The continental shelf off New Hampshire (NH) in the Western Gulf of Maine (WGOM) is extremely complex and includes extensive bedrock outcrops, marine-modified glacial deposits, marine-formed shoals, seafloor plains, and associated features that are composed of a range of sediment types from mud to gravel. Furthermore, the physiography and composition of the seafloor frequently changes dramatically over relatively short distances (tens of meters). The complexity of the WGOM seafloor results from the interplay of glaciations, sea-level fluctuations, and marine processes (waves and currents). High-resolution multibeam echosounder (MBES) bathymetry and backscatter surveys, along with ground truth consisting of archived seismic reflection profiles, bottom sediment grain size data, vibracores, and video were used to develop surficial geology maps based on the Coastal and Marine Ecological Classification Standard (CMECS). The surficial geology maps cover ~3,250 km2 and extend from the coast of NH seaward ~50 km to Jeffreys Ledge and depict major geoforms (physiographic features) and seafloor substrate (sediment size) classifications. CMECS provides a sound basis for classifying the texture of the seafloor; however, the geoform classifications need to be broadened for paraglacial environments in future studies.

The surficial geology maps presented here are a major refinement of the original maps produced in 2016 (see Ward et al., 2016a). The new maps reflect the results of a major field campaign conducted in 2016-2017 to obtain accurately located sediment samples and seafloor images to complement the original bottom sediment database. The new sites specifically targeted areas where high-resolution MBES bathymetry existed or where surficial features warranted further ground truth for evaluations. This work was designed to enhance the surficial geology mapping efforts and contribute to the development of new approaches for utilizing acoustics to remotely classify seafloor sediments and morphologic features (also supported by the University of New Hampshire Joint Hydrographic Center). The new surficial geology maps presented here depict the exposed bedrock, morphologic features, and sediment distribution on the continental shelf off NH, revealing features of the seafloor in exceptional detail that have not been previously described.

An important finding of this study was the extent and importance of marine-modified glacial features on the WGOM continental shelf. Extensive glacial deposits including drumlins, eskers, outwash, and moraines have been eroded and modified by wave and tidal currents as sea level fluctuated over the last 12,000 years. These features are potential sources of sand and gravel for future beach nourishment projects; however, more detailed subbottom seismic surveys and vibracores are needed for verification. Also, these potential resource areas are presently too far from shore and in too great a depth of water to be easily utilized. As the demand for sand and gravel becomes more acute and technologies advance, mineral resources farther offshore and in deeper water will likely become viable.