Magnetostratigraphy is the correlation and relative dating of rock layers (stratigraphy) using the magnetism in those rock layers. Magnetostratigraphy uses the inversions of the earth magnetic field that occurred during the course of geological history.
The inversions of the earth magnetic field divide the geological time scale into periods of normal polarity (in which the magnetic south pole is at the geographic north pole and vice versa) and reversed polar periods (in which the magnetic arctic is at the geographic north pole) . These periods are called chrons and the sequence of such periods is random. Therefore, by studying the magnetic polarity in a rock layer, one can say something about when the layer was formed. Also see
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