The data and interpretations in this report are based on National Oceanic and Atmospheric Administration (NOAA) hydrographic survey H12007 completed offshore of southeastern Massachusetts in central Nantucket Sound during 2009 and a U.S. Geological Survey (USGS) verification cruise completed during 2011. These data offer new geologic perspectives on the Sound's dynamic sea floor and show the composition and terrain of the seabed, providing information on sediment transport and benthic habitat. Together, information available through the reports in this series provides a fundamental framework for research and resource-management activities offshore of southeastern Massachusetts.

Surveyed depths within the study area range from less than 5 m at mean lower low water to more than 26 m. Depths are shallowest on Horseshoe, Cross Rip, and Halfmoon Shoals; depths are deepest along the thalweg axis of Cross Rip Channel.

The Holocene section is thin or absent in Cross Rip Channel and in the southeastern part of the study area, where high-energy sedimentary environments characterized by the processes of erosion or nondeposition prevail. In the channel, pavements of gravelly sediment and dense shell beds armor the sea floor, scour depressions occur around obstructions, and boulders are present on the exposed Pleistocene surface. Small-scale erosional bedforms present on the relatively flat areas in the southeastern part of the study area give the sea floor there a current-swept appearance.

Sand waves and megaripples cover the sea floor in more than 70 percent of the study area. Transverse morphologies dominate on the shoals, where sand supply is abundant; barchanoid morphologies dominate along the lower flanks of Cross Rip Channel, where sand supply is limited. Megaripples are prevalent throughout most of the southeastern part of the study area and, along with current ripples, are also present on the stoss slopes of the sand waves, showing that transport is active. Sand-wave and scour-mark asymmetry through most of the study area shows that net sediment transport is to the east and is flood-tide dominated.

Anthropogenic artifacts visible in the bathymetry include four shipwrecks, the largest of which is more than 30 m in length.