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U.S. Geological Survey Open-File Report 00-410

DISCUSSIONS PRINTED ON MAP SHEETS

Sheet 1 - Sea Floor Topography

This map is part of a three-map series of the Stellwagen Bank National Marine Sanctuary region. The maps are the products of a 1994-1996 survey that used a multibeam echo sounder to map 1100 square nautical miles of the sea floor. The other two maps show sun-illuminated (shaded relief) sea floor topographic imagery, and combined imagery of sun-illuminated sea floor topography and backscatter intensity (U.S. Geological Survey Open-File Report 00-410 Sheets 2 and 3). This map shows contoured topography at an interval of 5 meters. Contours are shown in blue except for topographic lows which are shown in brown. 

The project has published two map series of 18 quadrangles each (see location map) in which new sea floor topography and sun-illuminated imagery are presented at a scale of 1:25,000. For paper maps of contoured topography, see Valentine and others, 1997 (U.S. Geological Survey Open-File Reports 97-502 to 97-509, 97-682 to 97-685, and 97-726 to 97-731). For paper maps of sun-illuminated topographic imagery, see Valentine and others, 1999, 2000, and 2001 (U.S. Geological Survey Geologic Investigations Series Map I-2701 to 2718). For a CD-ROM of digital contoured topography (EPS, PS, Arc export, PDF formats), see Valentine and others, 1998. For a CD-ROM of digital sun-illuminated topographic imagery (EPS, PS, TIF, Arc export, PDF formats), see Valentine and others, 1999. 

On this map, the locations of the corners of the 18 quadrangles are shown for reference as ticks labeled outside the map edges and as large crosses within the map area. The great topographic detail of the sea bed revealed by this survey warrants the naming here of many geographic features. Some features were named in consultation with local fishermen. 

The Stellwagen Bank National Marine Sanctuary Mapping Project is a cooperative effort of the U.S. Geological Survey and the National Oceanic and Atmospheric Administration, with support from the University of New Brunswick and the Canadian Hydrographic Service.

 

Sheet 2 - Sun-illuminated Sea Floor Topography

This map is part of a three map series of the Stellwagen Bank National Marine Sanctuary region. The maps are the products of a 1994-1996 survey that used a multibeam echo sounder to map 1100 square nautical miles of the sea floor. The other two maps show new sea floor topography, and combined sun-illuminated (shaded relief) sea floor topography and backscatter intensity (U.S. Geological Survey Open-File Report 00-410 Sheets 1 and 3). This map combines contoured topography and sun-illuminated topographic imagery. The image shown here uses a sun elevation angle of 45 degrees above the horizon from an azimuth of 350 degrees and a vertical exaggeration of four times. In effect, topographic relief is enhanced by having the sun illuminate the sea floor from a position 10 degrees west of north so that shadows are cast on the southern flanks of sea bed features. Unnatural-looking stripes and patterns oriented parallel or perpendicular to survey tracklines are artifacts of data collection. Blank areas represent places where no data exists. The topographic imagery is identical to that shown on Open-File Report 00-410 Sheet 3. 

Major topographic features depicted in the maps were formed by glacial processes. In broad terms, these features 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 large basins, banks, ridges, and valleys. Many other features observed here 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. 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 of the U.S. Geological Survey and the National Oceanic and Atmospheric Administration, with support from the University of New Brunswick and the Canadian Hydrographic Service. For information on survey methods and on two map series of 18 quadrangles each (see location map) that show sea floor topography and sun-illuminated imagery at a scale of 1:25,000, see Open-File Report 00-410 Sheet 1 and references therein. On this map, the corners of the 18 quadrangles are shown for reference as labeled ticks and as large crosses on the map.

Sheet 3 - Backscatter Intensity and Sun-Illuminated Sea Floor Topography

This map is part of a three-map series of the Stellwagen Bank National Marine Sanctuary region. The maps are the products of a 1994-1996 survey that used a multibeam echo sounder to map 1100 square nautical miles of the sea floor. The other two maps show new sea floor topography and sun- illuminated (shaded relief) sea floor topographic imagery (U.S. Geological Survey Open-File Report 00-410 Sheets 1 and 2). This map combines the sun-illuminated topography with the backscatter intensity (shown here in color) of the sea floor. 

Backscatter intensity is a measure of the hardness and roughness of the sea floor as determined from the strength of sound waves reflected from the seabed during the survey. High-backscatter materials (red and orange) are coarse sand, gravel (including piles and ridges of boulders), and rock outcrops. Moderate backscatter (green) represents sand or muddy sand. Low backscatter (blue) represents sandy mud and mud. These interpretations apply best in regions of low regional topography, because steep slopes can divert the paths of some of the reflected sound waves away from the survey vessel. Thus, sea bed that slopes steeply away from the survey track can produce a lower backscatter intensity than if it were level. Unnatural-looking stripes and patterns oriented parallel or perpendicular to survey tracklines are artifacts of data collection. Blank areas represent places where no data exists. The topographic imagery is identical to that shown on Open-File Report 00-410 Sheet 2. 

Major topographic features depicted in the maps were formed by glacial processes. In broad terms, these features 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 large basins, banks, ridges, and valleys. Many other features observed here 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. 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 of the U.S. Geological Survey and the National Oceanic and Atmospheric Administration, with support from the University of New Brunswick and the Canadian Hydrographic Service. For information on survey methods and on two map series of 18 quadrangles each (see location map) that show sea floor topography and sun-illuminated imagery at a scale of 1:25,000,see Open-File Report 00-410 Sheet 1 and references therein. On this map, the corners of the 18 quadrangles are shown for reference as labeled ticks and as large crosses on the map.

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