Fossils are imprints left during the decay of a plant or animal. They are buried within sediments that are not disturbed for long periods. Mud is often a medium for fossil development since is can become shale or limestone over time. The study of various types of fossils have allowed us to learn about the structure and behavior of life many thousands of years ago.
Mold fossils are fossil impressions left after the animal or plant decomposes. They are negative images and often formed by the animal being left in acidic groundwater. Silt deposits form over the animal or plants impression after the shells or organic matter dissolves. An imprint is left. This "void" is called a mold.
Cast fossils are remnants of an animal or plant. They are slightly filled-in impressions that are a positive image of the animal, plant or fish. Cast fossils occur when organic matter dissolves, leaving a carbon film behind. Carbon copies can include leaves, fish, flesh and worms, as well as arthropods.
True-form fossils are body parts of an animal or remains of a substance. They are often formed when the organism is caught in ice, tree sap or tar. Complete features are left behind. True-form fossils can be millions of years old and perfectly preserved. Even soft tissue can remain intact.
Trace fossils are marks left by an animal or plant that has made an impression. Trace fossils include nests, burrows, footprints or any other markings of the animal's time on the earth. The structure of the animal or plant remains as a mineral form. The colors of the minerals that replace the form can be dazzling. Sometimes they are made into art and jewelry.