THERE has been considerable debate recently on how well sedimentary rocks record transitional field directions during a polarity reversal1-5. This has not been an easy question to answer because the input geomagnetic signal for a given sedimentary record is not known independently, and because the process by which remanent magnetization is acquired by sediments is difficult to duplicate in the laboratory. The geomagnetic excursion recorded by sediments outcropping around the shores of Mono Lake, California6-8, offers an unusually good opportunity to examine this question. We report here a study of the Mono Lake Excursion (MLE) at a new locality on the southeast shore where, although the records satisfy usually applied standards of palaeomagnetic quality, it is clear from comparison with records from other localities that the field directions during a time of low field intensity have not been faithfully preserved. The distinctive directions of natural remanence in the discrepant part of the record show that the southeast shore sediments were overprinted by the ensuing higher-intensity field acting on the still unconsolidated sediment, apparently by realigning some magnetic grains that had previously been locked in.