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(1) Development of a ‘New England type’ salt marsh was documented by pollen analysis in two cores from the William Floyd Estate on south eastern Long Island. New York. The objective was to test current concepts of salt marsh development that emphasize predictability of successional stages and stability of the high marsh. (2) Deposits were dated by 210Pb analysis, by correspondence of agricultural indicator pollen profiles with records of European settlement, and by correlation of salinity-indicator pollen taxa with documented changes in dynamics of a system of barrier beaches that lies offshore. (3) Vegetation changes at the core sites included xeric Quercus-Castanea-Carya forests with a Pteridium heath understory, followed by more mesic Fagus-Acer assemblages. Nyssa and a shrub transition zone subsequently invaded and were replaced by high marsh. At one of the core sites, a Typha marsh followed the shrub transition zone and was replaced by Phragmites. (4) Fossil spectra from periods when inlets were open to the sea had high Salicornia, Atriplex, Foraminifera, and hystrichosphere percentages and the presence of Limonium pollen. These taxa disappeared simultaneously when the inlet closed, and the marsh was invaded by Cyperaceae and shrub assemblages. (5) Evidence of sea-level fluctuations was provided by sequences that indicated a progression toward decreased moisture or salinity. (6) A response to the 18-6 yr tidal cycle was suggested by a shifting dominance of Gramineae and Cyperaceae pollen. (7) Results show that salt-marsh sediments can contain a detailed record of vegetation change and can be used to trace past sea levels and salinity conditions in detail. Vegetation changed greatly as a result of frequent and severe fluctuations in the physical environment. Vegetation changes were not predictable, and biotic processes played a minor role in determining the course of plant succession. [ABSTRACT FROM AUTHOR]