 In this video, I'm going to talk about two very important concepts to sedimentology and stratigraphy. The first is the principle of original horizontality, and the second is the idea that younger sediments are deposited over older sediments. Both of these were articulated in the 1660s by Nicholas Steno. The first one to write is down four sedimentary rock layers. The basic idea is that if you have a surface, so this is land, we're looking at it in terms of a cross-section, and you have sediments coming in, it will be deposited in layers that are approximately horizontal on top of this. I'll use a different color to represent the different layers. So layers in sedimentary rocks are initially, as they're deposited, about horizontal. They aren't exactly horizontal. They're always exactly flat because they're variations in the surface. But the idea is that if you see layers that are deposited or that are presently in an outcrop at an angle that's greater than the angle of repose for sediments, you can say, okay, they weren't actually deposited this way. There's been some tilting or tectonic processes tilting them. So the idea here is called as original horizontality. So this is the first of the two concepts for this video. The second one is pretty obvious. And that is for sediments to be deposited, there has to actually be a surface on them. And if you go back and look at the way I drew these, this is the oldest, and this one is the youngest. Basically, sediments can't be deposited in the middle of the flow or the middle of the air. They have to be deposited onto something that's actually older. So the second major concept is that you have younger layers overlie older ones. And this is one of the, this is really simple, but it's really, really important in terms of understanding time. It gives us a relative age for the different layers. So if we were looking at this in terms of time, we have the oldest to youngest. If something happened, or there was a change in environments between the deposition of the oldest layer and the youngest one, that would give us the relative timing of when one environment was before the other. So for example, say we have river deposits, and I'll say that they were deposited by sand. So they would make a sandstone here. And so we usually represent that by dots. And then maybe on top of it, you have some mud deposited. And in that mud, you have some roots growing, or you see fossil-wise roots in that mud. And then maybe you have sand again. You can interpret the sequence of events by using the fact that the older layers are on the bottom. And so you would say we have a river. So you have a river, and then you have something where you have mud being deposited, and that takes much less water flow, and roots were growing. So that suggests maybe there was a soil, and then you have sand again. So you can get a sequence of events by looking at the layers from the bottom to the top. Now of course if those layers are folded, and no longer originally horizontal, that you might actually get them all the way over turned, and then you would have the reverse sequence of events. But as a good sedimentologist, you'd be able to recognize which way is up, and in general roots grow down into the soil and not up. So if you have something of that sort, you can tell that the rocks are actually returned. So I'm going to show some examples of these in photos next. Okay, so we have these two principles, the principle of original horizontality, and the principle of younger over older. I'll show you two examples. We'll start with the Grand Canyon, one of the most beautiful places in the world. We'll import that picture here, not it on top of things. And so we're looking across miles and miles of the Grand Canyon, and what you can see here is I'll outline with the pencil. There's some layers and they're interacting with the topography, long here. And we have multiple layers, right? So I didn't trace that one very well, but you'll get the idea here. So because they're moving with topography, it's a little hard to tell, but these layers are all approximately horizontal. When you look at it over miles and miles, they aren't entirely horizontal. So we have that as a process, and then we can also interpret these as changes in environment. And so the layers here are older than the layers higher up, and I don't like that color on this beautiful Grand Canyon image. We'll just go back to a block. And the ones up here are younger. And so if you go on a trip down the canyon or you go hiking there, when you hike out of the canyon, you'll be walking up through the environments through time. So we can contrast that to, for example, Rainbow Basin, which is over near Barstow. It's a really gorgeous place as well. And in this particular case, you can see that the layers are not horizontal. In fact, they make this really nice sync line, which is the upside-down rainbow that gives the basin its name. I've been out there and I happen to have looked at the sedimentary structures, and I know that this way was originally up. So up was originally up in these rocks. But we have the sync line, and so in this case, we can say that they were folded. And this is actually the first place I ever had a mapping class. And one of the key things was to map the fold axis as well as the faults that are related to that. We still have older versus younger rocks. And interestingly, we also have an unconformity in here with very young rocks on top of that. So those are actually alluvium that are quaternary. The rocks in here are myosin in age. So this is a really useful process, and I want to keep using the analog of Mars a little bit as we go through. So I'm going to show you some layered sedimentary rocks on Mars. So we will paste that in down here, and it's a nice big image. So this is a picture from the Curiosity rover on Mars looking up at Mount Sharp, and you can see layers in here as well. So I happen to have traced these in satellite images of Mars, and I know that this particular layer is all one continuous layer. And then down here we have some layers that are sort of coming in through like this. And then again we have this unconformity that's cutting down here. So when we're looking at Mars and trying to interpret the change of depositional environments in Mars, which is one of the key aspects of this mission, we started at the bottom of the mountain, and we have our older sediments here, and we have younger ones in the zone in here, and then we have an unconformity again, where there was some erosion and change of environments, and it cannot cut through rocks that don't exist yet. So we know that these rocks are younger than the unconformity, and the ones up here above the unconformity are again even younger. So that is sort of the idea of how we will use the concepts of original horizontality and younger rocks over older rocks for the whole quarter. Thanks for watching.