The chronology of lunar volcanism is based on radiometric ages determined from Apollo and Luna landing site samples, regional stratigraphic relationships, and crater degradation and size-frequency distribution data for units largely defined prior to the end of the Apollo program. Accurate estimates of mare basalt ages are necessary to place constraints on the duration and the flux of lunar volcanism, as well as on the petrogenesis of lunar mare basalts and their relationship to the thermal evolution of the Moon. Here, we report on ages derived from crater size-frequency distribution measurements for exposed mare basalt units on the lunar nearside...