 So, when grains pile up on a bed and make a little bit of topography, that topography gets enhanced by the interaction between the grains and the flows. And that creates what we call bed forms. And these are structures like ripples or dunes that create the cross-lamination and cross- stratification that we see within beds. Okay? So, I'm going to use an example of a ripple. And we're going to talk in detail about how the ripple changes the characteristics of the flow and the sediment transport across that structure. Okay? So, my flow is going to go to the right here. And what happens is you end up with a nice spacing of ripples here. And we talked a little bit about how the flow varied with respect to the ripple in the last video. And so, basically you have water flowing here. It gets to the top of the ripple and it overshoots, and then it comes back down to the bed on the back of the next ripple, and the same process happens again. Okay? So this blue line represents the boundary layer. So there are places where the boundary layer is very close to the bed and places where it's not. And so we label the points where it separates from the bed, which is right at the cross to the ripple as a separation point. And where the flow comes in and interacts with the bed again, it attaches to the surface again. We call this the attachment point. And so we have our average flow speed here, but a lot of times in this zone here, the flow speed is so low that no sediment can actually be transported in this zone here. So particularly like right in here, our local flow speed is zero. So if you think about the Holstrom diagram, that means that basically no sand grains can actually get transported across that. However, we have the thin boundary layer on the upstream side, and the flow is pushing against that surface because it's sloping back upstream. So we get a lot of erosion of grains on this upstream side, and it happens between the attachment point and the separation point. You can think about it's oriented, so the flow from upstream is going right into it. And then we end up with deposition of any grains that saltate into this layer get stuck in the low flow zone, right? So we end up in this zone here with the stripes being an area of deposition. So there's still no transport of grains between this area of deposition and the attachment point, but you don't always get deposition there because you have to get a grain to be transported to that spot to get the actual deposition. Really what happens is the grains will be saltating, and most of them will end up in the flow in this zone very close to the separation point. And it's really very rare for a grain to saltate all the way over here. They can, depending on the flow, but they often don't. We can also have grains that are rolling, and if the grains are large enough that they're rolling, they'll reach the separation point and then they'll just fall down into this area of deposition on the ripple. Thanks for watching.