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Relative time ("chronostratic") -- subdivisions of the Earth's geology in a specific order based upon relative age relationships (most commonly, vertical/stratigraphic position). These subdivisions are given names, most of which can be recognized globally, usually on the basis of fossils. This is indicated by both the geographic distribution and the stratigraphic layering of sedimentary deposits.

1.5 and 1.6 of Harland et al.) and a few numerical estimates were available previously (but often for the duration of the entire scale rather than its individual subdivisions).

Examples are exposed in the Cordilleran region of North America, in north central Australia, along the Yangtze River in central China, and along the Lena River in Siberia.

Few Cambrian rocks from land environments have been documented, and most of those are of limited areal extent.

This gelogical time scale is based upon Harland et al., 1990, but with the Precambrian/Cambrian boundary modified according to the most recently-published radiometric dates on that interval, revising the boundary from 570 /- 15 million years to 543 /- 1 million years ago (Grotzinger et al., 1995).

Other changes have been proposed since 1990 (e.g., revision of the Cretaceous by Obradovich, 1993), but are not incorporated because they are relatively small. Blackwell Scientific Publications: Boston, p.1-512. ISBN 0-86542-069-6 [Chapter 4 provides an introduction to geologic time.

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