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Silent Sculptors - The Stones Shaping Gettysburg's Battlefield · PlanetGeo: The Geology Podcast

• 0:00
  SPEAKER_01: Totally.
• 0:00
  SPEAKER_00: Changed it to the point where, like, these are now famous locations.
• 0:03
  SPEAKER_00: Uh, right because bridge.
• 0:05
  SPEAKER_00: All these things that are famous locations because they're they're points in the battle, because there is a little bit of relief.
• 0:13
  SPEAKER_00: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it worked, and why it matters to you.
• 0:25
  SPEAKER_00: Yeah, you better straighten up.
• 0:27
  SPEAKER_00: You better straighten up.
• 0:28
  SPEAKER_00: We're getting into uh we're zooming in, man.
• 0:30
  SPEAKER_00: Zooming in on Gettysburg, the Gettysburg Basin.
• 0:34
  SPEAKER_01: What um I get warm fuzzies um thinking about this, thinking about the time we had in the field together, and and uh you know, also like the kind of blending of my own personal experiences over the last few decades, going to Gettysburg and then doing this with you.
• 0:51
  SPEAKER_01: It's just a a really cool thing for me.
• 0:54
  SPEAKER_00: So I love this.
• 0:56
  SPEAKER_00: This is very fun in this uh this episode that we're covering.
• 1:00
  SPEAKER_00: This is chapter three or episode three in our series of Gettysburg, the geology of the Battle of Gettysburg, really.
• 1:07
  SPEAKER_00: One was our field trip.
• 1:08
  SPEAKER_00: We saw a bunch of the rocks.
• 1:10
  SPEAKER_00: We're gonna talk a lot about some of those rocks today, the sediments today.
• 1:14
  SPEAKER_00: We're zooming in on the Gettysburg Basin.
• 1:16
  SPEAKER_00: Our last episode, we talked about the basins generally, like all of these Triassic Mesozoic basins up and down the east coast of the United States, what the tectonic setting is.
• 1:24
  SPEAKER_00: We had the sediment transport from the southeast through to the northwest with this basin in the middle.
• 1:28
  SPEAKER_00: We're gonna talk about a bunch of different stuff in our series.
• 1:31
  SPEAKER_00: You can find all this on the Camp Geo Mobile app, all the other episodes that we have.
• 1:35
  SPEAKER_00: We've got four more at least that we're we've got on there right now.
• 1:38
  SPEAKER_00: Um download the Camp Geo Mobile app, first link in your show notes there.
• 1:42
  SPEAKER_00: But we're gonna zoom in on the Gettysburg Basin, but the sediments in the Gettysburg Basin, Chris.
• 1:47
  SPEAKER_00: And we're gonna kind of hold off on the igneous rocks, which play a huge role in the battle, but we're gonna talk about that for the next episode because they're really important.
• 1:54
  SPEAKER_00: So we're zooming in on sediments here.
• 1:56
  SPEAKER_00: And is that hard for you?
• 1:58
  SPEAKER_00: Because you and I we like igneous rocks, we like metamorphic rocks.
• 2:02
  SPEAKER_00: We're talking about sediments for a whole episode.
• 2:04
  SPEAKER_01: No, I'm good.
• 2:05
  SPEAKER_01: Um we saw some really interesting things in these sedimentary rocks.
• 2:10
  SPEAKER_01: So, no, I think I'm I'm ready.
• 2:13
  SPEAKER_01: I'm ready for this, I'm excited for this.
• 2:14
  SPEAKER_01: We'll just this is the first time really that we're gonna talk at least a little bit about how the rocks played a role.
• 2:22
  SPEAKER_01: Yes, and that's gonna be a funny point, Chris.
• 2:24
  SPEAKER_00: Oh man, the the rocks played a role, and Chris, we uh to me, one of the key things that really stood out to me that I kind of known in an academic way, but but is hard to show and or hard to kind of sink your teeth into until you're on the rocks looking at it, is how minor differences in the rock types had dramatic have dramatic influence over the landscape because these are really old rocks that have been sitting there weathering for a long time.
• 2:52
  SPEAKER_00: And so to me, this was thing one thing that really stood out and stands out really well when you look at like high resolution like lidar data where you get this topography, this relief.
• 3:01
  SPEAKER_00: The relief is controlled by the rocks.
• 3:03
  SPEAKER_00: We kind of always know that, but they're relatively minor differences in the rocks.
• 3:07
  SPEAKER_01: They are that is that is a takeaway with this episode, and also from really when we did that stop and looked at these rocks and the subtle differences and how that created topographic variations, and and that was a really, really cool thing.
• 3:27
  SPEAKER_01: I wish that we had had a group of students with us to just point this out and say, I mean, look at this.
• 3:32
  SPEAKER_01: This is amazing.
• 3:33
  SPEAKER_01: Not a big difference, but it changed the topography of it.
• 3:36
  SPEAKER_00: Totally changed it to the point where like these are now famous locations, uh, right?
• 3:41
  SPEAKER_00: McPherson Bridge, all these things that are famous locations because they're they're points in the battle because there is a little bit of relief, a little bit of relief difference that mattered a lot.
• 3:49
  SPEAKER_01: So let's well, Jesse, let's let's let's jump into this then.
• 3:53
  SPEAKER_01: We're gonna talk about the Gettysburg Basin, particularly, how the different rocks may have influenced the battle.
• 4:00
  SPEAKER_01: And we're talking about rocks here, Jesse.
• 4:01
  SPEAKER_01: If you image number one here, that gift that you made.
• 4:04
  SPEAKER_01: Awesome job.
• 4:05
  SPEAKER_01: Um, we're looking at rocks that are in the 230, which is when the rifting first began.
• 4:11
  SPEAKER_01: So 230 to 190 million years ago range.
• 4:16
  SPEAKER_01: So a significant amount of time.
• 4:18
  SPEAKER_00: We're talking about yeah, yeah, yeah.
• 4:21
  SPEAKER_00: A big amount of time, a big amount of sediment, and we're again looking at this remnant as we talked about last episode.
• 4:26
  SPEAKER_00: We just have this remnant left over, right?
• 4:28
  SPEAKER_00: There's probably there's certainly missing pages of this book that have been ripped out.
• 4:32
  SPEAKER_00: From the bottom to the top, we're zooming in here, and again, we're zooming in on the Gettysburg Basin, which is one of these remnants of this larger Birdsboro Basin sequence.
• 4:42
  SPEAKER_00: And we're just we're right now zooming in on the sequence here, the sequence of sedimentation.
• 4:46
  SPEAKER_00: The major rock types that we're gonna care about for this episode is the background Gettysburg Formation, which is this reddish stuff.
• 4:55
  SPEAKER_00: We'll show a couple images of it, but it's this kind of reddish, sandy, silty stuff.
• 4:59
  SPEAKER_00: And then we have the Heidlersburg member, which is a layer in the Gettysburg Formation.
• 5:05
  SPEAKER_00: So it's kind of bounded on top and bottom by Gettysburg Formation, but it's a layer in the middle there, discontinuous layer that's really a gray, silty.
• 5:15
  SPEAKER_00: I don't know what would you classify this as, Chris.
• 5:17
  SPEAKER_00: Like uh, you know, it's a gray shale or argument kind of thing.
• 5:21
  SPEAKER_01: It is uh kind of kind of like that.
• 5:23
  SPEAKER_01: It's like a silty sandstone, some carbonates in there.
• 5:27
  SPEAKER_01: So gonna be more resistant to erosion, generally speaking, than what you get in the Gettysburg Formation.
• 5:33
  SPEAKER_00: That's right.
• 5:34
  SPEAKER_00: In different depositional environments here that we're we'll talk through as well.
• 5:38
  SPEAKER_00: And just the sequence, we're gonna walk through kind of the sequence here, starting from things at the bottom.
• 5:43
  SPEAKER_00: And these rocks on the bottom, which we do not have labeled, we have um basically a different composition down there in the bottom that is kind of a quartz sandstone-y kind of thing that's at the base of of all of these units.
• 5:53
  SPEAKER_00: The new Oxford you're talking about throughout these baits.
• 5:55
  SPEAKER_00: The new Oxford, yeah, that's exactly right.
• 5:57
  SPEAKER_00: That's kind of the starts of the process, but we're we don't see that rock unit in the Gettysburg battle area, and so we're gonna kind of skip over that for now.
• 6:05
  SPEAKER_01: Right.
• 6:05
  SPEAKER_01: Yeah, so if we take this this cross section that you made, spanning that 40 million years, showing the new Oxford, the Gettysburg, the Heidlersburg, and then the sills and dykes on top of this, uh in terms of age when you get this.
• 6:18
  SPEAKER_01: If we take that, the new Oxford is the oldest in this uh sequence of rocks that we're looking at.
• 6:24
  SPEAKER_01: It's the bottom, bottom most is almost always uh the oldest.
• 6:28
  SPEAKER_01: If you take that and you tip it, if we take that column and just tip it like a pizza box off to the left, what happens then is if you tip it to the left or to the west, we should say, then you're not gonna see that old layer in the west or on the left.
• 6:45
  SPEAKER_01: You're only gonna see the butt end of that on the right hand side or the eastern section.
• 6:51
  SPEAKER_00: So if you the way that and eroding it, remember tip it and cut it in half, and now we have tilted rocks.
• 6:57
  SPEAKER_00: And we talked about this on our field trip, which you can kind of see they're dipping to the northwest like 15-20 degrees in this part of the Gettysburg Basin, uh, and everything's tipping in that direction, right?
• 7:07
  SPEAKER_00: So there are kind of the stacked and then tilted and then eroded and cut in half.
• 7:11
  SPEAKER_00: And Chris, I'll move forward.
• 7:12
  SPEAKER_00: I just want to point on this timeline image the end of the Triassic, the end Triassic mass extinction, the transition into the Jurassic occurs in this sequence, and that's going to be really the topic for next episode when we talk about the diabase, the York Haven Sill, the Rossville dykes, those occur right near that boundary, and there's some really cool science coming out about those things right at the end of this boundary.
• 7:34
  SPEAKER_00: But we're spanning the Triassic-Jurassic boundary here.
• 7:37
  SPEAKER_00: So it's a great area to look at and study these key transformational events in Earth history as we go forward.
• 7:43
  SPEAKER_00: So the I skip ahead to this cross-section here, Chris, just to show this tilting.
• 7:48
  SPEAKER_00: When we when we look at this, when we think about it, think about a cross-section going from South Mountain.
• 7:52
  SPEAKER_00: Well, we're zooming in now, but from the northwest to the southeast through the Gettysburg Formation or through the Gettysburg area.
• 7:59
  SPEAKER_00: Let's take a geological cross-section, look, think about the rocks and how they're projecting down into the earth.
• 8:05
  SPEAKER_00: But then the way that we sometimes plot these things as geologists in the way that this little image is showing is vertically exaggerated.
• 8:12
  SPEAKER_00: So it accentuates the relief.
• 8:14
  SPEAKER_00: And you can kind of see this here.
• 8:15
  SPEAKER_01: Yeah, if I can interject here a second, Jesse, when I when I saw you make this, because you you did a good job, you made it.
• 8:21
  SPEAKER_01: I I looked at it, and I first thing I did, I pick up my phone and by Jesse, what are you doing?
• 8:24
  SPEAKER_01: That's tilted way too steeply.
• 8:27
  SPEAKER_01: It's way off.
• 8:27
  SPEAKER_00: It's tilted too steeply, there's way too much relief.
• 8:30
  SPEAKER_00: Um, and again, we you know, we we I built a bunch of these images based off of really excellent images in field guides and and other publications on this feature on the Gettysburg formation.
• 8:39
  SPEAKER_00: And so this is really typical in geology is to vertically exaggerate things to kind of accentuate the topography and the relief.
• 8:47
  SPEAKER_00: And, you know, this is a really nice zoomed-in image that shows that that shows that relief, and it's in part because of the vertical exaggeration.
• 8:55
  SPEAKER_00: So those sediment layers, they're definitely in this image tipped more than 15 or 20 degrees, but that's just because it's been vertically exaggerated, right?
• 9:02
  SPEAKER_00: But it shows that seminary ridge is underlain by a dike.
• 9:06
  SPEAKER_00: It shows that some of the other ridges are underlain by different rock types because the weathering matters a lot here.
• 9:15
  SPEAKER_01: Yeah, because uh the sediments here of the Gettysburg Formation, which again, we're dipping this to the northwest, and so because the Gettysburg Formation is the youngest sedimentary rocks there, that's where they're going to be exposed.
• 9:27
  SPEAKER_01: But the Gettysburg Formation is dominated by these shaley kind of mudstones.
• 9:32
  SPEAKER_01: And these are rocks that I just often describe to my students as these are soft, they're weak, they're wimpy, and so they preferentially weather out and form the valleys.
• 9:42
  SPEAKER_00: Yeah.
• 9:42
  SPEAKER_01: And that's why this forms when you go to Gettysburg and you see these, you'll see ridges off to one side and ridges off to the other side, but in the middle is this kind of very subtle but there valley, and that is like right away you're thinking, all right, this is the Gettysburg Formation, which is made up of these really red-colored, shaley, soft, weak, wimpy rocks.
• 10:06
  SPEAKER_00: Absolutely.
• 10:07
  SPEAKER_00: And it's such a it's very obvious in this region that that that is true, I guess, that the the weak rocks form lowlands or relatively low areas, and that slightly more well put together rocks, a little bit of sand in a couple layers, matters a lot here and forms these ridges.
• 10:29
  SPEAKER_00: So let's just paint the the kind of the picture here.
• 10:32
  SPEAKER_00: We're again we're we have these big mountains down to the southeast of us during the time of deposition 230, 220, 210 million years ago.
• 10:40
  SPEAKER_00: Sediment is kind of flowing through this basin.
• 10:43
  SPEAKER_00: The big sediment's getting captured in the basin, the really fine stuff, all the organics and stuff are are kind of making it out of the basin, flowing out to the northwest now.
• 10:53
  SPEAKER_00: And it's kind of getting captured in here.
• 10:54
  SPEAKER_00: We have alluvial fans, we have playas, this these big bajada regions, and we also, on top of that, this is taking a long time.
• 11:03
  SPEAKER_00: We have tens of millions of years of history preserved here.
• 11:06
  SPEAKER_00: There are climate variations that are occurring.
• 11:09
  SPEAKER_00: So we will have wetter periods, drier periods, wetter periods, drier periods, you know, normal kind of Milankovic cycle stuff.
• 11:17
  SPEAKER_00: And that's gonna affect these things, like the playa areas.
• 11:20
  SPEAKER_00: Sometimes the lakes will be deeper, and sometimes there won't be a lake there.
• 11:24
  SPEAKER_00: There's a lot of variability even in one location, right?
• 11:28
  SPEAKER_01: Well, Jesse, can I interject a second?
• 11:30
  SPEAKER_01: Because if you look at that that first cross section that you showed, you have the New Oxford formation on the bottom, but that's only going to be exposed in the southeast because of the way the beds are dipping.
• 11:40
  SPEAKER_01: But the New Oxford beds, they have some Arcosic sandstone.
• 11:45
  SPEAKER_01: So let's get a little bit into the weeds on that.
• 11:47
  SPEAKER_01: You've got this arcosic sandstone, whereas the Gettysburg Formation, mostly mudstones and shales, but there are sandstone lenses in that.
• 11:54
  SPEAKER_01: And you and I went to places where we saw that and it played a big part in the topography of the region.
• 12:00
  SPEAKER_01: The sandstone, though, is not arcosic, it's more of this quartz-rich sandstone.
• 12:05
  SPEAKER_00: So, like what is arcosic, Chris?
• 12:07
  SPEAKER_00: Maybe define arcosic first.
• 12:09
  SPEAKER_01: It's not a typical sandstone, like uh when we think of like these southwestern United States kind of quartz-rich sandstones.
• 12:17
  SPEAKER_01: Arcosic means it's moderately sorted, it's not well sorted like a quartz sandstone's gonna be, and it it has an abundant amount of feldspar in it.
• 12:26
  SPEAKER_01: And to the geoscientists, that means something.
• 12:29
  SPEAKER_01: If you have a sandstone that has abundant feldspar in it, that right away signals all right.
• 12:36
  SPEAKER_01: A couple things come to mind.
• 12:38
  SPEAKER_01: This was not transported very far before it was lithified.
• 12:42
  SPEAKER_01: And it was probably a drier climate because two things happened to feldspar, right?
• 12:50
  SPEAKER_01: Feldspar is not nearly as tough as quartz is.
• 12:53
  SPEAKER_01: And so the only way that that's gonna stand up to the rigors of weather and erosion is not having a lot of it, not a huge degree of transport before deposition.
• 13:03
  SPEAKER_01: And then also the drier climate, feldspar is really susceptible to chemically weathering into clay minerals and getting taken away.
• 13:11
  SPEAKER_01: And so, because you have abundant, that indicates those things to any like beginning geoscientist.
• 13:18
  SPEAKER_01: Yeah, the quartz-rich sandstone that indicates much greater degree of transport.
• 13:22
  SPEAKER_01: It's much uh, you know, the class are rounded more, it's well sorted, and all of that is gonna indicate.
• 13:30
  SPEAKER_01: We talked about this with sand, only the strong survive, and quartz is is strong.
• 13:35
  SPEAKER_00: Quartz is strong chemically and physically, exactly right.
• 13:38
  SPEAKER_00: You're kind of describing those two different weathering processes, and so that also plays a role when you look at this cross-section, and again, these are this cross-section and this timeline come from uh um a field guide basically from 2006, written by Roger Cuffey and a bunch of other people, that builds on a lot of information about the the region and the geology here.
• 13:56
  SPEAKER_00: But you can see that these sand and carbonate layers weather a little bit higher, a little bit more positively than the background shale of the Gettysburg Formation itself, the Gettysburg Formation proper.
• 14:09
  SPEAKER_00: And so those little variations become really important because only the strong survive, Chris.
• 14:15
  SPEAKER_00: Only the strong survive even when it's a sediment.
• 14:17
  SPEAKER_00: We'll talk about this in a later episode.
• 14:19
  SPEAKER_00: But when you look to the west from Gettysburg South Mountain, there's volcanic rocks up there for sure, but the highest stuff is quartzite.
• 14:26
  SPEAKER_00: The highest mountains in there are quartzite because quartzite's super resistant and really resistant up there.
• 14:32
  SPEAKER_00: So that only the strong survive matters in a sediment thing, and it also matters in a sediment rock.
• 14:37
  SPEAKER_00: We can apply that to sedimentary rocks as well.
• 14:40
  SPEAKER_00: For sure 100%.
• 14:41
  SPEAKER_00: The the environment of deposition here, real quick, the I I think we can kind of summarize this briefly, quickly, maybe.
• 14:49
  SPEAKER_00: The red, silty, mudstony stuff.
• 14:52
  SPEAKER_00: Well, maybe let's save that for later.
• 14:53
  SPEAKER_00: Should we start let's talk about the topography first, maybe?
• 14:56
  SPEAKER_00: Is that is that a better way to go and then talk about the rocks themselves?
• 14:59
  SPEAKER_00: I actually skip ahead to this.
• 15:00
  SPEAKER_01: So then we'll talk about what this, yeah, what did it look like, you know, from a environmental standpoint.
• 15:06
  SPEAKER_01: You know, what did this look like?
• 15:07
  SPEAKER_01: But let's deal with the rocks first.
• 15:09
  SPEAKER_00: Um Chris, this is um LIDAR.
• 15:12
  SPEAKER_00: You know, we've gotten tuned in to LIDAR by like Phil Prince and others.
• 15:16
  SPEAKER_00: And this is uh this image here, we we've basically taken a lidar map which shows incredible, incredible resolution of the relief, the topography, in this, especially with this kind of gray hill shade thing where you can see the little knobs that are hundreds of feet high, but they stand out like a sore thumb on these maps, right?
• 15:36
  SPEAKER_00: These these lidar maps.
• 15:37
  SPEAKER_00: It's really amazing.
• 15:38
  SPEAKER_01: I think a lot of people know this, but let's let's just it's worth explaining again anyway.
• 15:42
  SPEAKER_01: LIDAR is a way of imaging the surface of the earth, but it cuts through all of the canopy.
• 15:51
  SPEAKER_01: You have trees, it cuts through grass, it cuts through everything and just shows the rock below all that.
• 15:57
  SPEAKER_01: And so we get to see relief uncovered, essentially.
• 16:02
  SPEAKER_01: And it's so it's super, super valuable to geomorphologists and and people like this.
• 16:07
  SPEAKER_01: Uh, you know, lightars.
• 16:09
  SPEAKER_00: Basically, you can see uh we've drawn on this map, we've drawn the the battlefield arrangement.
• 16:15
  SPEAKER_00: Basically, we've drawn seminary ridge, we've drawn the union fish hook here.
• 16:20
  SPEAKER_00: And this is the the geological map that the previous image was a cross section of, and we can see that the highest relief areas here are on the diabase, and we've we've talked about that.
• 16:32
  SPEAKER_00: We saw that in the field on the field trip, but the diabase and it has this kind of more irregular weathering pattern.
• 16:38
  SPEAKER_00: There are high knobs, not there's knobbly parts to them, and then there's low parts to them.
• 16:43
  SPEAKER_00: But that's the diabase is characteristic of that.
• 16:46
  SPEAKER_01: In the sediments, we much more have these like ribs, long ribs that are running kind of northeast, southwest, and what would that, if you see these ribs on a lidar image within sedimentary rock?
• 16:58
  SPEAKER_01: So basically, these ribs are within a basin, an overall basin.
• 17:02
  SPEAKER_01: What would that mean?
• 17:04
  SPEAKER_01: More resistant to erosion.
• 17:06
  SPEAKER_00: More resistant layers, exactly.
• 17:07
  SPEAKER_00: It's paralleling that just slight changes.
• 17:10
  SPEAKER_00: And we saw this in the field, Chris, like you know, tens of centimeters thick sandy layers, a couple of those stacked together, a couple meters of sandy stuff can make a ridge here because the weathering has gone on so long.
• 17:21
  SPEAKER_00: And then we see these, well, we see faults kind of going through here too, but we also see these ridges that are cross-cutting everything.
• 17:26
  SPEAKER_00: They're kind of cross-cutting that background fabric in the rock that we can see.
• 17:30
  SPEAKER_00: And those are the dikes.
• 17:31
  SPEAKER_00: Those are the Rossville dye-base dikes that are cross-cutting everything.
• 17:34
  SPEAKER_00: And it's very easy to see on this map.
• 17:37
  SPEAKER_00: What I really appreciate is that the Rossville dike is cross-cutting the York Haven dye-base sill.
• 17:42
  SPEAKER_00: So we'd get age information, relative age information, even just from that.
• 17:46
  SPEAKER_00: So we we've layered this on, and this is going to be a really important one, a really important image to kind of keep in your head that the Union Fish Hook exists on all dye-based sill.
• 17:55
  SPEAKER_00: We'll talk about that next episode.
• 17:57
  SPEAKER_00: And then McPherson's Ridge is kind of one of these ribs within the sediments.
• 18:01
  SPEAKER_00: It's not a cross-cutting dike, but it's one of these more resistant ribs, and that's what we kind of see.
• 18:06
  SPEAKER_00: So we're seeing variations in shaliness to sandiness.
• 18:10
  SPEAKER_00: The composition of the sediment is influencing how it's weathering.
• 18:13
  SPEAKER_01: Okay.
• 18:13
  SPEAKER_01: Well, Jesse, if you go to that next image a second, this is actually something that we pulled from our own field excursion.
• 18:21
  SPEAKER_00: Yeah.
• 18:22
  SPEAKER_01: And uh, this is this is a great image showing this kind of soft, weak, really red, but it's a mudstone.
• 18:31
  SPEAKER_01: And you can see I think this image does a really good job of showing the facility of the mudstones, the very fine layering within it.
• 18:39
  SPEAKER_01: Hold on, say that again, Chris.
• 18:41
  SPEAKER_00: What is your word what word did you just say?
• 18:43
  SPEAKER_00: Facility, right?
• 18:44
  SPEAKER_00: What is that facility?
• 18:45
  SPEAKER_00: You know, facility fancy words.
• 18:48
  SPEAKER_01: Yeah.
• 18:49
  SPEAKER_01: I think a facility, like um, if you got a book laying on a counter, like a thick book, facility are the individual pages within the book.
• 18:57
  SPEAKER_01: This kind of like fine micro layering within a host of rocks, and that's what this shows.
• 19:02
  SPEAKER_01: Ficile, right?
• 19:03
  SPEAKER_01: It's very ficile.
• 19:05
  SPEAKER_00: You can break it apart easily, right?
• 19:07
  SPEAKER_00: And this is um this is the background rock to most of the almost all the valleys in this region, the big lowland valleys, you know, the area between Seminary Ridge, I mean Pickett's Charge went across this, right?
• 19:22
  SPEAKER_00: Went across this lowland area, was was underlain largely by this stuff.
• 19:28
  SPEAKER_00: This is the background sediment in a large part of the Gettysburg Basin.
• 19:32
  SPEAKER_00: This is the stuff that was contact baked.
• 19:35
  SPEAKER_00: Our first stop, we kind of saw this Hornfells alteration.
• 19:38
  SPEAKER_00: This is the stuff that was baked by the sill as it came in.
• 19:41
  SPEAKER_00: So this is kind of background rock, background thing.
• 19:44
  SPEAKER_00: And the geological environment here is it's red silty mudstone, so it's like playa, alluvial fan.
• 19:51
  SPEAKER_00: It's in this kind of playa environment.
• 19:54
  SPEAKER_00: Wet, dry climate cycles are imposed on this, but it's generally kind of everything we described last episode, where we talked about what this might have looked like, this playa area far away from the alluvial fan, so it's not capturing any conglomerate, and a lot of the courts has weathered, has has been captured by the alluvial fan.
• 20:13
  SPEAKER_00: And so I'm kind of imagining, Chris, we're standing in like Death Valley, and we're in the this sort of the one of the playa areas.
• 20:22
  SPEAKER_00: It's wet to our left, and to our right, there's a big alluvial fan system.
• 20:27
  SPEAKER_00: And what we're what we're seeing is.
• 20:28
  SPEAKER_00: Yeah, you can see the mountains in the distance, right?
• 20:30
  SPEAKER_00: You can see the mountains on the mountains on other mountains on one side, alluvial fan is near us, but the alluvial, we're far enough from the alluvial fan that the alluvial fan is capturing all the conglomerate in the sand, and we're really only getting like mud runoff from that.
• 20:44
  SPEAKER_00: So we're getting the mud from that system, but we're not right next to the mountains.
• 20:48
  SPEAKER_00: There's an alluvial fan, we're kind of laterally far away from the alluvial fan, but we're not in the lake either.
• 20:55
  SPEAKER_01: However, we're talking about a sequence of rocks here that can be upwards, it's not continuous, but it can be 18,000 feet thick.
• 21:02
  SPEAKER_01: In other words, you know, five kilometers or an excess of that.
• 21:05
  SPEAKER_01: Then you add on the new Oxford formation below this, the older stuff, that's another 6,800 feet.
• 21:11
  SPEAKER_01: So you're talking about well over 20,000 feet of vertically accumulated rocks within the Gettysburg.
• 21:17
  SPEAKER_00: Most of it's in the Gettysburg Formation, not all of it, but there's a lot of sediment here, right?
• 21:22
  SPEAKER_00: This is a long time period.
• 21:24
  SPEAKER_00: There's a lot of basin accommodation space to fill up this thing.
• 21:27
  SPEAKER_00: So so this is great.
• 21:29
  SPEAKER_00: And it's red.
• 21:31
  SPEAKER_00: This stuff's red.
• 21:32
  SPEAKER_00: Well, first of all, we have the we there are these climate cycles captured in it.
• 21:36
  SPEAKER_00: They're many meters, tens of meters, even thick, and they're called Van Houten cycles.
• 21:41
  SPEAKER_00: They're basically climate variations.
• 21:43
  SPEAKER_00: And if you let's imagine do that visual again where we kind of have an alluvial fan off to our right, and we have the lake, the playa lake off to our left, and we're kind of looking off to the mountains in the distance that are shedding their sediment towards us to where we're standing.
• 21:56
  SPEAKER_00: You have to imagine if the climate changes, we get a bit wet.
• 22:00
  SPEAKER_00: Wetter climate or get a bit drier climate, that lake off to our left will get on average higher or lower.
• 22:05
  SPEAKER_00: Yeah, lake high, lake, lake low.
• 22:07
  SPEAKER_00: Yep.
• 22:08
  SPEAKER_00: Yeah, and there'll be more or less mud coming off of that alluvial fan to our right.
• 22:13
  SPEAKER_00: We're still probably not going to capture the conglomerate.
• 22:15
  SPEAKER_00: Maybe big storm events give us some sand in our deposit, but it's not a lot.
• 22:20
  SPEAKER_00: There's small changes that are happening.
• 22:22
  SPEAKER_00: Sometimes we might be covered by the lake, sometimes we might get a bunch of sand from that alluvial fan, but we got this background mud still getting deposited there.
• 22:29
  SPEAKER_00: So that's the kind of climate variations.
• 22:31
  SPEAKER_00: That's the visual I want you to have when you're thinking about climate variations.
• 22:34
  SPEAKER_00: Wet, dry, wet, dry.
• 22:35
  SPEAKER_00: That's right.
• 22:35
  SPEAKER_00: That's what I was just going to say.
• 22:36
  SPEAKER_00: This is tens of meters.
• 22:38
  SPEAKER_01: Yeah, we're talking um these Van Houten cycles are recording sediments that are on the order of 15 to 25 meters thick.
• 22:46
  SPEAKER_01: So pretty extensive amount of time, but these were basically causing transgression-regression sequences that were driven by the climate variations.
• 22:56
  SPEAKER_01: So really what we're talking about are, you know, we would go from a Lacustarine lake phase, we'd go to a shoreline phase, we'd go to this kind of playa, dried up, mud flat kind of phase.
• 23:08
  SPEAKER_01: And you know, Jesse, we saw this.
• 23:10
  SPEAKER_00: And then back, right?
• 23:11
  SPEAKER_00: And then back again.
• 23:12
  SPEAKER_00: Back and forth and back and forth, right?
• 23:13
  SPEAKER_00: As the climate varies.
• 23:14
  SPEAKER_00: Yeah, yeah, yeah, yeah.
• 23:15
  SPEAKER_00: Yeah, it's totally cool to just I I love that visual of like visualizing that over tens of thousands of millions of years, right?
• 23:22
  SPEAKER_00: That cycle happening over and over and over.
• 23:24
  SPEAKER_00: Yeah.
• 23:25
  SPEAKER_01: I agree a hundred percent.
• 23:26
  SPEAKER_01: No, I just I think I'm gonna jump ahead a little bit and maybe you don't like this, I don't know, but you know, we saw this kind of shifting shoreline within this basin when we looked at the Heidersburg member on Plum Run.
• 23:39
  SPEAKER_01: We saw mud cracks, we saw ripples, we saw dinosaur footprints.
• 23:45
  SPEAKER_01: Um, and all of those are indicative of these kind of shifting shorelines, you know.
• 23:50
  SPEAKER_01: And so that was a really, really cool piece of the story.
• 23:53
  SPEAKER_01: Let's go back a second, Jesse, and talk about why is the Gettysburg Formation so red?
• 24:01
  SPEAKER_01: Because it is like it's it's beautifully colored red mudstones sprinkled in with some silty sandstones and so on.
• 24:10
  SPEAKER_01: Uh, you know, why so red?
• 24:12
  SPEAKER_01: Oh man.
• 24:13
  SPEAKER_01: Like iron.
• 24:14
  SPEAKER_01: I don't know if I'm oxidation state.
• 24:18
  SPEAKER_00: Yeah, that's right.
• 24:18
  SPEAKER_00: Um I very simply, it's rusted, right?
• 24:20
  SPEAKER_00: It's rusted.
• 24:21
  SPEAKER_00: And what do you need for things to rust?
• 24:23
  SPEAKER_00: Well, you need to be exposed to oxygen.
• 24:26
  SPEAKER_00: Typically, it happens when you're exposed to water and oxygen in the atmosphere or Earth's atmosphere.
• 24:30
  SPEAKER_00: So we need to have an oxidation state of iron that has more electrons removed from it.
• 24:36
  SPEAKER_00: We need iron in the three plus charge state.
• 24:39
  SPEAKER_00: So it's had three electrons removed as opposed to the two plus iron, which has only had two electrons removed.
• 24:45
  SPEAKER_00: So that exposure to oxygen, oxygen loves electrons, grabs electrons away from stuff, so more oxygen around in a very general sense can take more electrons from iron.
• 24:55
  SPEAKER_00: And this is really indicative of subarial, exposed to the air, exposed to the atmosphere.
• 25:02
  SPEAKER_00: When you get iron, background iron is kind of deposited in sediments very frequently, especially these fine-grained things.
• 25:09
  SPEAKER_00: When you get that iron deposited underneath of water, it's typically either grayish, which we'll talk about, or even green.
• 25:17
  SPEAKER_00: Green layers, these kind of red bed, green bed type of deposits, are iron changing its oxidation state, changing the minerals that are forming.
• 25:26
  SPEAKER_00: And here we're dealing with a lot of hematite, a lot of rusted version of iron because it's exposed to the atmosphere.
• 25:33
  SPEAKER_00: And so these red beds are indicative of like exposed on the surface, on the surface of the land is kind of a key that's yeah, diagnostic.
• 25:42
  SPEAKER_01: There's something interesting too here, Jesse, though, is that they also don't think that all of the red came during depositional, that it happened later on.
• 25:52
  SPEAKER_01: And that's because some of these beds within the formation have these like clam shrimp fossils, and these these specific organisms um they require standing water in order to live, which in other words, they were deposited as this kind of grayish green mudstone, and then circulating water.
• 26:11
  SPEAKER_01: So diagenesis happened, it got buried, but then you oxygenation happened later on, whether that was meteoric water, like rain or snow melt circulating down, becoming groundwater, flowing through and among the pore spaces, oxidizing it later on, turning it red after deposition.
• 26:27
  SPEAKER_01: I just think that's a it's I love that.
• 26:30
  SPEAKER_00: It's very cool.
• 26:31
  SPEAKER_00: I I we could we've spent episodes at this point on you know iron oxidation state and what it changes to color the rock.
• 26:37
  SPEAKER_00: Because Chris, in the first stop, or the second stop of the field trip, when we saw the scarn alteration deposits, you know, that was the sill is pushing some water, magmatic water, into those rocks, and that's pretty reducing.
• 26:50
  SPEAKER_00: So that changed from red to like this grayish-whitish rock, right?
• 26:54
  SPEAKER_00: The the scarns are not often as red.
• 26:56
  SPEAKER_00: They're maybe a little bit red, but they're much more like a different color, this grayish thing, which is a reducing form of the iron.
• 27:03
  SPEAKER_00: And so you could do the, you're exactly right, you could do these things diagenetically or at the actually at the depositional surface, but fluids flowing through these loose, unconsolidated, unlithified sediments after they've been deposited, but before they become a rock can change these things a lot.
• 27:18
  SPEAKER_00: So it gets really complicated figuring out exactly when this happened.
• 27:22
  SPEAKER_00: Chris, I've seen it in Western uh, well, in in central PA, where you got these big red beds, and then there's these thin green layers, green and gray layers in them that follow the layering.
• 27:33
  SPEAKER_00: And there's a kind of a debate about what that origin of that is, but some people would say that these are fluids associated with oil and gas reservoirs.
• 27:42
  SPEAKER_00: So that oil and gas, because there's so much organic material in there, that's a really reducing environment.
• 27:47
  SPEAKER_00: So those reducing fluids flowed through and turned the rock green, right?
• 27:51
  SPEAKER_00: Uh, like way later after deposition, even after it was formed into rock.
• 27:54
  SPEAKER_00: So you can't just say, oh, it's red, so therefore it was deposited subarially.
• 27:59
  SPEAKER_00: But on average, it's a good indication, right?
• 28:01
  SPEAKER_00: Along with a bunch of other stuff that we'll talk about here in a minute.
• 28:04
  SPEAKER_01: Well, I think that's a great transition, Jesse, what you just talked about into the Heidlersburg member of the Gettysburg Formation.
• 28:11
  SPEAKER_00: Because which just the Heidersburg set the stage.
• 28:14
  SPEAKER_00: The Heidlersburg is this, as you said, you said it really nicely, it's a discontinuous layer within the Gettysburg.
• 28:19
  SPEAKER_00: So you go Gettysburg Formation, Heidlersburg member, then Gettysburg Formation again.
• 28:22
  SPEAKER_00: And it's discontinuous, but it's pretty thick.
• 28:24
  SPEAKER_00: It's in there, kind of in the middle of this.
• 28:26
  SPEAKER_00: And in the Gettysburg battlefield, it's exposed on the northwest side of the town of Gettysburg.
• 28:33
  SPEAKER_00: McPherson's Ridge is right near the boundary between the Gettysburg Formation and the Heidlersburg member of the Gettysburg Formation.
• 28:41
  SPEAKER_00: And McPherson's Ridge, as we saw in the field trip, has these kind of sandier, blockier, chunkier packets in it, which makes the ridge.
• 28:49
  SPEAKER_00: So anyway, sorry to interrupt there, but I just want to set this up.
• 28:51
  SPEAKER_00: No, that's fine.
• 28:52
  SPEAKER_01: But what made me think of interjecting the Heidlersburg at this time is because the Heidlersburg is like shockingly not red.
• 29:01
  SPEAKER_01: You know, you you have it like it's right in the middle of that really red stuff.
• 29:06
  SPEAKER_01: So this is more of like a grayish green to, you know, almost like a a really, really darker color, but not oxidized.
• 29:13
  SPEAKER_01: And and this is because the Heidlersberg member was formed.
• 29:18
  SPEAKER_01: Well, there are a couple things going on.
• 29:19
  SPEAKER_01: This was a sediment-starved basin.
• 29:21
  SPEAKER_01: I'm sorry, this was a sediment-starved time within the basin.
• 29:26
  SPEAKER_01: So we were far away from the from the fluvial deposits, the alluvial fans, we were far away from this stuff.
• 29:32
  SPEAKER_01: And we only got then this kind of fine-grained sediment that was reaching this area.
• 29:36
  SPEAKER_01: But the color is also indicative of a reducing environment, which means maybe a little bit deeper water.
• 29:43
  SPEAKER_00: Yeah, that's right.
• 29:44
  SPEAKER_00: A lake deposit.
• 29:45
  SPEAKER_00: That's exactly right.
• 29:46
  SPEAKER_00: And and then in some of these reconstructions, especially the the reconstructions, this sort of compilation that Roger Fail did on the Birdsboro basin, you know, interprets the Heidlersburg member and actually says, Oh, this was a lake.
• 29:58
  SPEAKER_00: This was Heidlersburg Lake, actually formed here.
• 30:01
  SPEAKER_00: And so we're we're getting this big kind of lake deposit in the middle here.
• 30:05
  SPEAKER_00: Because it's gray, as you said, it's reduced, it's darn no water.
• 30:08
  SPEAKER_00: There's some ripples, there's beautiful ripples.
• 30:10
  SPEAKER_00: I mean, this image, image number five here, beautiful ripples in this thing.
• 30:13
  SPEAKER_00: Um, again, we saw this on a it was rain was coming down, but that almost accentuated the ripples, the dinosaur footprints that we saw in this Heidlersburg member, the stuff that was that came from a quarry just to the northwest of Gettysburg property, but used to make that bridge, the Plum Creek uh run bridge.
• 30:31
  SPEAKER_00: Beautiful ripple marks, beautiful.
• 30:33
  SPEAKER_00: I mean, amazing dinosaur footprints from this stuff.
• 30:35
  SPEAKER_00: It's totally cool.
• 30:37
  SPEAKER_01: That's right.
• 30:37
  SPEAKER_01: And again, that just indicates the shifting environments, these this kind of transgressive regression sequences driven by the Van Houten cycles, wet, dry, and so on.
• 30:47
  SPEAKER_01: Um, because you got all of that.
• 30:49
  SPEAKER_01: We got mud cracks, which is a wet, dry, wet, dry kind of setting.
• 30:54
  SPEAKER_01: And then you have ripple marks, which is shallow kind of.
• 30:57
  SPEAKER_01: I think the the ones we're looking at here, Jesse, these are these asymmetrical ripples.
• 31:01
  SPEAKER_01: So this was a current of water.
• 31:03
  SPEAKER_00: Yeah, yeah, yeah, yeah, yeah.
• 31:04
  SPEAKER_01: This was a moving current of water, but shallow.
• 31:06
  SPEAKER_01: And then you had the footprints that were preserved along what we believe to be a shoreline kind of environment.
• 31:12
  SPEAKER_01: This a shoreline of this playa setting, right?
• 31:16
  SPEAKER_01: And and um it just really, really cool.
• 31:19
  SPEAKER_01: I'm I'm so glad that I convinced you to to make this stop and we found it.
• 31:22
  SPEAKER_01: And you know, I had to I had to work that up a little bit, but it was good though.
• 31:26
  SPEAKER_00: I mean, just it was it was good, super cool.
• 31:29
  SPEAKER_00: I mean, uh, and so this is putting together, you know, this story, right?
• 31:33
  SPEAKER_00: You you're imagining again, imagine we still have some of these these um alluvial fans off to very you know, far off in the distance now, the hydrophilic lake, the lake has extended deeper water setting, but we got dinosaurs roaming around and stomping around in the mud, and we still have these climate cycles.
• 31:48
  SPEAKER_00: So the the the lake on average is deeper, much deeper, but it's still going up and down, up and down.
• 31:54
  SPEAKER_00: We're in this near shore environment here in many of these areas.
• 31:57
  SPEAKER_00: So it's a spectacular, uh I don't know, man.
• 32:01
  SPEAKER_00: There's so many cool parts to this, right?
• 32:02
  SPEAKER_00: It's a cool visual that you can paint by looking at the rocks themselves and figure out what was the definitional environment here.
• 32:09
  SPEAKER_00: And then it's also really amazing and impressive to see how small variations we're going from a red silty mudstone to a gray shale or argilite.
• 32:20
  SPEAKER_00: Those on the scale of rock variations, that's not a huge change, but it is a huge change, and it has a huge impact on this topography in the Gettysburg area and had a pretty big influence on the battle, how the battle played out in the end.
• 32:34
  SPEAKER_01: An argilite is a weakly metamorphosed mudstone, and that metamorphism is gonna make the rock a little bit stronger without changing sedimentary structures or things like this.
• 32:46
  SPEAKER_01: It's very it's you know, it's a low-grade metamorphic event, but it makes the rock stronger.
• 32:54
  SPEAKER_01: You know, that's what forms the edges that you're talking about.
• 32:56
  SPEAKER_00: Like a I mean, it it's a it's a mudstone, it's a squishy ooze.
• 33:01
  SPEAKER_00: So you know when you're like walking and uh all of a sudden you step into like a mud ooze and it squeezes through your toes or something like that.
• 33:08
  SPEAKER_00: That think of that turned into a rock.
• 33:10
  SPEAKER_00: That's really what this is.
• 33:12
  SPEAKER_00: And they you're right, they can be metamorphosed, not always, but they end up being kind of slatey type things, but it's not all organic matter like a black shell would be.
• 33:22
  SPEAKER_00: Yeah, it's think of the mud.
• 33:24
  SPEAKER_00: I mean, it it's muds and oozes that if you step in it and you think, ooh, that squished through my toes, it's probably an argilite when you turn it into a rock.
• 33:31
  SPEAKER_00: It's often gray.
• 33:33
  SPEAKER_00: And so it's great for capturing footprints, right?
• 33:35
  SPEAKER_00: I mean, that's you stick it's like you you stick in it, and then it goes slop, and then you pick your foot up, and your footprint is like imprinted in there really well.
• 33:43
  SPEAKER_00: And if that thing dries, then that dinosaur footprint would would be preserved for a long time, right?
• 33:47
  SPEAKER_00: So it's easy to see how this is a good preservational environment for footprints and ripples and mud cracks and all those totally cool features that you and I love about sediments.
• 33:58
  SPEAKER_00: Well, Jesse, is that a wrap?
• 34:00
  SPEAKER_00: Have we done the the Gettysburg Basin justice?
• 34:03
  SPEAKER_01: I think so.
• 34:04
  SPEAKER_01: Just makes sense to me.
• 34:06
  SPEAKER_00: A couple of dozen million years of sedimentation in this playa environment.
• 34:11
  SPEAKER_00: Very cool, very, very cool stuff.
• 34:12
  SPEAKER_00: Hey, Chris, next episode we are gonna move into well, we're gonna take a slight deviation, right, and talk about fanglomerate, the uh the latest sediments in this rock.
• 34:21
  SPEAKER_00: When some of the more beautiful ones, I don't know if they're a more spectacular story, but beautiful rocks, and then we're gonna come back and talk about the intrusions in the end triassic extinction event there.
• 34:32
  SPEAKER_00: So those chapters are those episodes are currently available to you on your Camp Geo Mobile app.
• 34:37
  SPEAKER_00: First link in your show notes there.
• 34:39
  SPEAKER_00: The Gettysburg Audio Visual Podcast series is there.
• 34:42
  SPEAKER_00: Download that, let us know what you think.
• 34:44
  SPEAKER_00: We're constantly updating it, constantly trying to make it better, adding new content all the time.
• 34:48
  SPEAKER_00: So download that, let us know what you think.
• 34:50
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• 34:55
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• 34:56
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• 34:57
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• 34:58
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• 35:00
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• 35:06
  SPEAKER_00: Cheers.