What Are Rock Fossils?

Sedimentary Rock

• Sedimentary rocks are one of three main types of rocks found on the Earth. According to Western Michigan University, sedimentary rocks are the only type of rocks that contain fossils. They are formed in layers as gravel, sand and organic material, to name a few, are deposited and then harden over thousands of years. Examples of sedimentary rocks that typically contain fossils are limestone, shale and sandstone.

Time

• When organic material, including living or dead plants or animals, is trapped between layers of sediment, they go through chemical changes. Over the years, the organic material can crystallize, for example, and can then be preserved in the sediment as it hardens into rock. According to Western Michigan University, fossils that are older than 10,000 years are considered prehistoric, while fossils younger than 10,000 years are considered historic.

Molds and Casts

• Rock fossils can be either molds or casts. Molds are a direct impression of what used to be an organism; it appears to be pressed into a rock. Casts are a duplicate of what used to be an organism; this duplicate formed when a mold filled with sediment. The sediment hardened into rock form in the three-dimensional shape of the organism.

Compression

• These types of fossils are found between layers of sedimentary rock. The organism was flattened when the overlying layer of sediment flattened the organism. Sometimes what is found is a literal carbon copy of the organism, because its organic parts convert to carbon and the exact, one-dimensional form of the organism is preserved in the rock.