What is a cast 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.