 The tidal flows create distinctive sedimentary structures that are related to the variations in flow through tidal cycle. So what I have plotted here is time, doing it as a vertical axis so we can sort of compare the time and I'm going to draw up some sedimentary layers here. And then the flow speed, with the red line being the zero flow speed and the arrow, if it's to the right it means the flow is going to the right, if it's to the left and larger that distance, the stronger the flow. So I started out with this brown time interval here where we have a flow going to the right depositing sand in ripples and I provided a scale bar here, so the bed, that's five centimeters, so the bed forms here are on the orders of two to three centimeters tall, which means that they're there in that ripple flow speed. And so we know, even though I haven't specified the grain size, you know it's sand or because the ripples only form in sand to silt size grains and it has to be bed load transported because it's a bed load transport that produces bed forms. So we have a fairly strong flow going this direction, so in orange I'll draw sort of the next time frame, so say the flow starts slowing down and at some point it can no longer transport sediment on the ripples so that stops and then I'm going to say that the tide is either a high or a low tide and so the flow speed in this zone is zero. So if we think about the Holstrom diagram, when the flow is very low you can accumulate mud, so I'm going to draw some mud that accumulates across the top of this ripple. So the duration of the tide at high and low flow is often pretty low. When you have a low tide some of it's exposed, which can last for a while but when it's in the submerged area, which we'll consider this on right now, well these ripples have to be submerged to get the mud later deposited on top. So now the flow of the tide changes directions and it will start increasing in speed in this direction. So the blue will be a flow in this other direction. So there's a little bit of topography on the top of the bed and the former crests of these ripples will be the place where the next ripple crests form. And so what happens is you start getting erosion as the flow speeds up on this side of the crest and you start getting deposition on the downstream side. The tops flatten out like this and you get deposition and then the top flattens out again and you get deposition. Can preserve a little bit of mud under the ripple in these ends here. So as this grows through time you flatten out parts and you start to develop the ripples. So then eventually what happens is you get ripple cross lamination. So now we have ripples in the sand moving in this direction. So if we look at the dip of the lamina here, as I defined it, we see the transports in this direction. In the blue zone we have transport in the opposite direction. And then you have bits of mud in places that get captured between those two zones. So we have two things that are very distinctive of ties. The mud, when it's incorporated inside the lamina, or sometimes it forms like on the dipping surface here, these are called mud drapes. And they're very distinctive because the mud requires a low flow speed to accumulate. And so they tell you you have a time of standing flow, which is what reflects the standing time. The two directions of flow shown like this are called herringbone cross stratification. Or in the case these are ripples, we call them cross lamination. And that comes, the name herringbone comes from a distinctive weave of jackets where you have layers going in one way on this way and the opposite direction in the next set of layers. So these are two distinctive forms of the cross stratification. So let's move on here and so let's say that we have a very strong flow coming this way. The tide is flowing very quickly in this direction. And let's say that this is fast enough that you actually end up forming dunes. So in this particular case, what you would see is it can be erosive. So the height of the dune is significantly more than five centimeters, which means that it's not a ripple. So the flow speed is significantly higher. So the grain size of the dune might be coarser than the ripples here. And maybe that flow speed is fast enough that somewhere where mud was deposited and maybe it had consolidated, you actually ended up with some clasts of mud sitting in that. So say the flow is fast enough, maybe you actually get mud clasts. So sometimes maybe they get deposited in all of these zones here. So I drew it in orange to represent maybe the mud deposited in the previous type. So you often end up with mud clasts. It takes a high flow speed to rip up the mud and you can actually transport that as clasts. So let's now say that our flow starts slowing down here and we're back in the zone where we would create ripples but can't create dunes. So now we still have the flow going this way but it's a smaller flow. And so what happens is that the flow will erode off the top here and deposit a thin layer of sediment that will just sort of roll down the front of the dune. But it's not really enough to transport the dune as a whole. And maybe if you have some fine sand you start getting a ripple migrating on the back of the dune a little bit here. But when the flow speeds lower it's usually not enough erosion power to completely remove and flatten out the dune to start getting the ripples. If it was a low flow for a long period of time it might do that but that's not necessarily the case. So then we get down again to the standing tide here and then you get mud accumulating across the whole surface. And then we go back to maybe ripples going in this other direction and so you might use can start end up propagating ripples in this direction but there's this extra topography here so then maybe there's a little bit more erosion across here and you get some lamina coming down here. So you eventually get ripples forming in this direction again like you did down at the bottom. We have this sort of mix of different grain sizes and transport directions and they can be sort of all mixed up together. So what I've drawn here is a little bit of this time line and the draw is a little bit of a mess. And a lot of times that's what you actually see in tidal environments. You see flow in multiple directions, you see multiple grain sizes and that's reflecting the variability of the flows. There's one more feature I want to try to draw for you and that is what we call a reactivation surface. In this particular case I showed accumulation of sediment with the flows going in both directions. Sometimes the flow in one direction is much stronger than in the other direction. That means that the record of flow in one direction is much stronger than the other. So let's say that we go down back to our standing tide, we accumulate mud and then I'm just going to have a little teeny flow in the other direction because of some change in the geometry offshore or maybe this is a strongly asymmetric tide. So if we only have a little flow in the other direction, we can erode off the tops. Now we're going to go back through our standing tide, I'm not going to draw that, and then say we go back to the ripples forming in this other direction here. But let's say that all of this part gets eroded and we start transporting a ripple in this side, this is the erosional part of the ripple so this part gets eroded but we end up with deposition on the down slope side. What we end up with preserved inside the ripples is the brown lamina are from an older flow. You have this a little bit of erosion and maybe some mud drapes and then you have this younger flow. This blue line in here is an erosion surface inside ripples and it can happen inside dunes as well. This blue line is an erosion surface that reflects a little bit of flow in the other direction and it cuts off the lamina above and below. So this lamina and this lamina are truncated and the lamina on top sort of downlap or onlap that surface. So this is an erosion surface that's inside the cross lamination but it's not one of these main bounding surfaces that divides the ripples. We call this a reactivation surface. What that means is that you had an active ripple. The flow changed and eroded the top of the ripple and then the ripple reactivated in the same direction again. So this is, it's much more subtle than these large changes in flow directions reflected by multiple lamina but it also indicates that you have a variation in flow direction through time. Thanks for watching.