 So I want to share with you some details of cross stratification. So as usual we have our flow going to the right and the surface right here represents the surface of the ripple. The fuzzy blue line represents the boundary layer with the flow and it is right along the bed on the upstream side of the ripple but it separates from the flow on the downstream side. And so we have those points, particularly the separation point at the peak of each of these ripples where the flow goes from along the bed to above the bed. We have an attachment point at every place where the boundary layer merges with the bed. So there's another one here, another one here. So we know from the Bernoulli effect and talking about sediment transport that when the boundary layer is very compressed there's a lot of shear and there's a lot of Bernoulli effect. And so the zone between the attachment point and the separation point is a place where there is erosion. So that's in this zone right here. The grains get transported from this erosion area downstream into an area where the boundary layer is separated from the flow and the flow speed is very low. And so you end up with deposition immediately downstream of the ripple crest and you get deposition only as far as the grains can sultate or roll down that slope. So most of the deposition occurs very close to the ripple crest. So over here I have this lamina that is thicker near the top and thinns at the bottom that reflects that deposition. So there's a gradation between where you stop getting deposition and maybe only a couple grains make it up this far and you end up with a whole bunch much closer. But there's often a zone where you have neither deposition nor erosion happening between some point in that flow shadow and where the boundary layer reattaches to the ripple at the attachment point. So there's neither deposition nor erosion in this area here. So if we look at what happens underneath the patterns of deposition and erosion track these processes really accurately. So now I've added this green line here right here that shows where the erosion happens on the ripple. If I sort of project that up this upper part of the ripple crest eventually gets eroded away as the ripple migrates downstream and that leaves behind what I've shown in this green line here which is the erosion surface that's created on the upstream side of this ripple. Now it stays right at the surface of the ripple until it gets deposition on top of it. And so that's when you get these grains here and this purple line represents that deposition and every time you have deposition you get a lamina that reflects the shape of the ripple crest that then sort of merges onto that erosion surface. So there's an erosion surface on every single one of these ripples and they stack on top of each other and you end up with a pattern that looks like this one here. So we have our ripples here so the ripple crest is the same but what this bottom one shows is the pattern of the lamina which are purple here and the green lines which are these surfaces right here. So each one of these surfaces that are separating the lamina reflect the erosion that happens at the attachment point a period of non-deposition and then the lamina on the downstream side of the ripples that get deposited on top of them. Thanks for watching.