Duxbury Beach, Massachusetts, is a retreating, transgressive barrier that is effectively managed to meet a range of competing land uses. While the barrier is heralded as a natural coastal setting, the entire landform is methodically engineered on an ongoing basis to best accomplish the goals established for the beach within a context of natural processes. Historical and geological data indicate that the natural barrier form includes numerous ephemeral tidal inlets (some of which have migrated) and overwash channels, and low discontinuous dunes. At present, the managed barrier has a continuous vegetated foredune and broad backdune. Management techniques have evolved over the past several decades based on growing experience and understanding of the coastal processes and of wildlife habitats. Although the foredune crest is reconstructed each spring, the entire beach is gradually being allowed to retreat to remain in equilibrium with rising sea level. The lagoonal shore is being widened through beach nourishment and through proposed creation of back-barrier salt marshes using silty dredge spoil. Uses of the barrier include town and public recreational beaches, off-road vehicle access, a right-of-way to isolated communities, flood protection of landward areas, and shorebird nesting habitat.

Abstract Plymouth, Kingston, and Duxbury Bays (herein referred to as Plymouth Bay) form a large reentrant (area = 46 km 2) 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.

Abstract The Provincelands in the Cape Cod National Seashore developed about 5 ka from wind- and waterborne Outer Cape Cod outwash sands as sea level rose to submerge offshore banks. A late Holocene chronology of sand dunes in the Provincelands is established from radiocarbon-age determinations on the basal organics from interdunal bogs and ponds. The Provincelands ponds, thought to be lakes trapped behind hooked spits, are shown here to have developed from bogs similar to those in the extant parabolic-dune field. An older, stabilized dune field in the location of the ponds is hypothesized. Episodes of alternating dune movement and interdunal-wetland formation in the Provincelands correlate with climatic changes in the North Atlantic and elsewhere during the last 1.2 ka, as interpreted from ice core, glacier movement, sea-surface temperature, tree-ring, pollen, and historical data. The dune field data suggest a periodicity of change of 0.2 ka. Before about 1.2 ka, dunes in the Provincelands were active. Between about 1.15 and 0.9 ka, bogs formed which, at least in the west-central area of the Provincelands, developed into ponds having continuous organic sedimentation until today. This evidence indicates that the west-central dune field stabilized after 1.1 ka. Between 0.9 and 0.7 ka, dunes were active in the north and east-central Provincelands, but from about 0.7 to 0.5 ka, bogs formed within that dune field, and at least one bog in the west-central area became a pond, both events indicating a warm and wet climate. During the Little Ice Age (0.5 to ∼0.2-0.1 ka), the Provincelands dunes were active, suggesting a cold, windy, and dry climate. Because of a warmer and wetter climate during the last 0.1 ka, bogs have formed again within the dunes.