Turkey Run State Park
Figure 1. View looking upstream in a small creek flowing into the Potomac River (see Fig. 2 for location). The hills are covered with a thin veneer of fine sediment deposited on Proterozoic and Paleozoic (i.e. 585-443 Ma) rocks. This image shows several ledges of this basement rock, which is the topic of this post.
Figure 2. Map showing Turkey Run Park relative to Washington DC. Further upstream, at Scotts Run (labeled on the map), we saw Proterozoic (2500 – 542 Ma) metamorphic rocks.
INTRUSIVE ROCKS
Figure 3. Outcrop of the Ordovician (488-423 Ma) tonalite, a medium-to-coarse-grained intrusive rock. Tonalite contains little or no quartz. This is a typical exposure in this area. This rock doesn’t form cliffs and the river isn’t contained within a narrow gorge as we saw further upstream. Large blocks have fallen away as the low and irregular bluffs erode. We’ll examine this tonalite in closer detail next.
Figure 4. Tonalite exposed further downstream from Fig. 1. Note the veins of light-colored minerals running through the rock. These are veins of quartz that filled fractures in the magma after it had cooled, but was still above quartz’s melting temperature. These veins are irregular because intrusive rocks aren’t layered like sedimentary rocks; they probably also reflect slow deformation of the semisolid magma on geologic time scales (i.e. millions of years).
Figure 5. Close-up of tonalite (image about 2 inches across), showing rectangular feldspar crystals and darker biotite and hornblende, which make up about half the composition. The darker minerals weather faster than the feldspar, leaving the latter protruding from the surface. This image also shows a slight foliation, running from the upper left to lower right. Foliation in igneous rocks can be syndepositional (i.e. as the magma cooled) or created when the solid rock is reheated enough to deform without breaking. I think these rocks fall in the first category.
Figure 6. This unusual image shows a layer of foliated tonalite sandwiched between two blocks that appear undeformed. This is probably an illusion caused by irregular weathering; however, Rock D reports this rock unit as containing fragments of older rock and previously solidified tonalite. The emplacement of a large batholith takes tens-of-millions of years to complete, during which time there was probably considerable crustal shortening associated with collisional plate tectonics. (Honestly, I wish I hadn’t taken this photo because it is really strange …)
Figure 7. Joint surface within the tonalite. Joints form when the magma has solidified and is brittle, tens if not hundreds of millions of years later. These joints were almost perfectly symmetrical, with 90 degrees between intersecting planes. Such an orientation suggests that the stress regime was uniform (horizontally and vertically). In other words, they occurred during uplift (isostatic stress regime) and not during an orogeny, i.e., they occurred a long time (geologically) after the events these rocks record.
METASEDIMENTARY ROCKS
Figure 8. This photo shows the approximate contact between the Ordovician intrusive tonalite and the older, overlying Cambrian (542-488 Ma) metasedimentary rocks (intrusive rocks come from deep within the earth). The boulders lying around are mixed lithologies, representing the two rocks. Whether the stream flowing towards the camera followed the contact (they are often weak points) or not is an open question.
Figure 9. Exposure of the Cambrian Sykesville Formation metasedimentary rocks into which the tonalite was intruded. These rocks were deposited in an ocean floor/deep-sea trench environment millions of years before they were buried deeply and heated enough to be metamorphosed. Their geologic age is uncertain but Rock D reports an age of 497-470 Ma. Considering that they had to be deposited and buried before being metamorphosed, it is reasonable to assume that they were deposited about 540 million-years ago. The tonalite age of 485-443 Ma is on firmer ground because this is an intrusive rock that can be dated by radioactive isotopes trapped in the minerals comprising it. Nevertheless, their ages overlap, which requires some explanation. Such large age ranges reflect the errors associated with dating rocks this old, but also the duration of orogenic events (I’ll get to that later). I tend to trust the oldest age reported because, when radiometric daughter products (used to calculate ages) escape from the rock, the apparent age will only decrease; thus I think the tonalite was intruded sporadically starting about 485 million-years ago. Note that the bedding is tilted to the right, which is to the west; I didn’t measure strike and dip, but the orientation is consistent with regional trends — dipping to the WNW at about 35 degrees.
Figure 10. Original bedding plane of the metasedimentary rocks. The lumpy surface is very similar to muddy sediments in modern submarine fan and trench settings, where sediment slides down steep slopes. Any fossils (if there ever were any) were destroyed during metamorphosis, which contributes to the dating problem I alluded to above.
FIELD RELATIONS
Figure 11. Exposure of the metasediments, showing light-colored layers within the darker beds of these rocks — exemplified by the thin layer just below the tree leaning to the right in the center of the photo (No, I didn’t tilt the camera; the tree is growing like that).
Figure 12. Photo of a block that fell away from the exposure seen in Fig. 11. This is mostly quartz but it contains several elongate fragments of metasedimentary rock. Although tonalite contains very little quartz, the original magma contained enough silica (Quartz is SiO4, pure silica) that magmatic fluids high in silica were infused into the overlying rocks as the magma cooled. Note that the country rock didn’t melt but was broken off during injection.
Figure 13. Exposure of a quartz-rich area within the metasedimentary rock.
Figure 14. Close-up of the exposure in Fig. 13, showing large quartz nodules. The red stains come from oxidation of iron-rich sediments in the country rock. The image is about two-feet across. The magma didn’t contain an abundance of rare-earth elements, but this looks like a pegmatite to me. Pegmatites form from excess quartz and other incompatible elements (e.g. lithium, tantalum, molybdenum) that remain in the last bit of semifluid as the magma cools; thus they are injected into the country rock as veins following weak zones (e.g. Fig. 11) to form large inclusions.
SUMMARY
We saw a lot today. More than 500 million-years ago, muddy sediments were deposited in a deep-sea environment, probably a subduction trench created as the Iapetus Ocean closed. This wasn’t a continental collision but more like we see in modern Japan or the Philippines. These sediments were buried over millions of years, finally heating enough under sufficient pressure to form new minerals but not obliterating their original sedimentary structures. About 485 million-years ago, magma rose from deep within the crust and forced its way into these ductile (but not molten) rocks, forcing high-pressure fluids rich in silicon along bedding planes, breaking off pieces of the older metasedimentary rocks and entraining them.
Today’s walk in the park took us to a time when an ocean basin was being subducted beneath a continent to form a supercontinent called Pangea.
These rocks record the Taconic Orogeny, the first in a series of collisional events that shook porto-north America to its roots. It was followed by continuous mountain building, culminating with the Allegehanian Orogeny, which ended approximately 260 million-years ago.
Radiometric ages tell us that the closing of Iapetus began at least 500 million-years ago and continued for 240 million years. However, the collision of tectonic plates isn’t defined by a single orogeny, as recorded in the rocks when continents are involved. I would like to point out that this massive tectonic event coincides with the evolution of animals, from the first mollusks to complex vertebrates like reptiles.
The evolution of life is closely correlated with plate tectonics so, the next time you see a rock in its natural state, take a moment to appreciate the debt we all owe to the earth …
Review of “Rabbit, Run” by John Updike
The author of this literary fiction must have been a student of James Joyce (Ulysses) because it is primarily a stream-of-consciousness portrayal of several months in the life of a young man who is trying to escape his pregnant wife and young boy. The plot is secondary to the writing style, which really placed me in the mind of a character I despised throughout, although several of the other characters found him charming and wanted to be around him. I thought about writing this review the way the book is written but decided that would be too much work. I can’t imagine keeping this up for 325 pages and, apparently, it was difficult for this iconic American writer too; like most of the books I’ve reviewed, it starts unravelling about halfway through.
The plot is as much based on the thoughts of the characters as their actions. This is a tragic story about a young man (Harry Angstrom) who dwells on the good old days, when he was a basketball star in a small Pennsylvania town, while selling vegetable peelers at a department store. After abandoning his family, he ignores the advice of everyone around him, even his beloved coach, who turns out to be mortal when viewed through Harry’s cynical eyes. Indecision and feelings of duty lead Harry to reversing course several times, ultimately leading to personal tragedy and humiliation.
Part of the book (there are no chapters) applies the close point-of-view method to several characters and this is where Updike’s skill as a writer shines. While maintaining the stream-of-consciousness style, he eloquently presents several characters’ perspectives on Harry’s predicament.
No detail in any scene is too small to warrant description, but this is where the story gets a little sketchy, especially towards the end. I think the author was running out of metaphors. Nevertheless, I was impressed with how he brought inanimate objects like streets to life, making them characters rather than backdrops. His detailed descriptions of the town where Harry lived are probably excessive, except that some of these paths were followed more than once, each time from a different spatiotemporal perspective, as Harry’s mental gymnastics evolved.
I don’t like tragedies personally because I hear them all the time in the news, but this book was published in 1960. I enjoyed reading about anachronistic devices, e.g. rotary phones with handheld receivers, from a time when I was a young child. Like I said, nothing is overlooked, not even a meal eaten with chopsticks in a Chinese restaurant in a small Pennsylvania town.
I am currently writing a science fiction novel, and I can already see Updike’s influence on my writing; however, I won’t be describing dappled shade as “…a darkness in defiance of the broad daylight whose sky leaps in jagged patches from treetop to treetop above him like a silent monkey.”
That is damn good writing, but maybe it’s meant for writers rather than the average reader.
This is excellent literary fiction.
Review of “U.S. Foreign Policy and the Crisis of Humanity” by Daniel Silver
In the words of comedian Louis Black, “Let the rant begin!”
I have read worse books but I can’t remember when. This is written as if the author were venting their spleen, even if the topic should be of interest to everyone. The number of typos, bad grammar, poor citations, run-on sentences, and every other lexicon error possible is staggering. Furthermore, the lack of references and missing footnotes etc for bold statements is unparalleled for what was sold to me as a serious political science analysis.
The topic is one I am familiar with, having lived through most of these events as an adult. I didn’t see any serious errors, mostly because the author relies heavily on direct quotes from other writers, many of whom I am familiar with. I appreciate the approach, which is kind of like a metadata analysis, summarizing published work within a specific frame. As you would expect, this can lead to misrepresentation of events and statements taken out of context; and that is what happens.
I agree with the author that, taken in whole, the post-WW II global financial infrastructure is a product of American economic imperialism and seemingly desperate attempt at world hegemony. However, the author presents a series of activities undertaken by different U.S. policies by administrations that certainly did not share ideals or goals, as if it was a smoothly executed grand scheme. He really has a grudge against the Council on Foreign Relations, and rebuts several articles that appeared in their magazine, Foreign Affairs. Without question, the CFR as well as many other influential academic organizations have been influential in developing a U.S. foreign policy based on Neoliberalism. I reviewed a book on this very subject in this blog. The consequences of applying hypercapitalism to developing economies are vividly described by the author and are undisputed, although the U.S.-led world order tends to blame the people within this large block rather than their own economic imperialism (see, I’m doing it too).
Despite feeling that I paid $15 to read someone’s master thesis (maybe undergraduate honors thesis), I enjoyed seeing all of these events and publicly declared policies described in one short book. Nevertheless, it is sloppy and thus his point is lost in confusion.
The objective of organizations like the CFR and the Trilateral Commission (one of the early leaders in Neoliberal international policies) isn’t a secret so it shouldn’t be treated as a conspiracy; I give the author credit for never once calling this a conspiracy.
He’s just ranting, and I don’t blame him …
Timothy R. Keen 
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