A cast fossil preserves the impression of the hard parts of an organism, such as a shell or exoskeleton. Cast fossils are most commonly found in sandstones and other porous rocks. Since hard parts do not decay as quickly as softer tissues, they are most likely to leave cast fossils.
As the softer parts of an organism decay and dissolve, a detailed impression of the organism is left behind in rock. Cast fossils result when minerals are deposited in these molds. Artificial cast fossils can also be created by using plaster of Paris or latex to fill an existing mold.