Extensive sampling has revealed a new feature of the Mono Lake geomagnetic excursion. The previously known eastward swing in declination and steepening of inclination are preceded by an even greater swing to westerly declination and shallow inclination. The duration of the entire excursion is estimated to be about 1000 years. Excellent agreement of paleomagnetic directions between four sites shows the excursion is a real expression of the geomagnetic field. Interpretation of natural remanent magnetization/anhysteretic remanent magnetization (NRM/ARM) ratios for one site indicates the field intensity may have fallen well below and then increased well above the dipole field intensity, suggesting the excursion is a manifestation of the nondipole field. The source can be modeled by a radial eccentric dipole at high northern latitudes and pointing outward during the first part of the excursion, and near the equator and pointing inward during the latter part. Movement of the source appears localized, displaying a complex pattern of eastward, westward, and even northward drift. The average moment of the hypothetical eccentric dipole during the excursion is comparable with the largest calculated for the 1945 field, and the maximum moment is almost twice as great.