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The Geology of the Battle of Gettysburg - A PlanetGeo Field Trip · PlanetGeo: The Geology Podcast

• 0:02
  SPEAKER_02: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you.
• 0:14
  SPEAKER_02: Christopher Bois, this is not our normal podcast.
• 0:17
  SPEAKER_02: This is our Gettysburg field trip, man.
• 0:19
  SPEAKER_02: This is uh this is a fun one.
• 0:20
  SPEAKER_02: This is we're gonna talk for like literally hopefully a minute here, and then go on our Gettysburg field trip.
• 0:26
  SPEAKER_02: Go out into the field with us.
• 0:27
  SPEAKER_02: And just a quick note here this is part one of a series that's available on our Camp Geo Mobile app where we can find the field trip.
• 0:35
  SPEAKER_02: You can find six more episodes additional to this where we do deep dives on individual topics about all kind of centered on the rocks, the landscape, the story of the geological story of the Battle of Gettysburg.
• 0:47
  SPEAKER_02: So the origin story here, how in the in your mind did this go about?
• 0:52
  SPEAKER_00: I was getting I was getting to the end of my my school year, summer's about ready to hit, and uh we had maybe two weeks to go, and I called you up.
• 0:59
  SPEAKER_00: I I just had an eye, I'm like, I we gotta just do this.
• 1:01
  SPEAKER_00: So I called you up and said, Jesse, I'm coming out.
• 1:04
  SPEAKER_00: We're gonna do this.
• 1:05
  SPEAKER_00: Um and and that's how it all started, and then it was like, oh crap, um, this is a monumental task.
• 1:11
  SPEAKER_00: It was a lot of work.
• 1:12
  SPEAKER_00: Those those two weeks were um a lot of late nights, I'll say that.
• 1:15
  SPEAKER_00: It was cram session, man.
• 1:16
  SPEAKER_00: But you know what?
• 1:17
  SPEAKER_02: I uh I've been meaning to I've I've been wanting to understand the geology of the Appalachians and this this region a bit more, uh deep at a deeper level, and this is a really good reason to do that.
• 1:28
  SPEAKER_02: But man, did I get a lot of value from this?
• 1:32
  SPEAKER_02: This I've been to Gettysburg before, you've been a lot before, and this was a totally new experience going there, understanding the geology, learning the geology, then walking around the battlefield, and it's a very somber place, but the geology is incredibly impactful and obvious, and it gave me a t completely new lens with which to see the you know the history aspect of it.
• 1:55
  SPEAKER_00: Um 100%, 100%.
• 1:57
  SPEAKER_00: It gave me that lens as well to see how exactly small, subtle uh changes in topography that are determined by the geology played really, really big into the outcome of the battle at Gettysburg.
• 2:09
  SPEAKER_00: That was that was really awesome for both of us to see and witness.
• 2:14
  SPEAKER_00: And uh yeah.
• 2:15
  SPEAKER_00: Um so and then we decided to not only just do the field trip, which included 18 stops, but it also we decided to take a deep dive into each of the rocks that played into the Battle of Gettysburg, and then one that not necessarily played into it, it's around, but it is historically very important, and we we did a dive into that one also.
• 2:39
  SPEAKER_02: That's exactly right, because all those episodes are available on the Camp Geo mobile app right now.
• 2:43
  SPEAKER_02: You can go there, first link in your show notes and purchase access, early access to that.
• 2:47
  SPEAKER_02: They will come out in the podcast feed over the next several months, probably, but you can get early access right now and see all the visuals with our audio visual uh with our visual podcast series as we normally do there.
• 2:58
  SPEAKER_02: So this is a long uh episode.
• 3:00
  SPEAKER_02: We're gonna turn to the field trip guide.
• 3:02
  SPEAKER_02: It's you and me with a GoPro out there in the field looking at rocks, talking about the geology.
• 3:06
  SPEAKER_02: It's a long one.
• 3:07
  SPEAKER_02: We decided to just release it all in one go because why not?
• 3:10
  SPEAKER_02: Um, and we have this as a visual podcast series as well on the app.
• 3:14
  SPEAKER_02: If you don't want to sit in front of your computer and watch it, you can take your phone and listen on the go.
• 3:18
  SPEAKER_02: Uh if you put Camp 2 mobile app there.
• 3:20
  SPEAKER_02: First link in your show notes.
• 3:21
  SPEAKER_02: So uh with that, Chris, shall we just get out there in the field?
• 3:25
  SPEAKER_02: Let's go.
• 3:26
  SPEAKER_02: Let's do it.
• 3:30
  SPEAKER_00: All right, some roadside geology at its best right here.
• 3:35
  SPEAKER_00: Looking at Fanglomerate, Chris.
• 3:36
  SPEAKER_00: Yes, our in York County, Pennsylvania.
• 3:40
  SPEAKER_00: It's pretty awesome.
• 3:41
  SPEAKER_00: Um back up a little bit, Jesse.
• 3:43
  SPEAKER_00: Back up a little bit.
• 3:44
  SPEAKER_00: Yeah.
• 3:44
  SPEAKER_00: I just want to show you that how the the environment was changing back and forth.
• 3:49
  SPEAKER_00: So, right there below the fanglomerate, you've got some decent looking, really red shale.
• 3:55
  SPEAKER_00: Um, yeah, so this is our first stop looking at fanglomerate, but we're gonna we're gonna go find some better outcrops because the stuff isn't very well put together.
• 4:05
  SPEAKER_02: So we're gonna find spectacular rock, Chris.
• 4:08
  SPEAKER_02: And the the story is very cool.
• 4:11
  SPEAKER_02: It's the last sort of stage of the basin formation, and that's a pretty cool outcrop right there.
• 4:17
  SPEAKER_02: Like that looks nice, but we can't get any can't get our hands on too many samples right here on this.
• 4:21
  SPEAKER_00: Yeah, just fall apart.
• 4:22
  SPEAKER_00: Yeah, yeah, yeah.
• 4:23
  SPEAKER_02: Weak rock, but really pretty.
• 4:25
  SPEAKER_02: Yep, excellent.
• 4:27
  SPEAKER_00: Alright.
• 4:35
  SPEAKER_02: Chris, the red light's blinking, man.
• 4:40
  SPEAKER_00: Alright, here we go.
• 4:41
  SPEAKER_02: Here we go.
• 4:42
  SPEAKER_00: Well, we are we're still in we're in York County right now.
• 4:46
  SPEAKER_00: Um Pennsylvania, heading to Gettysburg.
• 4:48
  SPEAKER_01: On the way.
• 4:49
  SPEAKER_00: And uh we are standing on the York Haven Diabase Silk.
• 4:54
  SPEAKER_00: Um, I don't know, Jesse, can you zoom in a little bit on the on the rock?
• 4:58
  SPEAKER_02: Yeah, let's look over here.
• 4:59
  SPEAKER_02: There's a couple.
• 5:00
  SPEAKER_00: You know, so diabase, just for I'm sure a lot of people understand what diabase is, but the most common rock in the world is basalt.
• 5:10
  SPEAKER_00: And um, but basalt is technically an extrusive igneous rock, and it's the most common rock in the world because it's what's extruded out in minnows ronages, and that's what covers the ocean here.
• 5:22
  SPEAKER_00: Well, diabase is just like basalt, but it's a shallower, intrusive rock, so it never became extrusive.
• 5:29
  SPEAKER_00: Yeah, that's the that's the distinction between it.
• 5:32
  SPEAKER_00: It it can be slightly coarser grain than a basalt rock where you can see the minerals because a little bit of slower cooling, okay, because it never never became extrusive.
• 5:42
  SPEAKER_00: That's right.
• 5:43
  SPEAKER_01: Yeah, yeah.
• 5:43
  SPEAKER_00: This is a famous um sill.
• 5:46
  SPEAKER_00: I mean, it's it's it's huge.
• 5:47
  SPEAKER_02: We'll see this later, and this uh this played a huge role in the Battle of Gettysburg as well.
• 5:52
  SPEAKER_02: Um and and a huge role, we'll have a whole chapter on this in our series because it played a role in perhaps in a massive extinction event, right?
• 6:00
  SPEAKER_02: And kind of sets the geological boundary point at around 200 million years old.
• 6:05
  SPEAKER_02: So we're kind of 200 million years old.
• 6:07
  SPEAKER_02: This is really important rock for the region.
• 6:09
  SPEAKER_02: Uh, it's really important rock for the Battle of Gettysburg, and this series of outcrops that we're gonna work our way along, right?
• 6:15
  SPEAKER_02: So you are standing right now.
• 6:17
  SPEAKER_02: I mean, I the weathering pattern, like what what do you think?
• 6:22
  SPEAKER_02: Or how how do you we've walked these outcrops so far, the these ones that we're gonna look at here, but how is this different than the sediments and the altered sediments that we'll see a couple hundred yards up the road?
• 6:32
  SPEAKER_00: Yeah, it's it's a very blocky kind of weathering, um, which is a testament to really kind of two things.
• 6:39
  SPEAKER_00: One, it's a really well put together rock, but it does have veins and joints in it.
• 6:44
  SPEAKER_00: And and those veins and joints kind of control the weathering patterns.
• 6:47
  SPEAKER_02: Yeah, here's a good example, Chris.
• 6:49
  SPEAKER_02: Show some veins.
• 6:49
  SPEAKER_02: We're just seeing these like thin black veins, but the rock itself is breaking in, you know, hand-sized or boulder-sized.
• 6:57
  SPEAKER_02: It's breaking apart into boulders, really, I would say.
• 7:00
  SPEAKER_02: Large cobbles or boulders.
• 7:02
  SPEAKER_00: Again, it's a testament to how well put together this rock is.
• 7:06
  SPEAKER_00: It's very resistant to erosion.
• 7:08
  SPEAKER_01: Yeah.
• 7:08
  SPEAKER_00: Um, and the other thing is, Jesse, that you know, this is this is a sill that's massive.
• 7:15
  SPEAKER_00: This is, you know, anywhere from 300 to 700 meters thick.
• 7:18
  SPEAKER_01: Yeah.
• 7:18
  SPEAKER_01: Oh, yeah.
• 7:19
  SPEAKER_00: So we're talking about outwards of 2,500 meters thick.
• 7:22
  SPEAKER_00: And by the way, hey, let's define this too.
• 7:25
  SPEAKER_00: A sill is an intrusivigneous rock that so it intrudes a rock above and below it.
• 7:32
  SPEAKER_00: And a sill is parallel to the rocks that it intrudes.
• 7:35
  SPEAKER_01: Right.
• 7:35
  SPEAKER_00: Whereas a dike is just like a sill, but it cuts across at an angle to the rocks that it intrudes.
• 7:42
  SPEAKER_01: Yeah.
• 7:42
  SPEAKER_00: So, you know, you textbook, in a in a geology textbook, you'll never see a sill like this because it's too big.
• 7:51
  SPEAKER_01: Oh, yeah, okay.
• 7:51
  SPEAKER_01: I see what you're saying.
• 7:52
  SPEAKER_00: Yeah, you need to see like sills on the order of several intruding rocks.
• 7:57
  SPEAKER_00: And and then you get a very like small zone of contact metamorphism with a sill like that.
• 8:03
  SPEAKER_02: A sill that's this size, good segue, Chris.
• 8:06
  SPEAKER_02: Good segue.
• 8:07
  SPEAKER_00: A like this is a very broad zone of contact metamorphism, and we're actually gonna walk up the road here.
• 8:14
  SPEAKER_00: We're gonna push pause in a second, walk up the road, and show you some of the rocks that are locally metamorphosed by this igneous intrusion, which is pretty awesome.
• 8:23
  SPEAKER_02: Yeah, and and and this is uh this sets the stage.
• 8:26
  SPEAKER_02: This intrusion sets the stage for a whole bunch of other stuff.
• 8:28
  SPEAKER_02: We'll talk about contact metamorphism just up the road, and this is just a a start out for our Gettysburg field trip here.
• 8:34
  SPEAKER_02: I mean, we're just seeing the initial rocks, we're not in Gettysburg yet, but but these rocks are regionally important, and we'll talk about some of that uh at the next stop.
• 8:42
  SPEAKER_02: All right, Mr.
• 8:47
  SPEAKER_02: Boy, what's happening?
• 8:49
  SPEAKER_02: Uh okay, we've moved a couple hundred yards down the outcrop.
• 8:53
  SPEAKER_02: We're now very close to an alteration halo, which you alluded to really nicely, but let's look at the background sediment.
• 8:58
  SPEAKER_02: First of all, we are in the Gettysburg Formation.
• 9:01
  SPEAKER_02: The Gettysburg Formation again plays a big role in the Battle of Gettysburg.
• 9:04
  SPEAKER_02: It's one of the two major, three major rock types there that influence the the battle itself.
• 9:10
  SPEAKER_02: But it's interesting to look at here, it's really well exposed, and we have some alteration and interaction between the diabase and the background sediments.
• 9:17
  SPEAKER_02: So these are older sediments, Gettysburg Formation deposited in the type here second.
• 9:24
  SPEAKER_02: Okay, oh yeah, perfect.
• 9:25
  SPEAKER_02: So this is one type of the Gettysburg Formation.
• 9:28
  SPEAKER_02: It looks at first pass.
• 9:30
  SPEAKER_02: This could you could convince yourself this is not a sediment, it could be you know some sort of basalt type thing, but upon close inspection, it's a mudstone, it's still breaking in a kind of a blocky way.
• 9:40
  SPEAKER_00: What color is it?
• 9:41
  SPEAKER_00: Like, you know, overall, what would you describe?
• 9:43
  SPEAKER_02: This is pretty purplish.
• 9:45
  SPEAKER_02: Uh, I mean, it's dark gray with hints of purple, I would say.
• 9:49
  SPEAKER_00: What would you be looking for?
• 9:50
  SPEAKER_00: Because it could be easily confused for uh uh in igneous rock like basalt or even the dye base.
• 9:57
  SPEAKER_00: What are you looking for to distinguish?
• 9:59
  SPEAKER_02: So, you know, you look for for sedimentary layering, which we see a little bit of here.
• 10:02
  SPEAKER_02: Um, it's it breaks along much flatter areas.
• 10:05
  SPEAKER_02: And Chris, if you just turn and look at this outcrop here, this outcrop is just breaking, it's more fissile, it breaks in much smaller chunks.
• 10:13
  SPEAKER_02: This is not a basaltic weathering pot pattern in a road cut outcrop, it's breaking into much smaller fist size and smaller chunks as opposed to big boulders, which we saw at the last outcrop.
• 10:23
  SPEAKER_00: Is resulting in covering up the actual contact, right?
• 10:26
  SPEAKER_02: Exactly, exactly.
• 10:27
  SPEAKER_02: So then what we're looking we're standing on over here, it's it's a little confusing because we were on one side of a contact, and now we're on the other side of the contact.
• 10:34
  SPEAKER_02: So we went through Gettysburg Formation.
• 10:36
  SPEAKER_02: We're sitting, this is kind of this background purplish stuff, and then as we get closer, there's diabase off to our right over here as we continue up the road.
• 10:45
  SPEAKER_02: There's more diabase, and in between there's a contact aureal, uh, what we call scarn deposit, which is a contact metamorphism deposit where the igneous rock intrudes in, and all the fluids, all the goodies that are coming off of that igneous body get pushed into the sediment, and the sediment has a very different chemical recipe for it.
• 11:06
  SPEAKER_02: So we have all sorts of interesting and creative, well, really dynamic reactions going on, and a bunch of those can precipitate minerals.
• 11:12
  SPEAKER_02: I mean, we interviewed Steph Morzak, who is a SCARN expert, and some of the largest ore bodies in the world are from SCARN deposits.
• 11:20
  SPEAKER_02: This is not that, but it is a little ore body.
• 11:23
  SPEAKER_00: I wouldn't say you would mind this, but circulating fluids can concentrate minerals that we want and need.
• 11:30
  SPEAKER_02: Exactly.
• 11:30
  SPEAKER_02: And so, Chris, there's a couple examples of this that I'll pull up here that basically we're looking these look in these rocks, they look very bleached.
• 11:38
  SPEAKER_02: So, this just looks like we took this background really dark gray, purpley rock, and we've we've bleached it.
• 11:44
  SPEAKER_02: We've put fluids in here that leached it out.
• 11:47
  SPEAKER_02: It's much lighter.
• 11:48
  SPEAKER_02: There's some green tints, there's some really pastel colors, a little bit light pink, I would say, even a little bit to white.
• 11:54
  SPEAKER_02: Um, and it just looks bleached.
• 11:55
  SPEAKER_02: And there's a bunch of different reactions that could go on.
• 11:58
  SPEAKER_02: In this particular instance, there's we also see some veins that are depositing minerals.
• 12:03
  SPEAKER_02: So we've got cracks with veins in them here that are fractures, and you can see they're filled with different minerals.
• 12:09
  SPEAKER_02: These aren't spectacular ones, but if we just kind of look at this rock, again, you can tell it's a background sediment, but we have little hints of epidote in here in these veins.
• 12:19
  SPEAKER_02: I wouldn't say this is particularly stunning epidote in this out this rock that I'm holding, but it just kind of looks white, bleached, brown, kind of rusted out.
• 12:27
  SPEAKER_02: Like we talked about, we've talked about that before, that rusting feature.
• 12:30
  SPEAKER_00: And epidote is often a metamorphic mineral, right?
• 12:33
  SPEAKER_02: Yes, very frequently.
• 12:34
  SPEAKER_02: In in especially around rocks that are mafic in composition, we get a lot of epidote.
• 12:39
  SPEAKER_02: Um, in this instance, in this deposit, we'll go look at a little hole in the in the wall that people have been digging because it's a famous mineral location for copper minerals.
• 12:47
  SPEAKER_02: And this is related to a big um mineralizing event that formed a lot of iron deposits, um uh copper deposits, little small-scale contact metamorphic deposits in this part of the world in Pennsylvania, um, when the sill intruded around 200, almost nearly 200 million years ago.
• 13:06
  SPEAKER_00: So, as a like a takeaway of this outcrop, then, is this is a really good this is a really good example of contact metamorphism from a dye-based sill intrusion.
• 13:18
  SPEAKER_02: Absolutely.
• 13:18
  SPEAKER_02: And a lot of these, Chris, we you know, we've we've talked about green shifts and green rocks.
• 13:22
  SPEAKER_02: A lot of these like altered bleached green rocks, you can tell that there's metamorphism that's happened to these.
• 13:27
  SPEAKER_02: And this is just metamorphism by the temperature brought in by the igneous intrusion by the dike of the sill, um, and then the fluids that went along with that circulating through the rock.
• 13:37
  SPEAKER_00: And again, not a lot of pressure, right?
• 13:38
  SPEAKER_02: Not a lot of pressure, no change in pressure, actually.
• 13:40
  SPEAKER_02: I mean, maybe a tiny bit, but not any substantial change in pressure.
• 13:44
  SPEAKER_02: Where contact metamorphism is just heating up and dumping fluids into the rock.
• 13:48
  SPEAKER_00: That's why a lot, a lot of the times then the sedimentary structures from the protolith rock, the parent rock before it was metamorphosed, they're preserved because it's like heat.
• 13:58
  SPEAKER_00: So we can see some of the stratification in the rocks.
• 14:00
  SPEAKER_00: That's a good point.
• 14:01
  SPEAKER_00: If it was originally metamorphosed rocks with lots of pressure and it was directional pressure, we'd be able to see that.
• 14:07
  SPEAKER_02: That's right, that's right.
• 14:08
  SPEAKER_02: Yep.
• 14:08
  SPEAKER_02: So let's go look at the adit and um zoom in on some epidote.
• 14:12
  SPEAKER_02: How about that?
• 14:13
  SPEAKER_02: Yes, all right.
• 14:21
  SPEAKER_02: Okay, Chris.
• 14:23
  SPEAKER_02: All right, there's a boys, Mr.
• 14:25
  SPEAKER_02: Bois.
• 14:25
  SPEAKER_02: Yeah, um, all right, we just moved tens of meters down the way.
• 14:30
  SPEAKER_02: We're now near the contact.
• 14:31
  SPEAKER_02: We have dye base off to this side.
• 14:33
  SPEAKER_02: That dye base sill that we talked about, the York Haven dybase.
• 14:37
  SPEAKER_02: Um, what as the magma cools down, the magma starts to release its fluids.
• 14:42
  SPEAKER_02: All magma has a little bit of fluid in it, and we what we mean by that is a magma is melted rock, it's silicate-based liquid.
• 14:49
  SPEAKER_02: Uh, fluid, we generally refer to that as uh what we call fluid, is generally like water-based liquid or carbon dioxide-based liquid.
• 14:57
  SPEAKER_02: So the magma is cooling down, crystallizing, giving off its fluids.
• 15:01
  SPEAKER_02: When that happens, there's a bunch of elements that love to be in the fluid.
• 15:04
  SPEAKER_02: Copper, uh copper is one of them.
• 15:07
  SPEAKER_02: Copper iron gets put into the fluids as well.
• 15:10
  SPEAKER_02: Bunch of a bunch of uh elements get put into the fluids.
• 15:12
  SPEAKER_02: Those fluids interact with again.
• 15:14
  SPEAKER_02: We are sitting on the Gettysburg formation, and so that those fluids are interacting with the Gettysburg formation, altering it.
• 15:21
  SPEAKER_02: Here's a pile of rocks here, Chris, that are all that really white, really chalky, altered, bleached, obviously, obviously not background sediment here.
• 15:31
  SPEAKER_02: Right on, and you, Chris, are gonna take us.
• 15:33
  SPEAKER_02: Should we go look at some rocks?
• 15:34
  SPEAKER_02: Got it.
• 15:34
  SPEAKER_02: Go take us in there.
• 15:35
  SPEAKER_02: Let's see what's going on.
• 15:36
  SPEAKER_00: Let's look at this.
• 15:37
  SPEAKER_00: All right, let me turn my lights on, everybody.
• 15:42
  SPEAKER_00: Okay, so this is what Jesse was alluding to uh earlier when he was talking about scarns, and this is like a very small localized example of a scarn.
• 15:54
  SPEAKER_00: Way in the back wall there, you can see that bright green.
• 15:57
  SPEAKER_00: I think it's showing up pretty well.
• 15:59
  SPEAKER_00: That bright green mineral.
• 16:00
  SPEAKER_00: Um, that's malachite, that's one of our favorite minerals.
• 16:04
  SPEAKER_00: It's probably one of my all-time favorite minerals.
• 16:06
  SPEAKER_00: Just love the the flavor of green that it that it gives off.
• 16:10
  SPEAKER_00: Um, and that's a copper hydroxide mineral.
• 16:13
  SPEAKER_00: So, as Jesse was saying, that you know, these these fluids are coming off the magma and they're circulating through this, and it and but those fluids also contain certain elements, in this case, a lot of copper, and it's concentrating them very close to the contact zone, then between the dye-base sill and these sedimentary rocks, um, heavily altering these sedimentary rocks and then concentrating uh minerals.
• 16:38
  SPEAKER_00: And in some cases, these are economic, not so much in this case, at least we don't think so.
• 16:42
  SPEAKER_00: No, it's it's too small, um, you know, but but we have interviewed people on our podcast, Stephanie, uh Mrozak, uh you know, that and she specializes in scarns on a much larger scale than this.
• 16:55
  SPEAKER_00: Exactly.
• 16:55
  SPEAKER_02: And Chris, these these min this mineralogy gets really complicated here because again, the minerals are changing.
• 17:02
  SPEAKER_02: They've experienced a bunch of different episodes.
• 17:04
  SPEAKER_02: They're magma interacting with the fluids, then the fluids interacting with the sedimentary rock.
• 17:08
  SPEAKER_02: Then, since 200 million years ago, there's been surface alteration, weathering, and hydration going on.
• 17:13
  SPEAKER_02: So the malachite and azurite are hydration products of the original copper ore minerals or copper-forming minerals, which are things like bornite, chalkasite, things like that that were originally deposited in these rocks and then later altered and made this kind of interesting little uh mineral collecting.
• 17:31
  SPEAKER_00: That's right.
• 17:31
  SPEAKER_00: Now, I would like to collect some of that back there, but you know what?
• 17:35
  SPEAKER_00: Those supports there that you saw in the video, they don't lend me a whole lot of.
• 17:40
  SPEAKER_02: Let other people do that.
• 17:42
  SPEAKER_00: I also will say this is tick heaven, apparently, because Jesse has already picked two ticks.
• 17:46
  SPEAKER_00: Oh, yeah, we got a couple of lower levels.
• 17:48
  SPEAKER_02: Hey man, it's Pennsylvania in the summer.
• 17:49
  SPEAKER_02: Gotta deal with the ticks.
• 17:50
  SPEAKER_00: Right on.
• 17:51
  SPEAKER_00: Excellent.
• 17:51
  SPEAKER_00: All right, let's go.
• 17:52
  SPEAKER_02: Let's do it.
• 17:52
  SPEAKER_02: Gettysburg, here we come.
• 18:04
  SPEAKER_02: You ready?
• 18:05
  SPEAKER_02: Stand close to the cannon, Mr.
• 18:06
  SPEAKER_02: Voice.
• 18:07
  SPEAKER_02: Okay, I will.
• 18:08
  SPEAKER_02: I will.
• 18:08
  SPEAKER_02: Okay.
• 18:09
  SPEAKER_02: Hey, we're here.
• 18:10
  SPEAKER_00: Here we are.
• 18:11
  SPEAKER_00: We're at Gettysburg.
• 18:12
  SPEAKER_00: I love this place.
• 18:13
  SPEAKER_02: I've been coming here for a long time.
• 18:16
  SPEAKER_02: It's uh very cool.
• 18:17
  SPEAKER_02: I've only been here, I've been here once before, and it was not with a geological understanding or appreciation or thinking about the geologist.
• 18:24
  SPEAKER_02: It was fun for me to tie it all together then to read all the articles, all the field guides, all the papers, and then come and this is the first stop that we've made to be here.
• 18:34
  SPEAKER_00: We're doing, you know, how the geology played into the the outcome of the battle of Gettysburg, but disclaimer first, we're not historians.
• 18:41
  SPEAKER_00: Oh, yeah, we are geoscientists, and and so but I feel comfortable.
• 18:45
  SPEAKER_00: I mean, focusing on the geology.
• 18:47
  SPEAKER_00: We're focused on the geology, but but you know, so just be nice if we if we don't get all the detail that the historians we're definitely staying out of the weeds about military history.
• 18:56
  SPEAKER_02: We're in the weeds with geology, but out of the weeds with military history.
• 18:59
  SPEAKER_02: So this stop, Chris, this is our first stop.
• 19:01
  SPEAKER_02: We're at Long Street Tower, um, is is just over there, and we'll go up there and get some footage as well.
• 19:07
  SPEAKER_02: Um, but this is an overview.
• 19:09
  SPEAKER_02: We can see from here, at least we can draw and then see up on the tower the geology, yeah, what we're looking at.
• 19:16
  SPEAKER_02: So let's show us a little map in a minute.
• 19:18
  SPEAKER_00: Yeah, so we're kind of standing about right here, and and I I labeled this as seminary ridge, but it's actually it's seminary ridge and Warfield Ridge, but wear field kind of angles down.
• 19:27
  SPEAKER_00: Yeah, it kind of angles down a little bit right there.
• 19:29
  SPEAKER_00: We're technically on Warfield Ridge, but we're just a little bit north of the roundtops right now, and so what we're standing on is this ridge.
• 19:36
  SPEAKER_00: It's lower.
• 19:38
  SPEAKER_00: This is where the Confederates were.
• 19:39
  SPEAKER_02: So, Chris, let's just set the stage here real quick.
• 19:42
  SPEAKER_02: We're we are standing to the west of Gettysburg, southwest of Gettysburg, the town.
• 19:46
  SPEAKER_00: Yeah, something here.
• 19:48
  SPEAKER_02: Yeah, and and the the classic, the old um, the famous Union fish hook, which is uh cemetery hill, the round tops we just described.
• 19:56
  SPEAKER_02: This is this fish hook that the shank of it, if you kind of think of the eye in the south, the shank runs kind of north-northeast basically, and then the hook hooks over to the right and drops down again.
• 20:07
  SPEAKER_02: So it's kind of a fish hook you're holding upside down with the hook side to the right.
• 20:11
  SPEAKER_02: Yeah, that's the fish hook.
• 20:12
  SPEAKER_02: We're standing on what's called seminary ridge or Warfield Ridge, which is just to the west of the fish hook.
• 20:18
  SPEAKER_00: And what we want to do is talk about like why is this a ridge?
• 20:21
  SPEAKER_00: Yeah, and why is this a ridge?
• 20:23
  SPEAKER_02: And then we'll go on all these stops, we'll see all these areas in detail, look at the rocks themselves, but this overview is really important.
• 20:29
  SPEAKER_02: That's right.
• 20:29
  SPEAKER_02: Um, and and just real quick, so finish on seminary ridge.
• 20:32
  SPEAKER_00: So seminary ridge here and Warfield Ridge.
• 20:34
  SPEAKER_02: Let me just show because you can nicely see while you're talking, Chris.
• 20:37
  SPEAKER_02: You can kind of see this ridge feature.
• 20:38
  SPEAKER_02: So keep talking.
• 20:39
  SPEAKER_00: And this ridge is it's uh it's part of what's called the Rossville Dike.
• 20:43
  SPEAKER_00: And again, we talked about this at one of our earlier stops.
• 20:45
  SPEAKER_00: Is that a dike is an igneous intrusion, it's a dye-based dike, and just like the rocks we looked at before in York County, um, but that cuts across the rock layers at an angle.
• 20:56
  SPEAKER_00: This is thinner, and it's it's not as massive as the York Haven or Gettysburg sill that we're gonna talk specifically about here today.
• 21:04
  SPEAKER_02: So these are intrusive rocks, and you think of them coming, you can think of it coming straight up out of the earth.
• 21:08
  SPEAKER_02: So this is a feeder, a feeder dike that's cutting across, fed some volcanic eruption up above what used to be rocks above our heads now, but we've eroded down in and they're really resistant, which is why it stands as a ridge, okay.
• 21:20
  SPEAKER_02: Right.
• 21:20
  SPEAKER_02: So this is again basalt, it's an igneous rock.
• 21:22
  SPEAKER_02: It's uh resistant to weathering, and we've got a long history of weathering here that creates, as we can kind of see behind over your shoulder here, we can see these valleys and the ridges in the background.
• 21:31
  SPEAKER_02: When we zoom in on that, we'll be able to see that.
• 21:33
  SPEAKER_02: But yeah, we're still landing on one of these ridges.
• 21:35
  SPEAKER_00: So then you know, so this was where the where the Confederates were, this is where the Union line was on this kind of cemetery uh hill here in cemetery ridge coming around this fish hook to Calps Hill.
• 21:46
  SPEAKER_00: This is a part of the Gettysburg Sill or the York Haven Sill that we talked about before.
• 21:52
  SPEAKER_00: It's massive, it's it is a big deal.
• 21:54
  SPEAKER_00: Um, and so in between is what's called the Gettysburg Basin.
• 21:58
  SPEAKER_00: Yeah, and this is just sediment.
• 22:00
  SPEAKER_00: Your rocks, some of them are locally altered because of this igneous intrusions, contact metamorphism.
• 22:05
  SPEAKER_00: We're gonna visit some of those here today.
• 22:07
  SPEAKER_00: But again, I got a ridge, and and this ridge, it has some lower spots to it, but generally speaking, it is the high ground.
• 22:13
  SPEAKER_02: Yeah, that's right.
• 22:14
  SPEAKER_02: Okay, and and so and Chris, if we if we zoom out from the battlefield central here, you've drawn not to scale South Mountain, which is a really important regional thing.
• 22:22
  SPEAKER_02: We are in the Piedmont.
• 22:23
  SPEAKER_02: This is the Piedmont province.
• 22:24
  SPEAKER_02: So everything in Gettysburg that we're focusing on, basically anything you can see in the screen, for the most part, if you're standing up at any of these battlefield locations, most of what you're seeing is the is the Piedmont province, which is basically sloping from the Appalachian mountain chain, sloping to the ocean.
• 22:40
  SPEAKER_02: That's generally the geology.
• 22:41
  SPEAKER_02: What we've drawn here is a really important one that we're gonna have episodes in this in this podcast series on is the South Mountain Baffle with, which is up to the the west and the northwest um of us now.
• 22:52
  SPEAKER_02: We can just make it out.
• 22:53
  SPEAKER_02: It's a pretty foggy day, but you can just kind of see it off to the horizon to the west and northwest.
• 22:57
  SPEAKER_02: Yeah, low they look like low hills.
• 22:59
  SPEAKER_02: Actually, Chris, they look really dramatic right now with all of the smoke.
• 23:02
  SPEAKER_02: I mean, they look substantial and intimidating.
• 23:05
  SPEAKER_00: Just for perspective, we're at about 600 feet above sea level, and the the top of those mountains in the distance that you can barely make out over there, they're the maximum is about 2,100 feet elevation.
• 23:16
  SPEAKER_00: So they're significant, significant, yep.
• 23:18
  SPEAKER_00: And that was significant in the Battle of Gettysburg too in terms of troop movements.
• 23:21
  SPEAKER_02: There's some cool rocks over there.
• 23:22
  SPEAKER_02: They're old rocks, old volcanic rocks, old quartzites that are the quartz sites are really resistant.
• 23:27
  SPEAKER_02: We'll get into that in more detail.
• 23:28
  SPEAKER_02: We're going there tomorrow.
• 23:29
  SPEAKER_02: It's gonna be really exciting, like very, very cool rocks.
• 23:31
  SPEAKER_02: I'm excited to check those out.
• 23:32
  SPEAKER_00: So but they prevented this mountain, which which bleeds into the Blue Ridge mountain and down into the Shenandoah, it's it's this long range, right?
• 23:43
  SPEAKER_00: The Blue Ridge range is 70 miles long, and and so the Confederates came this direction uh from the south, uh marching marching north, and they were obscured then by the Union, which came up the Piedmont Valley.
• 23:59
  SPEAKER_00: Right.
• 23:59
  SPEAKER_02: So this is the Great Valley over here, and the Piedmont contains all of the major cities, right?
• 24:03
  SPEAKER_02: Washington, DC, uh Philadelphia, a lot of them are in the this Piedmont province.
• 24:08
  SPEAKER_02: And the Confederates, the the Army of Northern Virginia, moved up through the Great Valley, the Shenandoah Valley, which is part of this Great Valley province, which just runs on the west side here of this mountain belt, which, as you said, obscured their troop movements.
• 24:20
  SPEAKER_02: And then they had to come through through a nice little fault.
• 24:23
  SPEAKER_02: Cast down gap, which we'll go visit and we'll we'll show some cool images of this.
• 24:27
  SPEAKER_02: We'll we'll try and get a visual of this, even though uh it's a little bit foggy.
• 24:30
  SPEAKER_02: But we'll look at this and the cast down gap is really important, right?
• 24:33
  SPEAKER_00: You said that it is very important because it formed uh the the way the faulting affected the rocks is it made for a wide gap and like um a gap that was easily traveled and easy to get artillery over and seriously fast.
• 24:48
  SPEAKER_02: Chris, look at that.
• 24:49
  SPEAKER_02: We're standing next to a cannon here, an artillery.
• 24:51
  SPEAKER_02: Trying to get this over 1,500 foot of relief of mountains is is no small task, especially with the huge uh you know, Civil War era troop movements.
• 24:59
  SPEAKER_02: Right on.
• 24:59
  SPEAKER_02: So these gaps were invaluable to both the the Confederates getting to Gettysburg, getting north, they hid behind it, they hid behind South Mountain to get north up into Pennsylvania, up into the the the um the Union country, and then eventually try and make the way to Harrisburg, but then came back south, used these gaps, and they also used those gaps as a retreat.
• 25:18
  SPEAKER_00: Yes, they came and they went through this.
• 25:20
  SPEAKER_00: Exactly.
• 25:20
  SPEAKER_00: Which is this is also important.
• 25:22
  SPEAKER_00: I said this is not to scale, and it's not even close, but this is three miles of the side.
• 25:26
  SPEAKER_02: This is definitely not to scale.
• 25:28
  SPEAKER_02: Highlight double click.
• 25:29
  SPEAKER_02: This is not to scale.
• 25:30
  SPEAKER_00: Three miles of office in this in this that cuts right through South Mountain.
• 25:34
  SPEAKER_00: It's beautiful, uh, super important.
• 25:36
  SPEAKER_00: So it's ironic too, Jesse is that so the Confederates came up this direction, right?
• 25:40
  SPEAKER_00: And they cut through the Cash Town Gap right here.
• 25:42
  SPEAKER_00: And and so they entered Gettysburg then from the north.
• 25:45
  SPEAKER_00: That's ironic.
• 25:46
  SPEAKER_00: The Confederates entered through the north, the Union entered through the south, which is like that's a it's an interesting turn of phrase.
• 25:52
  SPEAKER_02: Came north from the potomac down there.
• 25:54
  SPEAKER_02: Uh exactly, come from the south to the north, right?
• 25:57
  SPEAKER_02: That's right.
• 25:57
  SPEAKER_02: Okay, so let's we're gonna throw, we're gonna overlay some footage from up on the tower here, and so that we can actually try and see this stuff.
• 26:03
  SPEAKER_02: It's a little hard to capture this though.
• 26:05
  SPEAKER_02: The relief, I mean, Chris, it's yeah, looking around here, it's just it is it is very impressive to see the relief, even though it looks low, flat lying.
• 26:14
  SPEAKER_02: There's a lot of relief here.
• 26:15
  SPEAKER_02: It's actually pretty impressive, and it's cool to think we're standing on a two 201.2 million year old diabase dike.
• 26:22
  SPEAKER_02: We'll get into more details on that.
• 26:24
  SPEAKER_02: And that's the one of the more recent events that geologically happened here.
• 26:28
  SPEAKER_00: This is my favorite time in Gettysburg, and I've been here many times.
• 26:32
  SPEAKER_00: And and I don't mean any offense, Jenny.
• 26:34
  SPEAKER_00: I'm sorry, but to get to see it from this perspective, uh too.
• 26:39
  SPEAKER_00: He's having more fun with to do the research like this and then be able to see how the geology played a massive factor in how the battle went, why the battle happened here, and and then how it turned out.
• 26:53
  SPEAKER_00: The geology played a big deal in this, and this is what it's all about.
• 26:56
  SPEAKER_02: All right, let's go.
• 26:56
  SPEAKER_02: We'll keep we'll keep looking, we'll we'll go look for some rocks.
• 26:59
  SPEAKER_02: Let's go.
• 26:59
  SPEAKER_02: See ya.
• 27:08
  SPEAKER_02: Good, good, good.
• 27:10
  SPEAKER_02: It's raining, we're getting rained on.
• 27:11
  SPEAKER_02: All right, guys.
• 27:12
  SPEAKER_02: So uh all right, here we are.
• 27:14
  SPEAKER_02: Plum run or bloody run is in the background here.
• 27:17
  SPEAKER_02: This is the the stream that kind of divided the Confederate standpoint that we were last at on Seminary Ridge and and um on the ridges there to slightly to the west from the Union lines, and it runs right between uh the round tops, right between big round top and the round top.
• 27:34
  SPEAKER_00: Yep.
• 27:35
  SPEAKER_02: If we come over here, so the the rot just to set this stage, the rocks we're looking at are not in place.
• 27:40
  SPEAKER_02: We're looking at a bridge, uh rocks and a bridge, but they were quarried from the Heidelberg member, which is kind of in the middle of the Gettysburg formation unit sequence.
• 27:50
  SPEAKER_02: So these are about 210 plus or minus a bunch million-year-old sedimentary rocks, um, deposition rocks from the Heidelberg's member.
• 27:59
  SPEAKER_02: And uh, what we have, what's really cool, and I'm glad we stopped.
• 28:02
  SPEAKER_02: I wasn't completely convinced we couldn't.
• 28:03
  SPEAKER_02: Yeah, you worked.
• 28:04
  SPEAKER_00: I had to work at this one here.
• 28:05
  SPEAKER_02: I had to convince you.
• 28:06
  SPEAKER_02: We've got some dinosaur footprints and some other cool sedimentary features.
• 28:10
  SPEAKER_02: That's right.
• 28:11
  SPEAKER_02: So uh, if we come over here to this unit, this is really beautiful.
• 28:15
  SPEAKER_02: Get some rain off of this thing, but you can see kind of three lobes.
• 28:19
  SPEAKER_02: So this is kind of a grayish rock.
• 28:21
  SPEAKER_02: We're actually looking at an old bridge that was built in the national by the National Park Service, and this stuff, these stones that are from the top of this bridge came from a quarry from York Springs area, right around here.
• 28:32
  SPEAKER_02: Um, and like I said, this Heidelberg member is exposed just to the west.
• 28:36
  SPEAKER_02: So, in between Gettysburg National Park and the Gettysburg Monuments and Gettysburg Town, in between there and South Mountain, you would go across the Heidelberg member, then back into Gettysburg Formation, then hit South Mountain.
• 28:47
  SPEAKER_02: So these are nearby uh rocks quarried from very nearby.
• 28:51
  SPEAKER_02: But that's just very cool.
• 28:52
  SPEAKER_02: I mean, that's a dinosaur footprint.
• 28:54
  SPEAKER_02: These are smaller footprints, smaller uh creatures because they're Triassic, 210 million years old, before the well, I don't know, we kind of perceive of the age of the dinosaurs as being Jurassic, but they're around the before they got huge.
• 29:07
  SPEAKER_02: Yeah, before they got huge.
• 29:08
  SPEAKER_02: Um, and that that extinction event between the Triassic and the Jurassic, we talk about in relation to camp magmatism and the diabase sills and dikes that we see in the area.
• 29:18
  SPEAKER_02: Um, but this is just a great this is great outcrop.
• 29:21
  SPEAKER_02: If we move a little bit forward, we're not sedimentologists, we don't love sediments necessarily, but we do like sedimentary food.
• 29:26
  SPEAKER_00: I do, I like it, yeah.
• 29:27
  SPEAKER_00: Hey, here's Plum Run, real quick.
• 29:30
  SPEAKER_00: Oh, yeah.
• 29:30
  SPEAKER_00: Um, which was dubbed Bloody Run after the battle because for obvious reasons, I think.
• 29:36
  SPEAKER_00: A lot of fighting took place right in this vicinity.
• 29:38
  SPEAKER_02: Yeah.
• 29:40
  SPEAKER_02: And you can, I mean, again, the topography, we are definitely in a valley which you know, eastern Mar eastern Pennsylvania and Maryland is not known for topography, but you really feel it down in here that you are we are in a valley forested.
• 29:53
  SPEAKER_02: This creek would be very difficult to cross.
• 29:56
  SPEAKER_02: Um, and we've got some cool sedimentary features in here.
• 29:59
  SPEAKER_02: So here we've got a couple different ripple mark sequence.
• 30:02
  SPEAKER_02: So maybe just can you get a view of it correctly?
• 30:04
  SPEAKER_02: I'm trying.
• 30:05
  SPEAKER_00: Like it's we got.
• 30:06
  SPEAKER_00: I mean, we don't have the best weather today, but some ripple marks here that are that are kind of bigger as well.
• 30:12
  SPEAKER_02: These are bigger, these are maybe a couple inches in between the in um wavelength, so kind of in between the cross the trap, the the crest and the troughs.
• 30:19
  SPEAKER_02: Uh, we have a couple inches, four inches, something like that.
• 30:22
  SPEAKER_02: So they're kind of bigger wavelengths.
• 30:24
  SPEAKER_02: This again, it's a couple feet by four foot uh slab of rock that we're looking at the cut faces.
• 30:29
  SPEAKER_02: And the way these things are quarried, which is great for us, is they're often broken along the weak planes and rock.
• 30:34
  SPEAKER_02: So they expose uh when these things are quarried, you you cut them, but you break them, cleave them along the sedimentary surfaces.
• 30:42
  SPEAKER_02: So we preserve a lot of those surface features in the surface of sediments.
• 30:45
  SPEAKER_02: And this one, because I really like here's much shorter wavelength ripples, these are on the order of less than an inch wavelength, and they're a little asymmetric.
• 30:52
  SPEAKER_02: If I run my finger across them, I can feel there's a shallow upslope and then a steeply dipping down slope.
• 30:57
  SPEAKER_02: There's some kind of mud crack cracks in this thing as well.
• 31:01
  SPEAKER_02: This is a really uh this is a great sample of ripple marks.
• 31:04
  SPEAKER_02: It feels like a washboard when you run your hands across, you know, those old washboards, wash basins that you kind of scrape stuff across.
• 31:10
  SPEAKER_02: That's what it feels like.
• 31:12
  SPEAKER_02: And we've got some over here, Chris, that are mud cracks.
• 31:15
  SPEAKER_02: And mud cracks, so so ripple marks are flowing water or wave action moving sediment back and forth.
• 31:21
  SPEAKER_02: Anybody who's gone to the beach before, I think knows what a ripple mark is.
• 31:25
  SPEAKER_02: You can kind of see them when you get in just in the shallow waters.
• 31:28
  SPEAKER_02: You start to walk in a foot of water or less than a meter of water, and you feel that undulating um the undulating nature of the sediment underneath your feet.
• 31:36
  SPEAKER_02: It's that preserved in rock form.
• 31:38
  SPEAKER_02: Here we have some some mud cracks and that are running, and these are kind of big, big, big cracks that were later filled in with sediment, a little tougher sediment where we get little ridges.
• 31:49
  SPEAKER_02: And I'm not really sure, Chris.
• 31:50
  SPEAKER_02: What do you think?
• 31:50
  SPEAKER_02: These are uh you think these are raindrop?
• 31:52
  SPEAKER_02: They look like it's down um raindrop impressions.
• 31:56
  SPEAKER_02: I'm not a sedimentologist, so I wouldn't say for sure without seeing the outcrop, but I mean they kind of have that sense to me.
• 32:02
  SPEAKER_02: And then let's dip across the road.
• 32:04
  SPEAKER_02: Let's wait for a car coming here.
• 32:07
  SPEAKER_02: Oh, I would have gone.
• 32:09
  SPEAKER_02: You're still quite.
• 32:14
  SPEAKER_02: If we go across the bridge here, we're just going on the other side of the one-way road, and uh where was it?
• 32:20
  SPEAKER_02: It was over here.
• 32:20
  SPEAKER_02: I mean, avoid stepping in the puddle.
• 32:22
  SPEAKER_02: We've got a big puddle, but right here we can again see a beautiful dinosaur footprint that is exposed again in the cuts cut face.
• 32:32
  SPEAKER_02: We've got three kind of prongs to it, another little one over there.
• 32:36
  SPEAKER_02: Um, and you know, the we won't get into the details of the species of dinosaur and things like that, but it's amazing.
• 32:42
  SPEAKER_02: Actually, Chris, I have a neighbor who uh is a geologist, retired geologist who has a dinosaur footprint in the wall of his his chimney.
• 32:51
  SPEAKER_02: They like quarried the stone for his chimney that that he had masons make, and it has one side has the imprint, and one side has the cast in the mold or on either sides.
• 33:00
  SPEAKER_02: It's very, very cool.
• 33:01
  SPEAKER_02: Uh, but this is a spectacular example of this.
• 33:03
  SPEAKER_02: I this is amazing.
• 33:05
  SPEAKER_02: Here we go.
• 33:05
  SPEAKER_02: Sedimentary structures.
• 33:07
  SPEAKER_02: Touch it and think 210 million years ago there was a uh that's right, an now extinct creature walking across that.
• 33:12
  SPEAKER_02: It's just it's amazing.
• 33:14
  SPEAKER_02: It's amazing.
• 33:14
  SPEAKER_00: Sedimentary structures on Plum Run.
• 33:16
  SPEAKER_02: There we go.
• 33:17
  SPEAKER_02: Very cool.
• 33:28
  SPEAKER_00: This is so fissile.
• 33:30
  SPEAKER_00: It just looks like sand grain, it's crumbly, it's soft, it's weak, it's it's not gonna stand up well, and so it weathers out much faster and it doesn't form like steep embankments.
• 33:42
  SPEAKER_00: That's right.
• 33:42
  SPEAKER_00: So just kind of again, there's the details are in the rocks.
• 33:45
  SPEAKER_02: What we're starting to see is just rock that is really weak.
• 33:48
  SPEAKER_02: It's breaking apart in front of us, it's breaking apart into really small pieces.
• 33:52
  SPEAKER_02: You know, it looks like fingernail-sized pieces of rocks that are weathering down much shallower slope.
• 33:57
  SPEAKER_02: If I come over here, just much shallower slope, whereas further down, tougher rock, steeper slope, cliff former and slope former, as we talked about in our like geology of the Grand Canyon audiobook series.
• 34:09
  SPEAKER_00: And then and then we're gonna walk to McPherson's Ridge and talk about why again we get back to a cut and a ridge because that's kind of cool.
• 34:20
  SPEAKER_00: The detail here is subtle.
• 34:25
  SPEAKER_00: Yeah, all right, all right, all right.
• 34:26
  SPEAKER_00: So hey, so anyway, let's do it.
• 34:28
  SPEAKER_00: Now we're walking and we're just we're we're back in a cut, okay?
• 34:31
  SPEAKER_00: And that means that we're starting to have resistance again, standing up to the rigors of erosion.
• 34:36
  SPEAKER_00: It's weathering a little bit differently, but you look around, and these rocks here look very similar to where we said there's no cut because it's weak, it's soft, it's crumbly, and all of that.
• 34:46
  SPEAKER_00: Let's look down here a little bit further.
• 34:49
  SPEAKER_02: And note, Chris, where we are standing, the rocks are dipping in the direction downhill in the direction we're walking.
• 34:56
  SPEAKER_02: So they're 15, 20, 25 degrees.
• 34:58
  SPEAKER_02: Yeah.
• 34:58
  SPEAKER_02: Down, we're kind of walking down slopes.
• 35:01
  SPEAKER_02: Dipping toward the west.
• 35:02
  SPEAKER_02: Dipping towards the west, and we are walking towards the west.
• 35:04
  SPEAKER_02: So as we're walking, we are walking up in the stratigraphic section towards younger rocks that are dipping away from us.
• 35:11
  SPEAKER_00: So why is McPherson's ridge a ridge?
• 35:15
  SPEAKER_00: And here in is the answer.
• 35:17
  SPEAKER_00: If you look at the rocks here, and you look at the rocks over on this side, too, you can see that these are different.
• 35:23
  SPEAKER_00: These weather blocky.
• 35:25
  SPEAKER_00: Um, this is tougher stuff, and this is just a well-liquefied sandstone, a part of the Gettysburg Formation.
• 35:32
  SPEAKER_00: Um, and and it's just tougher.
• 35:35
  SPEAKER_00: And so it's this right here that kind of like added strength to the rocks below them, to the rocks above them, and it weathered slower differently, and it formed a ridge.
• 35:47
  SPEAKER_00: Okay.
• 35:49
  SPEAKER_02: This is very this is very impressive, and I it gives you a great appreciation for how a little bit of a very important amount of topography can be created by very small changes in the rock.
• 36:00
  SPEAKER_02: I mean, we're looking here and we have some sandy layers in the in the shale.
• 36:05
  SPEAKER_02: There's mostly shale, then sandy bits come in, and then it's more blocky sandy bits, but I mean it's only what uh 40 feet thick, maybe at least.
• 36:14
  SPEAKER_00: Yeah, this sandstone, maybe, but this ridge is not it's not huge.
• 36:18
  SPEAKER_02: Yeah.
• 36:18
  SPEAKER_00: And that's that's a point that we need to come back to here.
• 36:22
  SPEAKER_02: An analogy to a nick point in creating waterfalls where you don't need a thick layer, you need a resistant layer, and then it can create topographic change.
• 36:33
  SPEAKER_02: Is that a good analogy here that these rocks are up, are sloping up and they're they're protecting the shale that's underneath of them from weathering a little bit.
• 36:41
  SPEAKER_02: So there's shale directly underneath of the ridge, although it's not the ridge forming unit, really.
• 36:48
  SPEAKER_02: Does that make sense?
• 36:48
  SPEAKER_02: Is that a good analogy or is it kind of weak?
• 36:50
  SPEAKER_00: It it works.
• 36:51
  SPEAKER_00: It works.
• 36:52
  SPEAKER_00: I mean, I don't know.
• 36:53
  SPEAKER_00: When you say nick point, then I want to talk about a river flowing across this.
• 36:57
  SPEAKER_00: And I'm saying, oh, what would happen when it falls over the sandstone and into the shale?
• 37:00
  SPEAKER_00: Okay.
• 37:01
  SPEAKER_00: That's where your waterfalls are.
• 37:02
  SPEAKER_02: Fair enough, fair enough, fair enough.
• 37:04
  SPEAKER_02: But it it these up these gently dipping, and this kind of talks about brings us back to the topology of the battle in a way, is that the the layers are dipping to the west, and so the western slopes are often the shallower slopes.
• 37:17
  SPEAKER_02: And the east-facing slopes are very steeper as well.
• 37:19
  SPEAKER_02: That's a very important thing.
• 37:20
  SPEAKER_02: And so that's just generally the the sills are doing the exact same thing.
• 37:25
  SPEAKER_02: So we're gonna go after this, and we're gonna go look at Devil's Den, and we're gonna look at the union lines, and uh we'll see that dipping because the sill is dipping in the exact same orientation as the sediments that we're standing on.
• 37:36
  SPEAKER_00: But before we do that, can I go up top and just show like what this ridge looks like while you're down here?
• 37:42
  SPEAKER_00: All right, let's go do that.
• 37:47
  unknown: Okay.
• 37:48
  SPEAKER_02: Okay, I'm standing in the lowlands here, and we're looking at McPherson's Ridge.
• 37:54
  SPEAKER_00: This is kind of hard, right?
• 37:55
  SPEAKER_00: And there's a show.
• 37:56
  SPEAKER_00: There's a creek.
• 37:57
  SPEAKER_00: Can you turn around a second?
• 37:58
  SPEAKER_00: Just pan behind you there.
• 37:59
  SPEAKER_02: So McPherson's Ridge here, the lowlands, seminary ridge, that steep was kind of sitting on seminary ridge, and these trees are up on seminary ridge.
• 38:09
  SPEAKER_02: So if I pan all the way around and look at a lonesome Mr.
• 38:13
  SPEAKER_02: Bullheist standing there.
• 38:15
  SPEAKER_00: So I'm on the ridge.
• 38:17
  SPEAKER_02: You're on the ridge.
• 38:18
  SPEAKER_02: Go stand up, climb up just a little bit more at the road level.
• 38:21
  SPEAKER_02: Because and keep talking, because this is not a lot of really good.
• 38:24
  SPEAKER_00: I mean, it's not much though, right?
• 38:26
  SPEAKER_00: And that's like that's kind of it's enough.
• 38:30
  SPEAKER_00: We're on the ridge, you got the valley behind you.
• 38:33
  SPEAKER_00: It doesn't look like much.
• 38:35
  SPEAKER_00: And and like, you know, when you think about like modern warfare, we have helicopters and drones and and things like this.
• 38:42
  SPEAKER_00: This kind of topography that we've been talking about all day, um, it wouldn't even matter.
• 38:48
  SPEAKER_00: I don't think it would be even noticed.
• 38:49
  SPEAKER_00: The high ground, like what high ground?
• 38:52
  SPEAKER_00: Yeah, but back during this time it was 100 degrees out, we had wool uniforms, and they had heavy equipment, and it made all the difference.
• 39:03
  SPEAKER_00: These little subtleties.
• 39:05
  SPEAKER_00: And so that road that railroad cut is off over there, Jesse.
• 39:09
  SPEAKER_00: Yeah, that's where we just came from, and we just climbed out of that and came up on top of it for your own.
• 39:14
  SPEAKER_02: One of the early battlefields, and Chris, just to talk about that again.
• 39:18
  SPEAKER_00: Like, and so kind of like from where you are, Jesse, this this kind of this place went back and forth on day one for a while.
• 39:25
  SPEAKER_00: You know, the North had it, the the Union had it, the Confederates had it.
• 39:29
  SPEAKER_00: Um and you know, at one point there there was a whole bunch of Confederates down in the railroad cut, and Union soldiers stormed the railroad cut and ordered them to drop their muskets, and 230 Confederates surrendered.
• 39:42
  SPEAKER_00: Um, in the end, the Union got pushed back through the town of Gettysburg and took the high ground, and that's where the where most of the battle took place then, with Colps Hill snaking all the way around the fish hook down to the roundtops.
• 39:57
  SPEAKER_02: Yeah, to the south.
• 39:58
  SPEAKER_02: It's just very impressive how little topography mattered a lot.
• 40:02
  SPEAKER_02: And the small variations, I think we've seen the small variations in the rocks, small transition from shale to a thin couple tens of feet of sandstone, and then eventually back to shale.
• 40:12
  SPEAKER_02: And that made McPherson Ridge, which was a uh one of the early phases of the battle, and at least at some point a standing point for the Union forces.
• 40:21
  SPEAKER_00: Isn't that crazy?
• 40:22
  SPEAKER_00: Like that 30, 40 foot thick piece of sandstone, that's that that laterally continuous sandstone dip into the west formed a ridge that became important in a battle.
• 40:32
  SPEAKER_02: Amazing.
• 40:32
  SPEAKER_02: You know, it's just amazing.
• 40:34
  SPEAKER_02: Small changes, yeah.
• 40:35
  SPEAKER_02: Small things in geology matter a lot.
• 40:37
  SPEAKER_02: They do.
• 40:38
  SPEAKER_02: Amazing.
• 40:38
  SPEAKER_02: Yeah, they do.
• 40:39
  SPEAKER_02: All right, cool.
• 40:47
  SPEAKER_02: Oh yeah, little round top.
• 40:50
  SPEAKER_00: Little round top.
• 40:51
  SPEAKER_00: Yeah, it's really amazing.
• 40:52
  SPEAKER_02: This is a pretty amazing view from up here.
• 40:55
  SPEAKER_00: I think it gives really good perspective on at least this part of uh the you know the the dynamic that the sill played because we're standing on the dye-based sill.
• 41:05
  SPEAKER_02: We're standing on it, that's right.
• 41:06
  SPEAKER_02: And I'm gonna turn and just to tune our eyes to this this great massive weathering, huge blocks.
• 41:12
  SPEAKER_02: And Chris, set the stage for us.
• 41:13
  SPEAKER_02: You've got a drawing here that I had it in my shirt.
• 41:17
  SPEAKER_00: I'm trying to protect it from the rain, so it's not perfect.
• 41:19
  SPEAKER_00: I'm sorry, but it's gonna have to do so.
• 41:22
  SPEAKER_00: Here's the fish hook right here, here's the union line, um, and here's the little round top where we are.
• 41:27
  SPEAKER_00: Big roundtop is is uh further south of us.
• 41:31
  SPEAKER_00: Um, and so this is you know about 600 feet um elevation-wise, and there's there's something that's important that happened right near here um on day two, which is General Sickles had his line that connected, you know, just ran down the cemetery ridge to the roundtops, but he didn't like the vantage point because down in here uh it's lower elevation, it's about 70 feet lower, it's about 530 feet elevation.
• 42:00
  SPEAKER_02: Yeah, and just to kind of look at this, Chris, I think we're seeing kind of part of that.
• 42:04
  SPEAKER_02: We're standing on little roundtop, which is really high.
• 42:06
  SPEAKER_02: We're we're looking into these these low ones, and and just to the right over here is that low area that you're referring to.
• 42:12
  SPEAKER_02: And he kind of said, Oh, that's high, higher over there.
• 42:15
  SPEAKER_02: Let's go out to the west there and extend out, right?
• 42:18
  SPEAKER_00: Not very much higher, not higher, yeah.
• 42:20
  SPEAKER_02: Like to gain 30 feet or something like that.
• 42:23
  SPEAKER_02: Yep.
• 42:23
  SPEAKER_00: Um, and and so what that did then, if we can just talk about this, it's kind of important, is moving out into this what this somewhat higher ground out here.
• 42:33
  SPEAKER_00: We he so you had the peach orchard and and the wheat field behind him, um, is it created this salient point right here.
• 42:41
  SPEAKER_00: And it's it's so what that does is you can see it lengthened his line, and he went from a straight line here to this kind of triangular line, so much weaker because it was lengthened.
• 42:52
  SPEAKER_00: And the other thing that's important here is that the Confederates then look at the the angle of fire, they had an enphalading fire, which means they that you have a flank here and a flank here, and so they're they're firing down the line, yeah.
• 43:09
  SPEAKER_00: Uh, and and it's just untenable.
• 43:11
  SPEAKER_02: Um, so this is a good example, Chris.
• 43:13
  SPEAKER_02: And I think I like this drawing.
• 43:14
  SPEAKER_02: Let's just go back to the to zooming out here a minute to show this because we're looking out over um this this low area with a little bit of topographic leaf that we're seeing.
• 43:23
  SPEAKER_02: Standing on Little Roundtop, which is the high one of the high points, and Cemetery Hill is off to the north.
• 43:30
  SPEAKER_02: Can we show your map again?
• 43:30
  SPEAKER_02: Yeah.
• 43:31
  SPEAKER_02: Cemetery Hill is where on here for us?
• 43:33
  SPEAKER_00: Where are we cemetery hills up near the top of the of the fish hook up here?
• 43:37
  SPEAKER_02: Yeah, yeah.
• 43:38
  SPEAKER_02: And so high elevation, high elevation, and then we said sort of low elevation in here, which this is um this is the Warfield Dyke or the Warfield Ridge, which is uh there's a dye base dike, a younger Rossville dye base dike that runs this way, cemetery ridge, excuse me, seminary ridge, has an underlying Rossville dike, a cross cutting one that runs this way as well.
• 44:00
  SPEAKER_02: Youthful age.
• 44:01
  SPEAKER_02: This is the sill.
• 44:03
  SPEAKER_02: And the sill here, it's really we have to consider why is it high relief here, low relief there?
• 44:08
  SPEAKER_02: No, low relief in here, high relief up here.
• 44:11
  SPEAKER_02: Why is that relief change going on in the middle when it's all sill underneath of it?
• 44:14
  SPEAKER_02: And the sill is exposed.
• 44:16
  SPEAKER_02: If we looked at the geological map, the sill is exposed kind of like this.
• 44:20
  SPEAKER_02: So this ridge of hills is running from yeah, it it because the sill is dipping away from us, we're kind of looking at this fish hook, and the the sill is is running.
• 44:30
  SPEAKER_02: We're at the top of the sill, at the top of the fish hook, and the bottom of the sill at the bottom of the fish hook.
• 44:36
  SPEAKER_00: So what that means is down here in the southern end of the union line, the sill is right near the contact.
• 44:41
  SPEAKER_02: The lowermost part of the sill, the the the bottom of the sill intrusion, then, right?
• 44:46
  SPEAKER_00: And and what happens then is that is that forms a chilled margin on that on that igneous intrusion, which means it cools faster, it's finer grained, as opposed to the central part of the union line of cemetery ridge, it's in the middle of the sill.
• 45:00
  SPEAKER_01: Yeah.
• 45:00
  SPEAKER_00: And that's a massive sill.
• 45:02
  SPEAKER_00: So it would have cooled significantly slower and then significantly then coarser grained.
• 45:07
  SPEAKER_01: Yeah.
• 45:07
  SPEAKER_00: And then the northern end again, as it went through here, was on the top end of the sill.
• 45:12
  SPEAKER_00: That's right.
• 45:13
  SPEAKER_00: And and so you had then again finer grained igneous dye-base rock.
• 45:18
  SPEAKER_00: And what and and so what's the difference then between all right, how did the the high ground is fine grained, the high ground is fine grained, the lower ground is coarser grained.
• 45:28
  SPEAKER_00: Why is that?
• 45:30
  SPEAKER_02: Yeah, it's a very interesting phenomenon.
• 45:32
  SPEAKER_02: And just to kind of paint another paint this visual again for for audio listeners, you have to imagine this ridge is cutting the the dike, the sill is dipping down away from us about 25 degrees or so, dipping away from us, and we're kind of this the ridge is running at an angle to that across that.
• 45:50
  SPEAKER_02: So the ridge is running across the entire contact.
• 45:53
  SPEAKER_02: So you kind of imagine, so it's hard to describe this angular relationship.
• 45:57
  SPEAKER_02: Um, but at the bottom we have chilled, the center we have coarse grained, at the top we have chilled again and more fine-grained.
• 46:04
  SPEAKER_02: And this type of rock, Chris, is is mafic.
• 46:07
  SPEAKER_02: This is a mafic rock.
• 46:08
  SPEAKER_02: And so the coarse-grained minerals, most of the minerals are kind of easy to chemically weather.
• 46:14
  SPEAKER_02: Plagioclase is not like a super resistant mineral.
• 46:17
  SPEAKER_02: Clinopyroxene is not a super resistant mineral at the surface.
• 46:20
  SPEAKER_02: So, especially when it gets coarser grained, those minerals will weather out and those will leave bigger chunks in the rock, uh, bigger chunks out of the rock, which makes it a little bit more weatherable.
• 46:30
  SPEAKER_02: That's right.
• 46:30
  SPEAKER_02: Coarse-grained mafic rocks, gabbros, uh, especially ultramafic rocks, are really easy to weather.
• 46:37
  SPEAKER_02: And the salt is a little bit more resistant to weathering.
• 46:39
  SPEAKER_00: Yes.
• 46:39
  SPEAKER_00: You know, so like those minerals that you listed, those are higher temperature minerals.
• 46:44
  SPEAKER_00: Those were the first to crystallize.
• 46:46
  SPEAKER_00: And and so they're gonna be less stable because they're farther away from the environment in which they formed when they're exposed at the surface.
• 46:53
  SPEAKER_01: Exactly.
• 46:53
  SPEAKER_00: And minerals are happiest when they exist in the conditions in which they form.
• 46:58
  SPEAKER_00: And so those higher temperature, bigger minerals are gonna weather then fast.
• 47:02
  SPEAKER_02: Yeah, and so and so that this leads us into a nice couple, another few weathering features that create.
• 47:08
  SPEAKER_02: I mean, the devil's den, which we're gonna go down to next and see, but we can really see this nicely here.
• 47:13
  SPEAKER_02: This is a big blocky weathering outcrop here at the top of Little Round Top.
• 47:18
  SPEAKER_02: Like, can you point out some weather?
• 47:19
  SPEAKER_02: I'm gonna jump across here a second.
• 47:21
  SPEAKER_02: Okay, be careful.
• 47:22
  SPEAKER_02: I will.
• 47:23
  SPEAKER_02: Um, it is a little slip.
• 47:24
  SPEAKER_02: Big blocks are weathering.
• 47:25
  SPEAKER_02: We can see this kind of alligator skin texture to it, this really interesting weathering pattern on the edges of it.
• 47:31
  SPEAKER_02: That you know, crocodile skin, alligator skin kind of weathering pattern, which is one interesting phenomenon.
• 47:36
  SPEAKER_02: And then, Chris, another one.
• 47:38
  SPEAKER_02: Let's go.
• 47:39
  SPEAKER_02: I'll I'll show alligator skin.
• 47:40
  SPEAKER_02: You want me to show this right here?
• 47:41
  SPEAKER_02: Yeah, look what's going on here.
• 47:42
  SPEAKER_00: So you look at all right, so this was just a a singular block, really, that was jointed, and you can see how this jointing then, this part of the block after it had jointed, something happened, and this part dropped because it used to match up with this.
• 48:00
  SPEAKER_00: This was here, and you know, that's just like if you pan down to the soil down here below, right?
• 48:06
  SPEAKER_00: Um, hill slope weathering as as weathering takes place, it forms soil, but we're on a slope, and so that soil is getting washed away.
• 48:15
  SPEAKER_00: And so this is getting let down these blocks as the soil below them, it's letting them down gently, yeah.
• 48:24
  SPEAKER_00: Um, and and they don't do it evenly, and so now we got offset here.
• 48:29
  SPEAKER_02: Yeah, okay.
• 48:29
  SPEAKER_02: This is and I think that that ties in nicely to this alligator skin weathering pattern, which Chris, I'm gonna just jump over here a minute and look at a couple of raples right here that we have really nicely on these boulders.
• 48:42
  SPEAKER_02: We can see this alligator skin weathering pattern, interesting fractures going on, this really regular fracturing pattern, Chris.
• 48:50
  SPEAKER_00: And so what I'm coming around because because I want to like I mean, if you look at this one, actually come around here, yeah.
• 49:00
  SPEAKER_00: Okay, you look at look at that block there, look at that block there.
• 49:05
  SPEAKER_00: These were let down, these were fractured like crazy, and then they were just kind of gently let down on the rocks below them as the soil washed away.
• 49:15
  SPEAKER_02: Chris, this this boulder here shows a really nice example of this this weathering pattern.
• 49:19
  SPEAKER_02: So, this is a weathering pattern that we're talking about, this crocodile weathering on a micro scale, kind of a skin, yeah, micro weathering going on.
• 49:24
  SPEAKER_02: But this is a process by which these are expansion cracks because as this rock gets hydrated, um the minerals expand and contract and expand and contract with wet dry cycles, and that's happening all around.
• 49:37
  SPEAKER_02: You can kind of see this rock has been as a rind kind of chipping off of it.
• 49:42
  SPEAKER_02: This is being broken off in pieces, and as that that's happening on all sides, all around all of these boulders all the time, kind of a small scale like exfoliation, really, Chris.
• 49:51
  SPEAKER_02: Yeah, taking off the skin one layer at a time because the skin gets hydrated, altered, deformed, expanded, freezed off cycles, and then that's happening on the top, the bottom, the sides, all of these boulders, and gradually they're kind of shrinking and getting let down as you described before.
• 50:06
  SPEAKER_02: So it's really all about weathering, and you can imagine this is a has described in many of the accounts, a terrible place to have a battle.
• 50:16
  SPEAKER_02: Oh, yeah.
• 50:16
  SPEAKER_02: I mean, that this kind of topography here.
• 50:18
  SPEAKER_00: Let's let's talk about why, right?
• 50:20
  SPEAKER_00: You look at this rock, um, the way it's weathering, you can't just pick up rocks and make a wall.
• 50:29
  SPEAKER_00: Yeah, and and that's pretty much the case along this entire dive sill.
• 50:34
  SPEAKER_00: You didn't have the ability to make entrenchments or or to make walls for your defense, which is why, even though the union was in the defensive situ uh position, they took a lot of casualties, way more than they normally would have from a defensive position because they didn't have the ability to dig in.
• 50:52
  SPEAKER_00: They didn't have the ability to build walls because look at this stuff, it's very difficult to move.
• 50:57
  SPEAKER_00: So the only cover they got was behind boulders like this.
• 51:01
  SPEAKER_00: Yeah, yeah, and it's it's not continuous, it's not consistent.
• 51:04
  SPEAKER_02: Yeah, it's a sort of a uh hide-and-seek warfare, a guerrilla warfare sort of thing on the in these buildings, and uh it's it's all because of the rocks, it's all because of the geology, and the the change I think we can see pretty well from over here, and then we'll wrap it up at this location.
• 51:19
  SPEAKER_02: But get a view here, Chris, um, from Little Round Top as we look north, we can see we go high ground, low ground, high ground again, and that's the progression as we're working from the one edge of the sill to the inner part of the sill to the other edge of the sill.
• 51:34
  SPEAKER_02: And the dynamic.
• 51:35
  SPEAKER_00: So that inner part of the sill is gonna be coarser grained, it's insulated from all the chilled margins, and that's why it's low, it's weathered faster.
• 51:43
  SPEAKER_00: Yeah, exactly.
• 51:44
  SPEAKER_02: Yep, yeah, yep.
• 51:45
  SPEAKER_02: Very cool.
• 51:46
  SPEAKER_02: All right, shall we continue?
• 51:47
  SPEAKER_02: Let's go.
• 51:55
  SPEAKER_00: Okay.
• 51:56
  SPEAKER_02: Chris, we're in the the devil's den now.
• 51:59
  SPEAKER_00: Yeah, um, pan over to where we just came from.
• 52:04
  SPEAKER_02: Up there.
• 52:05
  SPEAKER_00: Little round top.
• 52:06
  SPEAKER_02: Little round top.
• 52:09
  SPEAKER_02: 150 foot above us.
• 52:12
  SPEAKER_02: Now we're down here in the lowlands.
• 52:15
  SPEAKER_00: So again, we've already like talked about this, but you know, there's no there's nothing you can move and create shelter for yourself.
• 52:23
  SPEAKER_00: You can't make breastworks here.
• 52:25
  SPEAKER_00: The only rocks, the only protection you're gonna get are rocks you can find to hide behind because everything's coming from that direction towards you down to Devil's Den.
• 52:33
  SPEAKER_00: Um, so I want to I have a diagram, Jesse.
• 52:37
  SPEAKER_00: Can you bear with me?
• 52:39
  SPEAKER_00: I want to pan up here a second.
• 52:40
  SPEAKER_00: Let's look at these rocks first, okay?
• 52:42
  SPEAKER_00: Um, but you can see what we talked about yesterday, right?
• 52:45
  SPEAKER_02: Well, first of all, the you start off with a jointing that's relatively Because I just I have to say, this is a really high quality diagram drawn for us here.
• 52:56
  SPEAKER_02: You didn't spend more than an hour on it.
• 52:58
  SPEAKER_00: This is the rock, okay?
• 53:00
  SPEAKER_00: But it's jointed at relatively, let's call them 90 degree angles.
• 53:04
  SPEAKER_00: Okay.
• 53:04
  SPEAKER_00: So rectangular to square kind of jointing, right?
• 53:07
  SPEAKER_00: And what happens then is this is just basic weathering.
• 53:12
  SPEAKER_00: The corners, imagine a wooden cube.
• 53:16
  SPEAKER_00: The corner of the cube has a higher surface area ratio to um to rock exposed.
• 53:26
  SPEAKER_00: Okay, and so it's gonna weather the fastest, the corners will, then the edges will.
• 53:33
  SPEAKER_00: And then lastly, what are you left with?
• 53:35
  SPEAKER_00: You're left with the surfaces that the that the jointing runs down.
• 53:39
  SPEAKER_00: So when you take that, uh if you take a square block and you weather the corners first fast, you rather weather the edges next fastest, and then the surfaces last, you end up with rounding over time.
• 53:53
  SPEAKER_02: Yeah.
• 53:53
  SPEAKER_02: And we that's what we have.
• 53:55
  SPEAKER_02: So, because I think this example right in front of us here that has a big crack down the middle is an excellent example of you know where a crack starts from and then where it ends up.
• 54:05
  SPEAKER_02: So we have this big crack going down the middle.
• 54:08
  SPEAKER_02: Um, this is just a huge bullet, this is a uh you know, an apartment-sized boulder that's broken in half.
• 54:14
  SPEAKER_02: That's one of these.
• 54:15
  SPEAKER_02: That's one crack.
• 54:16
  SPEAKER_02: And over time, you've drawn that's going to widen.
• 54:19
  SPEAKER_02: That's right.
• 54:19
  SPEAKER_02: So you take a square, you knock the edge, the corners off first, then you start to knock the edges off, and eventually you've got this spheroidal rounding thing.
• 54:27
  SPEAKER_02: And these boulders are all vaguely rounded.
• 54:29
  SPEAKER_02: We can see some of that beautiful, really intense alligator skin fabric up here as well.
• 54:33
  SPEAKER_00: Microweathering.
• 54:34
  SPEAKER_00: I'm not done.
• 54:35
  SPEAKER_00: I have another slide.
• 54:37
  SPEAKER_00: Okay.
• 54:38
  SPEAKER_00: So let's look at this.
• 54:39
  SPEAKER_00: Just an exception.
• 54:40
  SPEAKER_00: Exceptional artistic value here.
• 54:42
  SPEAKER_02: I mean, so you take the jointing from this side.
• 54:45
  SPEAKER_02: Chris's first drawing was some squares.
• 54:48
  SPEAKER_02: Chris's second drawing is some blobs with some specks around them.
• 54:50
  SPEAKER_02: Yes.
• 54:51
  SPEAKER_00: I mean, this is high quality.
• 54:52
  SPEAKER_02: I know.
• 54:53
  SPEAKER_00: But I think it does the thing.
• 54:55
  SPEAKER_00: So the weathering is gonna cause rounding of the blocks.
• 54:59
  SPEAKER_00: I had five blocks on the other side, I've got five now relatively spheroidal because soil.
• 55:06
  SPEAKER_00: And then what'll happen is the soil gets washed away.
• 55:10
  SPEAKER_00: And so this these rocks right here are gonna be gently lifted down on top of this.
• 55:14
  SPEAKER_00: Yeah, and then these two with them on that is gonna be gently lifted down on top of that.
• 55:20
  SPEAKER_00: And that's what we see here, right?
• 55:23
  SPEAKER_00: We see these rocks that have been just gently lifted down onto the rocks that had soil in between them.
• 55:32
  SPEAKER_00: That's the soil's now gone.
• 55:34
  SPEAKER_00: It's it's just an amazing feature.
• 55:36
  SPEAKER_02: We're seeing some beautiful weathering patterns here.
• 55:38
  SPEAKER_02: I mean, just exceptional.
• 55:40
  SPEAKER_02: Again, we're we're in the dives.
• 55:41
  SPEAKER_02: This is diabetes, this is the weathering pattern.
• 55:43
  SPEAKER_02: I am I'm struck, Chris, by the differences in weathering patterns that we've seen just today, the differences of rocks, it's very diagnostic of what will eventually become topography, how these rocks weather, eventually give us the topography that controls the topology of the entire battle.
• 56:00
  SPEAKER_02: That's right.
• 56:00
  SPEAKER_02: Um, and this is a great location.
• 56:03
  SPEAKER_02: This is this is just a humbling experience being in this location.
• 56:07
  SPEAKER_00: That's right.
• 56:08
  SPEAKER_00: It absolutely is.
• 56:09
  SPEAKER_00: I I don't want that to get downplayed.
• 56:11
  SPEAKER_00: This is a very somber place that's very humbling.
• 56:13
  SPEAKER_02: We're definitely not as I don't feel as enthusiastic as I do in a normal Planet Geo interview just because of where we are.
• 56:19
  SPEAKER_02: Nothing to do with the geology.
• 56:20
  SPEAKER_02: The geology is cool, uh, but this is you know, this is more serious tone to it for sure.
• 56:25
  SPEAKER_00: I do want to say, I apologize right now for my drawings.
• 56:28
  SPEAKER_00: I have almost zero artistic ability, so I do my best.
• 56:32
  SPEAKER_00: You have zero artistic ability that you refined over 25 years of teaching, and it just hasn't gotten that much better.
• 56:37
  SPEAKER_00: More than 25 now.
• 56:38
  SPEAKER_00: We're beyond that, my friend.
• 56:39
  SPEAKER_02: So yeah.
• 56:40
  SPEAKER_02: We'll just pan back to a little round top here to end us out of some of our diabetes.
• 56:54
  SPEAKER_02: Chris.
• 56:55
  SPEAKER_02: Jesse, doctor.
• 56:56
  SPEAKER_02: Hey, uh, this might be familiar to people.
• 56:59
  SPEAKER_02: Uh-huh.
• 56:59
  SPEAKER_02: Is it familiar to you?
• 57:00
  SPEAKER_00: Oh, yeah.
• 57:01
  SPEAKER_00: Well, you know it is.
• 57:03
  SPEAKER_00: We were driving down the road, we didn't know exactly where this was, and I'm like, I think that's it.
• 57:06
  SPEAKER_02: It's a pudding stone, man.
• 57:08
  SPEAKER_02: Yeah, let's get in on this.
• 57:09
  SPEAKER_02: This is the Massachusetts Infantry Memorial.
• 57:12
  SPEAKER_02: Um, it's Roxbury conglomerate.
• 57:14
  SPEAKER_02: We talked about this in our Geology of Pudding Stone episode.
• 57:19
  SPEAKER_02: It's a beautiful, it is it's really beautiful.
• 57:22
  SPEAKER_02: This is a this is a gorgeous rock.
• 57:23
  SPEAKER_02: There's the rhyolite class in it, really big Rhyolite class.
• 57:28
  SPEAKER_02: It's kind of altered.
• 57:29
  SPEAKER_02: You can see some fluid flowing through it, making it green.
• 57:32
  SPEAKER_02: This is a really pretty conglomerate.
• 57:34
  SPEAKER_02: It sure is.
• 57:35
  SPEAKER_02: And this commemorates um the the uh Massachusetts Infantry and the 20th Massachusetts Infantry, and it I apparently this boulder came from a sort of a local landmark in Roxbury.
• 57:47
  SPEAKER_02: Uh that was a playground, it was on a playground.
• 57:49
  SPEAKER_02: Um, so they they moved it up here.
• 57:51
  SPEAKER_02: And another interesting thing, Chris, this granite, the base is a granite.
• 57:55
  SPEAKER_02: Um, it's geochemically very famous, let's put it that way, even though maybe a lot of people don't know it.
• 58:01
  SPEAKER_02: Um, this is called the westerly granite, and it's famous because the United States Geological Survey makes uh takes rocks that are really homogeneous, makes powders out of them, and then sends them to labs all over the world and treats them as standards.
• 58:14
  SPEAKER_02: So it's a rock that we really know the composition of really well.
• 58:17
  SPEAKER_02: The United States Geological Survey makes a lot of these.
• 58:19
  SPEAKER_02: This is famous because the the G1 and G2, granite one and granite two geochemical sample standards from the United States Geological Survey, are this granite, the Western granite.
• 58:30
  SPEAKER_02: That's so I have this.
• 58:31
  SPEAKER_02: I have powders of this in my lab at Carnegie.
• 58:34
  SPEAKER_02: We had powders of this.
• 58:35
  SPEAKER_02: We used it to analyze uh when we're checking our measurements to make sure our neodymium column chemistry is working properly.
• 58:42
  SPEAKER_02: We used powder from the G2 granite, which is the westerly granite.
• 58:46
  SPEAKER_02: So this is a really famous outcrop from uh uh you know quarry quarry stone up there, the westerly granite with Roxbury um conglomerate sitting on top of it.
• 58:54
  SPEAKER_02: This is a gorgeous conglomerate.
• 58:56
  SPEAKER_00: I um it's not as good as the ones in Michigan, the Battle of Michigan, but hey, we're giving a shout out to Massachusetts right now.
• 59:03
  SPEAKER_02: Yeah, I'm sure it's a great, it's a great use for uh for a landmark.
• 59:07
  SPEAKER_02: I mean, beautiful rock for a spectacular landmark.
• 59:10
  SPEAKER_00: Doesn't have anything to do with the geology of the Battle of Gettysburg except that it's on the battlefield of Gettysburg.
• 59:16
  SPEAKER_02: Amazing monument, yeah, exceptional, very cool.
• 59:19
  SPEAKER_02: Yeah, we are good.
• 59:26
  SPEAKER_02: This might be our last stop at the Battle of Gettysburg.
• 59:30
  SPEAKER_02: At the battle.
• 59:31
  SPEAKER_02: At the battleground.
• 59:33
  SPEAKER_02: Yes.
• 59:33
  SPEAKER_02: So we're standing, Chris, now on Cemetery Ridge, along Cemetery Ridge.
• 59:37
  SPEAKER_02: We have the round tops down here to our right.
• 59:40
  SPEAKER_02: We're looking down the fish hook shank to the south.
• 59:43
  SPEAKER_02: The eye is down there, down to the south.
• 59:46
  SPEAKER_02: And as we look to the north, as we turn to our left, look to the north, we see the fish hook go up and kind of bend to the right around us here.
• 59:53
  SPEAKER_02: That's okay.
• 59:54
  SPEAKER_02: So we're kind of near the high stand.
• 59:55
  SPEAKER_02: We're back up on the probably the chilled margin of the the sill.
• 1:00:00
  SPEAKER_02: So higher relief on our topography.
• 1:00:02
  SPEAKER_02: We're above that low zone that was lower down in.
• 1:00:05
  SPEAKER_02: Um, what's really striking here?
• 1:00:06
  SPEAKER_00: Can I hold on, can I interrupt a second?
• 1:00:08
  SPEAKER_00: Sorry.
• 1:00:09
  SPEAKER_02: Um, I've never turned you down.
• 1:00:10
  SPEAKER_00: Yeah, you do.
• 1:00:11
  SPEAKER_00: Um, right there, though, this is the right very near the high water mark of the Confederacy during Pickett's charge on July 3.
• 1:00:19
  SPEAKER_00: Um, so they came across this low-lying field, uh, which Jesse's gonna talk about the difference in elevation here in a second, but that's the cops of trees right there.
• 1:00:27
  SPEAKER_00: Very important because that was like that was the goal.
• 1:00:30
  SPEAKER_00: Aim for that, that's where we're going.
• 1:00:32
  SPEAKER_02: We're maybe five feet downhill.
• 1:00:34
  SPEAKER_02: I mean, you know, a hundred feet downhill, but five feet in elevation off that hill.
• 1:00:37
  SPEAKER_02: Yeah.
• 1:00:38
  SPEAKER_02: That cops of trees.
• 1:00:39
  SPEAKER_02: And so what's really striking about standing here, I think, is the we've talked about this before, but really how little relief, how a small amount of relief had a massive impact.
• 1:00:49
  SPEAKER_02: There we're looking across.
• 1:00:50
  SPEAKER_02: If I turn and look west over my shoulder, we have seminary ridge.
• 1:00:54
  SPEAKER_02: We can kind of see it defined by this this stand of trees over there.
• 1:00:57
  SPEAKER_02: Seminary ridge, and there's not a lot of relief change across this.
• 1:01:03
  SPEAKER_02: It does not feel like there's a big slope, a big downhill valley between us.
• 1:01:08
  SPEAKER_02: From our standpoint here up on Cemetery Ridge, it's about 60 feet below us.
• 1:01:11
  SPEAKER_02: There, from Seminary Ridge to the bottom is about 30 feet broadly.
• 1:01:16
  SPEAKER_02: So, but it's a mile, it's a long distance with a gently sloping uphill, tough still.
• 1:01:23
  SPEAKER_02: Yeah, um, well, it it was brutally hot, brutally hot, exactly.
• 1:01:27
  SPEAKER_00: And you almost as hot and humid as you could get.
• 1:01:30
  SPEAKER_00: Yeah, like it was really not good.
• 1:01:32
  SPEAKER_02: Some buggy weather happens around these parts.
• 1:01:34
  SPEAKER_00: We have a and you you know what, Jesse?
• 1:01:36
  SPEAKER_00: Hold on.
• 1:01:36
  SPEAKER_00: You read something too that because of the complexity of movements, yeah, that nothing really got kick started in the morning until noon.
• 1:01:45
  SPEAKER_00: Nothing happened till noon at the earliest.
• 1:01:46
  SPEAKER_00: Let's just wait until the hottest part of the day.
• 1:01:48
  SPEAKER_00: Logistics, exactly.
• 1:01:49
  SPEAKER_02: So just to kind of frame this, we have a cross-section drawing.
• 1:01:52
  SPEAKER_02: So we're gonna look at this, Chris, and and this is a nice drawing you've made.
• 1:01:55
  SPEAKER_02: Thank you.
• 1:01:55
  SPEAKER_02: I appreciate that.
• 1:01:56
  SPEAKER_02: Um, but this kind of shows generally what we're looking at.
• 1:01:59
  SPEAKER_02: It's definitely it's not to scale the thicknesses, but we're standing on again your cave and sill.
• 1:02:04
  SPEAKER_00: Here, I'll switch to this one a second.
• 1:02:06
  SPEAKER_00: We're looking kind of this way to the fish hook again.
• 1:02:10
  SPEAKER_02: It's a candy cane drawing here.
• 1:02:11
  SPEAKER_00: We're looking, this is the orientation though.
• 1:02:14
  SPEAKER_00: So the fish hook where it curves around a calp's hill up on the top.
• 1:02:17
  SPEAKER_00: Can you point that at the second job?
• 1:02:21
  SPEAKER_00: Right.
• 1:02:22
  SPEAKER_00: So that's the orientation that we are facing right now.
• 1:02:25
  SPEAKER_02: We're looking up the fish hook to the hook end and it's hooking around to the right, to the tip of the fish hook.
• 1:02:31
  SPEAKER_02: Okay.
• 1:02:31
  SPEAKER_02: We are standing.
• 1:02:32
  SPEAKER_02: This is a cross-section, we're standing on the the York Haven sill still.
• 1:02:36
  SPEAKER_02: Um, and here, I don't like this angle, Jesse.
• 1:02:39
  SPEAKER_00: Why don't you turn that around and look at it upside down for me, please, so I can get you in the frame there.
• 1:02:43
  SPEAKER_00: There we go.
• 1:02:44
  SPEAKER_00: Just turn it upside down.
• 1:02:46
  SPEAKER_00: Like there.
• 1:02:46
  SPEAKER_00: There you go.
• 1:02:48
  SPEAKER_02: Upside down.
• 1:02:49
  SPEAKER_02: Okay, so we're standing on the York Haven Sill.
• 1:02:51
  SPEAKER_02: It's sloping to the west, it's dipping to the west.
• 1:02:53
  SPEAKER_02: It's a sill intruding with the sediments along the same plane as the sediments, so it's dipping in the same way the sediments to the west.
• 1:02:59
  SPEAKER_02: As we go across this field here, where Pickett's Charge came up to the west, we have the Gettysburg Formation, kind of undifferentiated Gettysburg Formation.
• 1:03:09
  SPEAKER_02: The Rossville Dyke, this dike that cross-cuts over there that defines Seminary Ridge, a thin dike that's coming up younger.
• 1:03:16
  SPEAKER_00: It's a seminary ridge on the on the dike.
• 1:03:18
  SPEAKER_02: Seminary ridge over here, the Rossville Dyke is coming up.
• 1:03:21
  SPEAKER_02: That's 201.25 million years old.
• 1:03:25
  SPEAKER_02: The York Haven still is 201.5 million years old.
• 1:03:29
  SPEAKER_02: Slightly older.
• 1:03:29
  SPEAKER_02: Slightly older, cross-cutting here.
• 1:03:31
  SPEAKER_02: And then you can see, Chris, you've nicely drawn a difference in the slight change in the sedimentation within the Gettysburg Formation that gives us McPherson's Ridge off to the northwest on the other side of the town of Gettysburg, which we're going to do.
• 1:03:43
  SPEAKER_02: So this is a really beautiful cross-section that kind of shows, and it also shows as we go across the York Haven Sill, as we go across Cemetery Ridge, we're going from the bottom of the sill to the top of the sill from the north to the south, and we go from these chilled margins to the interior zone to the chilled margins again.
• 1:03:59
  SPEAKER_02: And shows that nice sort of progression across this.
• 1:04:02
  SPEAKER_02: But this change in slope here where Pickett's charge was is pretty slight.
• 1:04:07
  SPEAKER_02: The change in slope elsewhere where we've looked is more dramatic.
• 1:04:10
  SPEAKER_02: Here it's pretty slight, but the weathering differences are still there, are still present.
• 1:04:14
  SPEAKER_02: And the reason that we're on a high stand is because we're standing yet again on the York Haven sill.
• 1:04:18
  SPEAKER_00: Yes.
• 1:04:19
  SPEAKER_00: Good.
• 1:04:20
  SPEAKER_02: So can I I want to with the spectacular drawing?
• 1:04:29
  SPEAKER_00: To wrap us out.
• 1:04:30
  SPEAKER_00: I just want to wrap this up here.
• 1:04:31
  SPEAKER_00: Um, so we have the fish hook of the Confederate uh position on the high ground, which is very subtle.
• 1:04:39
  SPEAKER_01: Yep.
• 1:04:39
  SPEAKER_00: But a couple things that I want to say about this is that the the geology of the hook, though, this right here, yeah, is super important.
• 1:04:47
  SPEAKER_02: Can we come?
• 1:04:48
  SPEAKER_02: Oh, let me just set the frame frame in here because we got again the fish hook we're talking about near the eye of the hook, down to the south.
• 1:04:53
  SPEAKER_02: We have the round tops.
• 1:04:54
  SPEAKER_02: Yep.
• 1:04:55
  SPEAKER_00: This is big round top, that's little round top, where vicious fighting took place on day two, July 2.
• 1:05:00
  SPEAKER_02: As we work north along the straight shank of the fish hook towards the north, we're heading through where that's where we're standing, about halfway up the fish hook, where the the high point of the Confederacy, the cops of trees, and then as we loop around, where are the the markers that we kind of see as we loop around this hook, Chris?
• 1:05:17
  SPEAKER_02: And where does the fish hook end basically?
• 1:05:19
  SPEAKER_00: So it this ends at Colps Hill, it kind of wrapped around the base of Colps Hill, like that.
• 1:05:24
  SPEAKER_00: So it really was more of an exaggerated hook at the end of the line right here.
• 1:05:28
  SPEAKER_00: Yeah, this is cemetery hill, followed by just some what we would now call cemetery ridge down here to the round tops.
• 1:05:34
  SPEAKER_00: So the the thing that I want to say about this is the hook and the geology of this having the high ground was super important also for the union advantage.
• 1:05:44
  SPEAKER_00: And here's why this hook gave General Meade the advantage of interior lines, and all that means is that he's able because it curves around this way, he's able to move troops with minimal distance and shoot.
• 1:06:00
  SPEAKER_00: Shift positions within his line, like this.
• 1:06:03
  SPEAKER_00: You can move them wherever you want.
• 1:06:05
  SPEAKER_02: Within the fish hook, you're sitting, you're saying within the fish hook, if you're centered in the fit in the fish hook bend, you can kind of get troops quickly to any side of the fish hook.
• 1:06:13
  SPEAKER_00: That's right, and I can move troops around this way too.
• 1:06:15
  SPEAKER_02: Back and forth on the inside of the fish hook.
• 1:06:17
  SPEAKER_00: You can adjust according to whatever General Lee was doing because of his advantage of the high ground, the geology of the sill, and giving him the military advantage of interior lines, which I think is pretty cool.
• 1:06:32
  SPEAKER_00: So I want to wrap up.
• 1:06:33
  SPEAKER_00: The last thing I'm gonna say is I don't want to offend the historians by simplifying things, but from a geologic standpoint, the Battle of Gettysburg was about the Confederacy kicking the Union forces off the sill.
• 1:06:50
  SPEAKER_00: Yeah.
• 1:06:51
  SPEAKER_00: That's the like the best summary I think that we can give.
• 1:06:54
  SPEAKER_02: Right.
• 1:06:55
  SPEAKER_02: And I think, Chris, that's a great summary.
• 1:06:57
  SPEAKER_02: And as we're standing here, as we're standing on Cemetery Ridge looking down the Pickett's Charge Hill, I think it's a good example.
• 1:07:04
  SPEAKER_02: Artillery mattered, right?
• 1:07:05
  SPEAKER_02: If if there were troops charging up this gentle slope over a mile that you could see coming for a long time, the artillery was invaluable and could bomb really harass them, decimate them on the way across, which is exactly what happened.
• 1:07:19
  SPEAKER_02: Yes, coming up this very shallow slope, shallow angle, artillery was pretty dangerous, yeah, and pretty useful.
• 1:07:25
  SPEAKER_00: You know, and another thing, uh you know, this is I the more we talk, the more we want to talk, and the more we want to say, but because of the humidity, too, um the smoke from on day three from the artillery from the south to kind of soften a cemetery ridge right here where we're standing, the smoke didn't go anywhere.
• 1:07:46
  SPEAKER_00: And so they had a very difficult time seeing where they were hitting because of this kind of smoggy, kind of just bottled it up.
• 1:07:55
  SPEAKER_02: Yeah.
• 1:07:55
  SPEAKER_02: So I mean, I think Chris, it's a it'd be worth just standing here and thinking about this, I think.
• 1:08:01
  SPEAKER_02: So looking from here across this valley, small, slight depression, actually, is uh it's it's moving, it's intimidating, it's somber, it's it's everything.
• 1:08:14
  SPEAKER_02: And as we look down, we see all the monuments down this monument row that are going right down cemetery ridge towards the roundtops down at the bottom there.
• 1:08:22
  SPEAKER_02: It's um the geology was super important here and hugely influential.
• 1:08:27
  SPEAKER_02: And I've really this experience is uh it made this is very I've learned so much through just learning about this, reading the guidebooks, leaving the field guides, looking at the rocks while we're here.
• 1:08:38
  SPEAKER_02: It's just great.
• 1:08:39
  SPEAKER_00: I want to I wanna like kind of tag on to that is that I have spent so much time here.
• 1:08:45
  SPEAKER_00: Yeah, with my wife, with my kids, we have ran every road, we've biked every road, we've done a ton of walking, we do the ranger walks, they're and they're amazing.
• 1:08:55
  SPEAKER_00: These people are so good.
• 1:08:57
  SPEAKER_00: But doing this, I said this to you earlier in the day.
• 1:09:00
  SPEAKER_00: Yeah, this gave me a much deeper understanding of what went on here when I intertwined the geology with the history, yeah.
• 1:09:08
  SPEAKER_02: And I love it.
• 1:09:09
  SPEAKER_00: It was just uh this was so much fun.
• 1:09:11
  SPEAKER_00: We're not done.
• 1:09:11
  SPEAKER_02: We're not done.
• 1:09:12
  SPEAKER_02: No, we got much more to do, much more to look at because there's cool geology around sort of all around this region.
• 1:09:17
  SPEAKER_02: I do want to say one two things to touch on that, Chris.
• 1:09:20
  SPEAKER_02: It's it's really interesting.
• 1:09:22
  SPEAKER_02: It makes it adds an additional kind of color to say, oh, not just that they came from that or that they were on that ridge over there, but why is that ridge there in the first place?
• 1:09:31
  SPEAKER_02: Why was that chosen?
• 1:09:32
  SPEAKER_02: What formed that ridge?
• 1:09:33
  SPEAKER_02: Um, why was that hill there?
• 1:09:36
  SPEAKER_02: I think if we turn this away, we're not gonna be able to see this, but if we look at a topographic map of this region, all of the high points in Adams County, in York County, all the way up in this southern part of Pennsylvania, are the diabase, the York Haven diabase makes up most of the really high points.
• 1:09:53
  SPEAKER_02: All the little knobs are in the outer edges of this diabase cylindrical.
• 1:09:56
  SPEAKER_02: So it kind of really explains the topography of this whole region.
• 1:10:00
  SPEAKER_02: The geology just explains it.
• 1:10:01
  SPEAKER_02: It's really amazing.
• 1:10:02
  SPEAKER_02: I lived in York County for three years, and I started to understand the geology, but now I have a much better grasp of that.
• 1:10:08
  SPEAKER_02: Not only the geology, but the geography as well and the topography.
• 1:10:12
  SPEAKER_00: You know, you you have a son that's almost two years old, and if we were born to be curious, and if you ever want to if you ever question that, then just spend time with a young person like that, and and how many times they say what and why and how, and you know, they're just asking, we're born to be curious, and and that doing this has satisfied a big part of curiosity for me and a deeper understanding.
• 1:10:37
  SPEAKER_02: So love it.
• 1:10:39
  SPEAKER_02: Well, that's a wrap for today and probably the battlefield, and uh we'll maybe get a couple better viewpoints.
• 1:10:43
  SPEAKER_02: Hopefully, the weather's better tomorrow, and we can get some nice overlooks and see South Mountain from high points.
• 1:10:48
  SPEAKER_02: Um, apart from that, then we're gonna move on to the geology, a little bit more regional geology from here.
• 1:10:58
  SPEAKER_02: Chris.
• 1:11:02
  SPEAKER_00: We're in the gap.
• 1:11:03
  SPEAKER_00: We do, we do.
• 1:11:04
  SPEAKER_00: Um, yeah, we just drove from Gettysburg up over through the Cash Town Gap.
• 1:11:11
  SPEAKER_00: Um, and it's just it's a it's a it's a striking thing.
• 1:11:14
  SPEAKER_00: It's so Chris.
• 1:11:15
  SPEAKER_02: Let me describe.
• 1:11:16
  SPEAKER_02: We look to the east.
• 1:11:17
  SPEAKER_02: We're we're standing at the high point of the gap.
• 1:11:19
  SPEAKER_02: Yep.
• 1:11:20
  SPEAKER_02: Um, where the road is sloping down to the west, down to the east from us.
• 1:11:23
  SPEAKER_02: To the east is Gettysburg.
• 1:11:25
  SPEAKER_02: To the west, we go into Chambersburg, Pennsylvania, Maryland, and then South Mountain and South Mountains all around.
• 1:11:32
  SPEAKER_02: We're surrounded on all sides by South Mountain.
• 1:11:34
  SPEAKER_02: Yeah, and we're standing on a sequence of outcrops that defines the Cash Town Gap, which is the Carbaugh Marsh Creek Fault, which the Carbaugh Marsh Creek Fault defines the Cash Town Gap.
• 1:11:47
  SPEAKER_02: Like the gap is there because of the fault, right?
• 1:11:50
  SPEAKER_00: Yeah, yeah, and it's like it's really impressive.
• 1:11:52
  SPEAKER_00: I mean, it's relatively straight.
• 1:11:54
  SPEAKER_00: We just drove it.
• 1:11:55
  SPEAKER_00: It's it doesn't have a lot of relief.
• 1:11:57
  SPEAKER_00: We didn't do a lot of climbing, not a lot of descent.
• 1:12:00
  SPEAKER_00: It's a legitimate gap and it's wide.
• 1:12:02
  SPEAKER_00: It's wide, yeah.
• 1:12:03
  SPEAKER_00: That's understanding the geology of this makes it easy to see why this was one of the main gaps that they chose for easy travel, fast travel, getting artillery through the gap.
• 1:12:16
  SPEAKER_02: And to set the stage, the Confederate forces were hiding to the west in the Great Valley behind us, to the west of where we are right now, and they used this gap to get to Gettysburg and also to retreat away from Gettysburg after the battle.
• 1:12:30
  SPEAKER_02: Um, and so there was some sort of little bit of pitched conflict in these gaps in the South Mountain Baffle in the South Mountain region that the Confederates were mainly using to traverse to and from the battlefield, basically.
• 1:12:44
  SPEAKER_02: Loud.
• 1:12:44
  SPEAKER_02: That was a big one.
• 1:12:45
  SPEAKER_02: Busy road, busy road, it's still a useful gap.
• 1:12:47
  SPEAKER_02: More later.
• 1:12:48
  SPEAKER_02: Let's just look at this real quick before we leave.
• 1:12:51
  SPEAKER_02: Is we have these quartz veining, lots of veining.
• 1:12:53
  SPEAKER_02: There's a lot of faults in South Mountain, and the Carball Marsh Creek fault being one of them.
• 1:12:57
  SPEAKER_02: And here we can just see loads of quartz veining associated with the fault movement.
• 1:13:01
  SPEAKER_02: We're seeing some nice planar features.
• 1:13:03
  SPEAKER_02: You're zooming on that a little bit planar breaks in this.
• 1:13:07
  SPEAKER_02: What is rhyolite?
• 1:13:08
  SPEAKER_00: This flow textured rylite.
• 1:13:09
  SPEAKER_00: We're gonna talk more about this later on because we're gonna get to a quieter place.
• 1:13:12
  SPEAKER_02: But just the way this is braking, this is breaking flat, which means this thing has been jointed, fractured, faulted.
• 1:13:18
  SPEAKER_02: It's not your average rhyolite.
• 1:13:20
  SPEAKER_02: That's not how rhymeite breaks normally, right?
• 1:13:22
  SPEAKER_02: So there's been stress imposed upon that rock.
• 1:13:26
  SPEAKER_00: Absolutely, and it's really super smooth.
• 1:13:28
  SPEAKER_00: Um, so we're gonna take some with us, yeah, and then we'll talk about these rocks later on.
• 1:13:33
  SPEAKER_02: Where it's quieter.
• 1:13:34
  SPEAKER_02: Yep.
• 1:13:34
  SPEAKER_02: All right.
• 1:13:43
  SPEAKER_02: We're in South Mountain.
• 1:13:44
  SPEAKER_02: Yes, we are.
• 1:13:45
  SPEAKER_02: In the middle.
• 1:13:46
  SPEAKER_02: In the mid in the middle, dang near the middle.
• 1:13:49
  SPEAKER_02: Uh, we're south of the Carbaugh Fault, uh, which is the Cashdown Gap fault that runs through there, or the the fault that defines Cash Down Gap.
• 1:13:56
  SPEAKER_02: We're kind of in the middle, we're in the Rhyolite sequence.
• 1:13:59
  SPEAKER_02: Chris, the the I think this is a good point to do a little bit of an overview though, of I think the geology of the South Mountain Gettysburg area, right?
• 1:14:07
  SPEAKER_02: South Mountain and east of here, and a little bit west of here that we're gonna talk about.
• 1:14:11
  SPEAKER_02: So we did a little drawing here real quick that will show.
• 1:14:14
  SPEAKER_02: Um, this is simplified, and it's simplified because it comes from a little bit south of here into Maryland.
• 1:14:22
  SPEAKER_02: Um, is a little bit of a simpler kind of cross section up here in South Mountain.
• 1:14:26
  SPEAKER_02: It gets a bit more complicated, but the overall story is generally the same.
• 1:14:29
  SPEAKER_02: So we're gonna focus here, Chris, on this one.
• 1:14:31
  SPEAKER_02: Um, we're looking at a diagram west on the left, east on the right, and the rocks are dipping gently to the west.
• 1:14:40
  SPEAKER_02: So point towards Gettysburg.
• 1:14:42
  SPEAKER_02: Gettysburg is to the east of us here.
• 1:14:44
  SPEAKER_02: Um, and the rocks that we're standing on are part of what's called an anticlinorium.
• 1:14:50
  SPEAKER_02: And what that means is it's kind of a regional anticline, you can think of it that way.
• 1:14:53
  SPEAKER_02: Big regional anticline, an A-type structure.
• 1:14:56
  SPEAKER_02: Rocks are bent up and down, forming kind of an A-type structure to it.
• 1:15:00
  SPEAKER_02: We're standing in the middle to the east of us.
• 1:15:03
  SPEAKER_02: If we go east, South Mountain, we when we exit South Mountain, there's a thing called a border fault.
• 1:15:09
  SPEAKER_02: And the border fault, it separates that's really this kind of half Grabman style thing where you imagine we've described it before as like a swinging down door, like a salute.
• 1:15:17
  SPEAKER_02: You decide saloon doors in the Tetons have the same kind of structure.
• 1:15:21
  SPEAKER_02: A normal fault that's dipping down, that's swinging, hinging down.
• 1:15:25
  SPEAKER_00: And the the reason the beds are dipping is because of the faulting.
• 1:15:30
  SPEAKER_02: That's right.
• 1:15:31
  SPEAKER_02: And there's some well, we talk about our episode on the the basins and these these uh Jurassic Trassic basins.
• 1:15:37
  SPEAKER_02: There's some debate about when the sedimentation happened relative to the faulting, but I think the general model is that the faulting happened after the sedimentation.
• 1:15:44
  SPEAKER_02: But when we were in Gettysburg, remember we're in Gettysburg looking west towards South Mountain, and the rocks were dipping down towards the west.
• 1:15:52
  SPEAKER_02: So we were kind of looking down the slope as we look to the west.
• 1:15:55
  SPEAKER_02: And that's because the South Mountain, the Gettysburg Basin, as we head west, we hit the south mountain rocks, and there's a fault there, and there's like 10 kilometers, upwards of 10 kilometers of movement on what's called the border fault.
• 1:16:07
  SPEAKER_02: That as we move from east to west, we go across these the Triassic Basin.
• 1:16:11
  SPEAKER_02: So point out the half graben.
• 1:16:13
  SPEAKER_02: The half grabin would be right here, this border fault.
• 1:16:15
  SPEAKER_02: As we hit South Mountain, and the topography changes dramatically when we cross that border fault.
• 1:16:20
  SPEAKER_00: The bedding planes then are if you can you show those as well.
• 1:16:23
  SPEAKER_00: They would be kind of dipping down to the west.
• 1:16:27
  SPEAKER_02: So we're kind of swinging down like this, 25-35 degrees, something like that.
• 1:16:30
  SPEAKER_02: Yep.
• 1:16:31
  SPEAKER_02: Um, in the Triassic Basin as we move towards the the South Mountain, which again was the border, the boundary, where the Confederacy moved in the Great Valley behind South Mountain to the west of South Mountain, and they used this as a major corridor to get down into Virginia, the Army of Northern Virginia moved up and down through that, up and advanced into the north and then retreated after the Battle of Gettysburg.
• 1:16:52
  SPEAKER_02: And getting across South Mountain was they utilized several gaps, the Cash Town Gap being one of them, which we'll talk we've talked about before and we'll talk about in more detail.
• 1:17:00
  SPEAKER_02: But the general geology of this region, South Mountain, down into the Blue Ridge, we have an anticlonorium where we have on we have the the rocks we're standing on right here are rhyolites, a big sequence of rhyolites, and they're about 550, 570, 580, somewhere in that range is when they were erupted onto the surface.
• 1:17:19
  SPEAKER_02: And it's a huge volume.
• 1:17:21
  SPEAKER_02: They're in the center, they're kind of in the center here of this anticlonorium.
• 1:17:25
  SPEAKER_02: We have younger sandstones and schists and phylites on top of them that are Cambrian age, so 500s-ish, 500 plus a little bit aged sandstones here, and those things make up the ridges.
• 1:17:38
  SPEAKER_02: So as we move south into Maryland, we end up having this doublet of ring of ridges where we have the Catoctin Mountains or the sandstone, the sandstone quartzite layer that forms a really resistant ridge.
• 1:17:49
  SPEAKER_02: And then to the right, we have uh South Mountain, which kind of peters out as we get into Maryland, but that's the other ridge, and in between we have the rhyolites.
• 1:17:57
  SPEAKER_02: And as we're where we're standing in South Mountain is really complicated.
• 1:18:00
  SPEAKER_02: The deformation, there's loads of faulting as we talked about before, faults cut across it.
• 1:18:04
  SPEAKER_02: These faults were reactivated and moved five, six times in their history.
• 1:18:08
  SPEAKER_02: The Cash Town fault or or the the carboff fault and the cache down gap there has been mobilized many times, moved up, down, up, down in different directions, and be been reactivated.
• 1:18:17
  SPEAKER_02: So it makes it really complicated.
• 1:18:19
  SPEAKER_02: But this is kind of a simplified view of the overview of the of the region.
• 1:18:23
  SPEAKER_02: And as we get to the west, as we move west, we go into a carbonate sequence of rocks that weathers that weathers a lot to form the Great Valley in this region.
• 1:18:34
  SPEAKER_00: And that would be this stuff right over here.
• 1:18:36
  SPEAKER_00: Exactly.
• 1:18:37
  SPEAKER_02: So the the Confederacy, yeah, when they came up, they came up through that Great Valley area, crossed over South Mountain to the east to get to Gettysburg, and after the battle, retreated back through South Mountain, made it to the and then it was kind of a race to get to the Potomac, a race to get across the Potomac, down back into Virginia, right down at the at the crossing.
• 1:19:02
  SPEAKER_02: So this is really spectacular.
• 1:19:03
  SPEAKER_02: This is a big fault.
• 1:19:04
  SPEAKER_02: This Carball Marsh Creek fault is a big one.
• 1:19:07
  SPEAKER_02: When we'll show kind of two faults here.
• 1:19:08
  SPEAKER_02: So this is a geologic map, recent geologic map that's been made by uh the by uh Pennsylvania Survey.
• 1:19:15
  SPEAKER_02: We're looking at South Mountain here.
• 1:19:17
  SPEAKER_02: So this is just South Mountain.
• 1:19:18
  SPEAKER_02: We see South Mountain.
• 1:19:20
  SPEAKER_02: I'm gonna try and zoom in here.
• 1:19:21
  SPEAKER_02: Um, this you can kind of just see okay, the pinks are rhylites generally, pinks and greens are rhylites, the browns are the Cambrian sandstones and and varieties of that, the the Cambrian rocks there.
• 1:19:34
  SPEAKER_02: As we go down, you can just you can see this massive offset, and this is the Cashtown gap.
• 1:19:40
  SPEAKER_02: This is the Carbaugh Marsh Creek Fault.
• 1:19:42
  SPEAKER_00: So hold on, before we go into that, why does a fault like this create a gap?
• 1:19:48
  SPEAKER_02: So, yeah, it's a good question, Chris.
• 1:19:50
  SPEAKER_02: Basically, it creates a weakness in the rock right there.
• 1:19:53
  SPEAKER_02: I've just highlighted this.
• 1:19:54
  SPEAKER_02: Creates a weakness, and we saw we were looking at rocks that were near this fault, quartz veins running through them, they're deformed, there's weak planes running through the rock.
• 1:20:03
  SPEAKER_02: That just makes it easy to road.
• 1:20:04
  SPEAKER_02: Yep, basically it makes it bust it up, bust it up, easy to road.
• 1:20:07
  SPEAKER_02: If you see if you see complicated geology with a straight topographic line through it, it's generally a fault.
• 1:20:13
  SPEAKER_00: That is like a general geologic principle.
• 1:20:15
  SPEAKER_00: When you see linearity in geology, you're right away thinking faulting.
• 1:20:19
  SPEAKER_02: That's exactly right.
• 1:20:20
  SPEAKER_02: Right.
• 1:20:20
  SPEAKER_02: So, Chris, if I go back to this map, I'm gonna try and zoom out a minute here.
• 1:20:24
  SPEAKER_03: There you go.
• 1:20:25
  SPEAKER_02: And Gettysburg, so Getty, this is South Mountain.
• 1:20:27
  SPEAKER_02: Gettysburg's kind of over here, and we're look the as you look to the west of Gettysburg, you can see South Mountain kind of wrapping around a series of ridges, a series of mountains.
• 1:20:35
  SPEAKER_02: Cash Town Gap is right there.
• 1:20:37
  SPEAKER_02: I've just highlighted it.
• 1:20:38
  SPEAKER_02: This was one of the Confederate retreat routes and advanced routes and retreat routes through the Cash Town Gap, and then another one down here that followed the Jack's Mountain fault, which kind of wraps up in here.
• 1:20:50
  SPEAKER_02: And the especially the model a bit okay.
• 1:20:53
  SPEAKER_02: This this good?
• 1:20:54
  SPEAKER_02: Yep, that's good.
• 1:20:55
  SPEAKER_02: Okay, yep.
• 1:20:56
  SPEAKER_00: Okay, so that just for perspective again, this is like three miles of displacement where this bottom part of South Mountain, the southern part of South Mountain, was shifted to the left, and the top part was shifted to the east.
• 1:21:09
  SPEAKER_00: So to the west into the into the east, and that that makes what's called a right lateral strike slip fault.
• 1:21:15
  SPEAKER_02: Yes, that's right.
• 1:21:16
  SPEAKER_02: Three miles of offset between those is really nice to see.
• 1:21:18
  SPEAKER_02: There is there are dramatic differences between the rocks on the south and rocks on the north, and there's difference in the folding that's happened, and there's definitely some protozoic or older than 550 million year old deformation in here.
• 1:21:31
  SPEAKER_02: The rocks are older, there's deformation in here.
• 1:21:33
  SPEAKER_02: That's that's that age, and the fault movement seems to be that way, but it's been reactivated, remobilized multiple times.
• 1:21:38
  SPEAKER_02: Um, so this is just a really complicated series of rocks, South Mountain, but it but the rocks are dramatically different here than the Trassic Basin, the Gettysburg Basin rocks that we were just looking at to the east.
• 1:21:50
  SPEAKER_02: And so that's why this is a topographic high with these amazing passes, really important passes.
• 1:21:55
  SPEAKER_00: Another interesting thing, Jesse, is that when you're on one side of the fault or one side of the road, yeah, you have one sequence of rocks.
• 1:22:04
  SPEAKER_00: That's right, and then on the other side, you have a completely different sequence of rocks, oftentimes, because the the offset was three miles.
• 1:22:11
  SPEAKER_00: Exactly.
• 1:22:11
  SPEAKER_02: And also there's chemical differences across these things.
• 1:22:13
  SPEAKER_02: There's low titanium, high titanium in the basalts and the rhyolites, and so there's chemical differences, but these basalts and rhyolites are all related to probably a big huge plume event or a big rifting event that formed the what we call the Proto-Atlantic Ocean 500 and some million years ago, um, and formed the Iapetus Ocean.
• 1:22:33
  SPEAKER_02: And so this was a big rifting event that happened in this region that formed these rocks, perhaps even a big caldera collapse event.
• 1:22:40
  SPEAKER_02: Um, huge volcanic eruptions.
• 1:22:42
  SPEAKER_02: Some of these, these, these rhyolytic units are a couple kilometers thick, so huge amounts of lava flowing out and really spectacular stuff.
• 1:22:51
  SPEAKER_02: So maybe we should go look at some of the rocks.
• 1:22:54
  SPEAKER_02: All right, let's do it.
• 1:23:00
  SPEAKER_02: I think there's a bullheist hiding in the hiding in the trees over here.
• 1:23:03
  SPEAKER_00: Yeah, what'd you find, Chris?
• 1:23:05
  SPEAKER_00: This is amazing.
• 1:23:07
  SPEAKER_00: Um I I don't know, I think it's fairly obvious, but these are columnar joints in Rhyolite.
• 1:23:14
  SPEAKER_00: Um like how did I don't know.
• 1:23:17
  SPEAKER_00: This is so cool.
• 1:23:18
  SPEAKER_00: I've I've never seen this before in Rhyolite.
• 1:23:21
  SPEAKER_00: Have you?
• 1:23:22
  SPEAKER_02: Uh no, I don't maybe once in Colorado, but I can't remember exactly where.
• 1:23:26
  SPEAKER_02: But no, not this you know, not this well formed.
• 1:23:32
  SPEAKER_00: Very common structure.
• 1:23:34
  SPEAKER_00: Um, so all right.
• 1:23:37
  SPEAKER_00: This is the Rhyolite we've been talking about, okay.
• 1:23:40
  SPEAKER_00: And columnar joining as the lava flow was cooling off, it was shrinking, it was solidifying all at the same time, and you end up usually with hexagonal then columns, and you can see these right here.
• 1:23:55
  SPEAKER_00: Here's a face right there, there's another face, there's another face, and they usually intersect at about a hundred degree angles or 100, I'm sorry, 120-degree angles.
• 1:24:04
  SPEAKER_00: Um, I don't know, it's just a it's a really cool igneous structure that you see in rocks.
• 1:24:09
  SPEAKER_02: Yeah, so cool.
• 1:24:10
  SPEAKER_02: I mean, we've done a whole episode on on this, uh, but it's basically a cooling structure, right?
• 1:24:15
  SPEAKER_02: So we can tell the cracks always run kind of perpendicular to the cooling, the cold side, so we can tell the cracks are running kind of up and down, tilting back away from us a little bit in this outcrop.
• 1:24:25
  SPEAKER_02: We're on a it's hard to see, but we're on a really steep outcrop, so they're tilting slightly back away from us.
• 1:24:30
  SPEAKER_02: But we can tell which sides of the flow we're cooling.
• 1:24:32
  SPEAKER_02: We can tell top and bottom because the the joints run perpendicular to that, yeah.
• 1:24:37
  SPEAKER_00: Um, that's a good point, you know.
• 1:24:39
  SPEAKER_00: Um, because they run perpendicular to the cooling surface, we know that this was extrusive.
• 1:24:43
  SPEAKER_00: I mean, we know that anyway, because this is a rhyolite, but sometimes they're not right, these are uh maybe shallow intrusions, and the cooling surface is really all the way around the rock, and so we can see that based upon the geometry, really, and the orientation of those columns.
• 1:24:58
  SPEAKER_02: That's right, really cool.
• 1:25:00
  SPEAKER_02: These um and Chris, these protozoic, just to mention here, these these precambrian rhyolites have amazing variety of textures.
• 1:25:07
  SPEAKER_02: We've seen probably three or four of them, but there's probably a dozen different textures that have been mapped in this south mountain area, so just an amazing array of the colour.
• 1:25:15
  SPEAKER_02: We haven't seen them all.
• 1:25:16
  SPEAKER_00: No, no, there's there's the the the rhyolytic uh brechas, yeah, which is where you kind of get this like bust as it's flowing and solidifying, it's still moving and it's busting up the rock, and you end up with this really brecchiated angular, like xenoliths in the in the rhyolytic flow itself.
• 1:25:35
  SPEAKER_00: We haven't seen that yet.
• 1:25:36
  SPEAKER_02: What is this one?
• 1:25:37
  SPEAKER_02: I mean, can you do you have a hand sable there is that you're holding a good one?
• 1:25:40
  SPEAKER_00: This is kind of like you know, we've looked at a uh quite a bit of different looking stuff here.
• 1:25:46
  SPEAKER_00: This is um this is what's called a modeled texture to this rock where we can see the separation of dark and light, kind of there.
• 1:25:55
  SPEAKER_00: I don't know, can you see the orientation in this?
• 1:25:58
  SPEAKER_00: Kind of this is weak, I would say.
• 1:26:00
  SPEAKER_00: Like I mean, we saw some really good stuff earlier.
• 1:26:02
  SPEAKER_00: That's right.
• 1:26:03
  SPEAKER_00: Um, but we're not there right now, so sorry about that.
• 1:26:05
  SPEAKER_00: But there it's kind of weakly oriented this way, right?
• 1:26:09
  SPEAKER_00: And sometimes this shows then the the directional flow.
• 1:26:13
  SPEAKER_00: Um, and we've seen the rhyolite that has phenocrysts of like uh a small uh smoky quartz, right?
• 1:26:21
  SPEAKER_00: And then bigger phenocrists of like white feld spots, yeah, right.
• 1:26:25
  SPEAKER_02: Um and then so huge riot.
• 1:26:27
  SPEAKER_02: I mean, you know we saw at the at the Carball Fault Lation, we saw the really laminated, yeah, um, laminated kind of asphalt deposits uh that are really, really finely laminated and faulted and they've been deformed.
• 1:26:38
  SPEAKER_02: These are beautiful though, and it's amazing the array.
• 1:26:40
  SPEAKER_02: So we're we're kind of on this big outcrop, the array of of different types.
• 1:26:45
  SPEAKER_02: This is all kind of one outcrop.
• 1:26:46
  SPEAKER_02: One this modeled rock type is this whole outcrop, but throughout South Mountain, it varies a ton.
• 1:26:52
  SPEAKER_00: Yeah, and and the other thing that we should talk about a little bit anyway is why is it so dark colored?
• 1:26:58
  SPEAKER_00: This is a rhyolite.
• 1:27:00
  SPEAKER_00: Most people associate light colored with a rhyolytic um uh flow, yeah.
• 1:27:05
  SPEAKER_00: Because it's felsic, it's made up mostly of quartz and feldspar, these really high silicate minerals.
• 1:27:11
  SPEAKER_00: Uh excuse me.
• 1:27:15
  SPEAKER_00: I'm still sick.
• 1:27:16
  SPEAKER_00: Um anyway, so the the darker color, this kind of like steel blue, bluish gray, and sometimes even purple color that we've seen today is really a result of iron that is like finely divided iron that's suspended in this lava and or in this, you know, this flow, this rhylitic lava flow, and then it oxidizes.
• 1:27:42
  SPEAKER_01: Yeah, that's right.
• 1:27:43
  SPEAKER_00: And it turns it various shades of um, you know, darker colors, like we said, the dark purple, the dark gray.
• 1:27:49
  SPEAKER_02: And we've talked about this before, Chris, that obsidian is usually really dark colored rock uh in many cases, and uh it's also because it's a glass, like there's a this is a glass fundamentally, and actually.
• 1:28:00
  SPEAKER_02: Interesting point there is that many of the Native American communities have quarries or there were quarries up in South Mountain that were looking for the obsidian because this stuff you can kind of see it a little bit here, it fractures, and there's really nice sharp patterns that you see in that rock.
• 1:28:14
  SPEAKER_02: You could you could shape this stone, and and early uh Native American communities certainly did.
• 1:28:18
  SPEAKER_02: And there's quarries all over the place.
• 1:28:20
  SPEAKER_02: It's the MVP.
• 1:28:21
  SPEAKER_00: It is the MVP.
• 1:28:23
  SPEAKER_00: Absolutely.
• 1:28:24
  SPEAKER_00: Yeah, and you know, it brings me to mind of something you said, and when you first said it to me, I was like, You're full of crap.
• 1:28:31
  SPEAKER_00: But it was classic Chris response to everything I said.
• 1:28:34
  SPEAKER_00: It was that you really can't identify a rock until you get it back in the lab.
• 1:28:38
  SPEAKER_00: Yeah, and when you when you do a petrographic analysis of this rock under thin section, under microscope, you're gonna see just almost all quartz and feldspar in here.
• 1:28:49
  SPEAKER_00: Yeah, um, and and then the the dark colors kind of like this oxidation bleeds into the into the minerals and it it kind of smears them and it can result in really kind of like tainting the whole rock a different color.
• 1:29:02
  SPEAKER_02: And Chris, that is even uh glass.
• 1:29:04
  SPEAKER_02: So this stuff was probably some sort of glass obsidian when it crystallized over 550 million years ago, but that glass is really unstable over geologic time scale.
• 1:29:14
  SPEAKER_00: Yeah, there's no such thing as old obsidian.
• 1:29:17
  SPEAKER_02: No, and because glass is basically a liquid, it's a liquid that flows over millions of years timescales, and so it has to re-it gets devitrified, unglassified basically.
• 1:29:28
  SPEAKER_02: It gets crystallized out, altered, you know, prenate pumpellate, all sorts of alteration minerals kind of come in and add some structure to this unstructured glass, which is kind of what we're seeing in some of the cases in this model.
• 1:29:39
  SPEAKER_00: Vitreous means glassy.
• 1:29:41
  SPEAKER_00: We talk about this with sometimes you describe the luster of a quartz mineral's vitreous, kind of glassy, like de-vitrified then is devitricified, you know, like it's unglassy, you know.
• 1:29:52
  SPEAKER_00: Yeah, but it's it's again it's a testament to the the lack of stability of glassy at the surface.
• 1:29:59
  SPEAKER_02: Totally, totally.
• 1:30:00
  SPEAKER_02: Uh this is amazing.
• 1:30:02
  SPEAKER_02: This is really cool.
• 1:30:03
  SPEAKER_02: This is spectacular.
• 1:30:03
  SPEAKER_02: I referred to this outcrop or these suites of outcrops in our um in our geology of of of um jointing like this of columnar jointing, and I've not been here before, not seen these, so it's really nice to see them because they're cool.
• 1:30:17
  SPEAKER_02: They're not far, they're in Pennsylvania in South Mountain.
• 1:30:20
  SPEAKER_02: Spectacular.
• 1:30:20
  SPEAKER_02: Hey, and 550 million year old greater than 550 million year old columnar joints.
• 1:30:26
  SPEAKER_02: That's pretty spectacular.
• 1:30:27
  SPEAKER_00: That it is, it's crazy um to think about that kind of time scale.
• 1:30:31
  SPEAKER_00: I've never seen anything like it.
• 1:30:32
  SPEAKER_02: Amazing, yeah, super cool.
• 1:30:34
  SPEAKER_00: All right.
• 1:30:43
  SPEAKER_02: Good to go.
• 1:30:44
  SPEAKER_02: Gotcha on.
• 1:30:44
  SPEAKER_02: Chris and our last stop, man.
• 1:30:46
  SPEAKER_02: That's right.
• 1:30:46
  SPEAKER_02: I don't know.
• 1:30:47
  SPEAKER_02: Is it the best rock?
• 1:30:49
  SPEAKER_00: Oh, that's tough.
• 1:30:53
  SPEAKER_00: Um, I don't know.
• 1:30:55
  SPEAKER_00: It's tough.
• 1:30:56
  SPEAKER_00: That's tough.
• 1:30:56
  SPEAKER_02: It's a tough one.
• 1:30:57
  SPEAKER_02: Let's see.
• 1:30:57
  SPEAKER_00: Let me let's zoom out real quick and we can talk about this stuff right here.
• 1:31:00
  SPEAKER_02: It's one of the more spectacular rocks I think I've probably ever seen, just from a look of it.
• 1:31:07
  SPEAKER_02: Um, this is fanglomerate, is what it's called.
• 1:31:11
  SPEAKER_02: This is uh the last sedimentary rock in the story of the Gettysburg Basin.
• 1:31:17
  SPEAKER_02: We are not in uh Pennsylvania anymore.
• 1:31:20
  SPEAKER_02: We're not in Pennsylvania anymore, Chris.
• 1:31:21
  SPEAKER_02: We are down in Maryland on the Maryland side of the Potomac, where there's a big exposure of this fangomerate, what's called calico marble, potomac marble.
• 1:31:30
  SPEAKER_02: It has some trade names.
• 1:31:31
  SPEAKER_02: It's a beautiful, uh, beautiful stone that down here near in and around Leesburg, Virginia, and in uh Point of Rocks area in Maryland around the Potomac, um, there's a big thick sequence of it.
• 1:31:44
  SPEAKER_02: But this fanglomerate stuff, which we just saw, I mean, what fanglomerate?
• 1:31:48
  SPEAKER_00: Yeah, so let's define that a second.
• 1:31:49
  SPEAKER_00: What is a fanglomerate?
• 1:31:51
  SPEAKER_00: Everybody knows conglomerate.
• 1:31:52
  SPEAKER_02: Conglomerate, yeah.
• 1:31:53
  SPEAKER_02: This to me is a brecia, um, really angular.
• 1:31:58
  SPEAKER_00: It's a little bit of both, right?
• 1:31:59
  SPEAKER_00: Yeah, a little bit of both.
• 1:32:00
  SPEAKER_00: It's not really that rounded, but it's not also strictly angular either.
• 1:32:04
  SPEAKER_02: Exactly.
• 1:32:04
  SPEAKER_02: I mean, it is full of class, huge class, as we saw.
• 1:32:08
  SPEAKER_02: We look at this thing.
• 1:32:09
  SPEAKER_02: I mean, it is various class size.
• 1:32:11
  SPEAKER_02: We see kind of pebbles to cobbles to sand grains down here because there's a mat, a bigger one, uh, you know, several inches long.
• 1:32:20
  SPEAKER_02: Um, we've seen in these some quartz.
• 1:32:22
  SPEAKER_02: We have some quartz pebbles in here.
• 1:32:24
  SPEAKER_02: Uh, we've seen boulders, like actual boulders, you know, football-size boulders in this fanglomerate.
• 1:32:31
  SPEAKER_02: And this is the the last sedimentary rock that's deposited in these basins.
• 1:32:35
  SPEAKER_02: We'll have a whole episode on it in our visual podcast series, but it's a breccia that is cemented with calcite dominantly.
• 1:32:42
  SPEAKER_02: And so around here, this thing has been baked by one of the dikes, and so it's a marble, calico marble, potomac marble.
• 1:32:49
  SPEAKER_02: So it's a meta fangless.
• 1:32:52
  SPEAKER_02: And these fanglomerates, you can find them all up, uh, kind of set up against the border fault on the west side of all these basins as we chase up through Gettysburg.
• 1:33:00
  SPEAKER_02: I've seen them up in York County.
• 1:33:01
  SPEAKER_00: Well, we saw some in the middle and we stopped, but that stuff was was like it was not nearly as well lithified as this because it hadn't been metamorphosis.
• 1:33:09
  SPEAKER_02: That's right, that's right.
• 1:33:10
  SPEAKER_02: And this is a sort of an alluvial fan type of uh deposit.
• 1:33:13
  SPEAKER_02: We can think of this.
• 1:33:14
  SPEAKER_02: And basically, what's an alluvial fan?
• 1:33:16
  SPEAKER_02: So this is a mountain stream coming out of the mountains, dumping into a valley, and the stream dries up and it just dumps all of its sediment right there.
• 1:33:23
  SPEAKER_02: So arid climates, stream uh high topography, high relief at least, and it's just pulling stuff out of the mountains, dumping it in the flat land below.
• 1:33:32
  SPEAKER_00: So they come ripping out of the mountains, they hit the flatland, slow down, and they start to deposit this stuff.
• 1:33:37
  SPEAKER_00: Um exactly.
• 1:33:39
  SPEAKER_00: So, like it's just a fanglomerate because it's an alluvial fan conglomerate.
• 1:33:44
  SPEAKER_02: That's right, that's right, exactly.
• 1:33:46
  SPEAKER_00: And these these were so abundant that they were coming out of the mountains and they were coalescing.
• 1:33:50
  SPEAKER_02: That's right, kind of coalescing multiple fans kind of aggregating together.
• 1:33:53
  SPEAKER_02: Here up around Leesburg, there was a particularly thick fan.
• 1:33:56
  SPEAKER_02: Um, and the the fanglomerate kind of changes character as we go around the basin.
• 1:34:00
  SPEAKER_02: So we can kind of think of this and draw this out in our heads is that we had this basin that was forming that to the southeast, so the Gettysburg Basin, we had mountains, or the high relief was to the southeast, and all of a sudden it was draining to the northwest at that time.
• 1:34:15
  SPEAKER_02: And uh, so we have the this big sequence, which now we have these four or five different basins together: Newark Basin, Gulf River Basin, Gettysburg Basin.
• 1:34:23
  SPEAKER_02: Um, those were all draining to the northeast.
• 1:34:26
  SPEAKER_02: Then something happened that created some relief down to the southwest area, basically, and that relief ended up dumping all of these fanglomerates onto it.
• 1:34:35
  SPEAKER_02: So we kind of had a shifting drainage basin direction here at the end, the latest stage of sedimentation in these Triassic, um, early Triassic basins.
• 1:34:44
  SPEAKER_02: Yeah, right near the top of the stuff in eastern North America.
• 1:34:47
  SPEAKER_02: That's exactly right.
• 1:34:48
  SPEAKER_02: It's not incredibly relevant to the Gettysburg War um or the Gettysburg Battle.
• 1:34:56
  SPEAKER_02: The troops had to cross it as they went into South Mountain.
• 1:34:59
  SPEAKER_02: There's fanglomerate in between South Mountain and Gettysburg, but I think what's kind of more historically interesting is if we turn right over here, Chris.
• 1:35:05
  SPEAKER_02: This is a quarry site that was used um to build some columns in the U.S.
• 1:35:13
  SPEAKER_02: Capitol building.
• 1:35:14
  SPEAKER_02: So this fanglomerate, Potomac Marble, Calico Marble, is uh a building stone, and it's really, really beautiful when it's polished.
• 1:35:21
  SPEAKER_02: And they used it to build some of the Capitol, the columns in the Capitol building.
• 1:35:25
  SPEAKER_02: There's a whole book written about this.
• 1:35:26
  SPEAKER_02: We'll go into more detail on the sort of story of these rocks.
• 1:35:29
  SPEAKER_02: But this is a very old quarry.
• 1:35:32
  SPEAKER_00: And we're not we're not gonna collect any rocks here because this is historical and historical site, absolutely respect, yeah.
• 1:35:37
  SPEAKER_02: For sure.
• 1:35:38
  SPEAKER_02: But this is it's a spectacular thing.
• 1:35:39
  SPEAKER_02: I mean, you can see this stuff is all over the place.
• 1:35:41
  SPEAKER_02: So just go look at just go look at the rocks, and there's big cobbles, little cobbles, sand grains.
• 1:35:47
  SPEAKER_02: I mean, it's beautiful, beautiful rock with this red sandy calcite matrix.
• 1:35:50
  SPEAKER_00: It's just a lot of biology covering this stuff up.
• 1:35:53
  SPEAKER_02: It is, unfortunately.
• 1:35:54
  SPEAKER_02: Lots of biology on this rock.
• 1:35:55
  SPEAKER_00: You can see here, um, this is a really good example of the angularity to a lot of these clasts, which is kind of like it's it's maybe more of a brecha tech technically than a than a conglomerate.
• 1:36:06
  SPEAKER_00: Yeah, that's at least in places.
• 1:36:07
  SPEAKER_02: At least in places, that's exactly right.
• 1:36:09
  SPEAKER_02: So, Potomac Marble.
• 1:36:11
  SPEAKER_02: There we go.
• 1:36:12
  SPEAKER_02: All right.
• 1:36:18
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• 1:36:19
  SPEAKER_02: We're at Plant Geocast.
• 1:36:20
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• 1:36:22
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• 1:36:25
  SPEAKER_02: There's a link in the show notes.
• 1:36:26
  SPEAKER_02: And if you want all of the deep dives on all of the content, all the topics here related to the geology of the Battle of Gettysburg, you can go to our Camp Geo mobile app.
• 1:36:35
  SPEAKER_02: First link in your show notes.
• 1:36:36
  SPEAKER_02: There's a Gettysburg series there that you can get access to.