It is important to realize that with new data about the subdivision or correlation of relative time, or new measurements of absolute time, the dates applied to the geologic time scale can and do change. Revisions of the relative time scale have ocurred since the late 1700s when geologists first started to realize there was a suprising level of consistency to the order of events preserved in the Earth's geology. The numerical calibration of the time scale has been also been continuously refined since about the 1930s, when radiometric dating started to become available, although the amount of change with each revision has become smaller over the decades as more precise data has become available (see fig.1.5 and 1.6 of Harland et al., 1990).
Like any good scientific measurement, every dated boundary between the geologic periods has a level of uncertainty associated with it, expressed as a "±X millions of years". These can not be included in the diagram for practical reasons, but if you are interested, the details can be found in references like Harland et al., (1990). It also includes a detailed description of the history and methodology involved in constructing a geologic time scale.
Because of continual refinement, none of the values depicted in this diagram should be considered definitive, but, on the other hand, many of them have not changed greatly in many years, and are very well constrained. For example, the Cretaceous/Tertiary boundary has been estimated at about 65±1 million years for decades, and has been tested many times, with almost all dates falling in the range between 64 and 66 million years. The overall duration and relative length of these geologic periods is unlikely to change much.
One of the largest changes in recent years has been revision of the Precambrian/Cambrian boundary from about 570 million years ago to about 545 million years ago (refer to Grotzinger et al., 1995), and changes in the upper boundary of the Cambrian period from about 505 million years ago to about 495 million years ago.
Geologic time is finely subdivided in even more detail than depicted here (refer to Harland et al., 1990 and Gradstein and Ogg, 1996), but most of the finer subdivisions (e.g., epochs) are only used by specialists, except in the Tertiary Period. Because of the vast difference in scale, the Tertiary subdivisions can only be shown on a more complete time scale.
Blatt, H.; Berry, W.B.N. and Brande, S., 1991. Principles of Stratigraphic Analysis. Blackwell Scientific Publications: Boston. 512pp. ISBN 0-86542-069-6. [This is a good introduction to many of the principles geologists use to understand Earth history. Chapter 4 provides an introduction to geologic time.]
Gradstein, F. and Ogg., J., 1996. A Phanerozoic time scale. Episodes, v.19, no.1&2. [For a more detailed account, also refer to: Berggren, W.A. et al., 1995. Geochronology Time Scales and Global Stratigraphic Correlation. Society of Economic Paleontologists and Mineralogists, Special Publication no.54. ISBN 1-56576-024-7.]
Grotzinger, J.P.; Bowring, S.A.; Saylor, B.Z. and Kaufman, A.J., 1995 (Oct.27). Biostratigraphic and geochronologic constraints on early animal evolution. Science, v.270, p.598-604. [One of the most recent revisions of the age of the Precambrian/Cambrian boundary.]
Harland, W.B.; Armstrong, R.L.; Cox, A.V.; Craig, L.E.; Smith, A.G. and Smith, D.G., 1990. A Geologic Time Scale, 1989 edition. Cambridge University Press: Cambridge. 263pp. ISBN 0-521-38765-5. [A very detailed account of the entire geologic time scale.]
© Andrew MacRae 1999