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Geologic periods can be determined or identified using the different fossils found in the sediments. However, in order for an organism to become a fossil a couple of things have to happen. The organism needs to die in or near water and then be buried rapidly. This preserves the specimen in the best possible way. Since igneous rock (rock formed from cooled magma), and metamorphic rock (rock that is changed by intense pressure and heat) do not hold any fossils, they are not the type of rock in which to look for fossils. Fossils are only found in sedimentary rock. Sedimentary rock is laid down in layers caused by wind, erosion, rain and bodies of water moving particles of minerals and depositing them one after another in what becomes large sheets of deposits. Trapped in these deposits are the organisms themselves that become the fossils we seek today. So how do these organisms tell us the age of the sediments in which they are found?
Paleontologists really like species that existed for short periods of time as they give us an indication of the age of the sediments they are found in. An organism that existed for no more than say a few hundred thousand years, and is found all around the world, indicates that the sediments it was found in are the same age even though the sediments may look very different. Sediments can be laid down under very different conditions throughout the world but the species itself is the same, so the different locations and sediments must be the same age. These types of species are referred to as index fossils or guide fossils. Since sedimentary rock is laid down a layer at a time, we can surmise that the top layer is the newest and the bottom layer is the oldest. There have been lots of movements in the earth's crust with upthrusts, down thrusts, plates shifting, etc., but a careful study of the layering can give us a pretty good idea of what is older and what is newer. We call this comparative or relative dating. So a fossil found all around the world that is lower in the layers of rock than those above it is considered older and indicates that the rock it was found in is older too. By observing and recording enough layers and the specimens found in each, you see the progression of the fossil record.