Recall that the principle of superposition simply states that the oldest layers of rock are found on the bottom of the strata and the youngest rocks are found on the top. Using the same principle of superposition, the relative age of fossils can be determined. This method also helps determine the relative age of rocks.  When the same kinds of fossils are found in rocks from different places, geologists can determine that the rocks are the same age. The fossils in the picture above are index fossils. Each index fossil existed during a specific age of time and they can help date the age of rocks. The youngest fossil in the picture is called a bivalve and it existed only during the Tertiary period. Since the bivalve was alive during the Tertiary period, its fossils are found in certain layers of rock. The bivalve fossil is an index fossil and its location can help determine the age of rocks. An index fossil is a type of fossil that is always found in a specific layer of rock all over the world. Fossils have helped piece together the positions of the different continents at different times in the past. Correlating fossils from rock layers can help geologists match the age of similar rocks in different and distant locations. The fossils in the rocks indicate that although the rocks are made of different material, they formed at the same time because they contain the same types of fossils. Click on and complete each of the following webquests to learn more about how fossils are important in determining the age of rocks: Geologic Time and Getting into the Fossil Record.

fossil Paleontologists are scientists who study fossils and life in the past. Fossils are the preserved remains or traces of organisms from the geologic past. Several types of fossils form after an organism has been buried. Fossils can also form without being buried if the organism becomes frozen or stuck in amber or tar. The different types of fossils are petrified fossils, molds and casts, carbon films, preserved fossils, and trace fossils.

Petrified fossils form when the remains of organic organisms turn into stone.  The process involves replacing the cell walls or other solid parts of the dead organisms with minerals. The minerals in groundwater can soak into the cavities or spaces of the organism.  When the minerals precipitate from the water or when the water evaporates, the minerals become a substitute for the organic matter.  Petrified wood is an example of a petrified fossil.

Many dinosaur bones are examples of molds and cast fossils.  A mold fossil is formed when the remains of an animal or plant is buried under layers of sediments which is later dissolved by underground water.  The mold is a hollow impression, the shape and surface markings, of the organism left inside a rock.  If the hollow spaces are filled with mineral material, it will form a cast fossil.

All living things contain carbon.  The formation of carbon film fossils begins when an organism dies and is buried under a layer of fine sediments, mud or clay.  A thin carbon rich film or imprint of the surface features of the organism remains after gases and liquids are removed from the decay organism.  The carbonized imprints of fish, leaves, and flowers have been found in sedimentary rock.

fossil imprint image Preserved fossils are the remains of organisms found in tar, amber, and even permafrost. Sometimes all or part of the organism looks exactly the same as it did when it was alive. A mammoth was found frozen in permafrost. Most bacteria can not survive very freezing temperatures; therefore the mammoth buried in frozen soil or ice did not decay.  If an insect is trapped in amber, its remains can become preserved when the sticky sap hardens.  The remains of animals can also stay well-preserved if they get stuck in tar. Scientists hypothesize that animals drinking water in a tar seep will inevitable become unable to move.  The tar seep is inescapable and over time the animal is covered by the tar.

Trace fossils are different from the previous fossils mention above because they are not the remains of an organism; but evidence of their activities found in sedimentary rocks.  Their fossilized movements such as tracks, footprints, borings, and burrows provide evidence of prehistoric life.  For example, trace fossils can provide us with information on where the organism lived, rummaged, and also their diet. Two more types of trace fossils are coprolites and gastroliths.  Coprolites are fossils of dung and stomach contents.  Gastroliths are stomach stones found in the digestive system of prehistoric reptiles.  They were used in grinding their food. Paleontologists use trace fossils to not only learn how animals from the past lived but to also compare their daily activities with modern animals. Therefore the existence of trace fossils in rock strata can help determine the type of environment and local conditions that occurred in the place and time that the impressions were created. There are records of trace fossils from different rock strata from locations all around the world. Trace fossils and other types of fossils are important in helping geologists understand the rock record.  They provide information about the formation of the geologic features on the surface of the Earth.

Write your answers to these questions. Then click the question to reveal the answer.

What is the law of superposition?

The oldest layers of sedimentary rock are found at the bottom or below the layers of successive younger layers of rocks.


What is a fossil?

Fossils are the preserved remains or traces of organisms from the geologic past.


What is an index fossil?

An index fossil is a type of fossil that is always found in a specific layer of rock all over the world.


What is a paleontologist?

Paleontologists are scientists who study fossils and life in the past.


What can fossils tell us about how geologic features have changed over time?

The fossils of trilobites have been found in mountains even though they lived in the ocean. Therefore, the tops of many mountains were once covered by oceans.


Of the three types of rock, which is most likely to contain fossils?