PlanetGeo: The Geology Podcast
PlanetGeo: The Geology Podcast
From Rifts To Rifles - The Regional Geology of the Battle of the Gettysburg
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Join us in this action-packed final episode of our Gettysburg series as we uncover the rock-solid weathering patterns that shaped one of the most famous battles in history! Standing on the historic battleground, you'll see how two supercontinent formation and breakup events have influenced the topography and geology of Gettysburg. Discover the impact of weathering and time on the eastern U.S landscape, and how these factors played a crucial role in the outcome of the Battle of Gettysburg. From the rugged Blue Ridge Mountains to the fertile soils of the Great Valley, we dive deep into the geological features that made Gettysburg a strategic focal point. Packed with geological insights and battlefield stories, this episode captures it all—rock formations, troop movements, and even a bit of friendly banter! So grab your rock hammer and join us as we dig into the legacy of Gettysburg. Geology wins again!
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I love standing on the Battle of Gettysburg site, knowing that the rocks in this area you can see two supercontinent formation and supercontinent breakup events. Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you. Okay, Chris. The end. Chapter 7. The last Wait a minute. Wait a minute. Are we recording right now? We're recording. Yeah. The red lights on, man.
SPEAKER_00Jeez. Oh well. I got you over here now. I'm I'm gonna switch you. I'm just gonna manually move you between it's only our like, you know, 250th episode here or something.
SPEAKER_01I mean, she'd think you'd figure it out by now. My goodness. One would think. I just don't know why I keep doing this with you. Um today, this is the last in our series on the geology of the Battle of Gettysburg. And this is uh we're we're zooming out, Chris. We're zooming out to look at a little bit more of the region. I won't say regional geology, but we are looking at a little, you know, we're gonna be looking at Virginia, Maryland, this southern Pennsylvania area a little bit more, and talking about some key features of this, of of the landscape and how it evolved and kind of how it led to Gettysburg being the focal point a little bit. How the geology controls it all again. Geology wins.
SPEAKER_00Yes, geology wins. So let me just give the rundown of this episode here. Is that okay? Can I do that, Doctor? You yeah, you may have doctor. I allow it. I'll allow it. All right, so we're gonna talk about weathering and time and how, like, over big spans of time, small little things about the rock then become very important in terms of what the topography of the of the land looks like. So, and then also how weathering can change over time. You know, one type of weathering dominates, and then as time goes on, another type of weathering begins to take over, and that becomes the predominant form of weathering. So we're gonna talk about that. The topography of what we get in the eastern US, the rocks are the same kind as a lot of the ones that I see. I'm gonna I leave here soon to leave my field course out west. A lot of the same rocks we're gonna see in Pennsylvania, we see out west, but they are very different looking out west. Yeah, and we're gonna talk about that as well.
SPEAKER_01Uh, because of the weathering and the the history, the relatively short history, the mountains out west are young. So, anyway, yes, we'll give you that right later.
SPEAKER_00But and the climate's very different. And and so, and then we're gonna I'm gonna give you an opportunity to to ramble on about the geology of the Great Valley, uh, which is actually a pretty cool story. So, I'm just gonna frame it such a negative tone right away. I'm not uh no, I'm just kidding. Um give me any opportunity to rip on my little friend over there, and I'll I'll take it. So, no, it's a it's a good story, and then we're gonna wrap up this whole series with just a really brief discussion about the roads. I mean, uh, Gettysburg, if you you look at a map and it literally is Gettysburg is the hub, and then you have the spokes on the wheel that just radiate outward. It's a ridiculous amount of roads. I mean it's like come into and go out of this.
SPEAKER_01Just ten roads, or if I forget the exact number. It's a lot of you know, it is spokes on a wheel, and there's a lot of spokes on this wheel. So, Chris, just uh let's turn to image number one real quick and just kind of go over the overview of the geology. We've talked in this series about a bunch of different geological features. First of all, we did our field trip where we saw a bunch of these rock types, we drove around this region, then we kind of dove into some of the specifics. We talked about these rift basins, these younger rift basins, of which the Gettysburg Basin, Gettysburg Remnant is one of them. And that on this map view is kind of on the southeast. We've seen this map before. We're looking at the Ridgeon Valley up to the northwest. We have the Great Valley as you come down towards the southeast, then South Mountain and the Blue Ridge. It's the north end of the Blue Ridge Mountains there. And then as you move east off of that, that's the last mountain belt before you kind of hit the Atlantic, you head east off of that, you're into these rift basins in what's called the Piedmont Province. And we've kind of worked through the geological story of all of those. Um, and now we're we're kind of zooming out again. So anyway, I just want to set that stage.
SPEAKER_00No, Jesse, I just want to interrupt one thing. Where on this gift that you've made here, um, where you call it the Catoctins, that's the Blue Ridge, right? That's what you're referring to with that?
SPEAKER_01So the the Blue Ridge, the Blue Ridge Mountains is a longer chain, and it's called South Mountain up to the north, and the Catoctins down there in kind of that Maryland, Northern Virginia area, and then you know the names of the mountains, the specific mountains kind of change as you go around. And it's the same thing for the Great Valley, too, Chris. Like we're gonna talk about the Great Valley. It's called a bunch of different things in a bunch of different places. The regional feature is called the Great Valley of the Appalachians there.
SPEAKER_00So anyway. So where do we come from? How did we get to this point then in this final episode of the of the geology of Gettysburg and how the geology may have definitely played a role in the outcome of the battle? Um, we know we talked about the Mesozoic basins, you know, the Birdsboro Basin, and then the all these sub-basins. We talked about the Gettysburg Basin specifically. We talked about the the sills and dikes, the Rossville dykes and the in the York Haven diabase sills, and the geology of those and and the end Triassic extinction and the possible correlation with that, which is a really awesome story. And then we just wrapped up with the geology of the very, very complicated South Mountain.
SPEAKER_01And complicated, i.e., interesting. South mountain. Absolutely. And but perhaps a caldera uh collapse geological feature there. Chris, the Okay, one really high-level thing, and I think this is a good you brought up a the juxtaposition with where you go out west. These are old mountains. Like, you know, the oldest. Well, we've talked about this before. When you're standing in Gettysburg and you look to the west, you're seeing supercontinent formation, supercontinent breakup, supercontinent formation, and supercontinent breakup. That's a complicated story, and it's an old story. A lot of these rocks, we talked about South Mountain, those rhyolites and the um sandstones and quartzites that sit on top of the rhyolites, those go back to 560 million years ago. And then they were folded and faulted and all this stuff. Even the Rift Valley, even the diabase is 201 million years old. So this is a really old mountain belt compared to, let's say, the Rocky Mountains, which are the uplift is much, much younger than that, and the the topography is younger.
SPEAKER_00And yeah, just how does that what does that look like, Jesse? Like what do young mountains look like? And we've often described this in prior episodes. Young mountains are muscular, young mountains are chiseled, they're they're angular, they're carved, they're absolutely beautiful. These mountains used to look like that, or or variations of that look, right? Yeah, uh maybe Andean or uh Himalayan kind of look, right? But now they I think they're shockingly beautiful still. I mean, you and I just came from there, and just how many times were we marveling at the beauty around us as we're driving up the Great Valley? And we're looking to our left and we're looking to our right, and it's amazing, but it's a very different topography that you these are older, so they're they're not as muscular, they're not as carved, they're lower in elevation, and that's how that kind of plays out.
SPEAKER_01Chris, I the way I feel this is I'm gonna say something nice about biology here, so get ready for it and prepare yourself for for that. The way I feel when I drive through, let's say, the Rockies, especially like the Canadian Rockies, which are these muscular, dramatic mountains. A lot of that drama, you know, when I look at those, I feel like this is we're in a higher risk environment. We are in a more you could get yourself into trouble in those mountain ranges. The Appalachians driving around them, they're beautiful, they're stunning, but you don't have that feeling of rawness, that feeling of ruggedness of raggedness. And I think a lot of that has to do with treeline. The fact that in the Rockies out west and any of the mountain belts out west, you know, the mountains go up above treeline. And so you get this rock and scree. It just looks rougher, it looks it looks more raw when you get above treeline. And that's a feature of the relief. Here in the Appalachians, we're never above treeline. The mountains are low, really low hills that are thousands of feet, a couple thousand feet at most, above in in sort of topographic relief. And that speaks to the age. They've just been weathered down over time. And so that's like a really first order conclusion is these are old, right? They're old because they're not high anymore. They've been eroded down, and that erosional force has been the most active force without much uplift at all for a long, long time now. We can dive into the details on that. Like there are some changes that happen as you get to old mountain belts versus young mountain belts in the erosional cycle. But first, Chris, maybe it's best to just review how tough various rock types are, like in a general sense. So when you think of weathering, what are the most resistant types of rocks? Or which one, which class of rocks broadly do you think of as resistant?
SPEAKER_00The thing that jumps to my mind with resistant rocks is igneous rocks are highly, highly resistant. Yeah. I mean, almost almost all of them, right? Yeah. But you know, as we talked about with in our last episode with the South Mountain and the Catoctons, there's a rock here locally that's more resistant than even the igneous rocks, like the diabase hills and the and the Rossville dykes. Quartzites are the most resistant rock locally. And they're tough, tough rocks. I mean, you and I have collected quartzites in various locations. The earth does not want to let it go very well. No, of course. It does not. Only the strong survives.
SPEAKER_01And if you make it, that's right. You play this out like sand, quartz-rich sand, is really resistant to chemical and physical erosion. That's why it can get enriched in sandstone deposits and beach deposits. Then you take that sand, you lithify it into a sandstone, and then you go even one step further and you actually metamorphose that thing into a quartzite where you you recrystallize those quartz grains into an interlocking network of quartz grains.
SPEAKER_00So what's yeah, with a quartzite, what's holding the quartzite together? It's made of quartz and it's held together by quartz. More quartz.
SPEAKER_01I mean, this stuff is tough, right? You you know, I always sort of think back to you know, rock saw. If you try and cut this with a rock saw, you know, that puts a lot of wear and tear on your blade. That's a tough rock. You're cutting through pure quartz that's crystalline, that's really well put together. It's one of the toughest rocks out there. So in this region, in you know, the Blue Ridge Mountains in South Mountain, the highest peaks, the highest ridges are all this Chiliwe group, the quartzites and conglomerates there, they're actually more resistant and tougher than the basalts in the rhyolites that make up the Catoctin formation in the South Mountain Rhyolite sequence. Right on. So it's a slight difference, but it is there, right?
SPEAKER_00Yeah, it absolutely is. So I would say that in this locally, it's gonna go. Quartzites are the toughest, the igneous rocks are gonna come next with the diabase and the rhyolites, and then you get down to these kind of shales and and siltstones, and even some of the sandstone. The sandstones would probably be next, and then the shales and siltstones are gonna be the weakest, softest, right, more easily weathered and eroded rocks that are present here.
SPEAKER_01That's right. And I think the difference is, you know, between, let's say, just the Chilliwee group, which are these rift sediments, rift sandstones that were metamorphosed into quartzites during the formation of Pangea, those things are really resistant. However, the sandstones, there's a bunch of sandstones in these terassic rift basins, they're not as resistant because they have not experienced a metamorphic event, really. They're part of the formation of the Atlantic Ocean, which there's no metamorphism that's happened to those since the formation of the Atlantic. So sandstone is weaker than quartzite. And that we can see that example here. I mean, almost in this image, you can see that you know, the the the blue ridges, South Mountain and the Catoctins there, those stand out. They're higher relief than stuff to the east that's in the Rift Basin, in the Gettysburg Basin.
SPEAKER_00So But Jesse, the one rock that I did not mention when I went down the list of like what's tough and what's not is we didn't talk about the carbonates. We didn't talk about the limestones. And and that fits in this important part of the story here. Um, but before I launch you into that, okay, can I just back out of this a second and just talk about how you know the weathering over time has changed in the Appalachians? It used to be dominated by physical weathering when these were young and muscular mountains. Yeah, was chemical weathering happening for sure.
SPEAKER_01Real quick, just explain physical weathering versus chemical weathering, give a couple examples, maybe.
SPEAKER_00Um, physical weathering, you know, processes like ice wedging or or what is often referred to as frost wedging, things like this, where rock is when water freezes, it expands by about nine percent, and then it thaws and water seeps deeper down into cracks. You get this freeze-thaw, freestyle, repetitive cycle, and it just breaks rock apart. Any kind of cracks, uh fractures, water's gonna like exploit those, and just it's gonna break the rock apart, but it's not changing the chemical nature of the rock, it's just breaking it up into smaller pieces. Mass wasting events do this do this as well.
SPEAKER_01Those are rock falls, landslides, debris flows, yeah, flight events that that move a lot of sediment downhill really quickly. Yeah.
SPEAKER_00And all of that is these are mechanical kind of weathering breaking the rock apart without changing any chemical nature to it. And that's what dominates, you know, like out west right now, the western US, the Rockies, those young muscular mountains. That's the process that's dominating. Not to say there's no chemical weathering that's happening, but right. And it is, but it's a it's not the process that everybody notices, you know. Where now these mountains have been knocked down, they're at much lower elevation now over just long millennia of weathering. And the there's a shift that you see then in now chemical weathering begins to be the dominant process. You still get you know mechanical, physical weathering that's taken place, but I would say I think it's safe to say now that chemical weathering is the dominant process that's occurring. So you see this shift over time, but it's a lot of time.
SPEAKER_01I kind of think of this like chemical weathering's always there. It's kind of set by the climate conditions. If it's warm and rainy and humid, then you're gonna have high chemical weathering rates. If it's a cold climate, icy climate, you're gonna have really low chemical weathering rates, but that's kind of set by the background climate. That's always happening at some rate. And then if you have a high topography, then the physical weathering, physical erosion, is gonna happen a lot. Rockfalls are really dramatic, they're happening a lot out west. They're rare, they're not non-existent, but they're more rare on the east coast here. And so that as physical weathering decreases, then the chemical weathering kind of takes over as the dominant player in the weathering process. Now, there's feedback loops between these things. You know, chemical weathering is more reactive when you have fresh rock exposed. So chemical weathering is faster in high terrain areas, but it's a larger piece of the pie of the weathering pie when you have low topography like this, especially in a relatively hot, humid climate, like we have, you know, wet climate on the east coast of the United States, right? And where there's a lot of free stall cycles going on and you have a lot of water in the system. That presents you brought up carbonate and carbonate limestones, dolomites. Those rocks out west, we've talked about this in our Grand Canyon series. Uh, we had kind of a whole episode talking about this. You know, limestones out west are often cliff formers, they form big dramatic cliffs. Here, limestones are forming the valleys, and that's a big dramatic difference. And I'll skip ahead to this image here, Chris, which is not a carbonate, but I think it shows this prevalence of chemical weathering. We saw this alligator weathering feature on the diabase on Little Round Top, and then in Devil's Den and all the diabase in that region. You see this type of alligator weathering, and this stuff is a chemical weathering phenomenon that you don't see in basalts out west, really. You don't see this type of weathering feature. This is like a long-term, this this is kind of telling us it's indicating that chemical weathering has been happening for a long time period here. It's taking a long time. Like chemical weathering has been operating on that particular surface for a very long time. That that boulder has not fallen down a cliff, broken apart, and exposed fresh rock. That's an old feature, that chemical weathering. It's been active for a long time. So I think this just highlights this the dominance of chemical weathering in this area.
SPEAKER_00You know, there's another positive feedback loop with this whole thing, too, is that as young mountains get knocked down to lower and lower and lower elevation, temperature goes up with that also. You know, that feeds into you know, the warmer it is, the more uh chemical weathering is gonna be a dominating force then in Can we actually take a quick deviation before we get to the carbonate thing in the in the Great Valley?
SPEAKER_01Does it make sense that carbonate erodes negatively is more erodible in the east where there's higher chemical weathering than in the west where it's physical? Like, do have we does that make sense? I mean, have we explained that enough? Do you think, Chris?
SPEAKER_00I I think so. It's you know, the I I think so. It's a dramatic difference. Limestone out west forms cliffs, limestone out east forms valleys.
SPEAKER_01I mean, you and I drove through across the ridge and valley, most of the valleys are limestone or shale, and all the ridges are quartzites or quartz-rich scaly layers, right? It it it it there's not much limestone that's forming a cliff. There's hardly any in the east coast, at least in the in this part of the world. So that's just because limestone's easy to dissolve. Like, I mean, we we can dissolve it with weak hydrochloric acid, and it just gets dissolved easier. So we've had episodes on karst topography, and you know, we've determined that Chris is very karsty. Um very karsty individual over there.
SPEAKER_00Um I don't even nobody knows what that even means, Jesse.
SPEAKER_01Okay, nobody knows it sounds like you. Because it sounds like crusty and you're exactly. It's a combination of crusty, you're kind of an arse. Like, you know, it's kind of like smudge fingers. It's sort of a negative, it's a derogatory term, but we don't exactly know what it means. Oh my gosh. Oh, so much that I want to say right now, but I'm gonna remain professional. So, okay, well, you're trying to trying to stifle some kind of comments over there. A quick thing about chemical weathering that I that really struck me, Chris, about you know, our our trip to Gettysburg and looking at the geology here is that the fact that chemical weathering has been such a dominant part of the weathering process for a very long time has led to minor differences in the rock types becoming dramatic differences in the landscape. Now, that to me, uh I'm curious where that point resonated the most with you. But for me, it was kind of the the hypothesis that I don't think has been proven. But the idea that in this York Haven sill, the edges are a little bit just a tiny bit more resistant to weathering, whereas the interior of the sill was less resistant to weathering. Hence the reason why you have the round tops are at kind of the bottom of the sill, and cemetery hill is at the top of the sill, and in the middle it's a little bit easier to erode, so it has this kind of valley feature to it, which played out really dramatically in the battle, which we talked about. Um that struck me as like, wow, these are really minor differences in the rock type, but they create topographic differences because chemical weathering is has worked for such a long time on these rocks that that you could actually generate these major chemical relief differences. Uh that that really struck me. I'm curious what you think about that, and if there's one that stood out to you that's different than that.
SPEAKER_00Yeah, for sure. Um, you took my example, and and but that's okay because there was another really to me what stands out in my mind is a stark observation when we were on McPherson's Ridge in the in the uh railroad cut. What you have is you've got this like shaley rock, highly chemically altered, it's red, it's oxidized like crazy, and you have these little like interfingering uh interbedded sandy siltstone that's kind of in the middle of it. And you take those rocks then that were deposited originally flat and you tilt them about 15 to 20 degrees. So not a not a huge tilt to them. They're not dipping that much. But where that sandy that more sandstone-rich rock exposed at the surface, it formed a ridge, not much of a ridge, because the rock wasn't that thick. We're talking just maybe 10 feet thick, maybe 20 feet thick locally, so not very much, but it was definitely enough to form a ridge. And uh so that stands out to me as a subtle difference in the rock formed uh what was at the time a very important ridge from the battle standpoint.
SPEAKER_01Yeah, it formed a high ground. That's a really good example. That was very that was striking to me too. This is minor changes in the rock. You you can definitely tell they're there, but you they were not thick layers that were influencing this topography. That that's a really good example as well of how much these minor differences have a bigger impact because it's been operating for such a long time, because there's such old terrains that chemical weathering has been active for so long, minor differences get made bigger in a way.
SPEAKER_00It reminded me of one of my favorite things to do, Jesse, is just take a bag of young students. Oh, I love a half of a bag of Brudos. Take a group of young students and just go for a walk. Which is basically what we did there. Let's let's all right let's look at McPherson's Ridge and let's get down into the rocks. Let's see where we can see a cross-sectional view of these rocks and and then what can we what story do those rocks tell? And I love to do this like just you can do it almost anywhere. I call it geology in a ditch. And you just I'm serious. Yeah you take a group but you're just getting down and dirty with what can you see? And it's amazing the things that you can see in a ditch or a little a little river cut. You know that's all you need.
SPEAKER_01Just take a look at the world around you geology is operating everywhere and it explains so much. It just it's such uh it gives you such a new appreciation such a great appreciation for how it works yeah I don't know how the world works and and why why that rock is there and why does it look like that you know that it's great. I love that geology in a ditch you might need to come up with a better branding than that.
SPEAKER_00Geology in a ditch I think it's actually a great brand and it's if you're listening to this and you you have an opinion on my name for geology in a ditch if I can improve on that send us an email but otherwise tell Jesse to leave me alone. Field trip go to a ditch with smudge fingers let's do it it's just the best all right okay so Jesse I think let's transition then so we've talked about you know weathering and time and subtle differences in the rock and how weathering plays out differently then uh let's move into a little bit about the geology of the Great Valley then and and this is it's a really a continuation this story of the of the Great Valley is a continuation of the geology of South Mountain.
SPEAKER_01That's true. And that's a good segue Chris because here I'm gonna slip to image number three here. This is a GIF that we've made this just a a really uh a very basic cross section after some images uh we took kind of kind of took the template from images in the roadside geology of Maryland Delvere in Washington DC some nice kind of overview illustrations this is not South Mountain in particular but it's kind of the if you move south from South Mountain into Maryland into Virginia the Catoctin Mountains in there um we talked about this sort of anticlonorium feature and this shows this really well that on the left side we have this kind of you take the rock layers we've got a bunch of different rock layers we've got the catoctin basalts we've got the chilli group sediments you've taken them like a stack of papers and you've folded them in half to form an A. And in this case the A has been kind of tilted over to the left so you kind of tilt it over to the left side. And then on the right side of this diagram we have the Gettysburg Basin which shows this Lystric normal fault and then we have some basin sediments and the diabase intrusions in there as well that have kind of taken that thing and broken it in half after this folding event. So we could tell that the folding was older than the faulting that happened just basically there. That sets the stage for the anticlinorium and in South Mountain there's faults on both sides of on the the east side of South Mountain has this Lystric normal fault. On the west side there's also a fault and the same goes as you move down into the catoctins here. But we have to kind of think well you don't just fold a piece of paper down and form an A and that's it. Like what happens to the other sides of that and if we zoom out and move a little bit to the west we're going to zoom out in this next image image number four here which shows on the right side of the diagram we've got kind of that same thing the blue ridge anticlonorium which has kind of been sketched out there. As we move west especially in Virginia there's the other side of the rumple the rumple the A-shaped rumple goes up and then it you have a downturned rumple which we call a syncline and in this case a synclonorium because it's a regional scale thing with a bunch of little ones on top of it. And this is what forms the Great Valley but it's I so there's this syncline feature this is this U-shaped feature where the rocks are dipping down and then back up and there's faults on both sides of them but importantly the rocks are made of carbonates and these carbonates are part of a a huge the great meaning limestones and dull stones sorry yes limestones dull stones shallow water carbonates that were deposited on a continental margin around 500 million years ago and so they're called this great American carbonate bank they're part of that sequence. So we had you know these rocks were deposited on top of the Chillawe group and then eventually folded together and now we have anticlonorium synclonorium this kind of can I interject a second Jesse yes please do so yes the Great Valley is in a synclonorium but that downfold that down wrong is not why the valley is there.
SPEAKER_00That's great that I think that's an important point is that the valley is there because the rocks that are exposed at the surface are these chemically susceptible weathered rocks. And so they because they weather out fast and dominantly you get a valley. So it's not because it's a downdrop uh fold that's you can actually get a syncline can actually be a ridge and an anticline can actually be a valley dependent upon the rocks that are exposed at the surface.
SPEAKER_01So that's a really good point the limbs of these folds like the rocks are are tilted dramatically they're tilted a lot and so we're not creating valleys just because the tilting or because of the folding that's a really good point. It has a lot has mostly to do with the the chemical what'd you say susceptibility that's a good word susceptibility of these rocks to chemical erosion.
SPEAKER_00And also Jesse just so because I think people are gonna recognize things the the Great Valley has local names you know up where you are just to the west of South Mountain it's the it's called the Cumberland Valley. You get down into Virginia now it's called the Shenandoah Valley. So it has these local names to them but overall geologically we refer to it as the Great Valley.
SPEAKER_01And the Great Valley extends all up and down the the east coast goes up into Canada here. And so that you can trace this Great Valley feature all the way up and there's different geology along it this geology that we're showing here is particular to this Northern Virginia Maryland a little bit into Pennsylvania region. But but this Great Valley thing is a major topographic feature in the region. And I think Chris that this kind of helps explain why Gettysburg was kind of a focal point of movement in this region, why all the roads are at Gettysburg and it has a lot to do I mean it's a long history and long complicated history has to do with like the economics of the Chesapeake Bay region. But there's a long history here that has to do with the Great Valley. It was a big feature for settlers really I don't know what struck you about the the human history in relation to the geology that we've talked about in this region so far? Like what what things stood out to you in that so let's say pre-Gettysburg battle history?
SPEAKER_00Okay yeah um that's a good question so thanks for asking that um I I think it was the perfect why did Gettysburg become what it became and and you know before the battle it was this kind of county seat and it was like this perfect set of circumstances where you have the rivers are nearby the Potomac rivers to the south the Susquehanna to the north you know the South Mountain and the Blue Ridge uh just to the west of Gettysburg but you have these gaps that come through it. Yeah a big barrier right there and the gaps being an important um the gaps were important to connect towns you know so it it just kind of to me it was this set of circumstances that led to Gettysburg looking how it does which is it's crazy how it it looks like a wheel it the it the roads resemble spokes on a wheel it's it's uh I I don't know I've I've never seen a situation quite like that where you have this hub and everything just kind of radiates out from it but it was it was kind of like this set of circumstances that led to that happening the rivers the mountains the gaps that happened and then and everything then became connected right and so it became important as as the area became more populated.
SPEAKER_01I found it very interesting that the Great Valley early settlers were kind of in the 1700s were moving from Pennsylvania from the you know from the coastal towns inwards inland and they were moving up rivers you know the Susquehanna was a major river system to move upwards and then it hit the Great Valley and kind of used the Great Valley to get down south so it kind of progressively migrated into the Great Valley there's the where South Mountain is is kind of the top of the anticlonorium so there's a gap between the blue ridges end there and then you pick up to the north in the in the ridge and valley and so this we kind of have this hook shape to the Great Valley and people would come into the Great Valley there and then head south all the way down into the Shenandoah Valley again because these blue ridges were the barrier South Mountain in the Catocton Mountains and the Blue Ridge was kind of a a little bit of a barrier there. And so it all came down to like how could you get your crops out and get some supplies in right that was kind of the main controlling thing here for developing roads and connecting rivers to each other basically with the road systems. So there's a there's an interesting history and we we kind of read a a long paper or a short book about sort of about the topic here. That's quite interesting I would say but but kind of explained this this the top the geology explains the topography the topography explains human movements in including troop movements.
SPEAKER_00Well and that also includes the agriculture Jesse because you know this is really really fertile soil and again that's controlled by the chemical weather and right at the surface and that played a part in this too is why people were moving into this area is this is really really fertile ground.
SPEAKER_01That's exactly right Chris and in Gettysburg is kind of we talked about this before it's kind of settled in this little it's nestled up against South Mountain. South Mountain kind of hooks to the right uh kind of goes up from the north and hooks to the right and Gettysburg's kind of in the middle there which means that a lot of those spokes on the wheel that we referred to are dirt are pointing towards gaps in South Mountain that will go through South Mountain and then end up hitting a city on the other side like so roads would go through a gap that kind of point right to it. So those spokes on a wheel were kind of I look at the map and think these gaps are pointing to intersection points and Gettysburg's at that intersection point. So if we go from Hagerstown is to the southwest if you go through the gap through Hagerstown it goes right to Gettysburg Chambersburg from the west you come through the Castown gap Carlisle up to the north comes through a gap and then you hit Gettysburg there and they all so it's kind of nestled in South Mountain and the gaps kind of point towards Gettysburg in a way which led to you know road development and and the and troop movements through as the Confederates were kind of hiding in the Great Valley you know the troops would kind of go through those gaps and it led to a very natural um sort of battle point at Gettysburg I guess.
SPEAKER_00Once again geology wins geology wins.
SPEAKER_01Yeah so Jesse I think we're about ready to wrap this up but I want to ask you let me let me flip to image number five here a minute Chris while we're while we're wrapping up here to just kind of show those features that we're talking about the the Valley and Ridge the Great Valley and I think some of those gaps that that we can we can see in the that are breaking South Mountain um anyway sorry we'll wrap um yeah what what's the takeaway for you with this episode what did we do?
SPEAKER_00What did we accomplish today?
SPEAKER_01This episode to me the takeaway is that these old mountains have a different weathering distribution that chemical weathering plays a big role because they're old and that weathering has been active for a very long time which means small differences matter a lot in the rocks in a way that out west uh or in the Alps or in young mountain belts the small differences between rocks don't matter nearly as much for the topography. So that that's my sort of takeaway I want to ask you the same question.
SPEAKER_00What's your takeaway from this episode and then also what's your takeaway from this entire series and our field trip and our deep learning about Gettysburg hey thanks for the heads up on that question because um did you need some warning this morning to take about this um so today is easy uh the takeaway is it it was a refresher on weathering today and I and I I love that um it's just nice to look at things with maybe a different perspective I think because we talked about weathering in very specific contexts and um you know I I just I always love contextual geology. That was always my number one concern in every chemistry class I ever took was there's no context to it.
SPEAKER_01Yeah no that's right.
SPEAKER_00You know where's the real world application here I want to walk out with that every day and and so that's my takeaway for today. So the overall takeaway with the series I guess that's pretty easy for me is that the geology of the rocks made subtle differences in the topography sometimes not so subtle but most often they were fairly subtle which was difficult for me like 20 years ago when I was looking at the battlefield with without the geologic context and how geology really then played a really big role in the outcome of the battle. It played a big role in why the Union suffered so many casualties even though they were the defending force because the Diabe Sill on Cemetery Ridge they couldn't make any entrenchments they couldn't build up rocks. There was no availability to do that so they they had the high ground but they didn't have the protection that should go along with the high ground you know so to me that's the that's the takeaway of of the series geology played a a really big role in this really important historic battle.
SPEAKER_01Yeah absolutely what about you what's your takeaway for the series yeah my takeaway for the series is that I love standing on the Battle of Gettysburg site and one of the ridges and looking out and knowing that the rocks in this area you can see two supercontinent formation and supercontinent breakup events and all those rocks are exposed and meaningful to this battle. You can stand there be interested in the Battle of Gettysburg be interested in the geology of the battlefield and that leads you down this this this learning of the history of our planet over 700 million years and two supercontinent cycles. I mean that to me is just a it's an it's an amazing inroad to learning a broader uh a broader set of geological history here so I that to me was was and that they all played a role you know that the that South Mountain played a role in the battlefield and that the geology of the South Mountain and and and all those rhyolites played a big role in actually how the battle played out so I like that it's all interrelated to really big geological concepts. So good answer Jesse well done my friend you too good answer good answer. Well uh I think that's a wrap you can get this and all of our previous episodes on this topic on our Camp GeoMobile app first link in your show notes you can access a bunch of other stuff including geology Yellowstone geology of the Grand Canyon Geology of the Grand Tetons etc in your show notes there if you're on YouTube or Spotify please leave us a comment we love seeing those give us a follow like subscribe share with your friends if you like this stuff share with somebody who might appreciate this send us an email or contact us link is on our website planetgeocast.com Chris I think we should do more of the geology of historical events like this particularly battles maybe what comes to mind when I say that oof I have to think about that that's that's too that's a that's a uh like you're saying what comes next right what would I want to do next um I I need to think about that I've been stewing on this one for a long time and you know that oh yeah totally I mean I could you could go in here it took me just calling you up and saying I'm coming on this these days um be ready because Bullheist is coming into town I mean I we could do more civil war type stuff and then do the you know a part of a broader huge series on the geology of the civil war or we could do something I mean something that focused on the appellations comes to mind oh yeah Jesse because why why yeah let's just do the whole civil war since Gettysburg was so easy for us let's just let's just go oh my gosh it's not fun unless we got years of work in front of us. I mean let's you know if we're not doing something big and cool what the heck's the point okay all right yeah all right yeah all right hey that's a wrap cheers peace