Data from seismic-reflection profiles, sidescan-sonar images, sediment cores, and surface samples outline the shallow stratigraphy and surficial sedimentary environments of the Boston Harbor estuary. Bedrock and till of pre-Wisconsinan age form an irregular acoustic basement surface that variably crops out or is buried as much as 35 m below the harbor floor. Where buried, this surface is overlain by discontinuous ice-proximal glacial deposits and by glacio-marine muds. These accumulated in late Wisconsinan time when ice retreat and marine submergence occurred simultaneously owing to crustal depression. During the immediate postglacial period, sea-level regressed due to crustal rebound, and texturally diverse fluvial and estuarine sediments were deposited in small channels that were cut into the subaerially exposed upper Wisconsinan drift. As the harbor was resubmerged in Holocene time in response to the eustatic rise of sea level, waves beveled the substrata, and localized deposits of marine sands and muds accumulated atop the trans-gressive unconformity.

The modern wave and current regime in the harbor area has produced three surficial sedimentary environments characterized by erosion, deposition, and sediment reworking, respectively. Areas of erosion are characterized by bedrock outcrops and coarse lag deposits that have been scoured by tidal currents and waves; they are common along mainland and insular shorelines, within large channels having strong tidal currents, and across much of the harbor entrance. Areas of deposition consist of sandy and clayey silts and are found on shallow subtidal flats and in broad bathymetric lows where tidal currents are relatively weak. Environments of sediment reworking are characterized by intermittent erosion and deposition and have diverse grain sizes; they are associated with many different bathymetric features and are indicative of highly variable bottom currents.