When sediments move and settle somewhere, they are being deposited. When, over a long time, layers and layers of sediments get deposited on top of each other, the weight of the top layers presses down on the bottom layers, forming them into rock called sedimentary rock. The oldest layers are on the bottom, and the youngest layers are on the top. Because sediments sometimes include once-living organisms, sedimentary rock often contains a lot of fossils.
