 Okay, so the bed forms that we get vary with the flow speed. So at low, relatively low, flow speed, fast enough to transport sand for example, we tend to get ripples, which we've been talking about here. So ripples are relatively small, and basically the wavelength at the distance from crust to crust is often 50 centimeters or less. They also don't grow very high, if they're pretty closely spaced those slopes can't get very steep. And so usually the height is less than a few centimeters. So we'll say the height is less than four centimeters here. So as the flow speed increases, the tops of the ripples tend to get eroded off very easily and the distance that grinds saltate gets large enough that it can be 50 centimeters or more. And so you end up with a change in the geometry. So as the flow goes up, we can say that the ripple crusts erode and the saltation length is greater than say 50 centimeters for many grains, not all of them. It depends a lot on the grain size. But what happens then is that the ripples flatten out and you tend to get much larger bit forms that reflect that longer saltation length and you get dunes. So these are subacreous dunes, separate from windblown dunes, but they tend to be, they have a very similar geometry to ripples, but they tend to be much larger. So one of the interesting things is because of the interaction of the sediment and the fluid dynamics, there's not a gradation in size between these ripples, have a maximum of 50 centimeters for the wavelength, but dunes have a minimum of about 80 centimeters in wavelength. And they can actually be meters or even tens of meters in some cases. So there's a distinct jump in size when the ripples start washing out because the flow speed gets too high and you form dunes. So then when you get to a high flow speed, the dune crusts start eroding away and that tends to flatten out the dunes and the grains are just moving downstream so fast that they're not really quite saltating anymore. There's not very much upward motion and you end up with flat bed, you lose your bed forms here and you end up with planar lamination, we usually call it upper planar lamination and that's because it's at the high flow speed. So one of the really nice things about this is that as flow speed changes, we get different bed forms that gives us different cross lamination, cross stratification and planar lamination in the rocks and so we can use the geometry of the layers in the rocks to tell us about flow speed. So that works in addition to the grain size variations with flow speed and so we have two indicators that really help us a lot to interpret the flow from sedimentary deposits and in sedimentary rocks. Thanks for watching.