Forming Fossils

First Fossils

What are molds and casts?

Molds and casts are types of fossils. After burial, a plant or animal often decays, leaving only an impression of its hard parts (and less often, soft parts) as a hollow mold in the rock. If the mold is filled with sediment, it can often harden, forming a corresponding cast.

Fossils are not always bones. These fish fossils are not actually bones, but rather imprints the fish made in the soil. (iStock).

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