Fossilization is dependent upon where organisms grow and how quickly they are covered by sediment. Rarely do paleobotanists find the fossil remains of whole plants. Usually only fossilized parts of plants are found. Fossilization occurs in many different ways. Three common methods of fossilization are compression, impression, and molding or casting.

Compression fossils are often formed in water, where heavy sediment flattens leaves or other plant parts. The weight of the sediment squeezes out water present in the plant tissue, leaving only a thin film of tissue. An impression fossil is an imprint of an organism that is left behind when the organism’s remains have been completely destroyed, leaving only the contour of the plant. Fossil molds and casts are formed when animal or plant tissues become surrounded by hardened sediment; the tissue then decays. The hollow negative created by the tissue is called a mold. When fossil molds fill with sediment over time, the sediment often conforms to the contours of the mold, resulting in a fossil called a cast.