- A fossil mold is an outline or negative image of an animal, or occasionally a plant. Some fossil molds preserve a complete image of the organism, while others only show a part of it. Often, mold fossils contain cast fossils shaped like the original plant or animal. Sometimes, however, the cast fossil has been removed or destroyed and the mold fossil is all that is left.
- Usually, mold fossils form under water. An animal falls to the bottom of a body of water. Usually, it is partially eaten by organisms, leaving only bones and teeth, or shell. Before the animal finishes decaying, however, it is covered by a layer of silt. Over time, more and more dirt builds up on top of the remains.
- As layer after layer of silt build up, the pressure on the mold increases, causing it to turn into stone. Ground water rich in minerals seeps in and dissolves the remains of the organism, usually replacing it with a cast in the same shape. Both the mold and cast fossils are now complete, but are buried far below the ground and under water.
- Before it is found, the fossil has to come to the surface. One of the most likely ways for this to happen is through continental upthrust. Where continental plates collide, one or both are often thrust upward, forming a mountain. Wind and water will then slowly erode the mountain, exposing fossils inside of it.
