Each period of geological time has had its special attributes and events; each has left a distinct and varied record that tells us something of the evolution of our planet. Deciphering the record is one of the main aims of geology, an aim that is more difficult to achieve the further back in geological time we search. Subsequent events mask the nature of the original record but the stratigraphical use of fossils, palaeomagnetism and the application of radiometric dating are increasingly useful in enabling us to ascribe relative and numerical dates to geological phenomena. The record is overall imperfect but continuous, since constructive and destructive processes are operating ceaselessly upon the earth’s crust. This represents a hugely dynamic state of affairs and the continuous expenditure of energy. Such is the nature of the crust that it is only in the continental parts that events more than 200 m.y.b.p. or so are recorded. The oceanic realm of earlier days remains more conjectural than does the continental.
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