A friend and colleague found a fossil and brought it in to show me. It is a small leaf in a rock. He asked me how the rock was formed, but I couldn't say as I know nothing of such things. The rock is layered. You can see the various layers by looking at the rock edgewise. In fact, the edge is so ragged that you can see fossils of leaves in several places. It looks like you could peel the layers of the rock. Indeed, the ragged edge shows that the bonds in one of the axes are weaker than those in the other two. I guess this is sedementary rock formed by layers of silt building up and then compressed into hardness. The rock is quite hard, not like compacted silt, but more like melted and recongealed silt. My question is what could cause the silt to form such rigid bonds in the horizontal directions and such weak ones in the vertical direction? Or have I got the whole picture messed up? Edit- Now that I think about it a little more, the bonds in the vertical direction are just as strong as the ones in the horizontal direction. The rock can only be split at certain places. Perhaps 50 per inch or so. It's as if the sedement collected and hardened before the next layer was deposited. But what could make it harden?