 So we've been driving around the West, pointing at rocks, there is a rock and saying this is a sand dune, this is a lake, this is an ocean, but we haven't spent a lot of effort on why do we say one is a sand dune and one is a lake and one is an ocean. Now in the case of the sand dune, when we looked at it, it was all sand. There were no little pieces, there were no big pieces, it was all intermediate sized sand pieces. They had these structures that one sees in modern sand dunes. If you found anything that looked like a fossil, it was little reptile tracks, and so they had all the characteristics that you see in a modern sand dune. Now here we're in rocks that are very different, the rocks behind us are sort of washing away and they're making mud that washes down around our feet, and there's a number of characteristics of these. The layers are different, how big the grains are in them is different, sort of everything about them is different, and we can say these were rocks that were underwater, and we could go through the whole list of reasons why we can say they're underwater, but there's one very easy one. If we look down at our feet and we look around at what's washing out of these rocks, what we find is oysters, and one can be very confident that if you find rocks full of oysters, you're not in the middle of the desert.