The study of polarity excursions, today, is analogous to the study of polarity chrons 40 years agowhen the principal polarity chrons of the last 5 Mywere in the process of being resolved by coupledK–Ar and paleomagnetic studies in volcanic rocks,and by studies of conventional piston cores from theoceans. The challenge of resolving brief excursions,that have remained obscure and poorly documenteduntil very recently, has only just begun.