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However, to read any clock accurately we must know where the clock was set at the beginning.
Most people think that radioactive dating has proven the earth is billions of years old.
It is important not to confuse geochronologic and chronostratigraphic units.
In the same way, it is entirely possible to go and visit an Upper Cretaceous Series deposit – such as the Hell Creek deposit where the Tyrannosaurus fossils were found – but it is naturally impossible to visit the Late Cretaceous Epoch as that is a period of time.
APWPs for different continents can be used as a reference for newly obtained poles for the rocks with unknown age.
For paleomagnetic dating it is suggested to use the APWP in order to date a pole obtained from rocks or sediments of unknown age by linking the paleopole to the nearest point on the APWP.
Two methods of paleomagnetic dating have been suggested (1) Angular method and (2) Rotation method.
Magnetostratigraphy determines age from the pattern of magnetic polarity zones in a series of bedded sedimentary and/or volcanic rocks by comparison to the magnetic polarity timescale.
A series of related techniques for determining the age at which a geomorphic surface was created (exposure dating), or at which formerly surficial materials were buried (burial dating).
By combining multiple geochronological (and biostratigraphic) indicators the precision of the recovered age can be improved.
Geochronology is different in application from biostratigraphy, which is the science of assigning sedimentary rocks to a known geological period via describing, cataloguing and comparing fossil floral and faunal assemblages.
A number of radioactive isotopes are used for this purpose, and depending on the rate of decay, are used for dating different geological periods.
More slowly decaying isotopes are useful for longer periods of time, but less accurate in absolute years.
Marker horizons are stratigraphic units of the same age and of such distinctive composition and appearance, that despite their presence in different geographic sites, there is certainty about their age-equivalence.