The BGS Geological Timechart is based on geochronology. This is the branch of earth sciences that deals with the concept of geological time and dating the sequence of events throughout the Earth's history. Intervals of geological time are given formal names and grouped into a hierarchy according to their length: Eon, Era, Period, Epoch, Age and Chron, in decreasing time intervals. Subdivisions are termed 'early', 'mid' or 'late' e.g. Early Jurassic.
Chronostratigraphy is similar, but is concerned with the relationship between time and the rocks deposited within those time intervals. Chronostratigraphical terms are applied to rocks deposited during specific intervals of time; in order of decreasing time, their hierarchical grouping is: Eonothem, Erathem, System, Series, Stage and Chronozone (with subdivisions into 'lower', 'middle' and 'upper') e.g. Lower Jurassic.
Although the age of rocks can sometimes be determined directly, by radiometric dating, in most instances rocks are dated indirectly, especially by means of their fossil content. Historically, named divisions of geological time and their associated chronostratigraphical divisions have been developed independently in different parts of the world, and a major task for the world stratigraphical community has been to establish a single, universal scheme for the subdivision of geological time. There is agreement at the levels of Eon, Era and (for the most part) Period, but regional terms continue to be widely used at the lower hierarchical levels. This is because in the geological past, just like today, different environments existed on different parts of the Earth's surface. Thus at any one time, deposits of different type and different fossil content were being deposited in different regions of the globe.
For parts of the geological column, correlation between these different regions is sufficiently well understood for the establishment of universal subdivisions at the Epoch/Series and Age/Stage level. For some intervals and some regions, however, global correlations are poorly understood and the rocks are assigned to divisions of regional application. For example in the British Isles (and in most cases also in north-west Europe) regional divisions are used for Cambrian series, Ordovician stages, and Carboniferous epochs and stages. A particular problem exists in relation to chronological subdivision around the Jurassic-Cretaceous boundary, where no fewer than three schemes are currently applied in different parts of the British Isles. For this reason some terms used by the BGS differ from Gradstein and Ogg (1996) in a few parts of the column.
On the BGS Geological Timechart, epoch boundaries (era boundaries in the Precambrian) have been assigned radiometric ages taken from the component timescales. Neither the dates nor even the boundaries themselves are fixed, however. Improved radiometric dating of strata is continually taking place and sometimes leads to improved age assignments for individual chronostratigraphical boundaries. Eventually, these piecemeal changes are incorporated into a revision of a complete geological timescale, such as used in this chart. The precise definition of a chronostratigraphical boundary may also be changed by international agreement, in which case it may lie at a level younger or older than that previously established. For example, the base of the Quaternary Period has been traditionally placed in north-west Europe at a level dated at around 2.3 million years before present (2.3 Ma), whereas a recently agreed international definition places the boundary at 1.8 Ma. Similarly, some stratigraphers place the base of the Cretaceous Period at the base of the Berriasian Stage (144 Ma), whereas others place it at the base of the Ryazanian Stage, at 142 Ma. Pending international agreement on the boundary, we here we follow Gradstein and Ogg (1996) in placing it at the base of the Ryazanian Stage.
COWIE, J W and BASSETT, M G 1989. International Union of Geological Sciences 1989 Global Stratigraphic Chart with geochronometric and magnetostratigraphic calibration. Supplement to Episodes, Vol. 12, No. 2.
FORTEY, R A, HARPER, D A T, INGHAM, J K, OWEN, A W, and RUSHTON, A W A 1995. A revision of Ordovician series and stages from the historical type area. Geological Magazine, Vol., 132, 15 - 30.
GRADSTEIN, F M, and OGG, J 1996. A Phanerozoic time scale. Episodes, Vol.19, 3 - 5.