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Sedimentary rocks are formed from layers of sediment and contain many fossils, including delicate features. As sediments are buried, they undergo pressurization and compaction, leading to the formation of fossils through mineralization and cementation. Fossils can also preserve footprints, tracks, and burrows.
Most of the earth’s surface is covered in a type of rock called sedimentary, because it forms from layers of sediment that pile up on top of each other. One of the characteristics of sedimentary rocks is the high amount of fossils they contain. All kinds of extinct plants and animals are preserved in the sand, silt or mud of distant eras.
Over time, animal corpses on sedimentary rock surfaces become covered in silt. Bones undergo mineralization, whereby minerals replace the original organic matter to leave a mold for the body. Over long periods of time, the organism is completely replaced by durable minerals. In the petrification of wood, cellulose and wood fibers are replaced by minerals such as silica.
Delicate features are also sometimes preserved in fossils. The eyes of flies and the delicate wings of butterflies have been found preserved in fossils. Most fossils, however, are thick shells or skeletons. Animals with durable fossils are also among the best studied and understood.
It’s amazing that something as soft and thin as sand can transform into something as stiff as a fossil. The reason is in the pressurization that the sediments undergo as they are buried more and more. For every 100 feet (31 meters) of depth, the average temperature increases by about 1°C. Average pressure increases about 1 ppsi (pounds per square inch), or about 7 kilopascals, for every foot (31 centimeters) of depth. The increase in pressure and temperature leads to a process called compaction, whereby the delicate particles in the sediment are worked tightly into each other and become rock.
The mud is purified by water during the compaction process, becoming rock. Sometimes the increase in heat leads the material to undergo chemical transformations, called cementation, which cause the formation of films on the surface of the sediment of minerals such as calcite, silica and iron oxide. This is how fossils are made. The minerals also occupy pockets of air between the sand molecules.
Fossils can also preserve things other than real bodies. Footprints, tracks, paths and burrows have been found. Since the coal itself is a compact vegetation, many fossils can be found within it.
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