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.
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