix on AZ-89 by a more westerly route.
The dramatic change in elevation from Phoenix to Flagstaff suggests that the earth’s crust was deformed substantially when the Colorado Plateau was uplifted (to the right in Fig. 3), much of the difference accommodated by faulting.
Blocks of crust the size of mountains slide up and down along the endless number of faults that characterize the Central Highlands, which is why its geological name is the Transition Zone (see Fig. 1), from low elevation to high. Most of the rocks we will see are Precambrian gneisses and granites. In other words, sedimentary rocks originally deposited far more than 500 Ma in the past, buried and subsequently altered by extreme heat and pressure more than a billion years ago, when they were penetrated and partially melted by molten material from deep within the earth (at least ten miles below the surface). Much later (within the last 100 million years) magma filled the faults crisscrossing the Central Highlands and flowed onto the ancient surface to form volcanoes and lava flows.
I will cross this Transition Zone at two locations, separated by about 40 miles, and attempt to identify variations in the rocks caused by a number of geological factors. That is the (hypothetical) purpose of this road trip.
There are six kinds of rocks and sediments we will be looking for on this trip, and we expect to find them in wildly confusing juxtapositions because of so many faults and the immense spans of time represented by the rocks, not to mention taking different routes on our journey.
- Quaternary gravel
- Quaternary and Tertiary lava flows
- Tertiary stream deposits of sand, silt and gravel with rounded pebbles
- Tertiary lake sediments of horizontal, whitish, fine-grained rock with layers of volcanic ash
- Precambrian granite
- Precambrian gneiss and schist
I will try to show examples of these with photographs but…well, roadside geology is a lot more dangerous than it used to be, with so many vehicles on the roads, and everyone in such a hurry…not to mention reading my blog on their cellphones (I wish).
We were spending Thanksgiving week in NYC, visiting our daughter, and went to see a unique collection of European artifacts from the Middle Ages (~1100 to 1500 CE) in an annex operated by the Metropolitan Museum of Art. They call the castle built in 1935 (this was common during that era) The Cloister. To our surprise and delight, this modern fortress was constructed on a ridge of metamorphic rock call Washington Heights, standing out high above the Hudson River.
Thick sequences of eugeosyncline sediments were deposited as the Iapetus Ocean was closing about 600 Ma (million years ago), weathered from rapidly eroding volcanoes (e.g. Japan today). Between 541 and 459 Ma, these sediments were buried deeply enough to be heated and semi-melted as the island arc collided with the mainland during the Taconic Orogeny. Figure 2 shows a schematic of the result of this process, which created the Manhattan Formation, the topic of today’s post.
It was a cold November morning when we hiked over the Manhattan Formation (red layer outlined by the rectangle in Fig. 2. The castle is right under the arrow for Washington Heights. When metamorphosed at several miles depth (a guess) under extreme pressure and heat, these immature sediments formed schist containing large garnet phenocrysts.
The overall dry gray schist has irregular layers of quartz showing rotation of inclusions (Fig. 4).
Hexagonal phenocrysts occur in widely separated areas (Fig. 5).
The garnet crystals (Figs. 4 and 5) were very large and well formed. The brownish surface indicates they are rich in iron, which is literally rusting (oxidizing).
The Manhattan Formation is a hard rock even though it has a preferred plane of inherent weakness as to all schists. For example the plane of weakness in this outcrop (Fig. 3) is approximately vertical, being aligned with original bedding as suggested in Fig. 2 (red layer).
As we recall from previous posts, the closing of the Iapetus Ocean occurred in pulses that began with the Taconic Orogeny and the deformation of the Manhattan Formation. A series of island arcs were thrust onto photo-North America until photo-Europe collided in the Alleghanian Orogeny about 250 Ma.
You don’t have to go far to find some rocks here in Fairfax County, Northern Virginia, and the rocks tell a fascinating story of earth-shaking scale; continents colliding and splitting asunder, right beneath our feet. Today’s post goes straight to the middle of it all, not far from our apartment.
It was a few degrees above freezing, but that doesn’t matter to the rocks or to me and my stalwart field companion. Figure 2 shows the field area and our approximate path along Sugarland Run, a meandering creek that passed through a narrow, rocky channel before entering a wide, boggy area as it flowed north (towards the Potomac I presume).
Towards the southern end of our trail (dashed red line in Fig. 2), we encountered very-large boulders adjacent to the creek (Fig. 3), which had a bed composed of angular gray casts showing little evidence of transport (Fig. 4).
We didn’t find any large boulders (some were as large as automobiles) that were definitively in place (i.e., visibly attached to subjacent basement rock), but these huge stones weren’t moved far at least not all of them. In several places, they were obviously pushed aside by bulldozers to make way for apartment buildings. Nevertheless, there is no obvious reason for some of them to have been displaced (e.g., Fig. 3), so it is reasonable to assume that the boulder fields represent the outer layer of unexposed rocks supporting the area, fractured and eroded but too resistant to have been weathered since exhumation, probably during the last ice age.
The drainage has been disrupted by construction, resulting in previous stream beds that have been cut off.
In keeping with the Rocks and (no) Roads principle of studying the earth as we see it, I couldn’t find any freshly broken rocks, but we can say for certain that this is a relatively uniform exposure of homogeneous, fine-grained, gray rocks with no obvious phenocrysts.
A couple of interesting characteristics of this rock are visible in Fig. 6: first, it weathers to a reddish color from an original gray tone (compare to Fig. 5, which shows boulders not exposed to constant water); secondly, the texture is fairly uniform. This last observation requires clarification. The hand sample in Fig. 6 was covered with whitish biological material of unknown composition, but if you stare at Fig. 6 long enough (or enlarge the image) you will notice that it does not consist of identifiable “grains” of sand. As I learned from my travels around Australia, when you can’t see sedimentary particles, and there is no apparent structure, you are looking at an igneous rock of some kind.
The lighter, bluish gray area in Fig. 2 indicates the extent of a rather special but widely distributed rock along the east coast of North America. According to several field reports, summarized in the Rock-D app, this is a high-titanium quartz-normative diabase. A traditional classification places it in the tholeiitic magma series. It has been reliably dated to have been emplaced between 200 and 174 Ma. Yet another unique rock type found within the Newark Supergroup.
In a previous post, we examined some of the sedimentary rocks deposited in subsiding basins during this time period. Meanwhile deep beneath the surface, magma squeezed into fractures in the weakened crust as Pangea began to split apart. Another piece of the puzzle falls into place. A single continent containing all of the earth’s land (look at a globe to imagine what that must have been like), was torn apart by slowly convecting rocks within the mantle (like a pan of water boiling), heated by the primordial molten iron of the earth’s core.
To help you visualize it, here’s a plate construction from just before all hell broke loose (geologically speaking) created by Fama Clamosa (I couldn’t find a solid reference but this is about what I’ve seen elsewhere).
Jump ahead 200 million years.
That completes our journey back in time. One last point I’d like to make, and explain the title of this post, is that Northern Virginia was ground zero (see Fig. 7) and every time I go for a walk or short ride, I’m reliving more than a billion years of the earth’s history. It is a humbling experience.
One last thought: our planet transformed from the Paleozoic Era (Fig. 7) to the modern world in 200 million years; at a spreading rate of about 1 inch per year (total widening of the basin) and a little math (200 million inches equals ~3200 miles), I see that Herndon, Virginia is about 1700 miles from the mid-Atlantic ridge, the point where it all began in the early Jurassic Period.
I’m feeling dizzy…
I’ve spoken before about the Iapetus Ocean that separated North America from Europe. It is more than conjecture because there are pieces of the sediments and volcanic rocks underlying it to be found along the east coast of N. America. These ophiolites are proof of the existence of ocean crust and upper mantle pushed onto what later became Maryland (albeit at great depths beneath the surface) during the closing of an ocean basin between approximately 500 and 300 Ma.
The green area in the inset map of Fig. 1 has been classified as the Sykesville Formation. These are sedimentary rocks, formed from a melange that collected behind an island arc. They contain a crazy mixture of sediments eroded from volcanic islands and exhumed rocks when these islands were smeared onto a continent at geological time scales (e.g. Japan and the Philippines today). This is a very slow process. However, this entire episode was nothing more than the prelude to closing of the Iapetus Ocean when two continental land masses collided, neither of which could be subducted beneath the other. Thus, these marine sediments were buried and subjected to incredible pressure and heat, producing the metasedimentary rocks of the Sykesville Formation.
The original sedimentary texture (layers of sediments deposited horizontally) has been overprinted by foliation, a metamorphic texture caused by compression deep beneath the surface, usually at high temperature. The nearly vertical, yellow line in Fig. 2 indicates this metamorphic foliation, which is probably close to the orientation of original bedding. The circled areas show striations on a foliation surface (lower right) and perpendicular (upper left).
After being metamorphosed at great depth these rocks were folded and faulted, so that nearly horizontal bedding and foliation were tilted to nearly vertical. Figure 2 is looking towards the NW (field estimate) and is consistent with similar deformed rocks observed at Great Falls. In other words, these metasedimentary rocks have been folded and faulted (there are several faults within the area) when two continental land masses collided.
A closer look reveals a hint at what occurred more than 300 million years ago.
The highlighter in Fig. 3 indicates a bleb of quartz (the gray mineral) with a distinctive pinching (note the lower and upper parts of the circled area). Although this does indicate very high temperatures and pressure, the rock has not melted and there is no indication of veining or other evidence of contact metamorphism.
Textures like those seen in Figs. 3 and 4 are associated with depositional/tectonic environments where clumps of rock are either being eroded by surface processes (e.g. rain and erosion) or plucked from a thick layer of overlying rock as it slides over a given sequence (in this case the Sykesville Formation) when it is buried deeply enough for the rock to be ductile. This poorly understood process produces allochthons like ophiolites. The textures of these rocks have been interpreted (using radiometric and textural data) as resulting from the latter process.
Take a long look at Fig. 4 and imagine sediments deposited in the Sea of Japan being jammed into Korea, buried deeply and sliding over each other for millions of years.
The earth is relentless…
We didn’t go very far from home today, just down the road to Frying Pan Park; the field area was the hill and a creek near a Baptist Church constructed in 1792 and still in original condition.
A small cemetery, with one gravestone as recent as 1938, marks the entrance to a series of trails wandering over the hilltop where Confederate soldiers bivouacked less than 20 miles from Washington DC. There were a few skirmishes but no battles.
The geologic map of the area identifies the rocks exposed on hilltops and along the creek (indicated in the center of Fig. 2) as being part of the Newark Supergroup. These rocks were originally deposited between 237 and 174 Ma in shallow basins defined by block-faulted mountains similar to the Basin and Range province of western North America. This was during the early stages of the breakup of the supercontinent Pangea. The sediments were thus immature, i.e., conglomerates, coarse sandstone, siltstone, etc, all mixed together in restricted basins and their deposition changing rapidly over time as the surrounding mountains eroded.
Not long after deposition in rapidly subsiding basins, magma from the upper mantle welled up and filled fractures within the crust. These diabase intrusions heated the sediments of the Newark Group and metamorphosed them by temperature. They were not deeply buried. Thus the rocks we found didn’t look that different from sandstones although they are technically metasedimentary rocks.
The beds are relatively flat here because the rocks were never subjected to compression, so they weren’t folded. Instead, as the crust split apart, they tilted slightly along normal faults to form grabens. In Frying Pan Park, they were horizontal. Zooming in on a bed we can see how sharp the edges are.
There are a couple of details to notice: First, the beds are about six inches thick; second, the rocks show a darkening that isn’t due to surface staining, seen in the block just right of center in the photo; third, the blocks have sharp edges. The darkening is caused by “cooking” of the original sediments when igneous rocks were injected into the pile of sediments. This process is called contact metamorphism. Another effect of coming in such close proximity to magma is that the mineral grains in the sediments become more tightly cemented, producing very hard rocks. Thus the sharp, knifelike edges.
In accordance with the “Rocks and (no) Roads” ethos, I don’t break open hand samples to examine the minerals. I accept what is available because this is about amateur geology, not data collection. There were no freshly broken samples so this is the best I could do.
The salt and pepper color is caused by organic stains having nothing to do with the mineralogy. What can be gleaned from this poor field sample is that the individual mineral grains are not rounded and none of them appear to be large, so this is an immature sandstone (greywacke) and not a conglomerate. If it were from an environment like a beach, the grains would be visibly smoother, even to the naked eye. (For example, check out orthoquartzite.)
The sediments that comprise the Newark Group collected in intermontane basins. The Sierra Nevada is an example of what the topography may have looked like when these sediments were deposited about 200 million-years ago.
After being buried several miles (nobody knows exactly how deeply) beneath the surface for 200 million years, these rocks were exhumed when ice sheets advanced into Pennsylvania during the last ice age, which began at least 2.5 million years ago. Northern Virginia was never covered by ice, but it was within a hundred miles of an ice sheet that reached two-miles in thickness. Two miles! The result was felt far to the south, where massive seasonal floods at the leading edge of the ice would have transported very large blocks of stone down rapidly eroding valleys.
This is a great picture. Four things leap out of this pastoral image of a NoVA forest: (1) The graffitied block reveals bedding much thicker than that seen in Fig. 4, suggesting a dynamic environment, possibly an alluvial fan, when these grains were washed down the sides of rising mountains (see Fig. 6) and came to their final resting place; (2) the larger blocks show a joint pattern that determined how the rocks would break down and weather when the third event occurred; (3) the rounded blocks juxtaposed on top of the jointed bedrock outcrops indicate fast-flowing water that physically eroded them and transported them some distance (less than a mile); and (4) the modern creek flows weakly over its boulder-strewn bed. This short stream, its watershed consisting of a few hilltops, didn’t transport these behemoths anywhere.
But this creek isn’t relict, it’s simply operating on a different time scale.
This boulder field, littering a dry fork of Frying Pan Branch, suggests that the smooth flow over the upstream reach shown in Fig. 3 is capable of transporting large rocks; however, it’s all relative from a geological perspective. If I were to guess, I’d say that the boulders in Fig. 8 haven’t moved in several thousand years. My reasoning is that the clear path seen in Fig. 3 suggests that there are no more large stones to roll downhill and dislodge others, like dominoes. But we never know what comes next.
A summary of the impacts of the most-recent ice age in Virginia is available at this web site. Take a look.
See you next time.