A timescale based on the periodic polarity reversals in the Earth’s geomagnetic field. Magnetic minerals within a rock retain an orientation induced by the field at the time the rock was formed (see natural remanent magnetism). Provided they include suitable minerals, strata from all over the world thus contain a record of the normal (as at present) or reversed state of the geomagnetic field at the time of their formation. This reversal pattern has been correlated between different successions of rocks to produce a sequence that, when combined with a dating method such as potassium–argon dating, has given a timescale measured in units of normal or reversed polarity. The scale was first established in detail for the last 4.5 Ma using data from terrestrial, mainly extrusive, rocks; it has now been extended back to the Upper Jurassic by means of the magnetic anomaly patterns in oceanic crust. The terms proposed by the ISSC for geochronologic units of the magnetostratigraphic timescale are polarity superchron, polarity chron (replacing the earlier term ‘epoch’), and polarity subchron (replacing the earlier term ‘event’). The corresponding terms proposed for rocks deposited during those polarity intervals are polarity superchronozone, polarity chronozone, and polarity subchronozone.