Over time, the layers of silt (sediment) that form by having been brought together by wind or water become harder and harder. Eventually the layers form rock. Geologists call that kind of rock "sedimentary rock." (Say "sedd-ii-MENh- ree.")

If you put your hand down onto a table and then put a book on top of it, there will be two layers. Then get another book, of a different color and thickness (for variety) and put that on top of the first book. The second book adds weight, and you can feel that on your hand. This is similar to the weight of new silt being deposited onto the older silt. You can put as many books on there as you like, until it becomes uncomfortable; each addition, however, will cause more pressure (weight) to be felt by your hand.

Another way to duplicate this is to squish some soil in your hand until it is no longer loose, put it on some waxed paper, then squish some more soil (maybe even put a little water in it), and layer that on top of the first batch of squished soil. If you push down hard on the top layer, it will make the bottom layer harder than it was. Just like Nature does it! There is a rock called "shale" which is formed in just this way, and you can fracture it with your bare hands. When you do this, you can see that the fracture reveals the layers of sediment that formed the shale.

Sometimes, when you fracture a piece of shale, you can find a fossil!