Flexi Says: Most fossils form when an organism dies. Their bodies get buried with sediment. Layers of sediment slowly build up. The sediment is buried with even more sediment. Over time, the sediment turns into sedimentary rock. The remains of the plant or animal also turns to rock. The remains are replaced by minerals. They literally turn to stone. Molds and casts are another way organisms can be fossilized. Have you ever walked in soft mud and left footprints? Once in a while, these traces of organisms can be preserved. In this case, nothing is left of the organism. A mold is an imprint of an organism that is preserved in rock. The organism's remains break down completely. There is nothing left of the original plant and animal. What is left is a mold, or impression, of the original organism. This mold is then filled with other rock. The fossil that forms in the mold is called a cast. Molds and casts usually form in sedimentary rock.
