 So, a very important principle in sedimentology and stratigraphy is called Walther's Law. So Walther recognized a relationship between the sequence of layers vertically representing changes in time and changes in rocks that happen laterally. And the idea is that if you have an environment, a set of environments, so we will make this, say the ocean and the beach here, if you deposit a layer of rocks within this environment, the ones that are deposited in the beach will reflect different processes in the ocean. Right? So this represents a new sediment. And on the beach you have waves crashing and swashing up, and as you go offshore, you're under continuous water. So the characteristics of the sediments vary depending on environment. So we can say that the sediments vary with environment. And that's because the process is very in space and time. So if you take a look at an ancient rock, so this is the modern, if we go and look at the ancient and we look at this, what we'll call this layer one, we look at layer one here. What we would see is this part of it represents the beach. This part may be from different colors since I have so many, but from here into maybe here represents shallow ocean, or we call that marine. And then further out we have maybe deep marine. So basically the deep marine is this brown, the shallow marine is the red, and the beach is blue here. So if we look at a layer of rock, we'll see that change in environment as we go laterally. So then we have the question is what happens if sea level goes up? There's a shift in the environment when sea level goes up. So if we have sea level going up here, our next layer of rock is going to have different characteristics. So I'll do it with color here. So layer two in this area is going to be red, and it'll go somewhere down to here. It's a shallow marine rocks. And then at some point we get to deep marine rocks, and it goes to brown. So if we look at our next layer here, layer two, we see that the rocks have changed in terms of the environment that they're representing. So if we look at our layers here, what we see through time is we have a beach, and then we have shallow marine rocks, and that tells us that sea level went up. If we look in this area here, we just see shallow marine rocks. Now it happens that they actually vary as they're super shallow and go deeper. So what we would likely actually see is that the rocks are getting deeper and deeper through time. If we look out here, we see shallow marine, and then deep marine, then out here we just see continuing deep marine rocks. So Walther's law and the key insight is related to how you have these shifts in environment through time, and those shifts are representing, if you go vertically, you see the same shifts as you go laterally. So one of the key caveats to this though is if you have non-deposition or erosion, a break in sedimentation, Walther's law doesn't work. So if you think about our previous example in the video with the discussion of time in sedimentary rocks, we were missing parts of layer three and parts of layer two when we had the uncomformity. There you would have a break in environments going vertically. So I'm going to type on the screen for the next part of the video so that you can really very clearly see Walther's law as text.