Cast and Mold fossil

One form of fossil forms when an organism dies and its body is covered by layers of sediment. As time passes, the organism itself, particularly when composed primarily of softer materials, is eroded and carried away, leaving a negative imprint of its body. This type of fossil is called a mold. A cast fossil forms when a mold fossil is filled with some form of mineral, usually through the seepage of water depositing the minerals within the mold. In time, the mold is filled and the materials deposited harden, creating a replica of the original fossil. This is called a cast fossil.