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To reconstruct the history of the Earth we need to know what happened and when – events and their dates – and we should like to know how it happened and why – processes and their rates. To date a historical event we need a timescale for reference – a calendar – and a means of placing events in this timescale – a clock. Direct access to the primary physical calendar, of time measured in years by means of elemental radiometry as clock, is possible in only a minority of geological problems. By far the richest historical source in the Phanerozoic Eon has been the stratigraphical analysis of sedimentary rocks by means of fossils, the approach pioneered by William Smith. The succession of fossil biotae found in the rocks is used to construct the calendar of relative time, the familiar geological calendar defining the standard chronostratigraphical timescale still in process of refinement today. Rocks are then dated through time correlations with this scale by means of their guide fossils (yon Buch) as clocks. The power to measure the rates of geological processes then depends on the time resolution achievable by means of fossils, the time intervals between distinguishable events, the finesse of the calendar.

The present-day state of play is reviewed, both in the refinement of the geological calendar and the finesse that has been attained. Comparison of the geological calendar with our familiar human historical calendar reveals some illuminating parallels as well as some important differences. Illustrative examples are taken from the Jurassic Period (170 Ma bp) and its ammonites as clocks.

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