Thursday, April 17, 2008

Virtual Billy Goat Trail trip

One of my most dedicated students (a) recorded our field trip last week to the Billy Goat Trail, and (b) transcribed it. Because I'm pretty much overworked at this point, I haven't been very blogophilic over the past couple of days. My apologies. So I'm going to offer you Jill's transcription of the Billy Goat Trail geology field trip instead. I've made a few small edits to clarify, but otherwise it's her transcription of our discussion/my lecturing at the various stops. I find it an interesting document... who would have ever thought anyone would pay this much attention to what I have to say?

--- T R A N S C R I P T --- B E G I N S ---

Billy Goat Trail Field Notes - April 8, 2008

Canal - C and O Canal stands for Chesapeake and Ohio. It's the canal that was originally intended to link the Chesapeake Bay watershed with the Ohio River. The Ohio River drains into the Mississippi River. So it's going to basically provide a watery link across the Appalachians. This specific structure right here as part of the canal, is what? It's a lock. The reason they went through all the trouble of building the canal where there's a river right there is you can't sail a river up a waterfall. Right here, there's a major waterfall [Great Falls] that prohibits navigation upstream and downstream. They built this canal where boats could sail upstream in a series of steps. These boats were actually pulled upstream. Technically, they weren't sailing. They were pulled along by mules. The mules were attached to the boat by a rope. The mules would pull the boat through these narrow, little chambers. Then these gates would swing shut at the downstream end and they would open up these gates at the upstream end. And, water would fill it up until the lock was filled with water at the upstream level, there. And then the boat would be pulled on out and do the same thing making steps uphill. Only when you do something like that, allowing a boat to float to a higher level, can you actually move a boat in the uphill direction. And they would do the same thing going downhill. Again, you can''t sail a boat down the waterfall very easily either. It's a little bit easier than sailing it up, but it's still not very safe. So, the C and O Canal was built for that purpose. It was originally the brainchild of none other than George Washington who originally tried to build the canal to get around Great Falls on the other side - on the Virginia side; called the Patowmack Canal. It's a very small canal. You can see its remains today. But, this (the C and O Canal) was a more successful canal - ultimately it was not completely successful. Ground was broken on it by John Quincy Adams about 5 miles downstream from here. He took that first shovel, and couldn't get that shovel into the ground. He tried again and again and he broke a sweat - it was very embarrassing and then he was a very staid individual and he rolled up his sleeves and played the part of like... "I'm going to get this!"...and so eventually that's when the canal construction began. The canal never made it to the Ohio River. It made it as far west as Cumberland, Maryland. In fact, the canal is now over 184.5 miles long. It's almost 185 miles long; it's quite long. At some point, the Baltimore/Ohio Railroad was started. The Baltimore/Ohio Railroad ultimately proved to be a more efficient means of extracting the natural wealth from the Appalachians; timber, and etc... And, the C and O Canal fell into disuse and eventually it was abandoned. At some point, developers were talking about taking this area and turning it into a highway that ran East/West along the Potomac River. Then there was this Supreme Court Justice who stepped in and said, "That's a lousy idea. It's a beautiful area. We should preserve it as a National Park." That Supreme Court Justice's name was William O. Douglas. He challenged a bunch of senators and editors of various Washington newspapers to join him on a walk. They went up to Cumberland, Maryland and they walked down the length of the canal to Georgetown where it ends. At the end, all of them were convinced that this was a place worth saving. And so, it became a National Park thanks to that one man saying, "we need to preserve this place." The reason we're able to come here today and actually look at rocks and experience the landscape is thanks to those efforts made by him and others inspired by him. So, that's why this area is still in a reasonably natural state.

The Billy Goat Trail starts about a quarter mile downstream. We're going to walk the Towpath where the mules once towed the barges up and down the canal. Until we get to the start of the trail.

It's a bridge. However, that's a really big abutment for a teeny bridge like that. The only thing going over that bridge is people. Yet they've got these massive abutments that are 40-50 feet thick. (You can see this one goes off into the woods that way: the abutment). Why would they go through the trouble of making a massive, racking structure just for the sake of a little footbridge? Because of flooding - yeah. This is not a structure for a bridge. This has to do with flooding. Explain yourself. If you're a canal engineer, and you spend years of your life and blood, sweat, and tears making that canal, you don't want the canal destroyed by a flood, right?...shutting down commerce from east to west. You want some kind of a fail/safe that you can activate in times of flooding. That's what this is. This is a flood control structure. You can see that there are grooves there, and in those grooves are slotted wooden structures, kind of like this one that I'm standing on. They're thin at the edge, thick in the middle, and that allows them to resist water slamming into them. And, think about that for a second. If you just walk, ...walk past this amazing view over there to the right, and you saw the Potomac River down 45 feet below you, -- during times of flooding the Potomac River level is up here - during times of flooding the discharge increases and the depth increases, so that the river is actually where you're up here standing, now, during times of highest flooding. And that could be totally destructive to your canal. So, this thing is put here so that in times of flooding they could act quickly and move these things in and make a big wall there. The flood waters slam into that wall and they could divert it off into the woods here and dump back into the Potomac River's main gorge. That main gorge is what we're going to be hiking along today and as we begin walking along the Billy Goat Trail, which I see is officially closed... as we begin walking on the Billy Goat Trail, keep your eyes peeled for evidence of flooding. You're going to see some evidence almost immediately after we start down the trail. Some of you are going to recognize that evidence. Some of you are going to walk right by it and not notice it. Our goal today is to turn up our observation meter so we are observing more. So anybody once you see some evidence that indicates flooding, call it to my attention and we'll stop and we'll discuss. (Question, John.) OK, there is sort of a wetland area right here - a little sag area where the water table is intersecting the surface - we're going to talk about groundwater in lecture next week. Basically, the groundwater is right on the surface so you get standing water there - a small little wetland. Good observation but that doesn't indicate flooding. (Jill - how about all those trees and brush just kind of pushed to the side?) Good, all right, it's a good observation - it's got something to do with trees...

OK. I'm expecting you guys to pay attention today. Probably you're going to want to take notes because ultimately what I'm expecting you to produce for me as a result of this field trip is a summary paper. This paper is going to be about 3 pages long - something like that and the paper is going to describe the geology of the Billy Goat Trail based on what we observe today. So this paper is going to be broken down, essentially, into an observation and then a geologist's interpretation of that observation. And then another observation and how a geologist interprets that. And so by talking about the physical evidence, and then separating it from the story that geologists tell based on that physical evidence, you're going to get an overall history of what happened to these rocks over time. (The paper is due in two weeks).

Head of the Trail - walking from the beginning of BGT.

Right, so during flood times, the water is coming from upriver, it's slamming into that flood diversion structure, and it ...over the landscape in this direction. So you'll see that the trees here are preferentially tilted in that direction. Do you see this one? How it's tilted in that way? This one, in fact, used to have this as its main trunk. That main trunk was killed and a little branch became the new trunk. Or look at this one over here. See how this one is pushed out in the same direction? Both of the original trunks broke off. Here was one, here was the second. Then branches became the new main trunk of that tree. Do you guys see that? -- tilted in a downstream direction. And, if you look around, you'll see plenty... now just tilted trees doesn't necessarily imply flooding. Trees that are all tilted in a common direction imply that they were all knocked down by a similar force. Knocked down but not killed. See how many tilted trees you can count.

Not creep - creep is on a slope. This was not creep. Tilting trees.

Knocked down by a flood, then it continued growing. Those do happen to be knocked down in the same direction but I'm not sure they were knocked down by a flood. Basically because those weren't knocked down last time I was here, and we haven't had a flood since then. I'm extrapolating that they were not knocked down by a flood. Furthermore, there's still dirt in the root. If you had a flood up here that was strong enough to knock down a tree it would likely have stripped away all that dirt.

We do have a couple of people coming through. We do want to clear - just step aside and make a path - part the Red Sea here.

John has made an observation that there's a round boulder up there. And that round boulder looks really different than most of these angular boulders that we see up here. John, is it also the same sort of stuff - does it look the same in terms of its composition? No, so maybe that could have come from somewhere else and the rounding suggests what, Elizabeth? It traveled a long distance (very good, Sal), OK. Remember the farther a sedimentary grain travels the more rounded it gets. So, flood waters may have deposited that, or maybe the Potomac River used to be flowing up here at this level. We'll talk more about that possibility later on when we see more of these boulders. It's a little premature to get into that, but it's - uh - a pretty big boulder. It's the sort of thing that wouldn't be picked up by the current and carried in a suspended load. It's more likely to be bed load along the bottom. So that indicates that that may be evidence that this used to be the bottom of the Potomac River before it incised to a deeper level.

What I'm stopping here for is where we're starting to see some more rocks. We're getting down closer to the river and because this area is more frequently subjected to flooding, that means there's less vegetation here. There's less dirt here. And, we can see more rock here. Your assignment over the next two minutes is to figure out what kind of rock this is. I'll give you two minutes - you're welcome to roam all around this area. What you want to do is you want to find nice, clean surfaces and try and identify the minerals, the texture, and ultimately the kind of rock that this is. Keep in mind that there is junk growing on the rock surface like this. What is this thing? It's a lichen, right. Lichen is a mix of algae and fungus that grows on rock surfaces. So, don't look at the lichens; they will deceive you. There are many different colors; these grey blobs are lichens, there are black ones, there are orange ones. You want to look for nice clean rock surfaces that don't have any lichens growing on them. OK, two minutes!

OK, what I would recommend everybody do is find yourself a nice, hunky seat. We're going to be here about 10-15 minutes, discussing. Somebody start us off with an observation about some of the different minerals that you've seen, or some of the textures that you've seen. Quartz. Big blobs of quartz here (she's got acid she's been dropping and the rocks aren't fizzing - not calcite). Some of those are very striking and obvious - very creamy looking - big blobs like right here, right here on that knob, etc. Good. What other minerals do we see here? Mica - muscovite micas, the silvery micas. Sometimes it's really obvious like, look at this, look at the shine on that, great. Nice and shiny mica. What can you tell me about all those flakes of mica? Are they oriented in random directions? Or are they all aligned in a common direction (Jill - they're in sheets). They're in sheets, says Jill. Would you agree with that John? How about you Elizabeth? OK? Yes, all the micas are aligned in sheets. And, obviously some of these are boulders broken off. Some are bedrock where the sheets are still in their original position. Like the one Jorge is sitting on - this one here - the one Elizabeth is sitting on. What is the orientation of those sheets in space? If you took your hand and made your hand a flake of mica how would you orient it in space? OK, good. Doug is showing us with his hand the orientation of all those flakes of mica in space. So what is that? When you get these layers of quartz and mica all basically strung out in these vertically oriented sheets? It's metamorphic foliation. When we studied metamorphic rocks, there are foliated metamorphic rocks and non-foliated metamorphic rocks. These are foliated. What does it take to produce foliated metamorphic rocks? Pressure, very good, Vivian. What kind of pressure? (Confining pressure is what happens to you when you're at the bottom of a new swimming pool: it may cause your ears to pop, but it doesn't realign your head in a new direction). The answer is differential pressure. So, what's happening here is that these rocks have been compressed, OK? Force is pushing on them this way, and then another force is pushing on them this way. So, all those original minerals got squished together, and they ended up lining up straight up and down as they were squeezed from the sides. So, this is a metamorphic rock. You guys have just figured out something really important about these rocks. What tectonic event creates regional metamorphic rocks that have foliation? Orogeny. So, these rocks have experienced orogeny. They've been squished from the sides due to that tectonic collision. Whoa! That's a pretty big insight to come to about these rocks. I'm sure this raises all kinds of questions in your head. Go ahead and ask some of those questions.

(Vivian - no, talus is often great big blocks like this - talus usually accumulates at the base of a cliff. You might be able to call some of this talus - like this could be a block of talus. This is not - this is bedrock. It's still attached to the solid earth. It's not that it's broken off and made into a piece of sediment like this. You'll see some areas today where you'll see some large accumulations of boulder piles, and I guess you could call that talus. Remember talus is specifically when it's falling into place.)

Jill - we're in the Coastal Plain? No, we're in the Piedmont. Is this a part of the Taconian Orogeny? Well, one way we can answer that question - Jill's bringing up the Taconian Orogeny. I want you guys to think back to when we talked about the geologic history of Virginia in lecture. We talked about this mountain building event that happened in the early Paleozoic/late Ordovician Period, we call it the Taconian Orogeny because it built up the Taconic Mountains in New York - um - what caused that Taconian Orogeny? (Jill - we've discussed this two days ago - maybe I'll put you on hold there, maybe somebody else can remember what caused the Taconian Orogeny?) A volcanic chain of islands bumping into us? Exactly.

Awhile ago, there was an ocean off the East coast of the United States. If you were able to go back in time 500 million years, and hover over North America, you would have seen something that looked roughly like this. Here you've got a smaller North American continent and it's missing some pieces. Notice that Florida's not there, California's not there, Alaska's not there. OK, those have all been added on more recently. 500 million years ago California, Florida, and Alaska were not yet part of North America. And, our location is marked right here. Now actually at that time what we'd really see is this (see map rotated) - North America was in a different position at that time. And, since 500 million years ago North America has rotated and moved north. OK, but at that time it was on the equator and it was rotated in a different position. So today what we call the East coast was really the southwest coast. Let's just call it the East coast and keep it simple. Does that work for you guys? OK. Notice what's offshore there. There's a subduction zone marked on the oceanic crust by a deep trench, and then next to that, paralleling the trench, is a chain of volcanic islands; a volcanic island arc. Subduction is bringing that volcanic island arc closer and closer to North America. It collides with North America. Jill is fortunate because she took my Prince William Forest trip on Sunday. We actually got to go and visit some of the rocks from those islands. They're preserved down by Quantico, Virginia. In between those islands and North America, a bunch of sediments got squished out. I want to remind you guys about the concept of an accretionary wedge. Accretionary wedge. What is an accretionary wedge? Right. Sediments that get scraped off the ocean floor at the sight of a subduction zone. OK, so remember in class I offered you the analogy of my arm covered in peanut butter, and my other arm scraping that peanut butter that went there? There's another analogy at the bottom of a bulldozer. So there's this big pile of oceanic sediments building up at this trench at the sight of subduction. And, those sediments then begin to squash between the volcanic islands and North America. I gave you guys the awful kitten analogy, right? So, this is the crushed-up kitten. These are these poor little oceanic sediments that are getting squashed between a Mac truck, North America, and a mini-Cooper, these volcanic islands. So, the kitten's little bones started off in many different orientations when they rotate to newer orientation which defines the foliation of the kitten. So, that's what you're looking at here, guys. You're looking at rocks that used to be sediments on the floor of an ancient ocean, and got crushed up and metamorphosed into the rocks that you're standing on now. So, if they're now metamorphic rocks and they used to be sedimentary rocks, what kind of sedimentary rocks were they? (Basalt? No, basalt is not a sedimentary rock - basalt is an igneous rock.) Did anyone see any grains when they were looking at these rocks - any grain size? Grains? Check this out, OK? What does this look like? You can see sand grains in there. There are sand grains in here, and sand grains are made out of what mineral? Quartz. Good. What is reacting to make the mica? What is reacting under elevated conditions of heat and pressure to make mica? It used to be greywacke. Greywacke is a mixture of sand and mud. Yeah, mud is made out of clay minerals. These clay minerals are not stable at high temperatures and pressures. So when they experience it, they turn into mica. Muscovite mica that's all lined up in the same direction. So these rocks used to be layers of sand and mud at the bottom of this ancient ocean basin (so-it's metagreyacke - Laura). Right, good. So, for the rest of the day, I'm going to call them metagreywacke And, I'm going to use that term over a more traditional metamorphic rock name like schist because I feel like it tells us more. All that "schist" tells you is it's a metamorphic rock. The term metagreywacke tells us a metamorphic rock and it used to be greywacke. So, it's got a double meaning there. Now, what can you guys tell me about how greywacke accumulates or where it accumulates? (Jill - an accretionary wedge. C. - An accretionary wedge just takes whatever is there and jumbles it into a big pile). Greywacke accumulates from submarine fans at the bottom of the sea. What is bringing sediment down to that deep location? What depositional force? Turbidity flow. You guys remember turbidity currents? Turbidity currents are these big, sediment rich flows that flow down across the bottom of the sea floor. When they slow down, what gets dropped first? Big grains. What gets dropped next? The finer grain stuff. And you end up with this overall sedimentary structure known as graded bedding. Anybody notice any graded bedding here today? (Here's an example...)

OK, so there may have been some preserved but then the river eroded out those boulders and transported them away, that's one reason. But we saw lots of rock left. OK - maybe there wasn't that much there to begin with? That's a possibility. They changed too much - they've been metamorphosed? Yeah! Metamorphosis tends to destroy those original sedimentary features, right. I mean, even though the mud isn't mud anymore, it's now mica. Yeah, metamorphosis has destroyed most of the graded bedding. If you go up and down the Piedmont, back and forth across the Piedmont, it's very, very rare to find graded bedding still preserved in the metagreywacke of the Piedmont. The only place that I'm aware of that you can still see it - no I take that back - there are two places that I know of where you can still find it. One place is here, and the other place is out near Sugarloaf Mountain. But everywhere else it's been destroyed. Like, Jill, did we see any at Prince William Forest Park? (Jill - uh, no). No, right, it was basically too intensely metamorphosed and the graded bedding is gone.

OK, let's try and bring this around full circle now at this point. If these sediments were originally accumulating as graded beds of greywacke, mixed of sand and mud, in an ocean basin, what ocean was that? The Iapetus Ocean - what the heck is that? Before the Atlantic. How does it relate to its name? The father of Atlas... The Atlantic Ocean is named for Atlas - the guy who held the world on his back. The ocean that came in the same place as the Atlantic but earlier is named for the Titan who fathered Atlas, and that was Atlas's dad, and that was Iapetus. So we call this ancient ocean basin the Iapetus Ocean. The Iapetus Ocean no longer exists. It's dead. The Iapetus Ocean was killed in a series of tectonic collisions. First, was a collision between these aforementioned volcanic islands and North America. Second, there was a microcontinent out there in the Iapetus Ocean - that crashed into North America. That microcontinent is now preserved as most of New England. Right, you can go up there and visit that ancient microcontinent. And then, finally, a much bigger land mass crashed into North America, finally killing off the Iapetus Ocean. What land mass is that? Yeah, Africa. Are you feeding them answers over there, John? OK, Africa crashed into North America, and that made a certain supercontinent that I'm certain that everybody knows, without John giving them a hint, -- Pangea. The moment when the Iapetus Ocean died was the moment Pangea was born. As soon as those continents butted up against one another, the Iapetus Ocean was gone.

We're talking about a geologic history here... we're talking about a collision. Exactly, very good, you've got the journalistic instinct. Who, what, when, where, why, when...so when did this happen? How can we answer that question? (By isotopic dating...) C - of what? What isotopic minerals would you choose to date here? The muscovite. That's right. The muscovite is a metamorphic mineral formed during the orogeny. So if you get an isotopic date on that it tells you when the orogeny happened. Well it turns out people have done exactly that. They've taken this muscovite mica and they've analyzed it, looking at the isotopes potassium 40 and argon 40 in that mica. And that gives you a date of 460 million years ago. That's the date of the Taconian Orogeny, Jill.

OK, so the Taconian Orogeny just to sum up here. The Taconian Orogeny was an episode of mountain building that occurred 460 million years ago. (Radioactive parent isotope is potassium 40 and argon 40 is the stable daughter product - question...)

We already noted back there that the rocks had been metamorphosed. Remember that metamorphism is one of the characteristic signatures of mountain building. You can identify a mountain belt even when the mountains themselves have eroded away by the presence of metamorphic rocks.

There were two other characteristics of mountain belts that we discussed in class. Vivian - what's one of them? Folding. And that's exactly what Doug noticed over here. He noticed that the metamorphic foliation has been folded up here. You guys see those sweeping folds going through these quartz layers here? Right here, you can see another one here. Down up, up and down again. Along the trail today, you will see dozens upon dozens examples of layers of folding. Sometimes it's a little hard to spot with the lichens growing all over them. You can see some here - you can see the layers go up and down and then up again. There's plenty - you guys are going to see some real nice, sexy examples of folding as we go along the trail. This isn't the most amazing spot, but since Doug noticed it, I wanted to point it out.

While we're on the topic, what's the third characteristic of mountain belts? Metamorphic rocks, deformed rocks (including folded or faulted rocks), and then the third characteristic is...? Come on guys, you can't take this for granted! Granite! Right. Granite. Remember granites are produced by partial melting when rocks get really hot. So, you want to keep your eyes peeled for granites along the trail today, as well. OK. What we're going to do...

Find yourself a spot where you've got a good, unobstructed view across the river to the other side. Remember, we're in Maryland, and we're looking across the river at Virginia. So, Virginia' on the other side. There's a feature I want to call your attention to here. Can everyone see there's a series of vertical gashes? Four of these gashes all in a row? All oriented in the same direction? If you look for the tallest tree over there, and then go down to the base of that tallest tree you'll see these deep gashes in the cliff face. Those are a series of igneous dikes. Dikes are what happens when a rock cracks open, magma squirts into a crack, then the magma solidifies into an igneous rock. Tell me something about the igneous rock that is inside these dikes. Is it more stable or less stable than the metagreywacke? Less stable. How do you know that Michael? More mafic - how do you know that from here? The color? You can see it looks a little bit darker. It's a mafic igneous rock? Ding! You're right. I'll give you a closer look at it here in a few minutes. But you can also see that these igneous dikes don't project out from the face of the cliff, they're sunk into the face of the cliff. Which means, that that rock 'rots' away more easily - more easily weathered. It's more easily broken down. Remember the Snickers bar that I made you suck on? Whatever is making up those dikes is more like the chocolate and less like the peanuts. It's easily etched away. Everybody with me on this? So, that supports the idea of it being mafic because mafic igneous rock has lots of iron and magnesium. Iron and magnesium like to oxidize. Now tell me this. How old are those dikes? Younger than 460 million years old. How do you know that? They're cutting through the metagreywacke. And, you can't have the dikes cut across the metagreywacke unless the metagreywacke already exists. Therefore the dikes must be younger than 460 million years old. Well it turns out their igneous dikes, so what can you do to them? You can date them isotopically. They've done isotopic dating on biotite that's present in those dikes, and biotite gives a crystallization age of 360 million years ago. Only 100 million years after the greywacke got metamorphosed to metagreywacke. Again, that number is 360 million years. Those dikes are 360 million years old - 100 years younger than the metagreywacke they cut across.

I want to point out that the second Appalachian mountain building event occurred 360 million years ago. This is the collision of that microcontinent with North America. So, as we said earlier, North American experienced a collision first with a mini-Cooper sized land mass of volcanic islands. Now, it's colliding with a good-sized sedan - the microcontinent. Eventually, it's going to collide with a Greyhound bus which, would be Africa. North America gets to collide with larger and larger land masses through time. This series of dikes over here occurred at the same time at that second episode of mountain building, sometimes called the Acadian Orogeny. You can see it well up in Acadia National Park, in Maine. (The highest point on the East Coast, still, is Klingman's Dome in Great Smoky Mountain National Park; 2nd highest is Mount Washington up in New Hampshire) (John - Avalonia up North and Carolinia in the Smoky Mountains?) (C.- That could well be true but...) We tend to divvy up these parts of the Piedmont and call them different terranes - I know there's a terrane called Carolina/Carolinina... but, I don't know if that's necessarily a microcontinent. I would only call it a microcontinent if it's made distinctly out of continental crust before it hit. Avalonia is the name of the microcontinent.

Doug did a great job earlier with his hand showing me the orientation of the foliation of these rocks. Again, we can all see the orientation down at our feet right now. It's oriented something like this. Now what I want you to do with your hands is show me the orientation of these dikes. I specifically chose this spot to view the dikes because we are looking directly down the barrel of these dikes. We're looking down that crack in the earth - it's coming straight towards us here. If we're looking down at our feet, we should expect to see the dikes right here. Where are they? What gives? There's a shift. It turns out the dikes are on our side, they're about 30 feet downstream. Let's go see them.

All right, look at this. Here's some almost vertical gashes in the rock. They have that same orientation. But, if you look (and this is actually a great time to be running this field trip because there's not leaves on the trees yet) if you look over there on the opposite side you can watch these go down and you would expect them to run into the middle of that cliff over there. But, that's not where you see the dikes on that side. Instead they're offset in an upstream direction on the Virginia side by about 30 feet. You guys see that? Pretty cool! What gives? Maybe, a fault? Let's discuss the evidence for faulting here. Oh, by the way. Here's an example right here - this boulder that my foot is on here. That is the igneous rock that makes up the dike. It's a kind of basalt - you remember basalt from lab, right - mafic and fine grained? And, what you see here, and I want everyone to come take a look at this after I move away, is that this basalt has visible flakes of biotite mica in it. Not muscovite mica, that silvery mica that we saw at the first stop, but instead biotite mica, which is jet black. You'll see these little shiny flakes of black biotite mica here in this special basalt. This basalt has a special name. It’s called a lamprophyre, because of those flakes of biotite mica in it, but, it's just a fancy name for a particular kind of basalt. Alright, again that name is lamprophyre. You'll see that in your handout that I gave you earlier. So, these are lamprophyre dikes. How old are the lamprophyre dikes again? 360 million years ago, which is the same age as the Acadian Orogeny. (That's coming from you John- one thing at a time, one thing at a time...) So, Laura please share with everybody your hypothesis on why the dikes do not line up from Virginia to Maryland. All right. Because there was a fault, and that fault offset the dikes on opposite sides of the Potomac River. Here's two diagrams. If you can't see these, move closer. Basically, here I have two different explanations for the offset of the dikes on either side of the Potomac River. The first explanation is that the dikes were originally straight and they were broken by a fault. What kind of fault would this be? Left-lateral or right-lateral? Right? Yes. Right because if you're looking across it looks like the other side has shifted to the right. Very good. The other explanation is that in fact, the dikes were not straight dikes. There's no rule that says if you crack open metagreywacke it must be a straight crack. The crack may have been jagged. Maybe that explains the offset well. Unfortunately the critical area we need to examine to answer this question is underneath the Potomac River. So, if we're going to answer this question, we're going to need to look around for additional lines of evidence. One piece of evidence has to do with the shape of the river. This is an aerial photograph of the Potomac River. We started off our hike today up here at the Great Falls Visitor Center. This white line going across the Potomac River is a dam where they divert water for D.C. Great Falls itself is this great, white blob here. And, then, we are right about here following the Billy Goat Trail along a very, very, very straight section of the Potomac River called Mather Gorge. Mather Gorge is what we're going to be hiking along for the rest of the trip today. Mather Gorge is named for Steven P. Mather, the original superintendent of the National Park Service. You'll find that the National Park Service has honored this guy endlessly. I think I've slept in four Steven P. Mather Memorial Campgrounds in National Parks around the country. They really love this guy. Anyway, Mather Gorge is named for him. Now, look at how straight Mather Gorge is. It is incredibly straight: It's as straight as an arrow. It's as straight as you would expect if there was a fault underneath the river there that had ground up the rock. Remember faults tend to break up rock into fault breccia. And that would be really easy for a river eroding into the landscape to erode away fault breccia opposed to solid bedrock. So, the actual shape of the river is suggestive of the fact that there may be a fault underlying the river at that location. Unfortunately, the only thing that we can use as a marker is these dikes. So, we don't have any other evidence of offset here because basically everything else is just smooshed up metagreyewacke. And, the place is the end of Mather Gorge which is here and here, where you might expect to see the fault exposed up out of the river, you can't really see any good evidence of it. Some geologists claim they've seen it up on the Rocky Islands that we walked by just before the fault diversion structure. I've been there and I've looked at the same outcrop and I don't see indisputable evidence of faulting there. I see a crack, but a crack doesn't mean a fault. ("Can you put divers in the river?" Sure you could put divers there at great expense and risk to the diver. Problem is at the bottom of the river there's all kinds of boulders covering up the bottom. And, there's silt and mud and big catfish and you're not really going to be able to get a good look at what's going on. The one thing you could do is you could back up the Potomac River for a couple of days, excavate away, and arrive at an answer to this question, but its not really that critical a question to arrive at an answer at.) Let me share another piece of evidence for you. Remember there's another way of explaining the offset in the dikes. It may be a fault but it could also be that the dikes were not originally straight. Here are two pictures of outcrops of the dikes. One is on the Virginia side and the other is on the Maryland side. Let's discuss the Maryland side, first. This is a photograph taken from Virginia looking at Maryland. You can see coming up from the river, one, two, three dikes. And up here, one, two, three, four dikes. One, two three. One, two, three, four. Three does not equal four. What's going on? Well, it looks like this middle one is actually branching. It splits into two arms there. When the rock cracks it was a jagged crack and the crack had two little fractures – two little arms that went on and those filled with magma. The other photo is over on the Virginia side. Again you see the lamprophyre dike here the metagreywacke host rock here. And you can see another one of those branching arms coming off the dike. The dikes are not in fact straight. Does that mean that there is no fault? No. A fault could break crooked dikes just as well as a fault could break a straight dike. So, do we have an answer to the question? No. We do not know which of these two hypotheses is correct. We have not been able to prove either one of them false, therefore, they both stand as possible explanations for the offset of these lamprophyre dikes. What questions do you have? Jill - what questions should we have? How about: "Sir, can we look at the lamprophyre, please?" Jill - can, I? OK, yes you may! Come here, stick your head in that hole and check out the lamprophyre up close, and see how it looks different than the metagreywacke. It's dark, fine-grained igneous rock and it has little flakes of biotite in it. It's going to be difficult to see from far away, you actually have to get about a foot away to see that. C I'm not lying to you, you can trust me. So, don't take my word for anything. Trust your own eyes and your own mind.

OK, why have I brought you over to look at this rock? It's fancy. Take it further. Jill - it's been fractured. There's some fracture. What do you see, Vivian? There's a lot of different joints in these rocks - remember joints are fractures along which no movement has occurred. Those are visible all over here making this very blocky landscape. Look at the other side. You can really see the joints. Good observation. But it's not why I brought you here. There's a nice big blob of quartz there. What kind of quartz is that? There's a lot of different kinds of quartz that we saw in our minerals lab. Smoky, rose, citrine, milky quartz - milky quartz, good. Milky quartz is generally whitish. Why is it whitish? Yes, it's got little tiny bubbles of water in it. That indicates how that quartz got there. It got there by hydrothermal fluids. OK, basically hot water in the earth had quartz dissolved in it and it precipitated out these big blobs of quartz. Very cool, this probably happened during the Taconian Orogeny, as well, when these rocks where nice and hot. Again, not why I brought you here. There's a beautiful fold here in graded bedding. Alright, do you see that really coarse-grained layer there that's been folded up? That used to be horizontal on the floor of the Iapetus Ocean deposited by a turbidity current and then during mountain building it got squished up and folded. Squished up and folded and it looks like the hydrothermal quartz was then placed as well. You can see another bed here below it pulling the same trick. Pretty cool, huh? Symptoms of mountain building.

Jill asks questions about source of sediments. Thank-you for being persistent with that.

John, pass me some clam shells. Did you guys notice all these clam shells all over the place all over here in these big sand piles? All right? What's up with that? So you're saying that these clams are the same age as the rocks? Turns out that these rocks have no fossils, whatsoever. For several reasons. One is, deposits in the deep ocean, there's not a whole lot alive down there. Second, these rocks may be older than multi-cellular life. So, they may not have any fossils in them for that reason. Third, they were metamorphosed so any fossils present would have been destroyed like most of the graded bedding. So, these are actually Recent clams. It's actually an Asian species of clam that's an invasive species colonizing North American waterways. It's a freshwater clam. So, these clams have come downstream from higher in the Potomac which means that they were deposited during floods. Just like the tilted over trees are evidence of flooding, so too are all these clam shells and sand deposits way up here above the level of the river. Unlike that big round boulder we saw earlier, this is the stuff that usually gets picked up by flood waters. This is like a little parachute very easily picked up by the waters and wafted around. Good. Let's go.

When does the Billy Goat Trail actually going to get tough? It's about to get Billy-Goaty. So what we're going to do is walk across an area called pothole alley. And as you walk across pothole alley you'll see why it got its name. And there's going to be lots of potholes there, you've got to be really careful. You want to use your hands and your feet. It's a good time to be putting away anything you've got in your hands and you've got your hands free to navigate the landscape. Jill - are you going to stop and talk a lot? C. - No I'm not going to talk at all. We're going to walk across it and then we're going to get to the other side and sit down on a nice broad plateau and have lunch overlooking Mather Gorge. OK?

(right after lunch) Maybe sand, maybe silt. Um, one of the things that you learn about these potholes is that if you take your hand and you reach inside and you run your finger around the inside you'll feel differences - that there are little ridges in there. There are different layers of quartz and mica. Quartz stays up in high relief because quartz is very resistant to erosion: it's hard. Mica on the other hand is soft and chemically unstable - it breaks down into clay at Earth's surface temperatures. So, what this is telling us is that something is preferentially etching away at the mica and leaving the quartz behind. Something really small has to get in there to do that job. Something like a grain of sand or like a grain of silt. So, pebbles may be part of the process, but, definitely sand or silt are part of the process. They're etching away at the mica and then maybe a pebble comes along and slams into these unsupported ridges of quartz and snaps them off. That would be one hypothesis, but the original etching is done by sand and silt. Based on these little ridges.

There's something else you may have noticed, and that is if you look across at the Virginia side, there's this very flat surface, basically parallel with the surface that we're on. Do you see that? Because if you look back up river, there's this sort of flat plateau, maybe not really flat, - it's etched into with all these potholes and stuff, but it basically continues across the Virginia side. That is one of those bedrock terraces ("straths"). They're older levels of the river that used to be the river bottom and then the river cut into a newer, deeper level. Some of the evidence that we have for this being the bottom of the river are these giant potholes. This sort of thing is not going to be scoured out in a flood. It's something where you've got the river working on it for centuries - maybe millennia. ...potholes... also, there are these great big boulders that we find up here. Boulders that were probably once bedload at the bottom of the Potomac River tumbling along, rolling downstream and then eventually when the Potomac cut into a deeper level, they were left high and dry up here on the surface. The next thing that we're going to stop and look at along the trail is one of those boulders that tells us about where the river was flowing from. On the other side (of the river) you see a hill. There is a hill on the other side that rises above this bedrock of terrace steps. That hill is called Glade Hill. On the top of Glade Hill you also find rounded boulders that have been transported downstream by the Potomac River. So the top of Glade Hill used to be the bottom of the Potomac River. So, the bottom of the Potomac River was above our heads and where we're standing now was still solid rock. Then the Potomac cut down to a deeper level. It carved out this bedrock terrace, made these potholes, deposited the boulders here, then it dropped again and cut down to a deeper level. The Potomac is incising over time. (This is not an entrenched meander because the Potomace does not meander here. There are areas where the Potomac does meander, like at the Paw Paw Bands. But, here of course the river is quite straight.)

OK, I want everyone to come and take a look at something. Wow. Alright. What I want you guys to do is I want you to stick your head in the cave. Tell me what you see! Stick your head in there and look at the ceiling. What are you seeing? Folds! You're seeing folds and what's being folded? Alternating layers of quartz and mica. The quartz is light colored milky quartz the mica is dark-colored biotite mixed in with muscovite. And as you look up there you can see that they're strung out in parallel layers. Light minerals - dark minerals. Light minerals, dark minerals, light minerals, dark minerals. It's a very coarse texture. We learned a name for that metamorphic texture - you got coarse alternating bands of light and dark minerals –gneiss. Gneissic banding. So, you got this foliation and remember that the foliating is formed due to differential pressure during mountain building. But what happens to the foliation, here, Laura? The foliation was folded. So you see that these alternating layers of quartz and mica that have been all folded up. And, that's an interesting thing because when you think about it, those layers themselves formed due to pressure in one direction. In order to get them to fold, you have to apply pressure from another direction. This is important stuff here. This tells us that these rocks have experienced more than one generation of deformation. They've been squeezed once. They got to sit still awhile, and they got squeezed again. OK? Questions on this outcrop?

OK., we're going to go down the path. We're not going to go far because we're going to see a stange, green boulder in the middle of the trail.

A Martian! This is an interesting rock, this is a greenstone, clever name. And, a greenstone is metamorphosed basalt. Where does basalt come from? Mafic lava. It's what basically happens when a volcano erupts mafic magma we call that basalt. If you want to see basalt forming today go to the Big Island of Hawaii or Iceland. If that basalt gets caught up in an orogeny, it gets metamorphosed and it becomes a greenstone. Basically, two metamorphic minerals grow - both of them colored green. And you met both of these metamorphic green minerals during our metamorphic rocks lab. Olivine is not metamorphic-that's igneous. I introduced you guys to 5 metamorphic minerals in that lab - garnet, kyanite, staurolite, and then these two. Chlorite - deep forest green, and pistachio colored green - epidote. Epidote indicates hot water in metamorphism. So what happened is this basalt flow got metamorphosed and it produced this greenstone. Now that brings us to the question of what are these little white blobs that are popping through here in different places? They are little round or ellipsoidal blobs of quartz.

Think of what this lava would be doing when it was first erupting. "Kitty eyes." Ignore her! Aren't they crystallizing. Sure, they're crystallizing and they're fine grained texture which makes them a basalt. What does a basalt do when it gets up to the surface and suddenly it's depressurized? Air bubbles... Remember lava often degasses at the surface causing little bubbles that we call vesicles and then those vesicles, those little swiss cheese like holes in the rock they can later get filled in with mineral deposits. In this case, quartz rich ground water flowing through this deposited quartz filling in these vesicles preserving the vesicles as... amydgules. Amydgules are these preserved gas bubbles. Now, I'm going into a lot of detail about this one boulder, even though this boulder is not from this area. This is like I mentioned, a visitor. It is one of these boulders that was deposited on the bottom of the Potomac River, before the Potomac River cut down to a deeper level. This is a piece of a very distinctive greenstone that is present out in the Blue Ridge Province. It's called the Catoctin Formation. I mentioned the Catoctin Formation when we talked about our Geologic history of Virginia, when I said that when Rodinia broke apart there were these big lava flows all over the landscape - flood basalts; that's the Catoctin Formation. Later on of course those flood basalts got metamorphosed during Appalachian Mountain building which made it green. Jill - so these are actually far deeper - have been layered deeper into the landscape, right? C. And the mountain building they got shoved up and erosion exposed them to the surface. The reason I go into all this detail about the identity of this boulder is I know where this boulder came from. I know that outcrops of amygular greenstone - the Blue Ridge province is west of here. So that indicates that when the Potomac River was flowing at this level, it was carrying sediments from the west to the east. Now that may seem obvious to you because of the Potomac today - it flows from west to east. But, we can say with some certainty based on the presence of this boulder right here that that boulder was doing the same thing in the past (Principle of Uniformity). It's a confirmation that the flow direction of the Potomac has been relatively constant at least since it was at this level. OK, I'm going to show you some other evidence if that. I'm going to point out some other boulders along the trail as we go along and they’re all going to have Blue Ridge identities. But, first I've got something even more spectacular to show you.

So I stopped here to show you this outcrop which might not look something too spectacular in the beginning, but once you understand what this thing is your eyes are going to pop out of your head and your jaw's going to drop. Get in close, take a look at this. What do we have here? "Rocks." (Eyes rolling) Sure... there's some nice folding. There's some potassium feldspar in there. See these peachy little potassium feldspars, here? They're opaque relative to the grey quartz here, no longer milky quartz, but grayish. Potassium feldspar, grey quartz - what is that? What are we looking at here? What are these little blobs of a mixture of coarse grained quartz and potassium feldspar? Granite! What's the third characteristic of mountain belts? Granite! You're looking at metagreywacke that's gotten heated up so much that part of the metagreywacke has melted. Not all of it, but some of it. Remember the idea of partial melting where you start off with a rock with a bunch of different minerals. Then if you heat it up, some of those minerals basically dissolve into liquid magma and some of them stay as a solid residue. So, ones that are more likely to melt are the felsic ones. Those that are less likely to melt are the mafic ones. So, essentially what you're seeing here is granite magma being sweated out from super-hot metagreywacke. This rock was originally deposited as sediment at the bottom of the Iapetus Ocean. Then it became metamorphosed and now part of it is becoming igneous. It's all three parts of the rock cycle right here in one outcrop. Jill - it's coming back to itself. C. - right, its coming back to itself. Right now its being weathered off and producing new sediments, so the cycle runs full circle, right? This is a granite being born. You've just got these little blobs of granite magma leaking out of this rock. You've got the midwife's perspective here watching this granite in the act of being born. This granite magma is liquid it's going to go upward in the crust like the blobs in a lava lamp and eventually it will join with other blobs and its going to cool together into a big granite pluton somewhere else. But, here it never made it that far. It just started to sweat out of the rock and then it stopped. So we are lucky enough here to have this snapshot moment of the rock cycle caught in the act: caught red-handed where metasedimentary rock is actually converted into igneous rock. It would have had to be really hot for this to happen. Probably around 400 degrees or 450 degrees; something like that. But because it's coarse grained it cooled slowly underground. Now it's up at the surface today, but originally it cooled down slowly deep underground. We some evidence in this area where we see boudinage. Little sausage shapes squeezed out. Remember we see that at about 10-15 kilometers depth. So this is a rock that formed about 15 kilometers beneath the surface when it was about 450 degrees. Now are there 15 kilometers or rock above us now? No, they've been removed. What removed them? Erosion, yeah. Erosion ground down these ancestral mountains and exposed their roots. The rocks that we're looking at here were once at the roots of the Appalachian Mountains. (Laura - So this surface here could be an unconformity surface?) If something else were deposited on top of it - right here we don't have anything else deposited on top of it - we see these occasional little boulders on top and if you go look on the top of Glade Hill there's a nice layer of boulders over there.) Let these people through and we'll continue our discussion.

Migmatite - partially molten - Jill. Um - migmatite. What can we do with igneous rocks, like we did with the lamprophyre? Isotopic dating. So we can isotopically date this granite. It turns out this granite gives us an age of 460 million years ago. Same age as the metamorphism – same orogeny. The Taconian Orogeny heated up these rocks and squeezed them. What was the cause of the Taconian Orogeny? The collision of a volcanic island arc with North America. So, good work guys! Isn't this a spectacular rock?!!

I have traipsed across this old planet a fair amount and I've seen migmatites in only two places. I've seen them up in Maine, and I've seen them here. So, you're really lucky that you're taking a Geology class where you're really close to a place where you can go and see a migmatite. Most students are not that lucky. OK!

He's showing us the difference between fresh and weathered metagreywacke - Jill - he's showing us... we just looked at the migmatite.

By the way the smell you smell right here is sulfur. This is a creek here that evidently ...some sort of pyrite deposit. There's iron in the creek which is rust - iron oxide. And, it smells sulfurous. Remember that pyrite is iron and sulfur. Here, it's being broken down here by the water.

Is that why they have the ridges here in general? - student. Ridges - Jill Ridges of quartz extruding out of mica. - example/observation

Remember I showed you that image of the scuba diver and he's standing in the river and the sea level is rising over him. OK, this is the river gravel that was deposited in that river. It's part of the Weverton Formation - it's early Cambrian. It's about 540 million years old or so. And again, I wouldn't expect you to know that by just looking at it. I only know that because I walk around thinking about geology a fair amount and I recognize it here. So, I'm correlating this boulder with outcrops to the west. There's some other boulders here as well. As well as this reddish stuff. There's these reddish sandstones. We're going to talk more about those just over the hill here. Here is a nice example of diabase which is going to be related to this red sandstone. This is a mafic igneous rock because I don't want to give away what I'm about to reveal down the trail. But, there is a variety of boulders here. All of these boulders can be sourced to outcrops in the west. Again, more evidence that the Potomac River is being pulled from the west to the east over time, carrying sediments along to prove it. OK? Like little passport stamps telling you where it's been. OK, a little bit further and we've got two more boulders to look at.

Three different sandstones. I've got samples from all three of them here. Somebody tell me the name of one of these sandstones. Quartz sandstone - the white one, almost pure quartz. The other is greywacke - dark grey - that's what the local bedrock was originally. This rock (sample) is a greywacke, not a metagreywacke - these (bedrock) are metagreywackes. The pinkish one is arkose. It's a mixture of sand, mud, and potassium feldspar. Big angular pieces of potassium feldspar. The kind we saw in that granite. In terms of maturity, the arkose and the greywacke are immature sandstones, and the quartz is mature sandstone. We've already learned that the greywacke is deposited in deep sea fans, sometimes called abyssal fans or submarine fans. Where is quartz sandstone deposited? Beaches, good. And where is arkose deposited? Very immature, it still has all its feldspars it hasn't broken down to clays which means it hasn't come very far. Rift valleys. Arkose is a characteristic of rift valleys.

We have two boulders here which can help complete our sandstone triumvirate. We've already got the greywacke down - check that off the list. This one here is a metamorphosed quartz sandstone, so its made out of quartzite. It's a really interesting one. Do you see those little circles on top of it? Those are the tops of fossilized worm burrows. These fossil worm burrows project down into the rock like this. They're cylindrical; called Skolithos. Some of you have noticed Skolithos trace fossils in lab. I've got a few samples out on the countertop. They look like little soda straws running through the rock. Again, I know where that came from. It's from the Antietam Formation. That's a barrier island beach sand that's found in the – the river has traveled from the west to the east - all the others boulders I've stopped to talk about have been located in the Blue Ridge. This is also a Blue Ridge rock. The Antietam National Battlefield is what its named for - the Antietam Formation. Characterized by these little fossil worm tubes.

This red beauty right here, and I encourage you to do so, - these little pink specks are potassium feldspar. This is an arkose. A big beautiful arkose from 10 miles away. This naturally outcrops 10 miles from here. 10 miles upstream at a place called Seneca Creek. Because it outcrops at Seneca Creek we call this the Seneca Sandstone. Seneca sandstone is an arkose. So, Michael found a piece of this early on. We've been walking over various boulders of it all along. Its source is so close by we actually have a lot of it here. What else have we seen that's red sandstone today? The first stop we made - along the canal. The locks were made out of this red Seneca sandstone. It turns out to be a great building stone. This Seneca sandstone is quite young. Because its young, its actually younger than Appalachian Mountain building. Which means it hasn't been metamorphosed. So, it's essentially, it's wet, poorly-lithified sand. So that means that when you cut it out into blocks, it cuts like butter. But, then once you take it out, it dries out, and once it dries out it becomes much harder. That's the ideal building stone. Easy to extract from the ground but hard once you make something out of it. The Smithsonian castle is made out of red, Seneca Sandstone. The "brown"stones in Dupont circle, too...

This is a very interesting chapter in Geologic History because, as Laura pointed out, arkoses get deposited in rift valleys. We've talked about putting Pangea together, killing the Iapetus Ocean through continental collisions, but we all know that Pangea didn't last. Pangea broke apart, and when it broke apart, what opened up? Rift valleys. Some of those rift valleys filled in with sediment and they didn't keep opening. Some of the rift valleys connected together and became a new ocean basin called the Atlantic Ocean. This is from the site of a failed rift. The rift began to open, but it didn't keep opening. It is located to the west of here, called the Culpeper Basin. The Culpeper Basin is a Triassic aged rift valley. When Pangea was breaking apart this big gaping hole opened up in the crust filling in with immature sediments like this arkose and then it stopped. That's where you find Dulles Airport today - it's in the middle of the Culpeper Basin. But, some other rift valleys, over in that direction (east) connected together and they were the weakest link. That's where the crust kept ripping over there. And it ripped and it opened wider, and wider, and wider and eventually sea water came in and it became a little ocean basin and then it widened and widened and widened and its still widening today. And, that's the Atlantic Ocean. So, basically that process began around 200 million years ago in the Triassic, and this is a Triassic sandstone. What organisms were alive during the Triassic? Dinosaurs.


--- T R A N S C R I P T --- E N D S ---

If you've made it to the end of this post, congratulations! I'm sorry, but I won't be able to refund the hour you just spent reading it... But since you're here, I'm interested in your feedback about this -- what elements you read about here caught your attention? Why? Thanks...

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5 Comments:

Blogger Tuff Cookie said...

Aw, I can't read this before the hike Saturday, that's cheating.

Nice work on Jill's part - I can't count how many times I've looked back at some scribbled note in my field book and wished I could remember exactly what someone was saying.

After spending most of the last three weeks on planes, I can't wait to get out and see this stuff.

April 17, 2008 3:03 PM  
Blogger Tuff Cookie said...

Okay, so I peeked. The "awful kitten analogy"? You're going to have to explain that one on Saturday.

April 17, 2008 3:11 PM  
Blogger Geology Happens said...

Callan, that is a heck of a post! Whoever transcribed your field trip was incredible. I have to echo tuff cookie's remark about my notes at the end of a field trip; they are usually brief and somewhat indecipherable, but always with some sketch that looks nothing like anything we saw.
This will be a great addition to all the stuff you have sent for our C&O bike trip this summer. Thank you

April 17, 2008 11:20 PM  
Blogger castlewon said...

Just have to say, what a great tour. I can see myself trying to learn this stuff. Thanks.

April 18, 2008 2:13 AM  
Anonymous Anonymous said...

Also don't want to peak, but I look forward to taking this to me on my next trip to Billy Goat. My thanks to Jill!

April 18, 2008 10:59 AM  

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