A dating technique developed in the field of geology that dates rocks on the basis of polar reversals. Such reversals occur when the magnetic pole moves to the opposite side of the globe; this has happened many times during the history of the planet. By taking a sample of in situ rock and comparing its modern orientation with that preserved in the rock itself as a reflection of polar orientation at the time of its formation, something of its age can be established by reference to a master chronology of global reversals. For archaeology the main use of this technique is for deposits containing early hominins. Reversals in the period 1.65 to 1.0 million years ago, for example, have helped to date beds II and III at Olduvai in Tanzania.