Byron-Bergen Swamp, an 800-ha mire complex in western New York, is a mosaic of hardwood–conifer forest, white cedar swamp, and open nonforested fens dominated by sedges, other herbs, and shrubs. The mire is a sloping, spring-fed rich fen in which marl deposition occurs in the open fens but not under forest. Two cores, 120 m apart, one in a marl fen and the other in a hardwood–conifer swamp, were taken to investigate the history of the mire. Sediment, pollen stratigraphy, and ¹⁴C chronology show that the Holocene record of local vegetation at each coring site was very different. One site was first a shallow marl pool (10 700 – 5600 years BP), then an open shrub–conifer vegetation, and finally a closed hardwood–conifer swamp. The other site progressed from a pine–spruce–tamarack swamp (10 700 – 8000 years BP) to a white cedar swamp (7500–3500 years BP) and then to a marl sedge fen (3500 years BP – present). The spatial arrangement of swamp forest and marl fen changed through time, responding to marl accumulation and lateral shifts in drainage pathways. Marl deposition occurred continuously at each site, although at different times, for several thousand years. The pattern of vegetation change that we found is not congruent with that predicted from classical hydrarch succession.Key words: marl, mire development, pollen stratigraphy, rich fen.