In addition to these principles that apply to all sedimentary rocks, a number of other important characteristics of sedimentary processes lead to the development of distinctive sedimentary features in specific sedimentary environments. The principle of faunal succession states that there is a well-defined order in which organisms have evolved through geological time, and therefore the identification of specific fossils in a rock can be used to determine its age.For example, the cobbles in a conglomerate must have been formed before the conglomerate. The principle of inclusions states that any rock fragments in a sedimentary layer must be older than the layer.The principle of superposition states that sedimentary layers are deposited in sequence, and that unless the entire sequence has been turned over by tectonic processes, the layers at the bottom are older than those at the top.The implication is that tilted sedimentary layers observed to day must have been subjected to tectonic forces. The principle of original horizontality states that sediments accumulate in essentially horizontal layers.Through careful observation over the past few centuries, geologists have discovered that the accumulation of sediments and sedimentary rocks takes place according to some important geological principles, as follows: Chapter 6 Sediments and Sedimentary Rocks
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