 Okay, back in the 17th century, there was a natural scientist by the name of Nicholas Steno and he was in a sense one of the first sedimentologists and he made the observation about natural systems when you're laying down sand and mud and he made three really really fundamental observations. One is the principle of original horizontality. In other words, if you lay down layers of sediment they're horizontal and the other really fundamental principle there is the principle of superposition. In other words, as they are laid down the younger ones are on top of the older ones. So you have the first layer, second layer, third layer and so on. The third one of Steno's principle is the principle of lateral continuity. In other words, these layers will go all the way to the edge of whatever basin you're in.