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Permineralization
Permineralized fossils occur when mineral deposits replace the organic matter in a deceased organism. Often this process occurs in areas with hard water. Hard water has high mineral content. When the water flows around the organism, minerals become deposited in the organic tissue. When all the organic tissue is gone, minerals deposits will remain, retaining the shape of the organism. Fully mineralized fossils are called petrified.
Impressions
Impressions are two-dimensional remains that occur when organic matter comes into contact and changes inorganic matter. These fossils are usually impressed in soft soil or clay. Over time this soil hardens, preserving the organism's outline or structure. These fossils do not retain any organic matter. Popular examples of impression fossils include footprints, plant prints and holes made by burrowing organisms.
Desiccation
In arid regions the warm, dry climate dehydrates organic matter, resulting in well-preserved fossils. This process is sometimes known as mummification. Desiccated fossils are so well preserved they have been known to retain their original skin and hair color. These fossils are also extremely fragile and can break if handled without care.
Molds
Molded fossils are natural three dimensional models of the organisms. They occur when the organism is pressed into soft, porous rock. The hardest parts of the organism, such as the skeleton, teeth or shell, make an impression in the stone. As the remaining organic matter decomposes, the hollow spaces are filled in by sediment. The result is a model of the organism that does not retain any of the organism's original matter.
Freezing
When organic matter freezes, the fossils that result are some of the best preserved. In animal and human fossil remains, organs, muscles, tendons and skin are often found intact. Freezing preserves organisms so well because it prevents soft tissue from decomposing, trapping the organism in a static environment.