How do Fossils get Inside Rocks?
Most of the earth’s surface is covered in a type of rock called sedimentary, because it forms from layers of sediment building on top of each other. One of the characteristic features of sedimentary rocks is the high amount of fossils that they contain. All sorts of extinct plants and animals are preserved in sand, silt, or mud from ages long past. Over time, animal corpses on sedimentary rock surfaces are covered by silt. The bones undergo mineralization, whereby minerals replace the original organics to leave a cast of the organism. Over great lengths of time, the organism becomes entirely replaced by durable minerals. In the petrification of wood, the cellulose and woody fibers are replaced by minerals such as silica. Even delicate features are sometimes preserved in fossils. The eyes of flies and the delicate wings of butterflies have been found preserved in fossils. The majority of fossils, though, are thick shells or skeletons. Animals with lasting fossils are also some of the mo
