Plymouth, Kingston, and Duxbury Bays (herein referred to as Plymouth Bay) form a large reentrant (area = 46 km²) along the south shore of Massachusetts approximately 57 km south of Boston. The location of the barriers, which front the bay, is controlled by bedrock, drumlins, and other glacial deposits that provide the sediment sources and pinning points for sand accumulation and the development of barriers and spits. A continual sediment supply derived from the reworking of glacial deposits, combined with a barrier alignment that funnels sediment into the bay, created an ideal sink during the past 6,000 years that resulted in accumulation of up to 35 m of sediment. The sediment distribution of the bay fill is controlled, in part, by the location of the inlet, major channels, and degree of sheltering within the bay.

Analyses of 42 km of high-resolution seismic and sidescan-sonar profiles, 18.5 km of ground-penetrating radar transects, 336 bottom samples, and 15 vibracores indicate that the present configuration of the embayment and position of the major channels are closely tied to the paleotopography of the region. The existence of a major drainage valley, formed during the late Tertiary and operative during deglaciation, is also recognized. The back barrier is comprised of extensive intertidal flats (62 percent of the back barrier is exposed at mean low water [MLW]), shallow bays and channels, and intertidal and supratidal marsh.

Modification of Plymouth Bay and its barrier system during the Holocene has been a product of cyclic barrier progradation followed by destruction and subsequent landward translation of the shoreline. The variety of the back-barrier stratigraphy reflects not only the cyclic barrier transgression, but also the distance from the main inlet channel. The thin nature of the barrier spits and the existence of numerous washovers and flood-tidal-delta deposits associated with the many historical inlets that occurred along the spits are evidence that the barriers are in a transgressive phase. Radiocarbon dates of basal peats in the northern part of the study area indicate that relative sea level has been rising at a rate of about 1.1 mm/yr over the past 3,700 years. A radiocarbon date obtained 200 m seaward of the foredune ridge at an elevation of the beach face indicates that the barriers have been migrating landward at an average rate of 0.27 ± 0.05 m/year.