An imprint fossil forms when an object is pressed into mud, such as a leaf or insect. As it slowly dissolves, the mud surrounding it can harden and a space is left. A mold of the original object is formed, which fills with sediment and becomes a fossil. This is an example of an imprint fossil. Fossils can also form when an object, such as a dinosaur bone, is buried underground for a very long time.
Water can deposit minerals in the tiny spaces in the bone, called pores, and it turns into rock.
This is known as petrification. Another way bones turn to rock is called replacement. This document has been accessed 44,586 times since November 12, 1996.