Stellwagen Bank National Marine Sanctuary Region off Boston, Massachusetts

INTRODUCTION (continued)

The sea floor mapping survey of the Stellwagen Bank National Marine Sanctuary region covers approximately 1100 square nautical miles of seabed located off Boston, Massachusetts and extending from Race Point Channel (just north of Cape Cod) to the southern part of Jeffreys Ledge (north of Cape Ann). It was conducted on four cruises over a two-year period from the fall of 1994 to the fall of 1996 using a multibeam echo sounder installed aboard the Canadian Hydrographic Service vessel Frederick G. Creed.

The sedimentary environments and biological habitats found on the sea floor are being identified and interpreted by using video and photographic imagery and sediment samples that have been collected on many cruises to the region since the mapping was initiated. Research results and products are presented here in the form of maps, posters, fact sheets, sonar images, and bottom photographs.

The great topographic detail of the seabed revealed by the sonar images warrants the naming here of many geographic features. Some features were named in consultation with local fishermen. The names are documented in the "GEOGRAPHIC NAMES" section.

The project has published two map series of 18 quadrangles each (see location map) in which new sea floor topography and sun-illuminated topographic imagery are presented at a scale of 1:25,000 (1 cm on the map represents 250 m on the sea floor). In addition, a map series showing the entire region on a single sheet at a scale of 1:60,000 is presented as a set of 3 maps; one shows contoured seabed topography only, a second adds seabed topographic imagery, and the third adds the backscatter intensity (or reflectivity) of the seabed. See the "LIST OF PUBLICATIONS" for a complete list of maps and CD-ROMs.

The sea floor observed in the Stellwagen Bank NMS region has been shaped for the most part by glacial processes, and the resulting topographic features have been modified since the melting of the ice and the return of the sea. Seabed shapes are interpreted here to represent a geologic history that developed in several stages. Ice containing rock debris moved across the region, sculpting its surface and depositing sediment to form the major basins, banks, ridges, and valleys. Minor features represent the latter stages of deglaciation. They are the result of processes at work when much of the area was covered by stationary rotting ice, and when at the same time small valley glaciers and ice falls were active in and near areas of high topographic relief. Subsequently, the sea invaded the region formerly occupied by ice, and glacial features were partly eroded and some new deposits formed. Today the sea floor is modified mainly by strong southwestward-flowing bottom currents caused by storm winds from the northeast. These currents erode sediments from the shallow banks and transport them into the basins. With time the banks become coarser, as sand and mud are removed and gravel remains; and the western flanks of the banks, as well as adjacent basins, are built up by deposits of mud and sand.

The Stellwagen Bank National Marine Sanctuary Mapping Project is a cooperative effort supported by the Coastal and Marine Geology Program of the U.S. Geological Survey and agencies of the National Oceanic and Atmospheric Administration (including the National Marine Sanctuary System, the National Marine Fisheries Service, the National Undersea Research Program, and the Office of Coast Survey). The acquisition and processing of the multibeam sonar data that form the basis of the maps was conducted with technical support from the University of New Brunswick and the Canadian Hydrographic Service.