The Process of Sedimentation: The Formation of Fossils

Both animals and plant fossils are found in some sedimentary rocks.

In some areas of the world, certain types of rocks contain fossils of both plants and animals that lived many millions of years ago. These now-permanent impressions give us a glimpse into what life was like in the Earth's ancient history.

Initial Formation

For a fossil to form, animals or plants first fell into mud or silt in water and settle. Over time, more layers of silt or mud covered the remains, compressing them between the layer below and the one above.

Preservation

The anaerobic environment in this sediment initially prevented the remains from decomposing. Pressure from additional layers of sediment that formed above the remains compressed them flat, forming an impression between the layers of silt.

Impression

Pressure and heat eventually changed the silt into layers of finely grained rock, such as shale and slate, commonly called sedimentary rock. Over many centuries, the matter in the remains was slowly dissolved, with the resulting space acting as a mold that filled with minerals, preserving the impression as a fossil.

Geological Changes

As time passed and the waters receded from geological changes, the once-wet regions became dry land. Further geological changes exposed the rock surface, making it possible to see the fossils within.

Collecting

Many people hunt fossils in geological formations. Removing large sections of rock, then split them along the layers, hoping to reveal the fossil of a fern, fish, trilobite or other life that existed millions of years ago.

References

• Northern State University: Fossils of the Badlands of South Dakota
• University of California Museum of Paleontology: The Green River Formation
• University of Alabama: Sediment and Sedimentary Rocks