How One Beach Reveals Two Completely Different Earths

Welcome to the Pembrokeshire Coast here in Wales. Just a fantastic location for scenery, looking at the ocean, and checking out some geology. I'm here at a place called Church Doors, which is this fantastic sea arch cut into these steeply dipping, almost vertical beds of carboniferous limestone. You can see how they've eroded out to form this fantastic arch here, again known as Church Doors. And we're gonna look at the geology a little bit here and explain things as best we can. Thanks for joining me. I'm John, this is Professor Sean Wilsey here at Church Doors in Wales. We're gonna take a look at these rocks, see what they have to tell us, and then we're gonna actually go through a cool little sea cave into the next cove and beach over and look at the rocks over there as well. So we can see that the layering in these rocks is quite pronounced, and the layering is all pretty much vertical or nearly vertical. These are all carboniferous limestone. So these were rock layers that were laid down at the bottom of a tropical sea that existed here during the carboniferous, or what we call in America, we call it the Mississippian period. And you can see the beds here. The tidal fluctuations are really extreme, so the color changes mainly because high tide reaches this point here, and so the rock here is totally covered in barnacles. So we can see that the beds here, steeply dipping. We'll talk about the story and how those beds came to be tilted at such an extreme angle here, but I'll go ahead and show you where we're going next. So we're gonna head across to this headland here. You can see the beds, if we kind of sweep around this way, you'd see those steeply dipping beds, especially over here by the staircase. You can see the layering there, and that continues on into the headland. But believe it or not, there's actually a little sea cave that cuts through this cliff face here and gets us into the next section. So let's head over there, and then we'll have a look at these rocks and explain them in a little bit more detail. So you can see the portal in the rock over here that we're gonna go through to get to the next beach. But before we head over there, I thought I'd show you in these boulders here on the beach, we can see some of the fossils in this limestone quite nicely. So here we have, let's find one down here. There's a nice horn coral cross-section right here. So you can see the geometry of that, looks like little spokes in the wheel. And then down here, we have a boulder with some crinoid stems. So these are all different invertebrates that would have lived in these shallow tropical seas during the Carboniferous, when the UK was at a much lower latitude, closer to the equator. Some more fossils in here. See some of those shapes in there, just full of them. Just the whole thing is just full of these invertebrate fossils. But let's go ahead and head through the little doorway over here. So here we are at the wall, this big cliff base that divides this little cove from the next. And here you can see the little portal, the little kind of tunnel. I don't know if this is manmade or natural, but this basically allows access from this side to the other. Again, you can see the steep, steeply dipping, basically vertical beds of this limestone here. So let's go ahead and head through this thing as best we can. Again, you can only access this at low tide. And the tidal fluctuations here are incredible. Something like eight meters, like 20, 25 feet or so of tidal fluctuation between high tide and low tide. So you can see how wet this is in here. And this thing would be completely flooded a few hours ago or a few hours from now. So you have to be very particular about when you come over here and time it just right. Kind of work our way through. We're working through the limestone, the Carboniferous limestone. And we got to make a little climbing move here to get over to the next cove. So here we have next little cove. And right away, what we can see over here is if we look up at the head of the beach, hopefully you can see there's a pretty marked difference in color. The rocks on the left side are much more red than the rocks on the right. And so we're going to head up there and take a look at those in a little bit more detail. So here we are up near the head of the beach, kind of looking back out towards the ocean. Couple of folks down here, the little gap, that little tunnel in the cliffs that we came through is back down there. It's a couple of larger sea caves right here. And now as we turn towards the head of the cove, we're looking at these steeply dipping, again, almost vertical, let's just call them vertical, fins of rock. And we can see the rocks over here on this side to the right are more gray. But as we move along here, right in front of me, they become much more red in color. So there's definitely some different rock types going on here. So we're going to take a look at these and explain these in a little bit of detail. So let's head over to these red outcrops first. Okay, so here we are at the, more or less the furthest end of the cove here, looking at these red rocks. So let's just start with some observations and then we'll give it some context. So let's look at these rocks up close, see what they have to tell us. And right away, the thing that kind of stands out is we've got particles of different sizes. We have clastic rocks. So these are not the same as the rocks we saw before. These are not limestones. These are clastic rocks, pebbles, up to gravel-sized particles, different shapes and sizes. We can see that the beds run through here. In places, the particle sizes are much finer. So we've got a little more of a fine sandstone over here. This is more of a coarse pebbly sandstone right here. And just looking at the size and the shape of these particles, the main interpretation I think we could take away from these, especially with a difference to the rocks we saw before, is that these are continental deposits. These are sediments that were laid down on the land, probably in some sort of stream of fluvial environment. We've got larger particles maybe here during periods of floods or higher energy events. And these more fine-grain deposits here. We'll kind of work our way across here. What's fantastic about this location is from a teaching perspective, you could have students work across the beach here and measure the thickness of the beds, describe them, interpret them. So basically put together a stratigraphic section, but doing so with the beds laid out this way vertically makes it easy to just walk perpendicular and use a tape measure or whatever to measure the thickness of the beds. Just really fantastic exposure this way. Oh, we can see some of the fine bedding details here. Kind of looking for cross bedding. Might be a little bit of cross bedding in there. Hard to tell for sure. And this, oh, here's a lovely bed here. So now we have these big gravels, right? You can see all these big gravels. Let's see if we can take that up to where it's dry here. So you can see these large particles, these clasts in the rock. Here's a nice rounded pebble right here. So these are definitively fluvial deposits. Another little section just above us here. Let's see, where's that at? There we go, right there. So definitely fluvial deposits. And these are all Devonian in age. This is the Devonian old red sandstone that's very prevalent in England and Wales. One of the major units you find here. And this boundary, we're gonna work our way through the red rocks into the more gray rocks here in a second. But we're looking at a contact here where we move from these red rocks, these sandstones into the gray rocks. It's gonna be a major division in time between the Devonian and the Carboniferous. And we're also gonna see, as we look at these rocks, a big difference in the depositional environments. Here we have these stream deposits, sands and gravels mainly. Here's another bed here with these gravel chunks in it here. And as we work our way over to the gray rocks, which are the limestones, we're gonna be seeing a change in environments from continental deposition, rivers and streams on land, to basically rocks deposited by the sea in a shallow marine setting. So probably some sea level rise or relative drop in the land is the cause of that. But this division between these red rocks and these gray rocks, or these sandstones and conglomerates and limestones is the boundary between the Devonian and the Carboniferous here in England and Wales. So just really fantastic exposures. I think the color change is pretty striking, but also the materials, right? We have these sands, gravels. Here we're actually seeing some of the cross bedding here. So these are definitely fluvial. Here's a beautiful view of some of the cross bedding right there. Yeah, fantastic. So this is, and then it shifts over. Now we're probably seeing a little bit of a change here. This looks like a little bit of limestone here. So this could be sort of the fluctuations between the land and the sea, possibly right here along this boundary. We'll grab the handy dandy acid bottle and see what we get here. This could just be a gray. No, no reaction. All right, so this is just a gray sandstone. I thought it might be the limestone based on the color, but just a little bit different color there in the sandstone. So the old red sandstone Devonian here laying on end. We haven't really talked about why these rocks are oriented this way. These rocks were of course laid down as horizontal beds, but then later after these two units were deposited, they were then subjected to intense compression that basically tilted them to these vertical orientations. And this was part of the Veriscan Orogeny, more or less the same orogeny that we find in the Eastern part of North America in the Appalachian Mountains. There it's called the Alleghenian Orogeny. Here it's called the Veriscan Orogeny, but it's the same event. It's essentially parts of Europe and Africa colliding with North America to produce huge mountains from that continental collision. So here, just working our way across the beach, we're working again from the Devonian. We're working our way up section or through younger rocks. More of these sandstones, maybe a little bit of mudstones, but mainly sandstones of different sizes, these red sandstones. And then we hit a pretty interesting boundary here because as I look to my right from here, everything's gray. And so presumably these are all the limestones like the ones we looked at in the other cove nearby. So these limestones here would be deposited beneath the seas. And so these would be all deposits from the bottom of the ocean. We can go ahead and test and see if these are truly calcite-rich. We'll give them a little hit with the acid bottle. Sure enough, we get that strong reaction there that indicates that these are calcite-rich. These are limestones. So basically this gully right here with the rubble more or less defines the boundary between the old red sandstone, the Devonian, and the gray limestones of the Carboniferous. We have a significant geologic bound, not only a contact between two units, but a significant geologic boundary in terms of the timescales and such. So we'll go a little bit further into the limestones. We've looked at those a little bit over by church doors, showed you some of the fossils there. Now we're looking at the actual bedding plains. So this is one huge bedding plain that's been tilted to vertical, which is just pretty incredible to think about. And these things are pretty fossil-rich. I mean, there's not fossils necessarily everywhere you look, but probably if you would look with the microscope and a little bit of magnification, you'd find a lot more of the invertebrate shell material in here. We'll see if we can find maybe a few more fossils. Again, the bedding is pretty remarkable. Some of it is a little bit shaly. So you can see that it's more thin bedded. And so it could be changes in sea level that produce that a little bit. Just fantastic. Look at these just clips here of these limestone just going on. Yeah, very cool. Yeah, we've got some small little fossils in here, it looks like. Let's see some of these. Might be little brachiopods maybe in this little slab. Another one right there. A couple more up here. Let me just get those in. There you go. Couple of those, fantastic. Just a really cool beach, a neat little access point, incredible scenery here, and amazing geology. Just part of the Welsh coast here, the boundary between the Carboniferous and over here in the red, we have the Devonian. Really amazing. Well, hey, thanks for joining me on this fun excursion. First time here, I wanted to check this out. It looked like a neat spot, and I wanted to share that with you as best I could. So really neat spot for us to look at these two different rock types, a major geologic boundary, but more importantly, a significant change in the depositional environment from rivers and streams on the land to shallow marine tropical waters in this area here. So thanks again for joining me. Appreciate your support of the channel. There's links in the description if you wanna support my geology videos, and we'll see you next time, friends. Take care.