 Okay in this video I'd like to talk about the sequence of bent forms that you get with increasing flow speed. So if you think about the Holstrom diagram, when the flow speed is very low versus high, down at the bottom you have no motion on the bed. So it depends on the grain size what flow speed that needs to be. And then as the flow speed increases with time you start to get grains that roll and saltate. And then as the grain size gets higher and higher the length of time the grains are off the bed increases and for small enough grains at very high flow speed you can get gains transported in suspension. And so bent forms are created by the dynamics of the roll and the saltation of grains because these are the, it's in this zone here that the grains in the flow interact with the bed and create bent forms. So bent forms are any topography on the surface of a bed. The sequence of bent forms has been studied extensively in flumes. So this is sketches that represent those flumes and often they start with a flat surface in no-flow water and then they increase the flow speed through time which is represented by these diagrams going upward. If you start with a flat bed once an irregularity is created it will start the propagation of those irregularities into a chain of ripples. So you often have a gradational change from a flat bed into the formation of ripples. And that is related to ripples are the stable bed form when you have enough flow to transport sediment as bed load. As the flow speed continues to increase the erosion on the ripples is so fast that they tend to flatten out and the turbulence in the flow creates bed forms with longer wavelengths called dunes. And this transition between these two bed forms is usually abrupt. And the wavelength of the dunes is significantly greater or the spacing between crests is significantly greater in dunes than it is in ripples and I didn't draw it quite to scale here but the height is also larger than for dunes than for ripples. Then as the flow speed increases even more the dunes start eroding quite more and more and the bed flattens out basically the downstream speed of the water flow is so fast that the grains are eroded off the tops of the dunes and deposited in the troughs very quickly. That's often a gradational change into what we call into this planar bed configuration. Then in flumes if the water is shallow enough and this line here and each one represents the water depth there is a feedback between the bed and the flow and you start to get deformation of the surface of the water that is in phase with the bed forms on the bottom. And so these are actually called anti dunes and that's because they actually migrate upstream instead of downstream. So what's happening with the anti dunes is so this is your sediment surface here. The flow is is hugging the surface and there's no separation zone on the downstream side and so what happens is that grains here get pulled out into the flow and the grains here get plastered into the upstream side of the dune. So the migration of the dune through time is actually in the upstream direction so that a new lamina would be something like this. So you have the water flow in this direction but then you have the dune migration in the opposite direction. So these anti dunes are not very stable bed forms and they only form if the surface of the water can mimic the shape of the bed form. So scientists have also done flume experiments with the flume closed on the top. So this sequence of bed structures inhibits the surface of the flow from changing. And what you see in this particular case is the same sequence where you start out with a flat bed and as the flow speed increases you start to get ripples and then dunes and then planar beds here but you don't actually get the anti dunes because the geometry of the experiments inhibit the formation of those dunes. So what we actually see in the rock record are ripples, dunes, and planar bedding. Yeah, it's a typical thing.