Eras are geological time units. All the divisions are systems. They refer to rocks formed during a particular interval of time.Dating PrinciplesThe geological column shows the relative ages of rocks and dates from absolute dating.Relative DatingRelative ages use comparisons rather than years. The Jurassic system is pre Cretaceous and post Permian. The main relative dating principles are: the law of superposition states that sedimentary layers are deposited one on top of the other, so that higher layers are younger than the lower ones the law of original horizontality states that all sedimentary rocks were originally laid down in a nearly horizontal position. The law of cross-cutting relationships states that faults or igneous intrusions must be younger than the rocks they cut through. The principle of included fragments states that any sedimentary rock that has fragments of another rock in it must be younger than the included rock fragments Dating Using FossilsA few fossils are very distincitve in particular systems. Ammonites, are restricted to Jurassic only. Therefore we can use fossils to say that a rock belong to a particular system. If we understand the sequence of fossils we can say whether one rock is younger or older than another.Absolute DatingAbsolute dates of rocks for all the geological systems can be obtained using the principle of radioactive decay. Radiometric dating methods are used to put actual dates on the geological column. For example, it is known that the Cambrian system contains rocks that are between 542 and 488 Ma.