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In addition, like any good scientific measurement, every dated boundary has an uncertainty associated with it, expressed as " /- X millions of years". These can not be included in the diagram for practical reasons, but can be found in Harland et al., 1990, along with a detailed description of the history of earlier-proposed time scales and the terminology, methodology and data involved in constructing this geological time scale. The time scale is depicted in its traditional form with oldest at the bottom and youngest at the top -- the present day is at the zero mark. This is a good starting point to get the basic principles.] Grotzinger, J. Geologic time is finely subdivided through most of the Phanerozoic (see Harland et al., 1990 for details), but most of the finer subdivisions (e.g., epochs) are commonly referred to by non-specialists only in the Tertiary. Methods fall into one of two categories: relative or absolute.Before more precise absolute dating tools were possible, researchers used a variety of comparative approaches called relative dating.The good dates are confirmed using at least two different methods, ideally involving multiple independent labs for each method to cross-check results.Sometimes only one method is possible, reducing the confidence researchers have in the results. “They’re based on ‘it’s that old because I say so,’ a popular approach by some of my older colleagues,” says Shea, laughing, “though I find I like it myself as I get more gray hair.” Kidding aside, dating a find is crucial for understanding its significance and relation to other fossils or artifacts.
These subdivisions are given names, most of which can be recognized globally, usually on the basis of fossils. In an exhaustive 2009 book, geologist Andrew Snelling summarized experiments that were performed on coal from Australia's Gippsland Basin.He wrote, "Brown coals from the onshore part of the basin were heated under conditions that simulated accelerated sedimentary burial conditions." This produced "liquid and gaseous hydrocarbons from the contained waxes, and leaf, pollen, and spore cuticles, all in a matter of two to five days." These hydrocarbons were identical to those harvested offshore, where they had been trapped above the coals from which they must have originated. Geological Association of Canada, Special Paper 39, p.379-396.When it comes to determining the age of stuff scientists dig out of the ground, whether fossil or artifact, “there are good dates and bad dates and ugly dates,” says paleoanthropologist John Shea of Stony Brook University.