In addition to being a descriptive science, geology also has historical aspects. We attempt to explain not only the processes which are ongoing, but also the sequence of events which have led to present conditions. In order to do so, he must not only have a grasp of the immutable, unchanging physical and chemical laws which control these on-going processes, but also a concept of change through time. Although there are many ways of defining and viewing time, to a geologist there are but a few practical ways: absolute (radiometric) time, relative time and magnetostratigraphy. Absolute time is measured in an actual number of years, expressed in dates such as 1,000,000 years B.P. (=before present). In relative time the emphasis is not on dates, but on a sequence of events. A period of erosion, followed by uplift and metamorphism is such an example of a sequence of events. 

Of these ways of reckoning time, relative time is much more important. Just as a series of dates is of little importance to a historian except to order events, (after all what does 967 CE mean in and of itself?), an absolute date by itself provides little information to a geologist. Only within a framework of relative time, of sequenced events, do patterns, events and even absolute dates acquire meaning.