Another type of impression is carbon. Leaves and soft-bodied animals are buried in soft mud. As the plant or animal matter decays, a coal-like film of carbon is left behind which shows the fine lines of the leaf or animal. Foot prints are another type of fossil record. Tracts of animals may be left in soft mud.
Some fossils are molds and casts.
A mold is a cavity left in the rock when groundwater dissolves and removes a shell or bone around which sand or mud already has hardened. If a mold later is filled with mineral matter or mud, the filling is a cast. A cast has the same shape as the original organism. However, when it is broken open, the lack of inner structure shows it to be a cast. Mineral matter may seep into shells, wood or bone openings.