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The principle of superposition states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it.Logically a younger layer cannot slip beneath a layer previously deposited.The principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut.

The principle becomes quite complex, however, given the uncertainties of fossilization, the localization of fossil types due to lateral changes in habitat (facies change in sedimentary strata), and that not all fossils may be found globally at the same time.Geologists can also give precise absolute dates to geologic events.Geology also plays a role in geotechnical engineering and is a major academic discipline.The following four timelines show the geologic time scale.The second scale compresses the most recent era, so the most recent era is expanded in the third scale.

Since the Quaternary is a very short period with short epochs, it is further expanded in the fourth scale.Geology can also refer generally to the study of the solid features of any celestial body (such as the geology of the Moon or Mars).Geology gives insight into the history of the Earth by providing the primary evidence for plate tectonics, the evolutionary history of life, and past climates.In geology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock.There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes.For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer.