Another field trip transcript
Here's another transcript of one of my field trips, again from uber-dedicated student Jill.
--- T R A N S C R I P T --- B E G I N S ---
Prince William Forest Park - Field Notes
Hike April 6, 2008
Paper due: Sunday, April 20th
Transcription Note - I apologize for the lack of words, at times. We were so near the river, the background noise of the river made it difficult to transcribe. Also, it rained during the morning. It was not the best day for a hike, but the weather outsmarted all of us that weekend.
Valley - Quantico Creek - Main Creek
South Fork - Main Tributary
(Tape #2?)
Field Notes - Weather: Raining...Callan cautions us about being nice to one another in the rain...
On Trail Observation - Top of high hill - rounded river gravel mixed in with sand & clay
On top of another hill - we will talk about it.
Volcano erupting signage. - Hawaiian volcano example. What it would have looked like 500 million years ago. Chain of volcanic offshore islands spewing out lava. As time goes by subduction is bringing those to the edge of North America until they eventually collide. Then all igneous rock - basalt rock gets slathered onto the edge of North America. Ex. - A snowball. New basalt rock is packed onto the edges - younger material added onto the edges.
Continents (low in density 2.7) never subduct. Oceanic crust subducts (2.9 vs. 2.7). Whenever the two butt heads, oceanic crust loses. Result is isotopically dated rocks which make up the continents and ocean floor. Ages are wildly different - 4000 million years (rocks which make up the continents) vs. 200 million year old (ocean floor). There is a huge difference. Continents are 40 times as old as the oceans.
Oceanic crust is constantly getting destroyed vs. continental crust which is constantly getting preserved.
Oldest rock is in the Northwest Territory of Canada.
Old Rag mountain. Rock from Old Rag is 1 Billion years old. 1 Page handouts. 1st event - Grenville Orogeny intruded granites into the crust. Granites are a symptom of mountain belts/building. And the granites cooled and became Old Rag. The Iapetus Ocean Basin was opened up. Cracks opened up into that granite into which mafic igneous rock (basalt) squirted into those cracks. Hiking up Old Rag - narrow little slots - which are those cracks. Floor is made up of dark colored, fine grained rock whereas the sides are coarse grained, light colored granite.
All that transpired before the stuff we are talking about today.
Volcanic rock is underneath us. We eventually will see it. First we will see sediments that accumulated at the bottom of the Iapetus Ocean right next to the volcanoes. Then at lunch today we will see some of this volcanic rock, itself.
We will get to see the rocks that made up volcanic islands.
We are at the boundary of sediments and the island arc further east.
Stop - signage. Small lake, dammed valley Sign about the Fall Line. 2 Virginia physiographic provinces meet. Region of the 1) Piedmont 2) Coastal Plain (as day goes by)
Boundary is the Fall Line. Coastal Plain is made out of very loose sediments, not stuck together in to a rock; loose gravel, loose sand, loose mud. Therefore, it is very easy to erode away. It is easy for water to strip it away.
Whereas the Piedmont is made out of hard rocks. (Like the ones we are gong to spend most of the day looking at). As water is draining from the Appalachian Mountains in the Atlantic Ocean, it is easy to strip away coastal rock and hard to strip away the Piedmont Rock. And, as a consequence, the boundary between the two you tend to get rapids and waterfalls. Those rapids and waterfalls line up on a line from Southwest towards Northeast - we call that line the Fall Line.
Here is the fall zone. The image on the left is a Geologic Map. You can see the difference in colors along the water. Coastal Plain layers vs Piedmont Plain layers. The Coastal Plain is draped on top of the Piedmont like a blanket. (see diagram). The deepest, therefore the oldest part of the Coastal Plain is a series of rounded river gravel. OK? Ring any bells? That is what we are looking at back there...these gravels. So, that is actually part of the Coastal Plain draped on top of the Piedmont. Right here we only find it on tops of the highest hills, but as we head east, it is found at lower and lower elevations. So, what we are going to see as we get down at the bottom of the hill - we are going to see our first little waterfall. Where the water of the South Fork of Quantico Creek which is the creek that carves this valley here where that is falling over some of these hard, difficult to erode rocks. And, then as we get down into Dumfries this afternoon, we are going to move out on the Coastal Plain itself where it is going to be very flat and the rock layers are very easy to erode. It is going to look very different - and it is a major theme we are going to discuss more over the course of the day. Since the sign is here, I figure I had might as well say something about it.
Stop. Our first actual outcrop of rock. I wanted to give you guys a chance to check it out. Outcrops are where we actually get to see some of the Earth is rock formations at the surface - and - you would think they would be more common, but they really are not. One of the reasons for that is we are trying to do Geology in the East. Out West, there are no plants, so all the beautiful rocks are exposed with these ugly forests that cover up the beautiful rocks. So, you only get to see the rocks where the forests have been stripped away. Like here, where a stream has cut down to reveal the rocks or sometimes when we build a road underneath. So, on the East Coast we are largely limited to the rock exposures that are in creek bottoms or in road cuts. We are at the creek bottom of the South Fork Quantico Creek coming out of that little lake from the dam upstream there. I want you guys to take 2 minutes. What you are going to do is examine them. I want you to make observations about this. I do not expect you to come up with the whole geologic history...
Little patches on the surface. Those are not rocks. Those are a symbiotic relationship between a fungus and an alga, right here where it is wet. Go.
So, yes. Yes, what is causing that? Something? The water. When these outcrops are first revealed they have nice angular edges like over here next to David. As the river flows over them with time they get worn down so they get nice and smooth. We will see some really cool smoothing out features downstream like the one, Kathy, you were noticing on that sign. Yeah, we will see some of that later on. Uh, Tee, what did you notice? OK, little sharp edges...OK...breaking...everything is parallel. We see this parallelism in the rock and that extends out to the stream making these little ridges of rock that extend out to the stream... (We will get to that - hold that thought). First, I want to talk about this parallelism that we are looking at here. Jill, what are we looking at here? Foliation. Right. Folio comes from the Greek word for plant or leaf - leaves on trees...(two choppers overhead go by...)
Foliation - originally folios to describe leaves and then eventually used to describe paper or books (a foliated structure...pages in a book parallel to one another is what we are seeing here in the rock). Something layered. What is it that is layered in the rock? Sediments from the breaking down of those mountains? Let us back up. You are right, but I want to make it a little bit simpler. What are rocks made out of? Minerals, right. What we are seeing here is the minerals all lined up in a certain direction. What force could line those minerals up? Pressure from what? A tectonic collision. The North American Continent pushing in this direction and the volcanic islands pushing in this direction. Let us go back to this idea of these two plates colliding - one is a volcanic island arc and one is the North American continent squishing together - this stuff got caught between - a sick analogy...a cute, little fuzzy kitten chasing a butterfly out in the meadow. Then it runs out into traffic on Route 66 and a big Mack truck comes along from one direction and a cement mixer comes from another direction and they collide head on and the kitten gets caught I the middle. The kitten changes its shape as a result of that collision - the kitten starts off with little kitten bones and fur all oriented in different directions, and as the two trucks collide, the bones get aligned perpendicular to the two trucks' collision direction. This is the mangled corpse of these sediments of /in the Iapetus Ocean. They have been crushed. They start off in all types of different directions. I will use my hands to represent two different directions - and then they rotate to the only stable configuration possible. Picture a bunch of papers on your desk. Push your hands toward each other and the papers end up lining upright the closer you put your hands together. The minerals here are all upright - all standing up essentially vertical that is..............See how straight this thing is? (A rock used as an example).
Break...then...
...bubble. OK. Did you guys see that in the back? OK, say again, speak up... "...crack in the rock and water filled in it, warm water and it crystallized into quartz." Right, and how do you know it was deposited by water? What is the clue that is telling you that? Hydrothermally deposited quartz. This white color is due to little tiny bubbles of water in that quartz mineral crystal. That is a sure indicator that that was originally deposited in a crack. Now, however, it is only the little chunk, just like that; then there is another little chunk there and another one there and there and there... They are a little bit pink because they have been stained by rust. Notice how they all line up in the same direction, too. Like this thing here is 7 cm wide by 40 cm long. Compression coming in East to West making everything line up North to South. Now, let us talk about the original sediments that got crushed here. (The so-called kitten). There are three rocks here in my hand; three different sandstones. 1) White quartz sandstone, 2) pinkish arkose (potassium feldspar) 3) greywacke - gray - all made of sand. This one is exclusively quartz. This one is a mix of minerals and has lots of potassium feldspar. Greywacke is a mix of sand and mud. It is a dark, gray color (clay as well). If I asked you to pick one of these three as the original source of these rocks before they got squished, which would you pick? Greywacke. See the color? If I put these three down, the one that blends in the most is the gray one, right? Now there are some other things influencing the color here, but yeah, if you zoom in on these rocks you will notice little grains which were originally sand and mud in this ancient, dead ocean - The Iapetus Ocean. When the tectonic collision happened, a lot of the mud reacted, becoming mica. Mica is another mineral, and a defining characteristic of mica is sheets; flaky sheets lining up one way once again indicating again that squeezing direction.
Zeta (sp?) was asking about some fractures cutting across, like here. See the fractures? There are a bunch of brittle features cutting across this rock. These rocks were squished in a flowing way and then later they were broken by brittle fractures. So, these brittle fractures may be related to the uplift of these rocks, over time. I do not know, or cannot say specifically if it is related to some tectonic cause.
But we know (with certainty) that that happened after the rocks had cooled down again. During mountain building, these rocks were hot and flowing. After mountain building, they cooled down and that is when they broke. To get these rocks to flow, they would have had to have been heated up to 350 degrees Celsius or so. Maybe higher. We will see evidence down the way of partial melting of these rocks. Partial melting yields granite magma. So, that is another symptom of mountain building. So, you have to get the rock up to 450 degrees Fahrenheit (when wet) in order to get it to melt. Using the Principle of Uniformitarianism, you can say they once were molten based upon how we know such rocks form today.
Greywacke. David - greywacke is from Old German for grey rock.
Making the distinction between a sedimentary rock and metamorphosed sedimentary rock...layers of sediment. You do not end up finding these big, long flaky bands of quartz...presence of all this mica. We do not get big deposits out there in the actual world where mica accumulates in big layers. We get layers of mud. So, the mica itself is a metamorphic mineral. Also, we do not see any continuity. As we look along the way here, we do not see that we can detect one layer of coarse grains or one layer of sand, mud, or anything like that. Instead, we see a sort of smeared-out looking feature. These rocks appear smooshed. What would you add to that, David? (The mica here is the metamorphic mineral, not a sedimentary mineral...B word....boudinage...)
Boudinage. French for sausage. Describe a rock layer that has been broken into sausage like segments. Right here, look at this you guys. 1,2,3,4,5 sausage links. This quartz vein is broken (as a brittle phenomenon). Also, the flow was pinched out along the breaks. There are pinched out ends, like a string of sausages all in a row. This occurs at 10-15km depth from crust...right at the transition between brittle behavior in the upper crust where rocks break, and more flowing behavior in the lower crust. Brittle means breaking. Flowing is like silly putty or bread dough.
So, I wanted to elaborate on something. See picture...4 parts. Something we can deduce about these rocks. A preserved sedimentary structure called graded bedding: layers of rocks that are coarse at the bottom and fine at the top. No graded beds here today, but at the Billy Goat Trail, you will see graded beds. That tells you how these sediments initially accumulated. I am correlating these rocks here with the rocks at the Billy Goat Trail, based on similarities in their mineral content, and my knowledge of the area. I'm saying these rocks exhibit all characteristics of Billy Goat Trail rocks except we do not see any graded beds preserved here. In the Billy Goat Trail, there are a few lucky areas where we see graded bedding preserved. Why do I care about graded beds? Graded beds are deposited by currents flowing along at the bottom of the ocean. (Picture of turbidity currents). These currents are very dense, sediment rich flows, that go along the bottoms and they slow down. As they slow down, all the sediments caught up in the rolled up water settled out. The stuff that settles out first is the big stuff. The stuff that settles out last is the light weight, really fine-grained stuff. So, you end up getting these graded beds forming. Those formed down in a location like this, down in the deep sea in what we call an abyssal fan or a submarine fan, where sediments are coming off some land mass piling up in the deep sea making graded beds of greywacke. Again, greywacke means nothing more than a mix of sand and dark mud. So, that is what this used to be. Then it got crushed up. When did it get crushed up? When the Iapetus was closing, good. As the Iapetus was closing (let me pull out another graphic here) it was a scraping up all that sediment. OK, the subduction zone was going down the hatch, but the sediment on top of the oceanic crust was getting scraped off. It was building up into a big pile; a big, jumbled pile of sediment. That is analogous to a bulldozer moving over the ground scraping up a big pile of dirt in front of it, OK? Where the bulldozer is like the island arc, and then in front of the continents are the sediments it is scraping up. OK? So, that is what we are on here. Really, these rocks here are the big pile of muck that got scraped off the subducted plate and then later it got squeezed between the islands and North America. So, we call this big pile an accretionary wedge. Now, Dean, you were talking earlier about California and San Francisco. San Francisco is still on an accretionary wedge. The difference between Prince William Forest Park and San Francisco is, Prince William Forest Park then had that accretionary wedge caught between two continents; Africa and North America, and it squished. Whereas San Francisco, it just filled out. It has not been squished between two continents, yet. Give it another 50-70 million years, something like that. OK. Questions? (Why the silt...? C. - What silt are you observing? Does that just look like it is going into the hills?) What was stable in the middle of the mountain belt is no longer stable. The mica is rotting away. In fact that is why over here, when I was running my fingernail through these little grooves, there is a groove there to run my fingernail through. The mica is soft and it rots away really easily, so it ends up etching out. And, the quartz is very stable and so it does not erode away easily and that is why it makes these little ridges. So, guys if you have not felt this for yourself, come put your finger on the rock and feel this yourself. That is why, like, on the drive down I was noticing these big white boulders on the drive in. Those big, white boulders are made of quartz - it is stable. It does not break down over time. That is why when you go to the beach; the beach is made out of quartz sand. It is not made out of feldspar sand, mica sand or anything like that. It is quartz. Quartz is the stuff that lasts. (Student questions Callan. C. -- Yeah, right. Black beaches are where you are really very, very close to a basalt source. And, there are not...there is not ....adequate time to break down all those unstable minerals, so the beaches built up making those black minerals there. So, we are finding that there are some black minerals even on beaches here on the East Coast, but, the majority of it, when it is a nice mature beach, is quartz sand.)
One of the exercises I had Jill do this semester, and I had Dave do last semester, as well as the rest of my Physical Geology class is that I give them a little, bite-sized Snickers bar and they have to suck on this Snickers bar. The chocolate dissolves away very readily in their mouths followed by the caramel. The nougat lasts about 10 minutes or so. But even after you suck on this thing for about half an hour, the peanuts are still there. Peanuts do not dissolve, right. Peanuts are like quartz whereas all the other ingredients in the Snickers are more like other less stable minerals.
(Student talking about some observation). C.- Actually that is a great observation. Let us take that one step further. Imagine, now this creek here is not a creek but a road and you are driving down it. You take this turn and you take it a little too fast. Which way does your body get pulled? Right, towards the outside of the curve. So, basically, during flood times the creek comes in and slams into that wall right there and strips away the plants and strips away the leaves and strips away the dirt and it exposes rock there. Whereas the rock that is underneath the hill here is not getting hit head-on by the force of that creek. So, it fills up with sand, dirt, and leaves over time. I did hold this rock down here and I just wanted to point something out, it is a little bit difficult to see because the stupid thing is all wet, alright, just like us. But, this is a foliated metamorphic rock. Does everyone see the plane of foliation? So, if I were to line it up here with the regional foliation, it would look like that. Alright, but this one is loose so we can pick it up and examine the plane of foliation itself. And, there are little black needles there on the surface. Do you see those little needle-like minerals? They are needle-like mineral growths. Do you guys see them there? It is almost like if somebody took a bunch of chopsticks and dropped them on a desktop. The chopsticks were in this random orientation because they would be parallel to the surface of the desktop. OK? If I took all your pens and dropped them on top of someone's notebook, they would all splay out on top of that notebook but some might be pointing this way and some might be pointing that way. That is what you are seeing here in this plane of foliation. These are amphibole minerals. And the amphiboles are randomly oriented within the plane of foliation. Pass it around. I know David wants to get a good look at that with his lens. Make sure he gets that. Great. Good. (David and Callan have discussion about amphibole, spelling, etc...David asks, does the random orientation of the amphibole suggest that was it done when the pressure has been released. C. - No, I think basically that what we have here is we have flattening. Remember what we talked about was the different types of deformation; folding, faulting, and I think I said squeezing or flattening. These rocks have been flattened. And what we see is that the dominant pressure direction was coming this way and then the rocks, in order to accommodate that (here we go – here is my little Nerf ball. Yeah, I left my kitten at home...) Um, they are getting squished in this direction. Right now the Nerf ball ends up basically elongating outward this direction and growing in this direction, as well. So, like, you think about three dimensions, these right here are growing and this one...whhish....gets squashed. So, I think what is happening here is that flattening stress is causing the amphibole as they are growing...they ca not grow in this direction. It is like, try growing if the building collapsed on top of you. But, you can grow in this direction.)...we will see a couple of them here today. ...Dave, unfortunately it is not...it takes certain elements to make the amphibole, so unless those elements are present in the original sediments, you do not make it. Dave - OK. Student - Do you want to leave this here? C. Yeah, that is why I brought it down. C. - No, I would not break it, I would just kind of stash it over there underneath a tree or something so maybe in a year from now I can find it.
Another segment...
...you take away the fact that we have determined that these used to be sediments in the Iapetus Ocean basin, and then they got squeezed due to mountain building. Due to that Taconian Orogeny...this episode of mountain building that we mentioned back in the shelter. Taconian stands for the Taconic Mountains of New York, alright. (see the one page handout that I gave you). Dave - What are the compass directions here? C. - Essentially, North/South and then, East/West - squishing. Now is it actually that? Well, no its North/Northeast, but close enough. Dave - it is a good observation to make in the paper the direction in which the squishing seemed to happen. C. - Yeah, I would say that would be a great observation to make in the paper. Um - it may not have been originally in that direction though, because remember a later collision happened. That later collision also squished. So, it is like remember the kitten, Mac truck, cement mixer pile-up we had earlier? Then along comes a tank and crashes into it, OK? And then that ends up rearranging everything again. Dave - could the magnetic poles have changed here? C. - Uh, yeah, but we have no magnetic signature in these rocks. We are just trying to get in our head how - Dave - You did say North and South...C. - Yeah, so we are using modern day directions but then again North America would have been rotated around in a different position, so it is a complicated question. Sounds like it is a simple question but it is really not! Another student asks a question. C. - You do not have to understand about magnetic poles to understand what we are talking about...Dave - ...unless it is present day...(Callan brings discussion back around to)...yeah, that is all we have to work with and that is what is going to be most readily available to you guys. So, again, the one thing to think is this foliation essentially lines up on the same line from the Appalachians, basically from Georgia...Dave - OK, yeah, that is a better way to go, yeah. C. -....so, that, that is due to this first collision...Dave - ...Appalachians...to the ocean, like that? The Appalachians have...to the Ocean, like that? C. - Well, the Appalachians would be parallel to this. So, essentially we are looking at Maine up that way, Georgia down that way...ok, West Virginia, that way, and then the Atlantic out this way. Uh, I was in the middle of making a statement there and I got derailed.
(Dave - If these clay minerals had all been aligned and micas formed in the first compression, and they were tilted up in whatever orientation, and a second collision took place and it came from a different angle, would all this seem likely to get reoriented or would it preserve some of the old orientation? C. - Probably you would have some preservation and some would get reoriented, depending on where you have little bits that poked out, those being more susceptible to being re-rotated.) Um, that being said the overall structural grain of the Appalachians is this North/Northeast to South/Southwest direction, so, I mean I think we can just sort of simplify things - well, it may be an oversimplification - we can simplify things by saying these collisions all essentially came in one after another from the same direction. First, these volcanic islands during the Taconian Orogeny. There was a second collision that happened, later on, we are not really going to talk about that today - called the Acadian Orogeny. And then finally, Africa hit and that was the Alleghanian Orogeny. So, all these different orogenies were episodes of mountain building.
Oh, I know what it was that I was going to ask! When does the Taconian Orogeny happen? Well, Dana, that is just great. How did you know that!? She says 460 million years ago is when this actual collision took place. And, she is right, but, she is just pulling that number out of thin air. You can not see it. Student - she got it out of the papers C. - OK? Yeah, you are on the right track. OK, radioactive decay. So, certain minerals when they form, they take in radioactive isotopes, and then if you can go and you can say that that mineral is a mineral that only formed during an orogenies, then you can say, "Ah ha!" All you have to do is look at the radioactive isotopes that remain in that mineral and compare it to what they decay into. So, in this case the mica here has been isotopically dated. What they did was they looked at radioactive potassium 40 that is present in that mica and they compared it to the daughter product - the stable daughter which is called argon 40 And, the mica as we said earlier formed during metamorphism ...was a good state for the orogeny. The date is 460 million years ago. We are going to see a granite today, and the granite has a date of 464 million years ago. OK, so it is basically the same age. And it tells us about the same event, and granites, you remember, are another symptom of mountain building. So, we have got a really good view on the orogenies then by dating these two independent, isotopic systems. The metamorphic mica here, and then the granite that resulted from partial melting. I will remind you of that again when we get to the granite, OK? Alright, that is a good point to keep in mind. Student - So what overall type of rock is......C. - No, this is not granite, granite is much lighter color and granite does not have this foliation. Um - so this is a metagraywacke. All right. Meta, the prefix meaning change, and, greywacke telling you what it originally was.
OK., I know everybody is hungry, so we are going to hoof it. We are going to walk down to the Cabin Branch Pyrite Mine. We are going to be walking across and along the South Branch of...
...stop and maybe point out this point bar......do not feel like you have to take notes. OK, I’m not going to go over anything really important...
(Tape #3) After lunch...
OK, so granite you remember is produced by the partial melting of other rocks - remember I showed you that other diagram where you had a bunch of starting minerals and then the light colored ones sweated out - they melted? Whereas the dark colored ones stayed behind. So, here, it is a metagraywacke that is being partially melted. And the minerals that are easiest to melt those metagraywacke are quartz, feldspar, and mica - some of those are basically melting out and they are leaving behind the darker colored minerals. Ok, so we are producing these granite blobs and these blobs of magma are moving up to the crust and eventually they are settling down and crystallizing into granite. Remember we call these blobs plutons. So, here in this diagram which is part of your handout, I have got a diagram showing you some igneous plutons, OK? So that bodies that were magma and have crystallized into solid igneous rock, like a granite - OK, here is one pluton, here is another pluton - they cooled underground. Now, basically these plutons are these wet batches of magma and they are moving up through the crust. What happens to the pressure that is on them the higher up they go? Yeah, it is released and as a result they erupt - basically the granite separates out and the stuff that is most readily removed leaves the granite. So, think about a bottle of soda - did anyone bring a carbonated soda? David did. David brought a ginger ale. So, when he popped the top on that, OK, it started off as just liquid, but when he popped the top he released the pressure on the liquid inside. And as a consequence, a gas that was dissolved in the soda came bubbling out. Carbon dioxide came bubbling out of the solution. The same thing is happening to this granite which, remember, originally was magma. As it gets up to shallower depths in the crust, gases and things start coming out of it. It is leaking fluids into the surrounding rock. OK, so it is intruding into this rock and so these fluids and gases are penetrating the surrounding rock. Some of those fluids and gases would probably be water vapor, another one would be carbon dioxide, another one would be hydrogen sulfide, um a bunch of different fluids, OK? And one of the things that these fluids are taking - with - them, (I think I have got my mouth.../sandwich repeat) - OK, the fluids are carrying with them metals. Alright, metals readily dissolve in those fluids that are carried out of the granite by the fluids. And as those fluids penetrate the surrounding rock, they cool down and the metals are deposited there. Frequently, the metals are - they glom onto sulfur. Sulfur joins up with lots of different metals and then it settles out in this big sulfide deposit which surrounds the granite pluton. OK? So, it is kind of like a halo or an aura surrounding the pluton is this big aura of sulfides deposits - sulfur mixed with metals. OK? So, some sulfide minerals contain galena, some of you are familiar with galena, it is beautiful, it is got this silver luster, and these little cubes. Um, another really important one is pyrite. Pyrite is nothing more than iron sulfide. The chemical formula for iron is Fe - the chemical for pyrite is Fe. OK, it is nothing more than iron mixed up with 2 sulfurs. For every one atom of iron, there are two sulfurs bonded to it. And that makes this mineral called pyrite. Then, what color is pyrite? Golden. Yeah it is sort of this golden color and it looks a lot like gold, if you do not know what gold looks like. Um, it is got that same golden luster. Well, they were mining this pyrite here. We have already said it is not gold so why are they mining it? What on earth is the point of pulling up fool's gold from the ground? Gunpowder. C. - Gunpowder. So, basically, they are not interested in iron, they are interested in the sulfur. The main thing they are using the sulfur for is gunpowder. It is also used in many other industrial applications like making soap and refining glass and other things like that. But, not nearly as exciting as warfare. Jill - Civil War? C. - The pyrite mine right here actually started in the aftermath of the civil war, and then it actually hit a fever pitch during WWI, when there was a really big demand for gunpowder. So, they were pulling lots and lots of pyrite out of this mine, processing it to extract the sulfur and then using it to make gunpowder. So, geologically why there is a mine here is that the granite is essentially sweating out all these fluids. The fluids are rich in dissolved metals. It is just like when you sweat, there is salt in your sweat. And if it dries out on your shirt, you get a little white crust left behind on your shirt, right? So it is the same thing her except for instead of salt crustiness left behind, you will see a metal crust. So, the same granites that were produced during the Taconian Orogeny were sweating out these deposits of pyrite into the crust. Later on, people came along and said, "Hey, we can make use of that, let's make a mine here. We will dig into the hills and dig out as much pyrite as we can." So where we are right now is we are sort of geologically on this dome surrounding one of theses plutons, OK, we are in this sweaty armpit region. So, what I want to do now, is I want to go and find some pyrite and look at it. So, what we are going to do is walk back over here to this area where there was nothing growing. And, we are going to go look for some pyrite. Does that sound workable? Eventually we will come back to this place, so if there is something heavy you do not want to carry, you can leave it here and then come back and pick it up again. (Student question - inaudible. C. - The granite pluton was. Yeah, so the granite - the body of magma which would eventually cool into a granite. We will visit that granite this afternoon). (Student - So it is coming out because it...C. - the granite was, yeah, and as it is getting to shallower depths in the crust, it is devolatilizing, so the various gases that are dissolved in it are coming out. So, it is starting off here during partial melting, then the granite is organizing itself into these blobs, they rise through the crust, as it rises it is sweating out into the surrounding rock all these mineral deposits. Alright, sometimes as it moves into the crust, the crust cracks open and you end up getting veins of pyrite or veins of hydrothermal quartz. Some of those veins of hydrothermal quartz have gold in them, actual real gold. Including in the northwestern corner of this park, and by the Billy Goat Trail in the Great Falls area. So, there are gold bearing quartz veins in this area and they are coming from the same source. They are essentially being sweated out of this granitic magma. (Student - ...keep that in mind if the dollar keeps going down. C. - That is right, we will start mining our National Parks.) OK, other questions? Let's go.
(New Spot.) Now, I'm going to start talking now about some of the environmental damage that the mine caused. In this area, where the ground was near the mine operations, they were filtering the lower grade ore, you know the stuff that did not have enough pyrite in it, and they just kind of dumped it, alright? And, that is what we are sorting away here, right? And that pyrite is then soaking out here at the surface of the Earth in water, and that water is oxygenated water. And, those two ingredients end of completing a reaction of water, oxygen, and pyrite. Water, of course, is hydrogen and oxygen. Oxygen is just oxygen. And, pyrite is iron and sulfide. So, basically, after that reaction you end up getting iron mixed with oxygen and hydrogen which is rust (FeOOH) and sulfuric acid (H2So4). So, two things are being produced here due to the weathering of the pyrite; rust and sulfuric acid. Rust is what is making the soil so darn red right here. It is staining everything red. Look at David's boots right now - they are getting all soaked in this red mud. Alright, the other thing is sulfuric acid. What is the effect of sulfuric acid that you see right in this area? Basically, most plants cannot grow in super acidic soils; soils that are essentially drenched in sulfuric acid. As a consequence, nothing grew here for a really long period of time. It was basically a day of awakening - completely environmentally degraded. So, the ground was basically an empty wasteland and then the Parks said, "OK, we have got to clean up this mess." So, they took a series of steps to essentially reclaim the land. OK, this is something that frequently has to happen where they do mining - reclamation - basically making it look like a decent landscape again. And, what they did was they brought in a lot of limestone. Limestone is made out of calcite, and that reacts with acid. So, basically it is a buffer through the sulfuric acid. And, when they laid down all these limestones in the area, some of them are dissolving away as soon as they get set down. They are taking away some of that acid. It is kind of like Tums in the landscape - that is a great way to think about it? Did you have some Tums this morning? David - no. So, it is like Tums through the landscape and it is working better in some areas and not as well in other areas. It is not working so well right here. There is still a lot of sulfuric acid right here in this area which is why you can actually go and pick up rocks there. There are no plants growing out. That is thanks to the acid. Same as that little patch there at the end of the trail - there is nothing growing there. That is so weird for the East Coast to have an area where there are no plants growing. That tells you there is something seriously messed up with the soil underneath.
One of the things I would like you to do, is I would like to have you test the pH of the waterways. You might want to clean off your hands first, because the way you are going to be able to read this thing is to check the color of the paper. Alright, this is a little pH paper here. It is going to change different colors depending on the pH of the water you put it in. Now, you want to make sure you are putting it in relatively clean water otherwise you will get sediment on this which is going to give you a false reading. OK? So, I'm going to give everybody a little strip here. You can choose to test this water here, somebody should go test the water of the South Fork, and somebody should retain their strip so we can test the water of the Main Fork of Quantico Creek. OK, so we want to collect data at several different points, to several different iterations at each point, so we have reliance on the data and then we will share everything we get.
Everybody tests. OK, what do you have? 6! What do you got? (Everyone testing.)
Results 5 or 6 - somewhere between a 6 and a 5....slightly acidic. He just put it on his tongue and it is a 7. So, your tongue is pH neutral, which is a pretty good thing for your body. So, he has a reading of between 5-6 so that is just slightly below neutral, so this area is slightly acidic. That is not as acid as it once was, but it is more acid than just what regular old water would be which is 7. So, as the number gets lower in pH level, the more acidic. Higher is more alkaline. End of segment.
...sweating out of a granite pluton. Then the fact that this was mined for awhile for the purposes mainly of making gunpowder, and one of the elements that make up pyrite is sulfur, and then the breakdown of that sulfur at the conditions found at the surface of the Earth here; mixing it with water and oxygen, making rust and sulfuric acid, and then the Park surface had to treat the area by putting down essentially the geologic equivalent of Tums with limestone chips in order to get rid of the acidity. Oh, and another point that we could make here that the plants that actually are growing here are pine trees. These are acid tolerant plants. Their needles, themselves, are acidic. So, their needles are dropping - so look underneath those pine trees. You see the carpet of needles underneath, right? Those needles are essentially making that soil more acidic and that makes it harder for other plants to grow there, OK? They make a special kind of acid called a tannic acid. It is the same thing if you brew your tea for too long - it is a sort of bitter thing and it makes your stomach hurt. That is essentially what is going on over there with those trees. OK, let's move.
Testing pH at new location. (Dave - So are these the rocks that they dumped? C. - Yeah, so this is the ........that basically have water coming through them. The water is...)
OK, so I imagine we all do not want to stand here for too much longer. Did everyone see that vein of pyrite that Topher found? Time to work our way back to the path.
New location.
...C. - I'm going to turn upstream on Quantico Creek. (Jill - Confluence of Quantico Creek). We are going to look for a place to cross Quantico Creek. We want to be on the other side of it.
New location.
C. - I want to point out we are at the confluence here. So, here is the South Fork of Quantico Creek, which flows under the bridge we just walked over. Right, here coming into the main stream of Quantico Creek, the two merge right here (Jill - the confluence) and they flow downstream. Where we actually want to go is where there are a bunch of branches across the creek down there. It is probably only 200 yards from where we are right now. All right, but, unfortunately there is no great way to cross the creek right here. So, we have got to go upstream a ways, until we get to a good creek crossing. As far as a bridge and cross over. David, if you want you can wade across, but, I do not want to make every...
New location.
...flow. Where were these lava flows accumulating? Jill - Volcanoes. C. - Volcanoes, where were the volcanoes. Jill - the island arc. C. - Yeah, the chain of islands offshore, you remember, ancestral North America about 500 million years ago. Then, subduction narrowed the ocean basin between them, and eventually they got added on to the edge of North America. Now, how do I know these are lava flows? Great. Color, texture, and maybe the mineral content. Yeah, so there are some good color indicators here. What color is this greenstone? It is a very descriptive name - greenstone. It is probably in the Old German for - just kidding. So, greenstone is metamorphosed basalt. Remember basalt is what is coming out of the volcanoes today in Hawaii. (Student - asks question. C. - Well, we call it magma if it is below the surface and we call it lava if it is above the surface. Once the lava cools we have to give it a rock name. The typical name we use for the dark colored rock is basalt). Basalt is what made up the oceanic crust and what made up these volcanic islands. So, when the basalt gets metamorphosed it undergoes chemical reactions and those chemical reactions turn it green. The main green mineral here is called chlorite - it is basically a green mica. There is also epidote. Right here there is a pistachio colored mineral. You guys see that one? It is sort of a bright green? Filling in little veins over here? It is epidote. Alright, so basically, I know Topher is going to ask about this - the "take home message" is that these were once lava flows that got metamorphosed. How do we know that? The Principle of Uniformity. When you see lava flow that gets metamorphosed in the world today they change color into a green color. The reason is that they grow chlorite minerals and epidote. ....yeah, well there is some other stuff. Remember these have gotten squished. So, a lot of the original layering is lost. They are foliated. They are foliated and again it is that squishing effect. OK, I want you guys to come and look at these rocks here. There are little white circles in the rocks. Oh, see these white blobs here? OK, what is going on here is the same thing we were talking about earlier. When you release pressure on a lava, it causes gases to come out of solution. Just like when David opened his Canada Dry the bubbles formed. When lava erupts at the surface bubbles form in the lava and gases come out, right? If those bubbles do not pop before the lava sets up into rock they are preserved as little holes in the rock, like Swiss cheese. We call those vesicles. The vesicles have gotten filled in with mineral deposits. Those mineral deposits are known as amygdules. Amygdule is for the Latin for "almond." So, these were originally decided to look something like almonds - set in a piece of bread, like that. There are some really nice ones over here with mineral deposits. Generally the minerals that are filling them in are quartz, well Let us just say, quartz. David - something about popping. C. - ...if they do not pop, it leaves hole. Later on that hole could get filled in with a mineral deposit. This is important. The bubbles form as gases are coming out of lava, then some of those bubbles pop, we do not have any evidence of it. But, some of the bubbles do not pop and those bubbles get filled in later on with mineral deposits which make these little white blobs in the greenstone. And, notice that these little white blobs are not perfect spheres. Up here they appear kind of like this. Why is that? They got squished - like a little kitten's eyeball. That is due to that tectonic squishing. You can see that they are all basically squished like this line up at the plane of foliation. Alright, they lay exactly parallel to the plane of foliation. That is why the sphere became a pancake. So, they are little pancake shaped fossil gas bubbles from a lava flow. We call them amygdule because they got filled in with mineral deposits. If they were still empty holes, we would call them vesicles. ( ...you can see epidote down here in veins... come down here with David and you can see them.)
Barely audible due to noise from river. C - ...ancient volcano island rock... ...Chopawamsic....Dave and Callan in a lot of discussion about the volcanoes, flows, etc...evidence of it all...Callan sticks to history of Virginia....at one time you could have walked from Ohio to Morocco. Now those rocks were once sediments in the ocean deposited way down...
...discussion about potholes....barely audible. The role in carving it out. When water come flowing through here, there is a vortex of water. Filled with sand, silt, and it acts as liquid drill bits. Layers of quartz and mica, quartz and mica. Quartz stands out in high relief. Sand gets in there and preferentially eats away the mica. So, that is a pothole and potholes are one of the ways that streams are cutting down in areas of waterfall where they are adjusting from one level to another level. Alright, Let us go ahead and work our way back to the trail and we are going to start climbing uphill, towards the bathroom...
OK, so here we have another tree that is tipped over, and it has brought up a nice, fresh sample for us. We can see here more of that gravel that we saw when we first started on the trail today. This is not a rock. This has not been stuck together into a rock - it is just loose gravel...David - it is a root ball, right? C. - Yeah, it is a root ball of a tree. You can pick up the loose grains of sediment, and let it run through your fingers. Actually, I encourage everyone to do this. You will feel that this is a mix of sand and clay, and the clay will feel very sticky on your hand, and then these nice, rounded pebbles and cobbles of mainly quartz, OK? Most of which is present here is quartz. Student - Sand and all that is what is left over after the quartz was left. C. - Well, basically, you said it yourself. This is a very poorly sorted sand pile. This is a mix of different grain sizes, which indicates that it was dropped very rapidly. Now, what does the rounding tell you? Jill - it was well sorted. C. - ...well traveled. It is not well sorted, it is poorly sorted. Yeah, the rounding tells you that this quartz cobble started off somewhere far away and then it traveled a long distance to get here and as it traveled it got more and more rounded. This one must have started off a little bit closer. Alright, because this one is a little bit more angular. And that is typical of river systems because river systems end up draining a whole area. Rocks are dropping into them from far away and nearby and they are both tumbling downstream together. And, Jill, that is how you interpreted this, right? Jill - yeah, well no, sorry, I was off in a zone. C. - Jill, how would we interpret this deposit, how did this form? Jill - Well, basically, it came off of an uplift, and it traveled downward, and it was deposited into a system of water...C. - OK, what kind of water? Jill - Probably very fast. C. - OK, good, why do you say that? Jill - Because, it looks like they are rounded, I mean...obviously they passed through...Student - ...it is the size of the rock...C. Yeah, it is the size. Student - it is a well-rounded big rock. C. - Yeah, it is obviously a well-rounded, nice big cobble of quartz. And, it takes a lot of water energy to move something this size. Jill - Yeah, definitely. C. - OK, good, so what kind of body of water has the energy to move big cobbles like this? Jill - River, a river. C. - Yeah. Rivers, OK? Because we just saw, in fact, I just destroyed my vocal chords trying to shout over - rivers have a lot of energy. Whereas a lake does not have so much, a swamp - less. The ocean has a fair amount of energy, but you generally do not get big cobbles like this in an ocean because as soon as rivers flow into the ocean, they slow down, and then they drop all these things, and then they carry maybe the sand and the mud further out into the ocean. Those are all that really make it into the ocean. So, this is a river deposit. And, when you think about it you might think it is a little bit weird, because we were just down at the river and we just hiked uphill, and now at the top of the hill we see these river deposits? What is going on here, Cathy? Cathy - The river was once up here? C. The river was once up here, she says. These were not recent flood deposits. So, these are ancient, and the reason I know that is I can come up with a rough date for these deposits based on fossils that we find within this same gravelly unit. Now, this gravelly unit here, do you think it is a particularly good setting for preserving fossil remains? Callan hits the “unit” with large cobble, again and again. C. - Do you think that is good for a fossil? All right. Think about the river here. As these things are moving along, all these boulders are smacking into one another and grinding around. This is a lousy environment for preserving fossils. It is a miracle that we have any fossils at all from this unit. The fossils that we do have from this unit are very poor and they have been broken up a bit but we can still identify them and they are dinosaur bones. Alright, we found 3 or 4 different dinosaur bones from this one unit. There are some sauropod fossils, sauropods are the big, sort of lumbering, vegetarian dinosaurs with the long necks. We found some of their teeth and some of their leg bones. OK, there is a species called Astrodon johnstoni, it is the state fossil of Maryland. Basically, Astrodon means star-tooth. Think about your molars in the back, and you have these little points for grinding. There is a series of five radiating ridges for grinding up vegetation. So, Astrodon johnstoni. Also, we found some raptor fossils in here. At any rate, these dinosaur fossils date back to the Cretaceous. Cretaceous is a period of geologic time that ended at about 65 million years ago. It started at about 120 million years ago. The best estimate for an age on this unit is about 100 million years old. One hundred million years ago a river was flowing along this area, and that river was meandering. It was cutting in at a cut bank and it was depositing materials on a point bar. This is an old point bar deposit. OK, remember we saw piles of gravels that look a lot like this being deposited down at Quantico Creek today. So, this river was no longer cutting down, it was simply meandering back and forth on the landscape. Now, something must have happened between what we were just talking about at the base of the hill, and the deposition of these river gravels. We have this great big mountain range that had gotten built up, right? The size of the Himalayas - what happened to that? It was eroded down to essentially a flat level, and over that flat level, this river was meandering back and forth depositing gravel. OK? Then at some point after that what happened? Sea-level dropped and what did the river do in response? It started cutting down again and carving new valleys like the valley that we spent most of the day hiking through, OK? So we have evidence here of a higher sea-level at some point where these rivers were meandering along depositing gravels as they flowed eastward from the west out toward the young Atlantic Ocean. The Atlantic Ocean, by the way at this point, was 100 million years old. The Atlantic Ocean was born 200 million years ago and these gravels were deposited 100 million years ago, therefore, that is about 100 million years into the history of the Atlantic. One of the reasons that I’m able to say that these rivers were flowing from the west to the east, is we find signatures of rocks that we know only come from the west. Like we find pieces of granite that we find from the Blue Ridge Province. And these Blue Ridge granites have blue-quartz in them. Which is an indication that these are from the Blue Ridge Province. Yeah, there is some nice blue-quartz in this sample to right up there by my thumbnail. It has sort of a purplish sheen to it. You guys see that? The other thing that sometimes we find here is quartz cobbles that have a trace fossil right in them. That tells us that that came from the west, the river brought it this direction deposited it here, therefore that river was flowing from west to east. The same direction the rivers are flowing today. Now when the Appalachians were real, real young, it was the opposite. The rivers were starting here in the highlands above our heads and draining off to the west. Student - that is why you find Appalachian rocks way out in the west - west of us...C. - Yeah, there is some fairly compelling evidence in fact that the Petrified Forest of Arizona was buried underneath Appalachian sand and mud. In Arizona - so we are talking Mississippi sized rivers draining these young Appalachian Mountains, transporting the sediments to the west, and then basically they snuffed out a forest out there in Arizona. We can go and pick up a certain mineral from those sands, and that zircon has a chemical signature that is more analogous to the Appalachians than it is to any local source out there like in the Rockies. So, it indicates again at that time the mountains were here and the lowland was there. Who has got their handout handy, the one with the colored map on the back? Callan explains map....Kp designation which stands for Cretaceous. The sub p there indicates the Potomac and this is called the Potomac group. They are exposed up and down the length of the Potomac and you find them on tops of the highest hills. Same unit atop Tyson's Corner. You find it on top of Mt. St. Alban where the National Cathedral is. You find it on top of Mount Pleasant in D.C. - river gravels, river gravels, river gravels....Say that three times fast. So, this surface on top of which the river gravels are deposited is a period of missing time. The last geologic evidence we have in this area is the intrusion of granites. That happened around 460 million years ago, and then the next thing that is recorded in this area is the deposition of this gravel on top of it, which happened 100 million years ago. So, 360 million years of geologic time are missing in this area. We can say nothing about them from Prince William Forest Park. You have to read the geologic record to find out what is missing from those 360 million years. OK? Student - How come no one could think about water or erosion? C. To erode away a Himalayan sized mountain range takes a fair amount of time, and so it took a long time to grind down those mountains to that level. Plus, during the Cretaceous, the world was quite warm and sea-level was quite high. There was very little glacial ice, if any, and at that point then you have this combine effect of having ground down the mountains plus sea-level being high and that is when it deposited gravel all over this area. Wow, so this is like a little mud stone, right. Student - I get extra credit. C. - I would not call this a schist, it does not have nice physical minerals, but I would not mind calling it a mudstone. This is a little clast of mudstone and this is very typical of some mudrock layers that are typical out in the Valley and Ridge Province and that would be consistent with the story of basically transporting eastward. That is rose quartz there...OK, the first thing we are going to do when we get back to the visitors center is use the restroom, and then we are going to go and visit a fossil tree and that fossil tree is growing during the same period and it was probably growing along the banks of the river.
This was deposited by a river after the Appalachian Mountains were ground down. It is Cretaceous in age. It never experienced mountain building. If this had gotten caught up in that collision in would not be a loose pile of gravel, mud, and sand, it would be a rock. What would we call this if this had gotten cemented together into a rock? A conglomerate. Nature's version of cement with big chunks set in a little fine-grained matrix.
And, I do not really know what that means, I mean....
...on the trail today you are not leaving behind your skeleton, but you are leaving behind traces of yourself. Right? So, the thing that this Skolithos worm tube tells us is that it is a piece of the Antietam Formation. Antietam Formation is a big sandstone unit that is out on the western slopes of the Blue Ridge. And you can find them near Antietam National Battlefield, where the bloodiest battle of the Civil War was fought. You can also find this same rock along portions of Skyline Drive and there is an area where David and I have hiked near 66. There is a nice big exposure of it near Front Royal. So, basically it is telling us that the river transport direction was westerly - consistent with the blue-quartz. Sedimentary transport from west to east.
...Fossilized Bald Cypress Tree. It is really not in any danger of being degraded or anything like that, but it is in danger of having stuff grow on it. It is the mineral itself that will break down. All quartz. The quartz was derived from Cretaceous river gravels...And as it met this wood, it percolated through the wood, soaked into it, a chemical reaction took place, and it precipitated silica in place of the wood. This similar process has occurred in the Potomac Formation. In Washington DC, when they were digging out the foundation for the Ronald Reagan Building, they found more of these there. Also, at the base of the Mayflower Hotel, they found these fossil tree trunks down there. And, what do all these areas have in common? They basically have these cretaceous, Potomac Group river gravel deposits. So, you can imagine growing along the banks of this ancient river, Cypress Trees. How would you recognize a Cypress Tree if you saw one today? Yeah, Bald Cypress, and they have got these weird structures that rise up out of the waters. They call them knees - Cypress knees. They poke up above. I noticed in a place up here there are little tension gashes and they are filled in with silica, too. You see that - these little gray, blobs cutting across up there? So, it is like the tree itself is being deformed. It is fracturing and then the silica is depositing in those cracks. Yeah, so David is asking a question I do not have the answer to which is where is all this silica coming from in the groundwater? The groundwater has various things dissolved in it at various places at various sources, you know, that is about as specific as I can get. David - Silica does not dissolve easily in rainwater. C. - Right. David - It has to be warm to dissolve.
C. -...when things die, they just tend to rot. Fossilization is a very rare circumstance, when an organism gets preserved over time. Yeah, so wood tends to rot, so things eat it, beetles eat it. Well, one of the interesting things that is contrasted in this specimen as opposed to the one they found underneath the Reagan Building, or underneath the future site of the Reagan Building, is that one is jet black. This one is very, very light colored, it has got rusty. It basically suggests more oxidized conditions in terms of its preservation. Student - Where is the other one? C. - The other one is in Rock Creek Park. If you take my Bedrock of DC Geology Trip, then I will show it to you. It is jet black and it is also got pyrite preserved in it. Pyrite is again something that breaks down in the presence of oxygen as we have seen today at the Cabin Branch Pyrite mine...where it broke down into rust and sulfuric acid...so, it is more of a typical preservational environment. Low oxygen is more likely to be preserved. So, this is somewhat anomalous, but it is a beautiful specimen. What I just love about it is the grain of the wood here.
(Last segment - Granite outcrop in creek)
...water to get in there. What happens to water in the winter? It freezes. The water freezes and expands in volume by 9%. So, that opens up that crack a little bit wider. That means it is more than likely going to break down pieces, and when those pieces get broken out you carve out a little valley in the rock, right here. The quartz itself is stable, right? But, the neighboring area is not necessarily as stable. So, if you look around this area, you can actually see this quartz vein has actually become this little valley here and it narrows down...Do not take my word for it, come see. Now I want somebody who has never taken a class with me ever before, what is this? A sausage! What is the word for sausage? A boudinage! Oui! A boudinage! Alright, there is a beautiful boudinage in the last quartz vein, there. Now in order for boudinage to happen, it has got to be hot. It has got to be under lots of pressure, right? That is something that I said happened about 10-15km depth in the crust. So, that boudinage must have taken place during mountain building when this rock was still deep and hot, after the granite had already solidified because you ca not break a granite. It went through the quartz vein until the granite was already solid, right? So sometime after 460 million years ago, but before 100 million years ago. (Dave - Does the orientation of that, uh, boudinage tell you anything about the forces? C. - It well could. I have not measured its orientation myself, so I have not even thought about trying to put that into a regional context. But, yes, orientation of different rock structures like foliation or dikes, um - joints which are what we call these little cracks in the rock - those all often tell us something interesting about the forces that went to work on the rock. Now where Adrianna (student) is standing, we see a really interesting feature. Alright, here this granite dike has been faulted. You see here? This crack is not just a crack, it is a crack on which the two sides have moved relative to one another. There is an offset here about 1 inch in this granite dike. And, look here, there is another segment and it is offset about a centimeter. And, then another one, and another one, and another one, and another one. Do you guys see that? It has basically been broken and the rocks over there where Adrianna is standing have moved probably about, you know, just judging from this alone, maybe about by 5 inches that way relative to the rocks where I’m standing. That is a brittle behavior, OK? That is strictly breaking the rock. Alright, you do not see any real evidence of flow, there, OK, unlike the boudinage. (Background discussion/question...C - No, because this water does not have a lot of quartz in it. In order to get quartz to dissolve in water as Davis was pointing out earlier, it has generally got to be kind of warm to dissolve quartz better than cold water like this.) Student - What is the difference between a dike and a vein? C. A vein is just one mineral - like we saw epidote vein in the greenstone and we see quartz veins here. But, a dike is many minerals because it is an igneous rock. It is a tabular mass of igneous rock. And this is - this crack opened up, magma squirted into that crack, solidified into rock, and later it was broken and faulted. David - Wherever you see lines and stuff, you have faults. C. - We can only call it a fault if we have clear evidence of offset on either side. Still, like this one here maybe a fault as well, but I do not have anything good that tells me there is an offset on either side. Jill - So, instead of displacement in a rock, it is just a brittle behavior? C. - You might note that it is a brittle behavior that accommodates a displacement. The displacement can happen by flow or it can happen by breakage. In this case, it is breakage. Jill - It is a displacement? C. - Yeah, it is displaced. (...a 2 second bit of talking over each other/discussion back and forth between David and Callan.) C. - Cathy had a good question like did this happen during the collision? Alright, it is a great question. I would say that this is such a strictly brittle behavior (we can even see like little shards of the dike in there) right there along that zone, that I would say that this happened sometime after these rocks cooled down. And, earlier, I evoke this tremendous mass of rock overlying this location, right? 10-15 km of overlying rock that has been removed. Now, so does that mean that this rock, right here, has been exactly at this point three dimensionally in space through all of time and that there were mountains 15 kilometers tall on top us and have been beveled down to exactly this point? Or, did this rock start off 15 kilometers down and then basically uplift as erosion went to work on the landscape? Or, was it both? So, maybe it started off 7 kilometers down with 7 kilometer tall mountains. The mountains were eroded away, that means the crust is lighter and it pops up. Then more erosion goes to work on it. So, that means the crust is lighter and it pops up. More erosion goes to work - finally exposing the granite at the surface. David - are joints in granite often a loading and unloading feature anyway? C. - Well, a lot of joints in granite are unloading features. A joint is what you call a crack in the rock along which no movement has occurred. But, oftentimes we see that they are parallel to the topography. And these are distinctly not parallel to the topography. They are vertical. If we were to see a similar joint set running through the rock like this way, you know these rocks were under lots of pressure - now on the surface they are under no pressure. Sometimes we see granite actually expand, and then that thing fractures as it expands out, and those fractures then run like this, like an onion skin. That is stuff you see up in the high Sierras in California. Topher (sp?) was just saying he would been out there, up in Lake Tahoe - and then up in Yosemite to see these big granite domes, like Half-Dome, which are sheeting off layer after layer because the granite is actually unloading, and the sheets are just popping off. Are you making a movie of me? Jill - yeah. C. Just do not put it on You-tube or anything, OK? Alright, David noticed something cool, Let us turn our attention over this way.
Look at this. There are two intersecting joint sets here. Now, a joint set is basically more than one joint that is oriented the same way. So, we have got one joint that is basically going like this. OK, very regular. And, you have got another joint set that is going like this through the rock, also very regular. Their intersection produces these little columns like square columns of granite that go downwards, right. So, like we might actually see some up movement - no, none of them are loose. Well, anyhow, these things may be related to the unloading but if they were related to unloading, I would expect to see a third joint going like this through it basically divvying it up into tubes. We do not see that, but I expect it has something to do with tectonic readjustment during the uplift process. Alright, and after the rocks are cooling down and getting uplifted as the overlying rock gets stripped away by erosion, they crack, you know that is a stressful experience for a rock. It happens vertically, sometimes like 7 kilometers or 10 kilometers, something like that. Great observation. Now, what is colonizing those cracks there? Student -What? C. - Colonizing the cracks? Student - Lichen. C. Yes, and moss. Yeah, this is lichen. This is a crustose lichen, and this is a foliose lichen. Foliose lichen has leaves, right, like folios. And, the crustose is like crust on the surface of the rock. Um, yeah, because those cracks end up holding water, that is a good place for moss. Moss likes that - it grows along those cracks. You know, if I was to take a picture of just the moss here, to show you where the moss wants to cover up the joints...... (river is too loud).
How old is this granite? 464 million years old, + or - 5. That means somewhere in the range of 459-469 million years. OK, what else have you guys noticed? There are plenty of other cool things to talk about here? I'm going to climb back over there. C. - Good. I heard nickpoint, I heard waterfall, I heard pothole. OK, the river here is adjusting from a higher level towards a lower level. You can see that it is carving out a nice, deep valley downstream, whereas upstream we do not see that big of a valley. There is a valley, certainly, but its not a serious a size as it is downstream. There is a series of waterfalls downstream from here, and the river is adjusting to a ....level, over and over and over again. Somebody brought up the term nickpoint. I think you did. The profile of a river is like where it starts off at a higher elevation and then it is descending towards a lower elevation. Those little nickpoints are where the waterfalls are. Um, this is a nickpoint right here. This nickpoint is retreating in an upstream direction. Over time, the river will adjust, and basically will be cutting down in an upstream direction, and then the downstream area, if it gets too steep will try to get flat, by mass wasting which will widen the stream valley. Oftentimes we talk about the Grand Canyon haven been cut by the Colorado River. It is only partly true. The Grand Canyon was cut deeper by the Colorado River, and the Grand Canyon got wider over time due to a landslide, not wasting events. So, gravity does not like having a whole bunch of rock supported by neighboring rock. It is more likely to collapse. So, that is the same thing here. The Quantico creek cuts downward, and over time the valley has been widened and widened and widened. Sometime on your own come back to downtown Dumfries and see how wide the valley is, because it is quite wide. Alright, questions? Jill - Did you say earlier that was an example of columnar jointing? C. - No, I was just saying that the intersection of these two joints just end up making these kind of vertical four sided columns of granite. Like you take a block of cheese you go chop, chop, chop and chop, chop, chop. You end up having these little columns of cheese. It is not columnar jointing. It is a completely different process.
Where the moss was the first one to colonize this joint, so you can see that this joint is filled in with moss, and here there is frost wedging to get this expansion of the dike, and various other process have made a little deposit of dirt here in the crack. And, a seed landed in that dirt, and the seed took root. Is this an ideal place for a tree? Probably not. But, it is growing where it ended up. In time of really high floods, you know it is probably more than likely to be stripped away. Notice that the majority of the trunks coming off of this thing are all tilted in a downstream direction. OK, that probably happened during flooding. Um, yeah so, probably not ideal...
Jill - I have a question about nickpoints. Is it like the vertical cut in the waterfall? C. - A nickpoint is basically something we describe on a river profile. The way we recognize nickpoints is we look for water dropping from one level to another. Jill - it is not like an event...C. - it is a feature. Jill - ...it is a feature. C. - And, I would only be comfortable here saying this is one nickpoint here, and there is another waterfall downstream where you get another sort of 10 foot adjustment. I would say that is a second nickpoint. And, one of the neat things at Great Falls is that as you hike along the Billy Goat Trail is, when you go up to Great Falls you can clearly see one, two, three jumps in elevation. There are three nickpoints bunched together at Great Falls. Here, they are more spread out. Jill - but there was an event that created it, right? Callan - Basically, as sea-level drops a new water fall develops there and then that starts working its way upstream. So long as it outpaces sea-level rise it is going to keep propagating upstream. Dave - ...sea levels are rising. C. - Well yeah, I mean sea levels are rising, so lower nickpoints could get drowned and then there is not going to be any more erosion going on. Jill - so basically they are faults...they are faults? C. - No, a fault is a break in the rock in which movement has occurred. These are simply levels in the rock - the river has eroded down to this level, then as base level drops, OK, the river is now eroding down to this level. So, a waterfall develops here and then it moves upstream over time. But, the actual rock underneath is not necessarily faulting. Jill - OK.
Yeah, this is an interesting blob here. I'm noticing this shape. It juts inward here and it juts outward there. It appears to be faulted where this side has moved over to the right relative to that side. That side moved to the left. The way we typically describe these things is when you have a fault - say there is a fault running through here about like that, OK? You look across to the other side, and you use the direction that side tends to move. In this case, the other side looks like it went to the left. It's a left-lateral fault.
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Prince William Forest Park - Field Notes
Hike April 6, 2008
Paper due: Sunday, April 20th
Transcription Note - I apologize for the lack of words, at times. We were so near the river, the background noise of the river made it difficult to transcribe. Also, it rained during the morning. It was not the best day for a hike, but the weather outsmarted all of us that weekend.
Valley - Quantico Creek - Main Creek
South Fork - Main Tributary
(Tape #2?)
Field Notes - Weather: Raining...Callan cautions us about being nice to one another in the rain...
On Trail Observation - Top of high hill - rounded river gravel mixed in with sand & clay
On top of another hill - we will talk about it.
Volcano erupting signage. - Hawaiian volcano example. What it would have looked like 500 million years ago. Chain of volcanic offshore islands spewing out lava. As time goes by subduction is bringing those to the edge of North America until they eventually collide. Then all igneous rock - basalt rock gets slathered onto the edge of North America. Ex. - A snowball. New basalt rock is packed onto the edges - younger material added onto the edges.
Continents (low in density 2.7) never subduct. Oceanic crust subducts (2.9 vs. 2.7). Whenever the two butt heads, oceanic crust loses. Result is isotopically dated rocks which make up the continents and ocean floor. Ages are wildly different - 4000 million years (rocks which make up the continents) vs. 200 million year old (ocean floor). There is a huge difference. Continents are 40 times as old as the oceans.
Oceanic crust is constantly getting destroyed vs. continental crust which is constantly getting preserved.
Oldest rock is in the Northwest Territory of Canada.
Old Rag mountain. Rock from Old Rag is 1 Billion years old. 1 Page handouts. 1st event - Grenville Orogeny intruded granites into the crust. Granites are a symptom of mountain belts/building. And the granites cooled and became Old Rag. The Iapetus Ocean Basin was opened up. Cracks opened up into that granite into which mafic igneous rock (basalt) squirted into those cracks. Hiking up Old Rag - narrow little slots - which are those cracks. Floor is made up of dark colored, fine grained rock whereas the sides are coarse grained, light colored granite.
All that transpired before the stuff we are talking about today.
Volcanic rock is underneath us. We eventually will see it. First we will see sediments that accumulated at the bottom of the Iapetus Ocean right next to the volcanoes. Then at lunch today we will see some of this volcanic rock, itself.
We will get to see the rocks that made up volcanic islands.
We are at the boundary of sediments and the island arc further east.
Stop - signage. Small lake, dammed valley Sign about the Fall Line. 2 Virginia physiographic provinces meet. Region of the 1) Piedmont 2) Coastal Plain (as day goes by)
Boundary is the Fall Line. Coastal Plain is made out of very loose sediments, not stuck together in to a rock; loose gravel, loose sand, loose mud. Therefore, it is very easy to erode away. It is easy for water to strip it away.
Whereas the Piedmont is made out of hard rocks. (Like the ones we are gong to spend most of the day looking at). As water is draining from the Appalachian Mountains in the Atlantic Ocean, it is easy to strip away coastal rock and hard to strip away the Piedmont Rock. And, as a consequence, the boundary between the two you tend to get rapids and waterfalls. Those rapids and waterfalls line up on a line from Southwest towards Northeast - we call that line the Fall Line.
Here is the fall zone. The image on the left is a Geologic Map. You can see the difference in colors along the water. Coastal Plain layers vs Piedmont Plain layers. The Coastal Plain is draped on top of the Piedmont like a blanket. (see diagram). The deepest, therefore the oldest part of the Coastal Plain is a series of rounded river gravel. OK? Ring any bells? That is what we are looking at back there...these gravels. So, that is actually part of the Coastal Plain draped on top of the Piedmont. Right here we only find it on tops of the highest hills, but as we head east, it is found at lower and lower elevations. So, what we are going to see as we get down at the bottom of the hill - we are going to see our first little waterfall. Where the water of the South Fork of Quantico Creek which is the creek that carves this valley here where that is falling over some of these hard, difficult to erode rocks. And, then as we get down into Dumfries this afternoon, we are going to move out on the Coastal Plain itself where it is going to be very flat and the rock layers are very easy to erode. It is going to look very different - and it is a major theme we are going to discuss more over the course of the day. Since the sign is here, I figure I had might as well say something about it.
Stop. Our first actual outcrop of rock. I wanted to give you guys a chance to check it out. Outcrops are where we actually get to see some of the Earth is rock formations at the surface - and - you would think they would be more common, but they really are not. One of the reasons for that is we are trying to do Geology in the East. Out West, there are no plants, so all the beautiful rocks are exposed with these ugly forests that cover up the beautiful rocks. So, you only get to see the rocks where the forests have been stripped away. Like here, where a stream has cut down to reveal the rocks or sometimes when we build a road underneath. So, on the East Coast we are largely limited to the rock exposures that are in creek bottoms or in road cuts. We are at the creek bottom of the South Fork Quantico Creek coming out of that little lake from the dam upstream there. I want you guys to take 2 minutes. What you are going to do is examine them. I want you to make observations about this. I do not expect you to come up with the whole geologic history...
Little patches on the surface. Those are not rocks. Those are a symbiotic relationship between a fungus and an alga, right here where it is wet. Go.
So, yes. Yes, what is causing that? Something? The water. When these outcrops are first revealed they have nice angular edges like over here next to David. As the river flows over them with time they get worn down so they get nice and smooth. We will see some really cool smoothing out features downstream like the one, Kathy, you were noticing on that sign. Yeah, we will see some of that later on. Uh, Tee, what did you notice? OK, little sharp edges...OK...breaking...everything is parallel. We see this parallelism in the rock and that extends out to the stream making these little ridges of rock that extend out to the stream... (We will get to that - hold that thought). First, I want to talk about this parallelism that we are looking at here. Jill, what are we looking at here? Foliation. Right. Folio comes from the Greek word for plant or leaf - leaves on trees...(two choppers overhead go by...)
Foliation - originally folios to describe leaves and then eventually used to describe paper or books (a foliated structure...pages in a book parallel to one another is what we are seeing here in the rock). Something layered. What is it that is layered in the rock? Sediments from the breaking down of those mountains? Let us back up. You are right, but I want to make it a little bit simpler. What are rocks made out of? Minerals, right. What we are seeing here is the minerals all lined up in a certain direction. What force could line those minerals up? Pressure from what? A tectonic collision. The North American Continent pushing in this direction and the volcanic islands pushing in this direction. Let us go back to this idea of these two plates colliding - one is a volcanic island arc and one is the North American continent squishing together - this stuff got caught between - a sick analogy...a cute, little fuzzy kitten chasing a butterfly out in the meadow. Then it runs out into traffic on Route 66 and a big Mack truck comes along from one direction and a cement mixer comes from another direction and they collide head on and the kitten gets caught I the middle. The kitten changes its shape as a result of that collision - the kitten starts off with little kitten bones and fur all oriented in different directions, and as the two trucks collide, the bones get aligned perpendicular to the two trucks' collision direction. This is the mangled corpse of these sediments of /in the Iapetus Ocean. They have been crushed. They start off in all types of different directions. I will use my hands to represent two different directions - and then they rotate to the only stable configuration possible. Picture a bunch of papers on your desk. Push your hands toward each other and the papers end up lining upright the closer you put your hands together. The minerals here are all upright - all standing up essentially vertical that is..............See how straight this thing is? (A rock used as an example).
Break...then...
...bubble. OK. Did you guys see that in the back? OK, say again, speak up... "...crack in the rock and water filled in it, warm water and it crystallized into quartz." Right, and how do you know it was deposited by water? What is the clue that is telling you that? Hydrothermally deposited quartz. This white color is due to little tiny bubbles of water in that quartz mineral crystal. That is a sure indicator that that was originally deposited in a crack. Now, however, it is only the little chunk, just like that; then there is another little chunk there and another one there and there and there... They are a little bit pink because they have been stained by rust. Notice how they all line up in the same direction, too. Like this thing here is 7 cm wide by 40 cm long. Compression coming in East to West making everything line up North to South. Now, let us talk about the original sediments that got crushed here. (The so-called kitten). There are three rocks here in my hand; three different sandstones. 1) White quartz sandstone, 2) pinkish arkose (potassium feldspar) 3) greywacke - gray - all made of sand. This one is exclusively quartz. This one is a mix of minerals and has lots of potassium feldspar. Greywacke is a mix of sand and mud. It is a dark, gray color (clay as well). If I asked you to pick one of these three as the original source of these rocks before they got squished, which would you pick? Greywacke. See the color? If I put these three down, the one that blends in the most is the gray one, right? Now there are some other things influencing the color here, but yeah, if you zoom in on these rocks you will notice little grains which were originally sand and mud in this ancient, dead ocean - The Iapetus Ocean. When the tectonic collision happened, a lot of the mud reacted, becoming mica. Mica is another mineral, and a defining characteristic of mica is sheets; flaky sheets lining up one way once again indicating again that squeezing direction.
Zeta (sp?) was asking about some fractures cutting across, like here. See the fractures? There are a bunch of brittle features cutting across this rock. These rocks were squished in a flowing way and then later they were broken by brittle fractures. So, these brittle fractures may be related to the uplift of these rocks, over time. I do not know, or cannot say specifically if it is related to some tectonic cause.
But we know (with certainty) that that happened after the rocks had cooled down again. During mountain building, these rocks were hot and flowing. After mountain building, they cooled down and that is when they broke. To get these rocks to flow, they would have had to have been heated up to 350 degrees Celsius or so. Maybe higher. We will see evidence down the way of partial melting of these rocks. Partial melting yields granite magma. So, that is another symptom of mountain building. So, you have to get the rock up to 450 degrees Fahrenheit (when wet) in order to get it to melt. Using the Principle of Uniformitarianism, you can say they once were molten based upon how we know such rocks form today.
Greywacke. David - greywacke is from Old German for grey rock.
Making the distinction between a sedimentary rock and metamorphosed sedimentary rock...layers of sediment. You do not end up finding these big, long flaky bands of quartz...presence of all this mica. We do not get big deposits out there in the actual world where mica accumulates in big layers. We get layers of mud. So, the mica itself is a metamorphic mineral. Also, we do not see any continuity. As we look along the way here, we do not see that we can detect one layer of coarse grains or one layer of sand, mud, or anything like that. Instead, we see a sort of smeared-out looking feature. These rocks appear smooshed. What would you add to that, David? (The mica here is the metamorphic mineral, not a sedimentary mineral...B word....boudinage...)
Boudinage. French for sausage. Describe a rock layer that has been broken into sausage like segments. Right here, look at this you guys. 1,2,3,4,5 sausage links. This quartz vein is broken (as a brittle phenomenon). Also, the flow was pinched out along the breaks. There are pinched out ends, like a string of sausages all in a row. This occurs at 10-15km depth from crust...right at the transition between brittle behavior in the upper crust where rocks break, and more flowing behavior in the lower crust. Brittle means breaking. Flowing is like silly putty or bread dough.
So, I wanted to elaborate on something. See picture...4 parts. Something we can deduce about these rocks. A preserved sedimentary structure called graded bedding: layers of rocks that are coarse at the bottom and fine at the top. No graded beds here today, but at the Billy Goat Trail, you will see graded beds. That tells you how these sediments initially accumulated. I am correlating these rocks here with the rocks at the Billy Goat Trail, based on similarities in their mineral content, and my knowledge of the area. I'm saying these rocks exhibit all characteristics of Billy Goat Trail rocks except we do not see any graded beds preserved here. In the Billy Goat Trail, there are a few lucky areas where we see graded bedding preserved. Why do I care about graded beds? Graded beds are deposited by currents flowing along at the bottom of the ocean. (Picture of turbidity currents). These currents are very dense, sediment rich flows, that go along the bottoms and they slow down. As they slow down, all the sediments caught up in the rolled up water settled out. The stuff that settles out first is the big stuff. The stuff that settles out last is the light weight, really fine-grained stuff. So, you end up getting these graded beds forming. Those formed down in a location like this, down in the deep sea in what we call an abyssal fan or a submarine fan, where sediments are coming off some land mass piling up in the deep sea making graded beds of greywacke. Again, greywacke means nothing more than a mix of sand and dark mud. So, that is what this used to be. Then it got crushed up. When did it get crushed up? When the Iapetus was closing, good. As the Iapetus was closing (let me pull out another graphic here) it was a scraping up all that sediment. OK, the subduction zone was going down the hatch, but the sediment on top of the oceanic crust was getting scraped off. It was building up into a big pile; a big, jumbled pile of sediment. That is analogous to a bulldozer moving over the ground scraping up a big pile of dirt in front of it, OK? Where the bulldozer is like the island arc, and then in front of the continents are the sediments it is scraping up. OK? So, that is what we are on here. Really, these rocks here are the big pile of muck that got scraped off the subducted plate and then later it got squeezed between the islands and North America. So, we call this big pile an accretionary wedge. Now, Dean, you were talking earlier about California and San Francisco. San Francisco is still on an accretionary wedge. The difference between Prince William Forest Park and San Francisco is, Prince William Forest Park then had that accretionary wedge caught between two continents; Africa and North America, and it squished. Whereas San Francisco, it just filled out. It has not been squished between two continents, yet. Give it another 50-70 million years, something like that. OK. Questions? (Why the silt...? C. - What silt are you observing? Does that just look like it is going into the hills?) What was stable in the middle of the mountain belt is no longer stable. The mica is rotting away. In fact that is why over here, when I was running my fingernail through these little grooves, there is a groove there to run my fingernail through. The mica is soft and it rots away really easily, so it ends up etching out. And, the quartz is very stable and so it does not erode away easily and that is why it makes these little ridges. So, guys if you have not felt this for yourself, come put your finger on the rock and feel this yourself. That is why, like, on the drive down I was noticing these big white boulders on the drive in. Those big, white boulders are made of quartz - it is stable. It does not break down over time. That is why when you go to the beach; the beach is made out of quartz sand. It is not made out of feldspar sand, mica sand or anything like that. It is quartz. Quartz is the stuff that lasts. (Student questions Callan. C. -- Yeah, right. Black beaches are where you are really very, very close to a basalt source. And, there are not...there is not ....adequate time to break down all those unstable minerals, so the beaches built up making those black minerals there. So, we are finding that there are some black minerals even on beaches here on the East Coast, but, the majority of it, when it is a nice mature beach, is quartz sand.)
One of the exercises I had Jill do this semester, and I had Dave do last semester, as well as the rest of my Physical Geology class is that I give them a little, bite-sized Snickers bar and they have to suck on this Snickers bar. The chocolate dissolves away very readily in their mouths followed by the caramel. The nougat lasts about 10 minutes or so. But even after you suck on this thing for about half an hour, the peanuts are still there. Peanuts do not dissolve, right. Peanuts are like quartz whereas all the other ingredients in the Snickers are more like other less stable minerals.
(Student talking about some observation). C.- Actually that is a great observation. Let us take that one step further. Imagine, now this creek here is not a creek but a road and you are driving down it. You take this turn and you take it a little too fast. Which way does your body get pulled? Right, towards the outside of the curve. So, basically, during flood times the creek comes in and slams into that wall right there and strips away the plants and strips away the leaves and strips away the dirt and it exposes rock there. Whereas the rock that is underneath the hill here is not getting hit head-on by the force of that creek. So, it fills up with sand, dirt, and leaves over time. I did hold this rock down here and I just wanted to point something out, it is a little bit difficult to see because the stupid thing is all wet, alright, just like us. But, this is a foliated metamorphic rock. Does everyone see the plane of foliation? So, if I were to line it up here with the regional foliation, it would look like that. Alright, but this one is loose so we can pick it up and examine the plane of foliation itself. And, there are little black needles there on the surface. Do you see those little needle-like minerals? They are needle-like mineral growths. Do you guys see them there? It is almost like if somebody took a bunch of chopsticks and dropped them on a desktop. The chopsticks were in this random orientation because they would be parallel to the surface of the desktop. OK? If I took all your pens and dropped them on top of someone's notebook, they would all splay out on top of that notebook but some might be pointing this way and some might be pointing that way. That is what you are seeing here in this plane of foliation. These are amphibole minerals. And the amphiboles are randomly oriented within the plane of foliation. Pass it around. I know David wants to get a good look at that with his lens. Make sure he gets that. Great. Good. (David and Callan have discussion about amphibole, spelling, etc...David asks, does the random orientation of the amphibole suggest that was it done when the pressure has been released. C. - No, I think basically that what we have here is we have flattening. Remember what we talked about was the different types of deformation; folding, faulting, and I think I said squeezing or flattening. These rocks have been flattened. And what we see is that the dominant pressure direction was coming this way and then the rocks, in order to accommodate that (here we go – here is my little Nerf ball. Yeah, I left my kitten at home...) Um, they are getting squished in this direction. Right now the Nerf ball ends up basically elongating outward this direction and growing in this direction, as well. So, like, you think about three dimensions, these right here are growing and this one...whhish....gets squashed. So, I think what is happening here is that flattening stress is causing the amphibole as they are growing...they ca not grow in this direction. It is like, try growing if the building collapsed on top of you. But, you can grow in this direction.)...we will see a couple of them here today. ...Dave, unfortunately it is not...it takes certain elements to make the amphibole, so unless those elements are present in the original sediments, you do not make it. Dave - OK. Student - Do you want to leave this here? C. Yeah, that is why I brought it down. C. - No, I would not break it, I would just kind of stash it over there underneath a tree or something so maybe in a year from now I can find it.
Another segment...
...you take away the fact that we have determined that these used to be sediments in the Iapetus Ocean basin, and then they got squeezed due to mountain building. Due to that Taconian Orogeny...this episode of mountain building that we mentioned back in the shelter. Taconian stands for the Taconic Mountains of New York, alright. (see the one page handout that I gave you). Dave - What are the compass directions here? C. - Essentially, North/South and then, East/West - squishing. Now is it actually that? Well, no its North/Northeast, but close enough. Dave - it is a good observation to make in the paper the direction in which the squishing seemed to happen. C. - Yeah, I would say that would be a great observation to make in the paper. Um - it may not have been originally in that direction though, because remember a later collision happened. That later collision also squished. So, it is like remember the kitten, Mac truck, cement mixer pile-up we had earlier? Then along comes a tank and crashes into it, OK? And then that ends up rearranging everything again. Dave - could the magnetic poles have changed here? C. - Uh, yeah, but we have no magnetic signature in these rocks. We are just trying to get in our head how - Dave - You did say North and South...C. - Yeah, so we are using modern day directions but then again North America would have been rotated around in a different position, so it is a complicated question. Sounds like it is a simple question but it is really not! Another student asks a question. C. - You do not have to understand about magnetic poles to understand what we are talking about...Dave - ...unless it is present day...(Callan brings discussion back around to)...yeah, that is all we have to work with and that is what is going to be most readily available to you guys. So, again, the one thing to think is this foliation essentially lines up on the same line from the Appalachians, basically from Georgia...Dave - OK, yeah, that is a better way to go, yeah. C. -....so, that, that is due to this first collision...Dave - ...Appalachians...to the ocean, like that? The Appalachians have...to the Ocean, like that? C. - Well, the Appalachians would be parallel to this. So, essentially we are looking at Maine up that way, Georgia down that way...ok, West Virginia, that way, and then the Atlantic out this way. Uh, I was in the middle of making a statement there and I got derailed.
(Dave - If these clay minerals had all been aligned and micas formed in the first compression, and they were tilted up in whatever orientation, and a second collision took place and it came from a different angle, would all this seem likely to get reoriented or would it preserve some of the old orientation? C. - Probably you would have some preservation and some would get reoriented, depending on where you have little bits that poked out, those being more susceptible to being re-rotated.) Um, that being said the overall structural grain of the Appalachians is this North/Northeast to South/Southwest direction, so, I mean I think we can just sort of simplify things - well, it may be an oversimplification - we can simplify things by saying these collisions all essentially came in one after another from the same direction. First, these volcanic islands during the Taconian Orogeny. There was a second collision that happened, later on, we are not really going to talk about that today - called the Acadian Orogeny. And then finally, Africa hit and that was the Alleghanian Orogeny. So, all these different orogenies were episodes of mountain building.
Oh, I know what it was that I was going to ask! When does the Taconian Orogeny happen? Well, Dana, that is just great. How did you know that!? She says 460 million years ago is when this actual collision took place. And, she is right, but, she is just pulling that number out of thin air. You can not see it. Student - she got it out of the papers C. - OK? Yeah, you are on the right track. OK, radioactive decay. So, certain minerals when they form, they take in radioactive isotopes, and then if you can go and you can say that that mineral is a mineral that only formed during an orogenies, then you can say, "Ah ha!" All you have to do is look at the radioactive isotopes that remain in that mineral and compare it to what they decay into. So, in this case the mica here has been isotopically dated. What they did was they looked at radioactive potassium 40 that is present in that mica and they compared it to the daughter product - the stable daughter which is called argon 40 And, the mica as we said earlier formed during metamorphism ...was a good state for the orogeny. The date is 460 million years ago. We are going to see a granite today, and the granite has a date of 464 million years ago. OK, so it is basically the same age. And it tells us about the same event, and granites, you remember, are another symptom of mountain building. So, we have got a really good view on the orogenies then by dating these two independent, isotopic systems. The metamorphic mica here, and then the granite that resulted from partial melting. I will remind you of that again when we get to the granite, OK? Alright, that is a good point to keep in mind. Student - So what overall type of rock is......C. - No, this is not granite, granite is much lighter color and granite does not have this foliation. Um - so this is a metagraywacke. All right. Meta, the prefix meaning change, and, greywacke telling you what it originally was.
OK., I know everybody is hungry, so we are going to hoof it. We are going to walk down to the Cabin Branch Pyrite Mine. We are going to be walking across and along the South Branch of...
...stop and maybe point out this point bar......do not feel like you have to take notes. OK, I’m not going to go over anything really important...
(Tape #3) After lunch...
OK, so granite you remember is produced by the partial melting of other rocks - remember I showed you that other diagram where you had a bunch of starting minerals and then the light colored ones sweated out - they melted? Whereas the dark colored ones stayed behind. So, here, it is a metagraywacke that is being partially melted. And the minerals that are easiest to melt those metagraywacke are quartz, feldspar, and mica - some of those are basically melting out and they are leaving behind the darker colored minerals. Ok, so we are producing these granite blobs and these blobs of magma are moving up to the crust and eventually they are settling down and crystallizing into granite. Remember we call these blobs plutons. So, here in this diagram which is part of your handout, I have got a diagram showing you some igneous plutons, OK? So that bodies that were magma and have crystallized into solid igneous rock, like a granite - OK, here is one pluton, here is another pluton - they cooled underground. Now, basically these plutons are these wet batches of magma and they are moving up through the crust. What happens to the pressure that is on them the higher up they go? Yeah, it is released and as a result they erupt - basically the granite separates out and the stuff that is most readily removed leaves the granite. So, think about a bottle of soda - did anyone bring a carbonated soda? David did. David brought a ginger ale. So, when he popped the top on that, OK, it started off as just liquid, but when he popped the top he released the pressure on the liquid inside. And as a consequence, a gas that was dissolved in the soda came bubbling out. Carbon dioxide came bubbling out of the solution. The same thing is happening to this granite which, remember, originally was magma. As it gets up to shallower depths in the crust, gases and things start coming out of it. It is leaking fluids into the surrounding rock. OK, so it is intruding into this rock and so these fluids and gases are penetrating the surrounding rock. Some of those fluids and gases would probably be water vapor, another one would be carbon dioxide, another one would be hydrogen sulfide, um a bunch of different fluids, OK? And one of the things that these fluids are taking - with - them, (I think I have got my mouth.../sandwich repeat) - OK, the fluids are carrying with them metals. Alright, metals readily dissolve in those fluids that are carried out of the granite by the fluids. And as those fluids penetrate the surrounding rock, they cool down and the metals are deposited there. Frequently, the metals are - they glom onto sulfur. Sulfur joins up with lots of different metals and then it settles out in this big sulfide deposit which surrounds the granite pluton. OK? So, it is kind of like a halo or an aura surrounding the pluton is this big aura of sulfides deposits - sulfur mixed with metals. OK? So, some sulfide minerals contain galena, some of you are familiar with galena, it is beautiful, it is got this silver luster, and these little cubes. Um, another really important one is pyrite. Pyrite is nothing more than iron sulfide. The chemical formula for iron is Fe - the chemical for pyrite is Fe. OK, it is nothing more than iron mixed up with 2 sulfurs. For every one atom of iron, there are two sulfurs bonded to it. And that makes this mineral called pyrite. Then, what color is pyrite? Golden. Yeah it is sort of this golden color and it looks a lot like gold, if you do not know what gold looks like. Um, it is got that same golden luster. Well, they were mining this pyrite here. We have already said it is not gold so why are they mining it? What on earth is the point of pulling up fool's gold from the ground? Gunpowder. C. - Gunpowder. So, basically, they are not interested in iron, they are interested in the sulfur. The main thing they are using the sulfur for is gunpowder. It is also used in many other industrial applications like making soap and refining glass and other things like that. But, not nearly as exciting as warfare. Jill - Civil War? C. - The pyrite mine right here actually started in the aftermath of the civil war, and then it actually hit a fever pitch during WWI, when there was a really big demand for gunpowder. So, they were pulling lots and lots of pyrite out of this mine, processing it to extract the sulfur and then using it to make gunpowder. So, geologically why there is a mine here is that the granite is essentially sweating out all these fluids. The fluids are rich in dissolved metals. It is just like when you sweat, there is salt in your sweat. And if it dries out on your shirt, you get a little white crust left behind on your shirt, right? So it is the same thing her except for instead of salt crustiness left behind, you will see a metal crust. So, the same granites that were produced during the Taconian Orogeny were sweating out these deposits of pyrite into the crust. Later on, people came along and said, "Hey, we can make use of that, let's make a mine here. We will dig into the hills and dig out as much pyrite as we can." So where we are right now is we are sort of geologically on this dome surrounding one of theses plutons, OK, we are in this sweaty armpit region. So, what I want to do now, is I want to go and find some pyrite and look at it. So, what we are going to do is walk back over here to this area where there was nothing growing. And, we are going to go look for some pyrite. Does that sound workable? Eventually we will come back to this place, so if there is something heavy you do not want to carry, you can leave it here and then come back and pick it up again. (Student question - inaudible. C. - The granite pluton was. Yeah, so the granite - the body of magma which would eventually cool into a granite. We will visit that granite this afternoon). (Student - So it is coming out because it...C. - the granite was, yeah, and as it is getting to shallower depths in the crust, it is devolatilizing, so the various gases that are dissolved in it are coming out. So, it is starting off here during partial melting, then the granite is organizing itself into these blobs, they rise through the crust, as it rises it is sweating out into the surrounding rock all these mineral deposits. Alright, sometimes as it moves into the crust, the crust cracks open and you end up getting veins of pyrite or veins of hydrothermal quartz. Some of those veins of hydrothermal quartz have gold in them, actual real gold. Including in the northwestern corner of this park, and by the Billy Goat Trail in the Great Falls area. So, there are gold bearing quartz veins in this area and they are coming from the same source. They are essentially being sweated out of this granitic magma. (Student - ...keep that in mind if the dollar keeps going down. C. - That is right, we will start mining our National Parks.) OK, other questions? Let's go.
(New Spot.) Now, I'm going to start talking now about some of the environmental damage that the mine caused. In this area, where the ground was near the mine operations, they were filtering the lower grade ore, you know the stuff that did not have enough pyrite in it, and they just kind of dumped it, alright? And, that is what we are sorting away here, right? And that pyrite is then soaking out here at the surface of the Earth in water, and that water is oxygenated water. And, those two ingredients end of completing a reaction of water, oxygen, and pyrite. Water, of course, is hydrogen and oxygen. Oxygen is just oxygen. And, pyrite is iron and sulfide. So, basically, after that reaction you end up getting iron mixed with oxygen and hydrogen which is rust (FeOOH) and sulfuric acid (H2So4). So, two things are being produced here due to the weathering of the pyrite; rust and sulfuric acid. Rust is what is making the soil so darn red right here. It is staining everything red. Look at David's boots right now - they are getting all soaked in this red mud. Alright, the other thing is sulfuric acid. What is the effect of sulfuric acid that you see right in this area? Basically, most plants cannot grow in super acidic soils; soils that are essentially drenched in sulfuric acid. As a consequence, nothing grew here for a really long period of time. It was basically a day of awakening - completely environmentally degraded. So, the ground was basically an empty wasteland and then the Parks said, "OK, we have got to clean up this mess." So, they took a series of steps to essentially reclaim the land. OK, this is something that frequently has to happen where they do mining - reclamation - basically making it look like a decent landscape again. And, what they did was they brought in a lot of limestone. Limestone is made out of calcite, and that reacts with acid. So, basically it is a buffer through the sulfuric acid. And, when they laid down all these limestones in the area, some of them are dissolving away as soon as they get set down. They are taking away some of that acid. It is kind of like Tums in the landscape - that is a great way to think about it? Did you have some Tums this morning? David - no. So, it is like Tums through the landscape and it is working better in some areas and not as well in other areas. It is not working so well right here. There is still a lot of sulfuric acid right here in this area which is why you can actually go and pick up rocks there. There are no plants growing out. That is thanks to the acid. Same as that little patch there at the end of the trail - there is nothing growing there. That is so weird for the East Coast to have an area where there are no plants growing. That tells you there is something seriously messed up with the soil underneath.
One of the things I would like you to do, is I would like to have you test the pH of the waterways. You might want to clean off your hands first, because the way you are going to be able to read this thing is to check the color of the paper. Alright, this is a little pH paper here. It is going to change different colors depending on the pH of the water you put it in. Now, you want to make sure you are putting it in relatively clean water otherwise you will get sediment on this which is going to give you a false reading. OK? So, I'm going to give everybody a little strip here. You can choose to test this water here, somebody should go test the water of the South Fork, and somebody should retain their strip so we can test the water of the Main Fork of Quantico Creek. OK, so we want to collect data at several different points, to several different iterations at each point, so we have reliance on the data and then we will share everything we get.
Everybody tests. OK, what do you have? 6! What do you got? (Everyone testing.)
Results 5 or 6 - somewhere between a 6 and a 5....slightly acidic. He just put it on his tongue and it is a 7. So, your tongue is pH neutral, which is a pretty good thing for your body. So, he has a reading of between 5-6 so that is just slightly below neutral, so this area is slightly acidic. That is not as acid as it once was, but it is more acid than just what regular old water would be which is 7. So, as the number gets lower in pH level, the more acidic. Higher is more alkaline. End of segment.
...sweating out of a granite pluton. Then the fact that this was mined for awhile for the purposes mainly of making gunpowder, and one of the elements that make up pyrite is sulfur, and then the breakdown of that sulfur at the conditions found at the surface of the Earth here; mixing it with water and oxygen, making rust and sulfuric acid, and then the Park surface had to treat the area by putting down essentially the geologic equivalent of Tums with limestone chips in order to get rid of the acidity. Oh, and another point that we could make here that the plants that actually are growing here are pine trees. These are acid tolerant plants. Their needles, themselves, are acidic. So, their needles are dropping - so look underneath those pine trees. You see the carpet of needles underneath, right? Those needles are essentially making that soil more acidic and that makes it harder for other plants to grow there, OK? They make a special kind of acid called a tannic acid. It is the same thing if you brew your tea for too long - it is a sort of bitter thing and it makes your stomach hurt. That is essentially what is going on over there with those trees. OK, let's move.
Testing pH at new location. (Dave - So are these the rocks that they dumped? C. - Yeah, so this is the ........that basically have water coming through them. The water is...)
OK, so I imagine we all do not want to stand here for too much longer. Did everyone see that vein of pyrite that Topher found? Time to work our way back to the path.
New location.
...C. - I'm going to turn upstream on Quantico Creek. (Jill - Confluence of Quantico Creek). We are going to look for a place to cross Quantico Creek. We want to be on the other side of it.
New location.
C. - I want to point out we are at the confluence here. So, here is the South Fork of Quantico Creek, which flows under the bridge we just walked over. Right, here coming into the main stream of Quantico Creek, the two merge right here (Jill - the confluence) and they flow downstream. Where we actually want to go is where there are a bunch of branches across the creek down there. It is probably only 200 yards from where we are right now. All right, but, unfortunately there is no great way to cross the creek right here. So, we have got to go upstream a ways, until we get to a good creek crossing. As far as a bridge and cross over. David, if you want you can wade across, but, I do not want to make every...
New location.
...flow. Where were these lava flows accumulating? Jill - Volcanoes. C. - Volcanoes, where were the volcanoes. Jill - the island arc. C. - Yeah, the chain of islands offshore, you remember, ancestral North America about 500 million years ago. Then, subduction narrowed the ocean basin between them, and eventually they got added on to the edge of North America. Now, how do I know these are lava flows? Great. Color, texture, and maybe the mineral content. Yeah, so there are some good color indicators here. What color is this greenstone? It is a very descriptive name - greenstone. It is probably in the Old German for - just kidding. So, greenstone is metamorphosed basalt. Remember basalt is what is coming out of the volcanoes today in Hawaii. (Student - asks question. C. - Well, we call it magma if it is below the surface and we call it lava if it is above the surface. Once the lava cools we have to give it a rock name. The typical name we use for the dark colored rock is basalt). Basalt is what made up the oceanic crust and what made up these volcanic islands. So, when the basalt gets metamorphosed it undergoes chemical reactions and those chemical reactions turn it green. The main green mineral here is called chlorite - it is basically a green mica. There is also epidote. Right here there is a pistachio colored mineral. You guys see that one? It is sort of a bright green? Filling in little veins over here? It is epidote. Alright, so basically, I know Topher is going to ask about this - the "take home message" is that these were once lava flows that got metamorphosed. How do we know that? The Principle of Uniformity. When you see lava flow that gets metamorphosed in the world today they change color into a green color. The reason is that they grow chlorite minerals and epidote. ....yeah, well there is some other stuff. Remember these have gotten squished. So, a lot of the original layering is lost. They are foliated. They are foliated and again it is that squishing effect. OK, I want you guys to come and look at these rocks here. There are little white circles in the rocks. Oh, see these white blobs here? OK, what is going on here is the same thing we were talking about earlier. When you release pressure on a lava, it causes gases to come out of solution. Just like when David opened his Canada Dry the bubbles formed. When lava erupts at the surface bubbles form in the lava and gases come out, right? If those bubbles do not pop before the lava sets up into rock they are preserved as little holes in the rock, like Swiss cheese. We call those vesicles. The vesicles have gotten filled in with mineral deposits. Those mineral deposits are known as amygdules. Amygdule is for the Latin for "almond." So, these were originally decided to look something like almonds - set in a piece of bread, like that. There are some really nice ones over here with mineral deposits. Generally the minerals that are filling them in are quartz, well Let us just say, quartz. David - something about popping. C. - ...if they do not pop, it leaves hole. Later on that hole could get filled in with a mineral deposit. This is important. The bubbles form as gases are coming out of lava, then some of those bubbles pop, we do not have any evidence of it. But, some of the bubbles do not pop and those bubbles get filled in later on with mineral deposits which make these little white blobs in the greenstone. And, notice that these little white blobs are not perfect spheres. Up here they appear kind of like this. Why is that? They got squished - like a little kitten's eyeball. That is due to that tectonic squishing. You can see that they are all basically squished like this line up at the plane of foliation. Alright, they lay exactly parallel to the plane of foliation. That is why the sphere became a pancake. So, they are little pancake shaped fossil gas bubbles from a lava flow. We call them amygdule because they got filled in with mineral deposits. If they were still empty holes, we would call them vesicles. ( ...you can see epidote down here in veins... come down here with David and you can see them.)
Barely audible due to noise from river. C - ...ancient volcano island rock... ...Chopawamsic....Dave and Callan in a lot of discussion about the volcanoes, flows, etc...evidence of it all...Callan sticks to history of Virginia....at one time you could have walked from Ohio to Morocco. Now those rocks were once sediments in the ocean deposited way down...
...discussion about potholes....barely audible. The role in carving it out. When water come flowing through here, there is a vortex of water. Filled with sand, silt, and it acts as liquid drill bits. Layers of quartz and mica, quartz and mica. Quartz stands out in high relief. Sand gets in there and preferentially eats away the mica. So, that is a pothole and potholes are one of the ways that streams are cutting down in areas of waterfall where they are adjusting from one level to another level. Alright, Let us go ahead and work our way back to the trail and we are going to start climbing uphill, towards the bathroom...
OK, so here we have another tree that is tipped over, and it has brought up a nice, fresh sample for us. We can see here more of that gravel that we saw when we first started on the trail today. This is not a rock. This has not been stuck together into a rock - it is just loose gravel...David - it is a root ball, right? C. - Yeah, it is a root ball of a tree. You can pick up the loose grains of sediment, and let it run through your fingers. Actually, I encourage everyone to do this. You will feel that this is a mix of sand and clay, and the clay will feel very sticky on your hand, and then these nice, rounded pebbles and cobbles of mainly quartz, OK? Most of which is present here is quartz. Student - Sand and all that is what is left over after the quartz was left. C. - Well, basically, you said it yourself. This is a very poorly sorted sand pile. This is a mix of different grain sizes, which indicates that it was dropped very rapidly. Now, what does the rounding tell you? Jill - it was well sorted. C. - ...well traveled. It is not well sorted, it is poorly sorted. Yeah, the rounding tells you that this quartz cobble started off somewhere far away and then it traveled a long distance to get here and as it traveled it got more and more rounded. This one must have started off a little bit closer. Alright, because this one is a little bit more angular. And that is typical of river systems because river systems end up draining a whole area. Rocks are dropping into them from far away and nearby and they are both tumbling downstream together. And, Jill, that is how you interpreted this, right? Jill - yeah, well no, sorry, I was off in a zone. C. - Jill, how would we interpret this deposit, how did this form? Jill - Well, basically, it came off of an uplift, and it traveled downward, and it was deposited into a system of water...C. - OK, what kind of water? Jill - Probably very fast. C. - OK, good, why do you say that? Jill - Because, it looks like they are rounded, I mean...obviously they passed through...Student - ...it is the size of the rock...C. Yeah, it is the size. Student - it is a well-rounded big rock. C. - Yeah, it is obviously a well-rounded, nice big cobble of quartz. And, it takes a lot of water energy to move something this size. Jill - Yeah, definitely. C. - OK, good, so what kind of body of water has the energy to move big cobbles like this? Jill - River, a river. C. - Yeah. Rivers, OK? Because we just saw, in fact, I just destroyed my vocal chords trying to shout over - rivers have a lot of energy. Whereas a lake does not have so much, a swamp - less. The ocean has a fair amount of energy, but you generally do not get big cobbles like this in an ocean because as soon as rivers flow into the ocean, they slow down, and then they drop all these things, and then they carry maybe the sand and the mud further out into the ocean. Those are all that really make it into the ocean. So, this is a river deposit. And, when you think about it you might think it is a little bit weird, because we were just down at the river and we just hiked uphill, and now at the top of the hill we see these river deposits? What is going on here, Cathy? Cathy - The river was once up here? C. The river was once up here, she says. These were not recent flood deposits. So, these are ancient, and the reason I know that is I can come up with a rough date for these deposits based on fossils that we find within this same gravelly unit. Now, this gravelly unit here, do you think it is a particularly good setting for preserving fossil remains? Callan hits the “unit” with large cobble, again and again. C. - Do you think that is good for a fossil? All right. Think about the river here. As these things are moving along, all these boulders are smacking into one another and grinding around. This is a lousy environment for preserving fossils. It is a miracle that we have any fossils at all from this unit. The fossils that we do have from this unit are very poor and they have been broken up a bit but we can still identify them and they are dinosaur bones. Alright, we found 3 or 4 different dinosaur bones from this one unit. There are some sauropod fossils, sauropods are the big, sort of lumbering, vegetarian dinosaurs with the long necks. We found some of their teeth and some of their leg bones. OK, there is a species called Astrodon johnstoni, it is the state fossil of Maryland. Basically, Astrodon means star-tooth. Think about your molars in the back, and you have these little points for grinding. There is a series of five radiating ridges for grinding up vegetation. So, Astrodon johnstoni. Also, we found some raptor fossils in here. At any rate, these dinosaur fossils date back to the Cretaceous. Cretaceous is a period of geologic time that ended at about 65 million years ago. It started at about 120 million years ago. The best estimate for an age on this unit is about 100 million years old. One hundred million years ago a river was flowing along this area, and that river was meandering. It was cutting in at a cut bank and it was depositing materials on a point bar. This is an old point bar deposit. OK, remember we saw piles of gravels that look a lot like this being deposited down at Quantico Creek today. So, this river was no longer cutting down, it was simply meandering back and forth on the landscape. Now, something must have happened between what we were just talking about at the base of the hill, and the deposition of these river gravels. We have this great big mountain range that had gotten built up, right? The size of the Himalayas - what happened to that? It was eroded down to essentially a flat level, and over that flat level, this river was meandering back and forth depositing gravel. OK? Then at some point after that what happened? Sea-level dropped and what did the river do in response? It started cutting down again and carving new valleys like the valley that we spent most of the day hiking through, OK? So we have evidence here of a higher sea-level at some point where these rivers were meandering along depositing gravels as they flowed eastward from the west out toward the young Atlantic Ocean. The Atlantic Ocean, by the way at this point, was 100 million years old. The Atlantic Ocean was born 200 million years ago and these gravels were deposited 100 million years ago, therefore, that is about 100 million years into the history of the Atlantic. One of the reasons that I’m able to say that these rivers were flowing from the west to the east, is we find signatures of rocks that we know only come from the west. Like we find pieces of granite that we find from the Blue Ridge Province. And these Blue Ridge granites have blue-quartz in them. Which is an indication that these are from the Blue Ridge Province. Yeah, there is some nice blue-quartz in this sample to right up there by my thumbnail. It has sort of a purplish sheen to it. You guys see that? The other thing that sometimes we find here is quartz cobbles that have a trace fossil right in them. That tells us that that came from the west, the river brought it this direction deposited it here, therefore that river was flowing from west to east. The same direction the rivers are flowing today. Now when the Appalachians were real, real young, it was the opposite. The rivers were starting here in the highlands above our heads and draining off to the west. Student - that is why you find Appalachian rocks way out in the west - west of us...C. - Yeah, there is some fairly compelling evidence in fact that the Petrified Forest of Arizona was buried underneath Appalachian sand and mud. In Arizona - so we are talking Mississippi sized rivers draining these young Appalachian Mountains, transporting the sediments to the west, and then basically they snuffed out a forest out there in Arizona. We can go and pick up a certain mineral from those sands, and that zircon has a chemical signature that is more analogous to the Appalachians than it is to any local source out there like in the Rockies. So, it indicates again at that time the mountains were here and the lowland was there. Who has got their handout handy, the one with the colored map on the back? Callan explains map....Kp designation which stands for Cretaceous. The sub p there indicates the Potomac and this is called the Potomac group. They are exposed up and down the length of the Potomac and you find them on tops of the highest hills. Same unit atop Tyson's Corner. You find it on top of Mt. St. Alban where the National Cathedral is. You find it on top of Mount Pleasant in D.C. - river gravels, river gravels, river gravels....Say that three times fast. So, this surface on top of which the river gravels are deposited is a period of missing time. The last geologic evidence we have in this area is the intrusion of granites. That happened around 460 million years ago, and then the next thing that is recorded in this area is the deposition of this gravel on top of it, which happened 100 million years ago. So, 360 million years of geologic time are missing in this area. We can say nothing about them from Prince William Forest Park. You have to read the geologic record to find out what is missing from those 360 million years. OK? Student - How come no one could think about water or erosion? C. To erode away a Himalayan sized mountain range takes a fair amount of time, and so it took a long time to grind down those mountains to that level. Plus, during the Cretaceous, the world was quite warm and sea-level was quite high. There was very little glacial ice, if any, and at that point then you have this combine effect of having ground down the mountains plus sea-level being high and that is when it deposited gravel all over this area. Wow, so this is like a little mud stone, right. Student - I get extra credit. C. - I would not call this a schist, it does not have nice physical minerals, but I would not mind calling it a mudstone. This is a little clast of mudstone and this is very typical of some mudrock layers that are typical out in the Valley and Ridge Province and that would be consistent with the story of basically transporting eastward. That is rose quartz there...OK, the first thing we are going to do when we get back to the visitors center is use the restroom, and then we are going to go and visit a fossil tree and that fossil tree is growing during the same period and it was probably growing along the banks of the river.
This was deposited by a river after the Appalachian Mountains were ground down. It is Cretaceous in age. It never experienced mountain building. If this had gotten caught up in that collision in would not be a loose pile of gravel, mud, and sand, it would be a rock. What would we call this if this had gotten cemented together into a rock? A conglomerate. Nature's version of cement with big chunks set in a little fine-grained matrix.
And, I do not really know what that means, I mean....
...on the trail today you are not leaving behind your skeleton, but you are leaving behind traces of yourself. Right? So, the thing that this Skolithos worm tube tells us is that it is a piece of the Antietam Formation. Antietam Formation is a big sandstone unit that is out on the western slopes of the Blue Ridge. And you can find them near Antietam National Battlefield, where the bloodiest battle of the Civil War was fought. You can also find this same rock along portions of Skyline Drive and there is an area where David and I have hiked near 66. There is a nice big exposure of it near Front Royal. So, basically it is telling us that the river transport direction was westerly - consistent with the blue-quartz. Sedimentary transport from west to east.
...Fossilized Bald Cypress Tree. It is really not in any danger of being degraded or anything like that, but it is in danger of having stuff grow on it. It is the mineral itself that will break down. All quartz. The quartz was derived from Cretaceous river gravels...And as it met this wood, it percolated through the wood, soaked into it, a chemical reaction took place, and it precipitated silica in place of the wood. This similar process has occurred in the Potomac Formation. In Washington DC, when they were digging out the foundation for the Ronald Reagan Building, they found more of these there. Also, at the base of the Mayflower Hotel, they found these fossil tree trunks down there. And, what do all these areas have in common? They basically have these cretaceous, Potomac Group river gravel deposits. So, you can imagine growing along the banks of this ancient river, Cypress Trees. How would you recognize a Cypress Tree if you saw one today? Yeah, Bald Cypress, and they have got these weird structures that rise up out of the waters. They call them knees - Cypress knees. They poke up above. I noticed in a place up here there are little tension gashes and they are filled in with silica, too. You see that - these little gray, blobs cutting across up there? So, it is like the tree itself is being deformed. It is fracturing and then the silica is depositing in those cracks. Yeah, so David is asking a question I do not have the answer to which is where is all this silica coming from in the groundwater? The groundwater has various things dissolved in it at various places at various sources, you know, that is about as specific as I can get. David - Silica does not dissolve easily in rainwater. C. - Right. David - It has to be warm to dissolve.
C. -...when things die, they just tend to rot. Fossilization is a very rare circumstance, when an organism gets preserved over time. Yeah, so wood tends to rot, so things eat it, beetles eat it. Well, one of the interesting things that is contrasted in this specimen as opposed to the one they found underneath the Reagan Building, or underneath the future site of the Reagan Building, is that one is jet black. This one is very, very light colored, it has got rusty. It basically suggests more oxidized conditions in terms of its preservation. Student - Where is the other one? C. - The other one is in Rock Creek Park. If you take my Bedrock of DC Geology Trip, then I will show it to you. It is jet black and it is also got pyrite preserved in it. Pyrite is again something that breaks down in the presence of oxygen as we have seen today at the Cabin Branch Pyrite mine...where it broke down into rust and sulfuric acid...so, it is more of a typical preservational environment. Low oxygen is more likely to be preserved. So, this is somewhat anomalous, but it is a beautiful specimen. What I just love about it is the grain of the wood here.
(Last segment - Granite outcrop in creek)
...water to get in there. What happens to water in the winter? It freezes. The water freezes and expands in volume by 9%. So, that opens up that crack a little bit wider. That means it is more than likely going to break down pieces, and when those pieces get broken out you carve out a little valley in the rock, right here. The quartz itself is stable, right? But, the neighboring area is not necessarily as stable. So, if you look around this area, you can actually see this quartz vein has actually become this little valley here and it narrows down...Do not take my word for it, come see. Now I want somebody who has never taken a class with me ever before, what is this? A sausage! What is the word for sausage? A boudinage! Oui! A boudinage! Alright, there is a beautiful boudinage in the last quartz vein, there. Now in order for boudinage to happen, it has got to be hot. It has got to be under lots of pressure, right? That is something that I said happened about 10-15km depth in the crust. So, that boudinage must have taken place during mountain building when this rock was still deep and hot, after the granite had already solidified because you ca not break a granite. It went through the quartz vein until the granite was already solid, right? So sometime after 460 million years ago, but before 100 million years ago. (Dave - Does the orientation of that, uh, boudinage tell you anything about the forces? C. - It well could. I have not measured its orientation myself, so I have not even thought about trying to put that into a regional context. But, yes, orientation of different rock structures like foliation or dikes, um - joints which are what we call these little cracks in the rock - those all often tell us something interesting about the forces that went to work on the rock. Now where Adrianna (student) is standing, we see a really interesting feature. Alright, here this granite dike has been faulted. You see here? This crack is not just a crack, it is a crack on which the two sides have moved relative to one another. There is an offset here about 1 inch in this granite dike. And, look here, there is another segment and it is offset about a centimeter. And, then another one, and another one, and another one, and another one. Do you guys see that? It has basically been broken and the rocks over there where Adrianna is standing have moved probably about, you know, just judging from this alone, maybe about by 5 inches that way relative to the rocks where I’m standing. That is a brittle behavior, OK? That is strictly breaking the rock. Alright, you do not see any real evidence of flow, there, OK, unlike the boudinage. (Background discussion/question...C - No, because this water does not have a lot of quartz in it. In order to get quartz to dissolve in water as Davis was pointing out earlier, it has generally got to be kind of warm to dissolve quartz better than cold water like this.) Student - What is the difference between a dike and a vein? C. A vein is just one mineral - like we saw epidote vein in the greenstone and we see quartz veins here. But, a dike is many minerals because it is an igneous rock. It is a tabular mass of igneous rock. And this is - this crack opened up, magma squirted into that crack, solidified into rock, and later it was broken and faulted. David - Wherever you see lines and stuff, you have faults. C. - We can only call it a fault if we have clear evidence of offset on either side. Still, like this one here maybe a fault as well, but I do not have anything good that tells me there is an offset on either side. Jill - So, instead of displacement in a rock, it is just a brittle behavior? C. - You might note that it is a brittle behavior that accommodates a displacement. The displacement can happen by flow or it can happen by breakage. In this case, it is breakage. Jill - It is a displacement? C. - Yeah, it is displaced. (...a 2 second bit of talking over each other/discussion back and forth between David and Callan.) C. - Cathy had a good question like did this happen during the collision? Alright, it is a great question. I would say that this is such a strictly brittle behavior (we can even see like little shards of the dike in there) right there along that zone, that I would say that this happened sometime after these rocks cooled down. And, earlier, I evoke this tremendous mass of rock overlying this location, right? 10-15 km of overlying rock that has been removed. Now, so does that mean that this rock, right here, has been exactly at this point three dimensionally in space through all of time and that there were mountains 15 kilometers tall on top us and have been beveled down to exactly this point? Or, did this rock start off 15 kilometers down and then basically uplift as erosion went to work on the landscape? Or, was it both? So, maybe it started off 7 kilometers down with 7 kilometer tall mountains. The mountains were eroded away, that means the crust is lighter and it pops up. Then more erosion goes to work on it. So, that means the crust is lighter and it pops up. More erosion goes to work - finally exposing the granite at the surface. David - are joints in granite often a loading and unloading feature anyway? C. - Well, a lot of joints in granite are unloading features. A joint is what you call a crack in the rock along which no movement has occurred. But, oftentimes we see that they are parallel to the topography. And these are distinctly not parallel to the topography. They are vertical. If we were to see a similar joint set running through the rock like this way, you know these rocks were under lots of pressure - now on the surface they are under no pressure. Sometimes we see granite actually expand, and then that thing fractures as it expands out, and those fractures then run like this, like an onion skin. That is stuff you see up in the high Sierras in California. Topher (sp?) was just saying he would been out there, up in Lake Tahoe - and then up in Yosemite to see these big granite domes, like Half-Dome, which are sheeting off layer after layer because the granite is actually unloading, and the sheets are just popping off. Are you making a movie of me? Jill - yeah. C. Just do not put it on You-tube or anything, OK? Alright, David noticed something cool, Let us turn our attention over this way.
Look at this. There are two intersecting joint sets here. Now, a joint set is basically more than one joint that is oriented the same way. So, we have got one joint that is basically going like this. OK, very regular. And, you have got another joint set that is going like this through the rock, also very regular. Their intersection produces these little columns like square columns of granite that go downwards, right. So, like we might actually see some up movement - no, none of them are loose. Well, anyhow, these things may be related to the unloading but if they were related to unloading, I would expect to see a third joint going like this through it basically divvying it up into tubes. We do not see that, but I expect it has something to do with tectonic readjustment during the uplift process. Alright, and after the rocks are cooling down and getting uplifted as the overlying rock gets stripped away by erosion, they crack, you know that is a stressful experience for a rock. It happens vertically, sometimes like 7 kilometers or 10 kilometers, something like that. Great observation. Now, what is colonizing those cracks there? Student -What? C. - Colonizing the cracks? Student - Lichen. C. Yes, and moss. Yeah, this is lichen. This is a crustose lichen, and this is a foliose lichen. Foliose lichen has leaves, right, like folios. And, the crustose is like crust on the surface of the rock. Um, yeah, because those cracks end up holding water, that is a good place for moss. Moss likes that - it grows along those cracks. You know, if I was to take a picture of just the moss here, to show you where the moss wants to cover up the joints...... (river is too loud).
How old is this granite? 464 million years old, + or - 5. That means somewhere in the range of 459-469 million years. OK, what else have you guys noticed? There are plenty of other cool things to talk about here? I'm going to climb back over there. C. - Good. I heard nickpoint, I heard waterfall, I heard pothole. OK, the river here is adjusting from a higher level towards a lower level. You can see that it is carving out a nice, deep valley downstream, whereas upstream we do not see that big of a valley. There is a valley, certainly, but its not a serious a size as it is downstream. There is a series of waterfalls downstream from here, and the river is adjusting to a ....level, over and over and over again. Somebody brought up the term nickpoint. I think you did. The profile of a river is like where it starts off at a higher elevation and then it is descending towards a lower elevation. Those little nickpoints are where the waterfalls are. Um, this is a nickpoint right here. This nickpoint is retreating in an upstream direction. Over time, the river will adjust, and basically will be cutting down in an upstream direction, and then the downstream area, if it gets too steep will try to get flat, by mass wasting which will widen the stream valley. Oftentimes we talk about the Grand Canyon haven been cut by the Colorado River. It is only partly true. The Grand Canyon was cut deeper by the Colorado River, and the Grand Canyon got wider over time due to a landslide, not wasting events. So, gravity does not like having a whole bunch of rock supported by neighboring rock. It is more likely to collapse. So, that is the same thing here. The Quantico creek cuts downward, and over time the valley has been widened and widened and widened. Sometime on your own come back to downtown Dumfries and see how wide the valley is, because it is quite wide. Alright, questions? Jill - Did you say earlier that was an example of columnar jointing? C. - No, I was just saying that the intersection of these two joints just end up making these kind of vertical four sided columns of granite. Like you take a block of cheese you go chop, chop, chop and chop, chop, chop. You end up having these little columns of cheese. It is not columnar jointing. It is a completely different process.
Where the moss was the first one to colonize this joint, so you can see that this joint is filled in with moss, and here there is frost wedging to get this expansion of the dike, and various other process have made a little deposit of dirt here in the crack. And, a seed landed in that dirt, and the seed took root. Is this an ideal place for a tree? Probably not. But, it is growing where it ended up. In time of really high floods, you know it is probably more than likely to be stripped away. Notice that the majority of the trunks coming off of this thing are all tilted in a downstream direction. OK, that probably happened during flooding. Um, yeah so, probably not ideal...
Jill - I have a question about nickpoints. Is it like the vertical cut in the waterfall? C. - A nickpoint is basically something we describe on a river profile. The way we recognize nickpoints is we look for water dropping from one level to another. Jill - it is not like an event...C. - it is a feature. Jill - ...it is a feature. C. - And, I would only be comfortable here saying this is one nickpoint here, and there is another waterfall downstream where you get another sort of 10 foot adjustment. I would say that is a second nickpoint. And, one of the neat things at Great Falls is that as you hike along the Billy Goat Trail is, when you go up to Great Falls you can clearly see one, two, three jumps in elevation. There are three nickpoints bunched together at Great Falls. Here, they are more spread out. Jill - but there was an event that created it, right? Callan - Basically, as sea-level drops a new water fall develops there and then that starts working its way upstream. So long as it outpaces sea-level rise it is going to keep propagating upstream. Dave - ...sea levels are rising. C. - Well yeah, I mean sea levels are rising, so lower nickpoints could get drowned and then there is not going to be any more erosion going on. Jill - so basically they are faults...they are faults? C. - No, a fault is a break in the rock in which movement has occurred. These are simply levels in the rock - the river has eroded down to this level, then as base level drops, OK, the river is now eroding down to this level. So, a waterfall develops here and then it moves upstream over time. But, the actual rock underneath is not necessarily faulting. Jill - OK.
Yeah, this is an interesting blob here. I'm noticing this shape. It juts inward here and it juts outward there. It appears to be faulted where this side has moved over to the right relative to that side. That side moved to the left. The way we typically describe these things is when you have a fault - say there is a fault running through here about like that, OK? You look across to the other side, and you use the direction that side tends to move. In this case, the other side looks like it went to the left. It's a left-lateral fault.
--- T R A N S C R I P T --- E N D S ---
As before, I would be pleased to hear any comments / insights / suggestions you might have.
Labels: field trips, nova, piedmont, teaching
2 Comments:
thanking you for taking the trouble to post this. So fascinating and I would never have had a chance to hear
Interesting about knickpoints, having first learned about the concept in the desert west, I never really thought about waterfalls!
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