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Capitol Curiosities - Fanglomerate and the Gettysburg Basin · PlanetGeo: The Geology Podcast

0:00
SPEAKER_02: You know, visiting that quarry is like uh emotional experience in some way.
0:04
SPEAKER_02: It's the history of of something like that.
0:06
SPEAKER_02: In the quarry going back many hundreds of years to make the Capitol building.
0:11
SPEAKER_02: It's it's a cool story.
0:12
SPEAKER_02: Very, very cool, powerful story.
0:16
SPEAKER_02: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it worked, and why it matters to you.
0:28
SPEAKER_01: Hold on a minute.
0:29
SPEAKER_01: Let me get situated here, Jesse.
0:31
SPEAKER_02: Get your NASA get your NASA screen sorted, man.
0:34
SPEAKER_01: I g I know, I gotta figure this out.
0:36
SPEAKER_01: I I'm switching it up a little bit.
0:38
SPEAKER_02: Oh no.
0:39
SPEAKER_02: Um it never works well when you switch it up.
0:41
SPEAKER_01: It does.
0:42
SPEAKER_01: I switched it up last the last two times we recorded.
0:44
SPEAKER_01: Oh, really?
0:45
SPEAKER_02: You've been just trying out all sorts of iterations over there?
0:48
SPEAKER_00: Mm-hmm.
0:48
SPEAKER_02: Oh.
0:49
SPEAKER_01: Yeah.
0:50
SPEAKER_01: So now I have you over here.
0:51
SPEAKER_01: So when I want to look at you, I'm gonna look this way.
0:54
SPEAKER_02: Okay.
0:54
SPEAKER_02: And why what are you looking at when you look away from me?
0:58
SPEAKER_01: I have the script.
0:59
SPEAKER_02: Oh, okay.
1:00
SPEAKER_02: In front of images and stuff.
1:02
SPEAKER_02: Okay, perfect.
1:03
SPEAKER_02: Perfect.
1:03
SPEAKER_02: Well, you gotta get it, uh you gotta get it sorted out because this is a good rock.
1:06
SPEAKER_02: This is a fun rock.
1:08
SPEAKER_02: This is near and dear to my heart rock that we're talking about.
1:11
SPEAKER_01: Yeah, it is.
1:11
SPEAKER_01: I'm surprised though.
1:12
SPEAKER_01: I'm a little I'm a little irritated because you didn't you you never really you never told me this.
1:18
SPEAKER_01: Before our field trip to Gettysburg?
1:20
SPEAKER_01: No, you never told me that this was a rock that was near and dear to your heart, and that hurts me a little bit.
1:27
SPEAKER_02: I I need to update you when I have a new rock that's near and dear to your heart.
1:30
SPEAKER_01: Yeah, absolutely.
1:31
SPEAKER_01: Like Well, I took you to one nearby.
1:36
SPEAKER_02: You know, you are you are my rock friend, Chris.
1:39
SPEAKER_02: Uh it's true.
1:41
SPEAKER_02: I should I should be sending you more photos of cool rocks that I well I just want to know.
1:45
SPEAKER_01: I want a heads up when when you have a new rock that's on your radar that you're like, I really this is my new my new favorite rock.
1:53
SPEAKER_01: I something I should know.
1:55
SPEAKER_02: Yeah, that's true.
1:56
SPEAKER_02: All right, I'll send you photos as I'm um because I my workshop is is up and running now again.
2:01
SPEAKER_02: So uh I got the rock saws plugged in and got power to it.
2:04
SPEAKER_02: So I'll send you photos as they come off the saw.
2:06
SPEAKER_01: I liken it to like let's say you come home after work and Tess asks you how your day was.
2:12
SPEAKER_01: Is there anything new?
2:13
SPEAKER_01: And you're like, nah, it was my day was okay, everything's new.
2:16
SPEAKER_01: And then three days later, she finds out that you got written up for doing something stupid and you forgot you didn't tell her that.
2:22
SPEAKER_01: That's kind of how I feel with this.
2:23
SPEAKER_02: Okay.
2:24
SPEAKER_02: Wow, all right, yeah, that's fair enough, man.
2:26
SPEAKER_02: I don't want to offend my work wife, Chris Belhois, over there.
2:30
SPEAKER_02: That's right.
2:31
SPEAKER_02: My rock friend.
2:32
SPEAKER_02: That is right.
2:33
SPEAKER_02: Um, yes.
2:33
SPEAKER_02: So the the story for this rock, this is a okay, we're gonna talk about fanglomerate.
2:38
SPEAKER_02: Fanglomerate, okay.
2:40
SPEAKER_02: Chris, what does that word mean to you?
2:43
SPEAKER_02: And have you to two parter here?
2:45
SPEAKER_02: What does that word mean to you?
2:46
SPEAKER_02: And have you heard it in other contexts outside of these Mesozoic basins, the New England, you know, this Northeast um geology?
2:55
SPEAKER_01: All right.
2:55
SPEAKER_01: Well, that's easy for me to answer.
2:57
SPEAKER_01: Fanglomerate means that Chris Bullheis is a fan of conglomerate.
3:02
SPEAKER_01: Um that's that's true.
3:04
SPEAKER_01: That's actually I mean, it is true, but what it really means is if it's a conglomerate that is formed in a specific setting, geologic setting, an alluvial fan conglomerate is what it means.
3:16
SPEAKER_01: So we take those words, mash them together, and you come up with a fanglomerate.
3:20
SPEAKER_01: Was this on my radar?
3:21
SPEAKER_02: Yeah, was it right on your radar?
3:23
SPEAKER_02: Because I I this is a not a rock word that most people know, but when you say it, people's ears perk up because it's kind of a fun word.
3:30
SPEAKER_02: People say you say fanglomerate, we're gonna look at fanglomerate, like, oh, what's that?
3:33
SPEAKER_02: I want to see that one.
3:34
SPEAKER_02: I want to go like take me to some fanglomerate, you know.
3:37
SPEAKER_01: And by the way, it is worthy of you making the effort to go find some fanglomerate.
3:42
SPEAKER_01: Totally.
3:42
SPEAKER_01: Um, it's an amazingly beautiful rock.
3:44
SPEAKER_01: Um, no, this was actually not on my radar at all.
3:47
SPEAKER_01: This seems to be a locally uh maybe almost a colloquial kind of uh um turn of phrase.
3:54
SPEAKER_01: Is that accurate or not?
3:55
SPEAKER_02: It's certainly not common, that's for sure.
3:57
SPEAKER_02: I've heard it, I've seen fanglomerate in one other instance, it's like up in the Yukon Mountains, where there's again these kind of debris flow deposits.
4:04
SPEAKER_02: So, what these are are mud flow or debris, really debris flow deposits that are formed in these alluvial fan settings.
4:12
SPEAKER_02: And the fanglomerate rock, it's not a major part of the Gettysburg battle story, it's not exposed in the battlefield, on the battlefield.
4:22
SPEAKER_02: It's exposed in the region and it has some really cool history.
4:26
SPEAKER_02: This particular fanglomerate.
4:28
SPEAKER_02: And Chris, I came upon this fanglomerate at first when I was living in Washington, DC.
4:32
SPEAKER_02: Living in the city is great, but I also needed to get outside a little bit.
4:36
SPEAKER_02: So at one point I was like, I'm gonna go on a rock collecting like weekend, you know, uh, somewhere around.
4:41
SPEAKER_02: So I was just looking at cool rocks in the region.
4:43
SPEAKER_02: Where could I go collect some of these?
4:44
SPEAKER_02: And I stumbled across these rocks that were called calico marble was the the name, or Potomac Marble, because it's it's exposed.
4:53
SPEAKER_02: The one we saw, one of the ones we saw was exposed along the Potomac River, and they're really famous, not super famous, but like regionally famous.
5:01
SPEAKER_02: And these are these are pretty popular rocks.
5:03
SPEAKER_02: Uh, they have a great history behind them.
5:05
SPEAKER_02: But that's how I kind of stumbled upon it.
5:07
SPEAKER_02: And so you go hiking and you can find some of the places, and really the interesting part here, Chris, there's a cool geological story to them, but also they were used in rebuilding the Capitol, and they have this potomac marble, this fanglomerate, was used to build some of the grand columns that are in uh the U.S.
5:25
SPEAKER_02: Capitol building that was rebuilt after the War of 1812.
5:28
SPEAKER_02: Um that's right.
5:29
SPEAKER_01: And I just want to say, I just want to say to for everyone that you're dumping out some words that seem very contradictory.
5:36
SPEAKER_01: It's a fanglomerate, you just dropped out that it's the it's called the you know the Potomac Marble.
5:41
SPEAKER_01: We're gonna get into all of that and why it's it's called that and and so on.
5:45
SPEAKER_01: Um so image number one, I'm just real quick.
5:49
SPEAKER_02: There's a there's a great book about this rock, and a really good book about this rock.
5:54
SPEAKER_02: It's called the Potomac Marble, that's the title of the name, and it it gives it's kind of the lost history of the Potomac Marble.
6:00
SPEAKER_02: The author is trying to find the uh quarries basically that were used to quarry the stone.
6:05
SPEAKER_02: So it gives a really nice history of this rock, and that's um uh a really cool resource there.
6:09
SPEAKER_02: So I just want to point people to that.
6:11
SPEAKER_02: Anyway, sorry, interrupted you.
6:12
SPEAKER_01: Yeah, no, that's fine.
6:13
SPEAKER_01: Uh so image number one here is just a a Wikimedia image of an alluvial fan actually in Iran.
6:20
SPEAKER_01: Yeah, we're looking at this as an aerial view looking down, yep.
6:23
SPEAKER_01: And you have these fast-moving streams coming out of this mountainous region, and then it hits a flat, broad plateau or just this valley below, and the stream loses energy very abruptly, and then deposition happens.
6:36
SPEAKER_01: And so, what you have then is most rivers are tributary networks where small rivers lead to bigger rivers and and so on.
6:45
SPEAKER_01: This is actually a distributary network.
6:48
SPEAKER_01: So, what happens is the the water loses energy when it comes to the flat land, deposition happens, and it happens rather fast, and it happens within its own channel.
6:56
SPEAKER_01: And so the channel fills up, then the river's diverted into a different cuts a new channel, which fills up and it cuts a new channel, and so you it just kind of like splays outward from this one central point, and so it looks kind of like these oriental fans, and and that's a good analogy, yeah.
7:14
SPEAKER_01: Yeah, but the tip of the fan points upstream, the tip of the fan points up into the mountainous regions, right?
7:21
SPEAKER_02: Um, and and so it fans out into the valley in this really symmetrical pattern, as you're describing, because it just is a space-filling exercise.
7:28
SPEAKER_02: It'll pour some to the left until it fills up too much there, then it pours some to the right, then it pours some in the middle, and it just dumps these things out.
7:35
SPEAKER_02: And so, you know, we've talked about alluvial fans before on the podcast.
7:40
SPEAKER_02: They can be really ripe for flooding events because you get a bunch of water upstream in the mountains or snow melt, water that also melts snow, even you can get catastrophic flooding that dumps out into these alluvial fans.
7:51
SPEAKER_02: And there are often places where there is water coming out in an otherwise arid environment.
7:55
SPEAKER_01: So people have built structures around these things and irrigation, it's not exceedingly smart to yeah, exactly.
8:04
SPEAKER_02: But you know, that's where you can grow crops, as you can see here.
8:06
SPEAKER_02: You know, this is like a uh cropland.
8:09
SPEAKER_02: Um, so these things, these rocks that are dumping out that form this fanglomerate.
8:14
SPEAKER_02: And this fanglomerate, Chris, is exposed all over these basins, these Mesozoic rift basins on the west side, though, so near the the border faults.
8:26
SPEAKER_02: So the west side of the basins and we've kind of different.
8:30
SPEAKER_02: It's different, yes, exactly.
8:32
SPEAKER_02: And it's not flat, it's not a a sedimentary layer that's homogenous thickness across the basin.
8:39
SPEAKER_02: Most of them are on the west side.
8:41
SPEAKER_02: There's small little deposits on the on the right side, but the west side, the left side of these basins are where the bigger deposits are.
8:48
SPEAKER_02: And that's where they're thicker, yes.
8:50
SPEAKER_02: Why is that interesting, I guess?
8:53
SPEAKER_02: Well, why is it at odds with what we talked about before, maybe.
8:57
SPEAKER_01: So, first of all, this is the youngest rock that we've been talking about so far to this point.
9:04
SPEAKER_01: This is the the uppermost sedimentary rock.
9:06
SPEAKER_01: We just finished last episode talking about the Gettysburg Basin, and we talked about how the drainage it drained toward the northwest.
9:14
SPEAKER_01: Well, this actually drains to the southeast, it's on the western side of this basin.
9:20
SPEAKER_01: Let me move the image number two.
9:24
SPEAKER_02: It's draining the opposite direction.
9:26
SPEAKER_02: That's a really kind of key point, I think, and and it tells an interesting story.
9:31
SPEAKER_02: This image, we've got that same image that we saw before.
9:33
SPEAKER_02: We were talking about the basins generally.
9:35
SPEAKER_02: We have high relief to the southeast of where we are now in the Gettysburg Basin.
9:40
SPEAKER_02: We have low relief, and the water is draining and the sediment is draining to the northwest in this in this instance.
9:47
SPEAKER_02: And so you just described this opposite pattern.
9:50
SPEAKER_02: How do you get that?
9:51
SPEAKER_02: How do you what what's the image that we kind of paint in our heads for how you get a kind of a reversal maybe of sedimentation direction?
9:58
SPEAKER_01: Well, we have to have highlands off on the other side, further to the northwest, which is providing the relief, you know, and the higher energy for these streams to come out and then hit that basin, and then the water abruptly slows down.
10:15
SPEAKER_01: Um so we had some highlands off to further to the northwest, and we haven't really talked about that to this point because the drainage up until now was actually coming from the opposite direction.
10:27
SPEAKER_01: Yeah.
10:27
SPEAKER_02: So that's exactly right.
10:29
SPEAKER_01: These these alluvial fans are thickest at the base on the western margin, the northwestern margin of the basin, and then it thins as it goes toward the southeast.
10:41
SPEAKER_02: Right, right.
10:43
SPEAKER_02: We must have had some high relief.
10:45
SPEAKER_02: And those the the rock, we can look at this rock.
10:48
SPEAKER_02: This rock is a I'm gonna skip ahead, Chris, to this image just because it's a this is a zoom in, this is a cut face of fanglomerate.
10:56
SPEAKER_02: It's a beautiful rock.
10:57
SPEAKER_02: I mean, if you're not looking at the images, it's kind of a red matrix, but it's full of angular and kind of rounded to sub-rounded clasts.
11:06
SPEAKER_02: The clasts are dominated by carbonate rocks.
11:08
SPEAKER_02: So we think that these little, these highlands that kind of came up in the southwest that dumped sediment back towards the east that made these alluvial fan deposits, these fanglomerate deposits, we think they were carbonate dominated, that these mountains were full of carbonate because that's what's in the class that we see.
11:24
SPEAKER_02: I mean, the class are beautiful, they're highly varied, they're shot full of veining features, they've got all sorts of alteration in them, they're widely different colors, and then they're cemented together by this red calcite-rich or carbonate-rich matrix.
11:39
SPEAKER_02: So this is a really carbonate-rich rock.
11:43
SPEAKER_02: The whole thing is full of carbonate.
11:45
SPEAKER_02: There's not a lot of silica-rich stuff in here.
11:48
SPEAKER_02: No, there's quartz veins and things going through it for sure, but there's, you know, it's a lot of carbonate.
11:53
SPEAKER_01: It is a lot of carbonate.
11:54
SPEAKER_01: Some of the clasts are actually quartzite as well, but most of them are these marbleized uh clasts.
12:01
SPEAKER_01: Yeah, so Jesse, there's another interesting thing that that I want to bring up regarded as fanglomerate, is you said it right from the onset that these were debris flows.
12:12
SPEAKER_01: And that means that you have this really viscous, thick, non-homogeneous event coming out of the mountains, hitting the flatland below.
12:23
SPEAKER_02: Well, the visual I think of when I think of debris flow is, you know, people have probably seen news images of these, they're catastrophic in in many ways.
12:30
SPEAKER_02: But think of a a clogged up river system that's kind of slowly flowing down a mountain river channel.
12:39
SPEAKER_02: It's not actually rushing, it's not water dominated, it's mud dominated.
12:44
SPEAKER_02: So it's coming and there's no stopping it.
12:47
SPEAKER_02: You can't divert these things, they run over anything in their path.
12:50
SPEAKER_02: Like, knock I remember there's this image, I think it was from somewhere in the Rocky Mountains, of a mud flow, debris flow that just took out a little bridge, you know, took out a little pedestrian bridge, and you could see the image of it, and it's this big, it's kind of like a plugged-up river system that's just coming down carrying all sorts of crap in it.
13:08
SPEAKER_01: Well, and that's a good point, Jesse, because if you all right, let's say that you take this very um watery, sediment-laden, and then it comes to a rapid decline in velocity and deposition happens.
13:21
SPEAKER_01: That leads to what we call graded bedding in geoscience.
13:24
SPEAKER_01: And uh, like an analogy of this would be taking a five-gallon bucket and filling it up with a bunch of different class sizes, like fill it up with gravel, fill it up with sand, fill it up with some silt, throw some some uh clay-sized material in there, right?
13:39
SPEAKER_01: And then just dump all that mixture into a swimming pool.
13:43
SPEAKER_01: And what happens is uh the gravel plops to the bottom right away, then the sand settles on top, then the silt, and eventually days later, you'll even get the clay dust size pieces that will settle on top of that.
13:54
SPEAKER_01: So you get this like uh layered uh appearance where it goes coarse on the bottom to fine on top, and that's just called graded bedding.
14:03
SPEAKER_01: But with this situation, the debris flows, these very thick, viscous uh events that happen, you get the opposite that can occur, and that's what we see in this fenglomerate.
14:13
SPEAKER_01: We get a layer, but it goes fine on the bottom to coarser on top, and we call that reverse graded bedding.
14:20
SPEAKER_01: Like I think that this is easy to conceptualize if you take a jar full of mixed nuts.
14:27
SPEAKER_01: You know, you got up maybe some sunflower seeds, some cashews, and you got some Brazilian nuts, you got like the really widely varying nut sizes in the same way.
14:36
SPEAKER_02: Which hold on, Chris, which ones are you put picking out of there first?
14:39
SPEAKER_02: You know, I know I know you're you're the one.
14:41
SPEAKER_02: Oh, I'm going to the cashews out, don't you?
14:44
SPEAKER_02: And then leave the rest for everybody else.
14:47
SPEAKER_01: 100%.
14:48
SPEAKER_01: Unbelievable.
14:49
SPEAKER_01: But if you take that and you shake it up vigorously for a period of time, just imagine what you're gonna get.
14:55
SPEAKER_01: It's gonna then the class are gonna kind of separate out.
14:58
SPEAKER_01: The nuts are gonna separate out.
15:00
SPEAKER_01: And what you're gonna get is the smaller nuts are gonna settle to the bottom of the jar.
15:06
SPEAKER_01: And it'll progress all the way up to the largest, like the Brazilian nuts on top.
15:12
SPEAKER_01: And that's what we get with this reverse graded.
15:15
SPEAKER_01: It's really thick, it's really viscous, it's very kinetic, and you got a lot of motion, so these clasts are banging into each other.
15:21
SPEAKER_01: What's happening is these smaller pieces then of the clast are gonna settle down through the pore spaces between the bigger grains, and they're gonna settle to the bottom.
15:31
SPEAKER_01: And that's how you get this reverse graded bedding.
15:33
SPEAKER_01: But the difference is it has to be this you have to have a lot of class, a lot of energy, and you have to have very viscous material which kind of supports the heavy things.
15:43
SPEAKER_01: It forms this kind of matrix that it just kind of like it's the base that that supports everything above it, right?
15:49
SPEAKER_01: Right, right.
15:50
SPEAKER_01: Uh and that's so we get that really cool feature in a fanglomerate.
15:54
SPEAKER_02: That's a really good description, Chris, of these beautiful features.
15:57
SPEAKER_02: And so I, you know, kind of zooming out a little bit, where are they?
16:00
SPEAKER_02: There's some to the east, as you said, they're mostly exposed along kind of the western side of these basins, these Mesozoic basin remnants.
16:07
SPEAKER_02: They're often pretty small, like actually fairly small deposits.
16:12
SPEAKER_02: We saw the first stop in our field trip, we saw a pretty small one up in, you know, kind of York County, Adams County area.
16:18
SPEAKER_02: I've seen outcrops there as you come off of the border fault.
16:22
SPEAKER_02: So usually like the border fault is the boundary, but as we'll talk about in more detail later in this series, the border fault separates kind of the older Precambrian rocks from these Mesozoic, Trassic, and Jurassic rocks uh to the east.
16:34
SPEAKER_02: So you got to be into the east part a little bit.
16:37
SPEAKER_02: They're little outcrops, they're little low, rubbly outcrops.
16:40
SPEAKER_02: Uh, we saw one in a little tiny road cut, right?
16:42
SPEAKER_02: Like that's that's your typical fanglomerate deposit.
16:44
SPEAKER_02: Again, they're carbonate, they weather negatively here often.
16:48
SPEAKER_02: Um, and super, super cool outcrop.
16:51
SPEAKER_01: Because we couldn't outcrops.
16:52
SPEAKER_01: Like, you know, it wasn't worthy of like, hey, I need to take a sample for my classroom because it it just fell apart.
16:59
SPEAKER_01: It just falls apart too easy.
17:01
SPEAKER_02: That's exactly right.
17:01
SPEAKER_02: And they're little, little outcrops kind of sporadically all along the western edge, and they're really discontinuous.
17:09
SPEAKER_02: I mean, check out a geological map and you'll see these things are really discontinuous.
17:12
SPEAKER_02: They're kind of hard to find, they're scroungy outcrops, tough, tough, tough stuff to find.
17:17
SPEAKER_01: Well, I think I think that's intuitive that they're they're kind of discontinuous.
17:21
SPEAKER_01: I think that's intuitive because these are events, these are debris flows.
17:27
SPEAKER_01: These are not gonna be laterally continuous like you get with normal sedimentary rock deposits.
17:33
SPEAKER_01: But Jesse, can you do one thing before you launch into whatever you're gonna launch into here?
17:37
SPEAKER_01: I get I get the feeling that I better settle in.
17:40
SPEAKER_01: Um can you please define, though, the difference between a normal alluvial fan and a fanglomerate a second?
17:47
SPEAKER_01: Because we have both on this gift that you created here.
17:50
SPEAKER_02: Yeah, there's both.
17:51
SPEAKER_02: I think you know, I I tend to think of like normal alluvial fan as they're they're still storm deposits, really, right?
17:58
SPEAKER_02: Like they're they're water kind of bringing rounded cobbles and sand down.
18:02
SPEAKER_02: These fanglomerates are much more big debris flow.
18:06
SPEAKER_02: These ones in this area are really restricted um clasts, so the clasts are are pretty homogenous.
18:13
SPEAKER_02: There's a bunch of varieties of them, but they're pretty homogenous.
18:16
SPEAKER_02: It's not a huge basin that's being pulled continuously into this system.
18:21
SPEAKER_02: Um, and so I think they're much more mud flow deposits.
18:24
SPEAKER_02: However, there's a there is overlap.
18:26
SPEAKER_02: I mean, you know, some of these conglomerates, and there are fanglomerates at the base of these Triassic sediments that occur as well.
18:34
SPEAKER_02: And, you know, there's some discussion in the field guides that we've read that, okay, some of them have gray matrix, some of them have red matrix, where the classic ones kind of have this reddish matrix to them and are carbonate-dominated, and other ones might be a little bit more muddy dominated.
18:49
SPEAKER_02: So there's variations on this kind of fanglomerate thing.
18:52
SPEAKER_02: When we use the word fanglomerate, we're referring to like these late stage sediments that have this reversed depositional direction that are carbonate dominated, and usually they have this red matrix.
19:03
SPEAKER_02: And Chris, we saw that in the soil.
19:05
SPEAKER_02: I mean, just driving around to these outcrops, you just notice all of a sudden, oh, wait, the soil's really red here.
19:10
SPEAKER_02: You could see that just in the road cuts, it not even in your outcrop, just the soil is very red when you're over these fanglomerates.
19:16
SPEAKER_01: Hey, one thing, Jesse.
19:18
SPEAKER_01: Uh, just had a thought.
19:19
SPEAKER_01: The last time that you and I have seen reverse graded was in the Absaricas.
19:26
SPEAKER_01: Oh, yeah.
19:26
SPEAKER_01: Looking at these Lahar deposits, I'm heading into Yellowstone, you get the same thing, but it's it that should make sense because you have this uh lahar is going to be a really, really thick, slurry, high viscosity mixture that has a bunch of varying class sizes in it, which is gonna lead to the same kind of matrix supporting it, and and you get this reverse grading of small to to big on top of the same kind of uh process.
19:51
SPEAKER_01: Exactly.
19:52
SPEAKER_02: Yeah, same same exact kind of process.
19:54
SPEAKER_02: So the sizes of these things, the typical fanglomer deposit, it's pretty small, half to six kilometers maybe in size.
20:00
SPEAKER_02: It's tens of meters to maybe a couple hundred meters thick.
20:04
SPEAKER_02: Small mountain streams pouring out of valleys, you know, with this direction, tapping a different, a different source, a different sediment source, really.
20:11
SPEAKER_02: There's one exception, and this brings us to the historically interesting and really beautiful rock, which is what's called the Leesburg Fanglomerate, the Leesburg marble, the Leesburg member of the Bull Run Formation.
20:23
SPEAKER_02: So this is it's all, as the name implies, centered around Leesburg, Virginia.
20:28
SPEAKER_02: Leesburg's just off the Potomac on the southwest side of the Potomac, like kind of the same latitude kind of as Washington, D.C.
20:39
SPEAKER_02: So that Maryland, the Potomac defines the Maryland, Virginia border there.
20:42
SPEAKER_02: Leesburg's set right up against the Catocton Mountains.
20:46
SPEAKER_02: It is underlain by a very large alluvial fan, 35 kilometers in aerial extent, thickness of about four kilometers.
20:54
SPEAKER_02: So this is like repetitive debris flows.
20:56
SPEAKER_02: This is a huge sequence of fanglomerate.
20:58
SPEAKER_02: And this is the one we went to at our last stop, Chris.
21:01
SPEAKER_02: It's just a spectacle, and that's where this rock uh photo is.
21:05
SPEAKER_02: This is a Leesburg fanglomerate sample, and uh it's just spectacular, spectacular rock.
21:10
SPEAKER_01: It is, and you know, Jesse, one noteworthy thing when we were at this outcrop is there are a lot of noteworthy things because it's a beautiful, beautiful rock, is the angularity of the clasts.
21:25
SPEAKER_01: It almost was like a a fan breccia instead of a fanglomerate, you know?
21:31
SPEAKER_02: Oh, totally, totally.
21:32
SPEAKER_01: And the angularity of the class, you have to remember these are these are limestone class or or marbleized clasts.
21:39
SPEAKER_01: Um that's not a really tough rock, you know, it's easily rounded and so on.
21:44
SPEAKER_01: And the angularity then suggests, well, high energy because the class were big, but the angularity of them suggests that the source for the rocks, or the source for the clasts was very close by.
21:56
SPEAKER_01: It was not a lot of transport between it, wasn't eroded.
22:00
SPEAKER_01: Very long before deposition happened.
22:02
SPEAKER_02: The other unique characteristic about this Leesberg sample, and I I've I've read sort of mixed arguments for the cause of this, but I'm going to skip ahead to the image number four here.
22:12
SPEAKER_02: And you know, the first fanglomerate that we saw was this kind of low, ragged roadside cut.
22:19
SPEAKER_02: I mean, it's a pretty recent road cut, I mean, modern kind of road cut, and it was pretty ragged, it was weathering out, it was rubbly.
22:27
SPEAKER_02: Like you said, you wouldn't think, oh, I can take a big sample of this.
22:30
SPEAKER_02: Like, you know, it's kind of broken apart.
22:33
SPEAKER_02: Cool looking rock, but not spectacular.
22:35
SPEAKER_02: The quarry that we went to from the 1800s, which we'll talk about this story in a minute, but the the sort of 1800s era quarry was really well put together, and you have these beautiful quarry walls still there.
22:46
SPEAKER_02: This is this is a much more well put together rock, the Leesburg member, the Leesburg fanglomerate, the really big, really thick one.
22:54
SPEAKER_02: And I saw kind of different arguments for why that is, and this kind of relates to this name, this naming kind of um confusion.
23:02
SPEAKER_02: Like it's called the Potomac marble, but we've been referring to it as a carbonate panglomerate, right?
23:08
SPEAKER_02: Uh which is a sedimentary sedimentary morphos.
23:10
SPEAKER_02: Yeah, exactly.
23:11
SPEAKER_02: And I saw I've seen different arguments for this that are like, oh, it's just a misnamed rock, you know, it's it's not actually marble, that's the trade name for it, which we run across that all the time when we hear about granite cover tops.
23:22
SPEAKER_01: Hey, let's back up just a second, just a second, Jesse.
23:25
SPEAKER_01: Uh just basic stuff.
23:26
SPEAKER_01: But if you metamorphose limestone, limestone is to me not anything special, it's not a pretty rock by any stretch of the imagination.
23:35
SPEAKER_01: But when you metamorphose it, the calcite grains recrystallize, they grow and they cement together all at the same time, and it becomes beautiful, and you get these veins that are running through it.
23:47
SPEAKER_01: It's just a gorgeous, gorgeous rock.
23:49
SPEAKER_01: So just metamorphosing limestone turns it into marble, and that's what you're kind of alluding to here.
23:56
SPEAKER_01: Why do we call it the Potomac marble?
23:59
SPEAKER_02: There if it's what we described.
24:01
SPEAKER_02: There's different arguments, I think.
24:03
SPEAKER_02: One is that it's not actually a marble, just a little bit more resistant form of this fanglomerate, and it's just a, you know, the trade name has taken over, and it's it's not really a marble.
24:13
SPEAKER_02: The other argument is that well, we haven't really talked about this yet, but this whole area, all of these basins, all the sediments in the Gettysburg Basin, the Birdsboro Basin, they were intruded by basaltic sills and dikes, which we're going to talk about next chapter, next episode.
24:30
SPEAKER_02: They were intruded by these dikes and these events.
24:34
SPEAKER_02: And in this area around Leesburg, there are large versions of those, of the sort of York Haven sill.
24:41
SPEAKER_02: There's large versions around there.
24:42
SPEAKER_02: So there might have been a little bit of thermal metamorphism that also happened that affected so like little bits of metamorphism that might take you from a pure sediment to a slightly metamorphosed sediment.
24:53
SPEAKER_01: Yeah, like a meta conglomerate, then.
24:56
SPEAKER_02: Exactly, a meta fanglomerate, maybe.
24:58
SPEAKER_02: And again, this stuff is the it's carbonate class, which to me I've rarely seen rocks that have carbonate matrix material.
25:06
SPEAKER_02: That's not the most common thing in the world.
25:08
SPEAKER_02: No, it's more what makes this rock stand out.
25:11
SPEAKER_02: Yeah.
25:11
SPEAKER_02: So anyway, uh it's not probably technically a marble, but there, you know, it is more around Leesburg area, it's much better more put together than most of the fanglomerate outcrops uh across the region, I would say.
25:24
SPEAKER_01: Right.
25:24
SPEAKER_01: Yeah.
25:25
SPEAKER_01: You know, it it is a map.
25:26
SPEAKER_01: This is the biggest of the big in terms of fanglomerate deposits, this kind of potomac marble, which means that well, you know, the source of the rock was present for a very long time.
25:37
SPEAKER_01: Uh I don't know.
25:38
SPEAKER_02: Upwards of millions of years, perhaps, that these debris flows were coming out and successively dumping.
25:44
SPEAKER_02: I mean, four kilometers of fanglomerate, that's a lot.
25:46
SPEAKER_02: That's a lot of debris flow.
25:48
SPEAKER_02: That's a I I just think of the kind of catastrophic events that these represent.
25:53
SPEAKER_02: That's a lot of catastrophic events over a long time period.
25:56
SPEAKER_02: That's right.
25:56
SPEAKER_02: Very cool.
25:57
SPEAKER_01: Um, I think another thing that is noteworthy, Jesse, is that you and I we felt that the historical significance of the rock was so like I think that's a powerful story if you skip to the next image and and we can start getting into this a little bit.
26:12
SPEAKER_01: Uh this is uh why the rock was quarried right here.
26:16
SPEAKER_01: Is uh when they had to rebuild Washington, D.C.
26:19
SPEAKER_01: in eight after um 1814, they want this to be a sturdy rock, but they also wanted it to be beautiful, right?
26:27
SPEAKER_01: To project this kind of like, what do we want to be?
26:29
SPEAKER_01: Right, right.
26:30
SPEAKER_01: So uh the artistic nature of this rock or the beauty of the rock was equally as important as how sturdy the rock is.
26:38
SPEAKER_01: So for you and I, we didn't want to take any of this rock.
26:42
SPEAKER_01: It's it's too significant.
26:44
SPEAKER_01: It's historically just too important, and and it was just enough to just see it and understand it.
26:52
SPEAKER_01: It was just I I loved it so much.
26:54
SPEAKER_02: And and this this, like I said, this book, The Potomac Marble by Paul Crongold covers it really well.
26:59
SPEAKER_02: There's a couple of YouTube videos of this author talk to speaking about this and and um the the sort of the story of this rock, 1814, the British Disordered the Capitol.
27:10
SPEAKER_02: At that point, President Monroe and Benjamin Latrobe, who was the architect of the Capitol, official title there, basically were rebuilding this thing.
27:19
SPEAKER_02: And they they went and found this quarry stone that could hold up, it would accept a polish, which was really important.
27:26
SPEAKER_02: Marble, again, going back to the sort of Greek and Roman architecture, marble's this stately, you know, stone that that people have used as building stone.
27:35
SPEAKER_02: And so it checked a lot of boxes there.
27:37
SPEAKER_02: Getting it out of these areas.
27:39
SPEAKER_02: I mean, this is upstream the Potomac River across a couple locks.
27:42
SPEAKER_02: They had to find a quarry that was near the Potomac so they could transport it.
27:46
SPEAKER_02: The locks were such that you couldn't get boats, especially heavily laden ones, downstream very much of the year, like only 50 days a year or something like that.
27:55
SPEAKER_02: So it's not really like quarrying like we think.
27:58
SPEAKER_02: You know, this is this is difficult.
27:59
SPEAKER_02: And there were probably multiple quarries.
28:01
SPEAKER_02: We've saw one of the allegedly older quarries that Paul Kringold sort of rediscovered and and traced chased down some of these locations.
28:10
SPEAKER_02: And so um they're kind of a lot in that Leesburg area where this fangomerate outcrops, and there are big outcrops of it.
28:17
SPEAKER_02: I mean, I I'm always really impressed that there's huge stretches of the Potomac that are bordered on both sides by fanglomerate as it comes across the Catoctins there.
28:26
SPEAKER_02: So it's a really beautiful area, really spectacular geology as well.
28:30
SPEAKER_02: It's hard to see and hard to find.
28:32
SPEAKER_02: You kind of got to scrounge to find it a little bit, but it's beautiful when you do.
28:36
SPEAKER_01: It's covered up by a lot of biology.
28:38
SPEAKER_01: A lot of biology getting in the way.
28:40
SPEAKER_01: Oh my goodness.
28:41
SPEAKER_02: A lot of that moss covering up our beautiful rocks.
28:45
SPEAKER_02: I mean, Chris, was that what that was uh that you know, visiting that quarry is like um it's more than cool.
28:51
SPEAKER_02: It's a emotional experience in some ways.
28:55
SPEAKER_02: The history of of something like that.
28:56
SPEAKER_02: I mean, a quarry going back many hundreds of years to make the Capitol building is it's a cool story, very, very cool, powerful story.
29:04
SPEAKER_01: You know, I couldn't help but wonder about how brutal it must have been getting those rocks out of there and you know, kind of towing them down the tow bridge or the tow path.
29:15
SPEAKER_01: I mean, um my goodness.
29:18
SPEAKER_01: Just I don't know.
29:19
SPEAKER_01: Can you imagine it's I can't actually I tried.
29:23
SPEAKER_01: I can't I can't do a very good job.
29:24
SPEAKER_01: It must have been a brutally hard process, though.
29:27
SPEAKER_02: Just uh completely terrible.
29:29
SPEAKER_02: Um it kind of controls the path of the Potomac River a little bit, you know.
29:33
SPEAKER_02: The Potomac River comes out a point of rocks there where there is some fanglomerate, hits a big outcrop and hangs a big takes a big bend to the southwest because the fanglomerate's kind of resistant there.
29:43
SPEAKER_02: Yeah.
29:43
SPEAKER_01: So it's gonna take the path of least resistance, and that ain't it.
29:46
SPEAKER_02: That's not it.
29:47
SPEAKER_02: That's exactly right.
29:48
SPEAKER_02: Um I mean, we could go into more detail here, but I don't think we should.
29:52
SPEAKER_02: I think we could just cut it off at that, Chris, and point people to this book.
29:57
SPEAKER_02: Again, Potomac Marble.
29:58
SPEAKER_02: That's a great book to read if you if you're interested in learning more about this story and about this particular beautiful rock.
30:04
SPEAKER_02: And Chris, I promise to update you next time I have a a favorite rock that I'm thinking about.
30:09
SPEAKER_01: Okay.
30:10
SPEAKER_01: I hope so.
30:11
SPEAKER_02: I hope so.
30:11
SPEAKER_02: That was upsetting.
30:12
SPEAKER_02: So well, I'm sorry.
30:14
SPEAKER_02: I'm sorry that I upset you.
30:15
SPEAKER_02: I don't like to do that.
30:16
SPEAKER_02: I really don't like upset Chris, you know?
30:19
SPEAKER_02: Nobody does.
30:20
SPEAKER_02: No, I gotta try and avoid that.
30:22
SPEAKER_02: Avoid that next time.
30:23
SPEAKER_02: Well, what do you think?
30:24
SPEAKER_02: Is that a wrap on this?
30:25
SPEAKER_01: That's it.
30:26
SPEAKER_01: That's it.
30:26
SPEAKER_02: Hey, this is episode four in our series on the geology of the Battle of Gettysburg.
30:33
SPEAKER_02: We've got a couple more to go, three more to go.
30:35
SPEAKER_02: You can find all those three on our Camp Geo Mobile app right now.
30:38
SPEAKER_02: If you download the Camp Geo Mobile app first link in your show notes, there you can get access to this with all the images, plus the rest of the series is available there.
30:47
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30:48
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30:52
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30:58
SPEAKER_02: Cheers.