 Sequence of rocks observed in estuaries are different than those in deltas. Estuaries form when sea level is going up and flooding a river valley. So if you were to look at a sequence of rocks that represents accumulation in an estuary, you would see that evidence of the increase in sea level. So if we were going to draw a stratigraphic column through time using Walther's law, the ocean water is going up. And I've mentioned that this is a flooded river valley. So if we took a point right here, originally it was an environment with a fluvial channel. And so there might have been some mud deposited, but then the channel itself is erosional on one side. And if it has pebbles, it could have quite coarse-grained sediments. And then we have it finding up, representing the migration of a point bar in a meandering river. So this shows the grain size change. We have our mud here. And then we can have trough cross stratification, maybe some upper planar cross lamination here. Some trough cross stratification. And then at the top of the point bar, it's often rippled. And if we had fluvial deposition for quite a while there, we might have another channel. Again, finding upward with a sequence of structures, including, say, trough cross stratification and ripple cross lamination. So these represent the fluvial deposits. When the ocean starts going up, you start to see the evidence of tides. I drew this as a tidally influenced estuary here. And so as the sea level goes up, maybe you end up reworking some of the sandstones. And you see the two directions of flow. So I'm going to draw my current ripples showing opposite directions of flow here, which will be a shorthand for herringbone cross stratification. It could be that you also start getting more mud. Even though there's a river coming in, if the tide's coming in strongly, you can get mud settling from suspension. So we will add a little bit of mud in here. And it can be exposed at low tide. And so we might have some mud crocs. And then maybe we actually have an area where marsh develops. And so we have some root casts in the fine grained area. Maybe there's a little bit of sand in there. So this part in here, the mud cracks and the roots and the plants is consistent with either an estuary or possibly a floodplain deposit. But then the tides keep working the sediment back and forth. So then maybe you end up with this bar, like the one the star is right next to. And in that bar, you'll see evidence of tides again, including the ripple cross lamination. Sometimes you can also get trough cross stratification. And then maybe there are some mud drapes in that zone as well. And reactivation surfaces would be another feature. As the water level keeps going up, the water depth might get finer. You still have transport in two directions. I should say the water depth will get greater, not finer. The grain size gets finer. Maybe you see a lot of mud cracks in this zone here, but the estuary floods and you get mud, but there are no longer any mud cracks and there are no longer any roots because the water is too high for that area to be exposed. But especially if there are times when there are floods with the river bringing in a lot of sediment, you can still get some of these sandstones and you might still see evidence of two directions of flow. So if we look at this in terms of the change in depositional environment through time, we're going from something that's dominantly fluvial to something with tides, and maybe more evidence of mud cracks and roots at the bottom and deeper water deposits at the top. So if we looked at the environments through time, an estuary shows the sequence of environments getting deeper with time. That's the opposite of what you see with the delta, building the sediment out through time. You can see these same rock types, these same faces, but the order would be different because the delta's environments get shallower through time. So deltas and estuaries can transfer back and forth between each other, so estuaries almost always form when sea level is rising, but if it falls again, the river will extend out towards the ocean again and it could start building a delta on top of the estuary. And if that was going to happen, we'd sort of see the opposite of these faces again. We'd start to see the shallower environments with the mud cracks and then go back into the fluvial faces with the river channels. Thanks for watching.