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Maps Showing Sea Floor Topography, Sun-Illuminated
Sea Floor Topography, and Backscatter Intensity of
Quadrangles 1 and 2 in the Great South Channel Region,
Western Georges Bank

Page C. Valentine, editor

U.S. GEOLOGICAL SURVEY GEOLOGIC INVESTIGATIONS SERIES MAP I–2698

2002, Version 1.0

Prepared in cooperation with the National Oceanic and Atmospheric Administration

DISCUSSIONS PRINTED ON THE MAP SHEETS

Sheet A—Sea Floor Topography of Quadrangle 1

Introduction

The Georges Bank Mapping Project is a cooperative effort of the U.S. Geological Survey and the National Oceanic and Atmospheric Administration (NOAA), with support from the University of New Brunswick and the Canadian Hydrographic Service. A multibeam echo sounder survey of the Great South Channel region was conducted in November 1998. This map, one in a series of two mapped quadrangles (see location map), presents the results of this survey, in which sea floor topography is depicted at a scale of 1:25,000. For a map showing sun-illuminated sea floor topographic imagery, see the companion map by Valentine and others (2002) on this CD-ROM.

Survey methods

The survey was conducted using a multibeam echo sounder installed aboard the Canadian Hydrographic Service vessel Frederick G. Creed, a SWATH (Small Waterplane Area Twin Hull) ship that surveys at speeds up to 15 knots. During the survey, the ship's position was determined with an accuracy of 10 m or better using a global positioning system (GPS) receiver in conjunction with differential GPS corrections transmitted by U.S. Coast Guard radio beacons. The multibeam echo sounder data were collected by means of a Simrad Subsea EM 1000 Multibeam Echo Sounder (95 kHz) that is permanently installed in the hull of the Creed. In water depths between 5 and 200 m, the EM 1000 generates 60 aimed beams, spaced at intervals of 2.5 degrees, that insonify a swath of sea floor measuring in width approximately 7.5 times the water depth. Horizontal spatial resolution is on the order of 10 percent of the water depth at 15 knots; vertical resolution is approximately 1 percent of the water depth or better. Software developed by the Ocean Mapping Group, University of New Brunswick, was used to process and edit the echo sounder bathymetric and differential GPS navigation data; tidal corrections based on NOAA's Nantucket tide gauge were used to reference depth data to mean lower low water.

Topographic contour mapping

Bathymetric data were contoured using the Arc/Info geographic information system software (Environmental Systems Research Institute, Inc., version 7.0.3). Processed data were formatted into a point file using the Arc/Info "point generate" routine. The point file was transformed to a Mercator projection, with the longitude of the central meridian at 68°55' W. and the latitude of true scale at 40°50' N. The "point grid" routine was used to create a grid from the point file and to assign depth values to individual grid cells. The cell size of the output grid was 9 m. Topographic contours at 5-meter intervals were generated using the "lattice contour" routine. Most of the contour lines are shown here unedited. However, in areas of very smooth sea floor, some contours displayed distortions caused by problems encountered during data acquisition at nadir (directly below the vessel's keel) and by refraction effects at the outermost edge of the swath. These distortions were smoothed by employing a user-defined low-frequency "focal median" filter routine on the grid created by "point grid". A square focal median filter using a 3-cell by 3-cell size was used. The resulting contours were compared with features displayed in shaded-relief seabed imagery of the same data, and were edited manually with "Arc/Edit" to remove small artifacts that remained after filtering. Large east-west-trending bedforms (typically 5 to 10 m high, but ranging up to 20 m) characterize parts of the sea floor in this region. The multibeam echo sounder had difficulty imaging these features because they are oriented at right angles to the ship's tracks and display steep sides and narrow crests. As a result, some of these bedforms are depicted by slightly irregular contours. Each of the quadrangles was contoured independently, and contours that extend into Quadrangle 2 were edited manually to match at the boundary.

Reference Cited

Valentine, P.C., Middleton, T.J., and Malczyk, J.T., 2002, Sun-illuminated sea floor topography of Quadrangle 1, map B of Valentine, P.C., ed., Maps showing sea floor topography, sun-illuminated sea floor topography, and backscatter intensity of Quadrangles 1 and 2 in the Great South Channel region, western Georges Bank: U.S. Geological Survey Geologic Investigations Series Map I–2698, scale 1:25,000, on a CD-ROM.

Sheet B—Sun-Illuminated Sea Floor Topography of Quadrangle 1

Introduction

The Georges Bank Mapping Project is a cooperative effort of the U.S. Geological Survey and the National Oceanic and Atmospheric Administration (NOAA), with support from the University of New Brunswick and the Canadian Hydrographic Service. A multibeam echo sounder survey of the Great South Channel region was conducted in November 1998. This map, one in a series of two mapped quadrangles (see location map), presents the results of this survey, in which sea floor topography is depicted in sun-illuminated (or shaded relief) view at a scale of 1:25,000, with topographic contours overprinted in blue. The image shown here uses a sun elevation angle of 45 degrees above the horizon from an azimuth of 0 degrees and a vertical exaggeration of four times. In effect, topographic relief is enhanced by having the sun illuminate the sea floor from the north, so that shadows are cast on the southern flanks of seabed features. Some features in the image are artifacts of data collection. They are especially noticeable where the seabed is smooth, and they include small highs and lows and unnatural-looking features and patterns that are oriented parallel or perpendicular to survey tracklines, which run north-south. Blank areas (black on the image) represent places where no data exists. Large east-west-trending bedforms (typically 5 to 10 m in height, but ranging up to 20 m) characterize parts of the sea floor in this region. The multibeam echo sounder had difficulty imaging these features because they are oriented at right angles to the ship's tracks and display steep sides and narrow crests. As a result, the crests of some of these bedforms display small gaps or lows that do not exist. For a depiction of the sea floor topography without imagery and for an explanation of survey and topographic data-processing methods, see the companion map by Valentine and others (2002b) on this CD-ROM.

Regional seabed features

The Great South Channel separates the western part of Georges Bank from Nantucket Shoals and is a major conduit for the exchange of water between the Gulf of Maine to the north and the Atlantic Ocean to the south. Water depths range mostly between 65 and 80 m in Quadrangle 1. Minimum depths of 55 to 60 m occur in the northeast and southeast-central parts, and maximum depths of 85 to 90 m occur in the western part. The major topographic features depicted in the Great South Channel were formed by glacial and postglacial processes. Ice containing rock debris moved from north to south, sculpting the region into a broad shallow depression and depositing sediment to form the irregular depressions and low gravelly mounds and ridges that are visible in parts of the mapped area. Many other smaller glacial features probably have been eroded by waves and currents at work since the time when the region, formerly exposed by lowered sea level or occupied by ice, was invaded by the sea. The low, irregular and somewhat lumpy fabric formed by the glacial deposits is obscured in places by drifting sand and by the linear, sharp fabric formed by modern sand features.

In many respects, the seabed in Quadrangle 1 is similar to that in Quadrangle 2 (Valentine and others, 2002a on this CD-ROM). In both quadrangles the seabed consists of coarse-grained glacial deposits (gravel, sandy gravel, and coarse sand), and, in part, modern sands. In the central part of Quadrangle 1, a series of sinuous shallow depressions extends westward and southwestward from an 80-meter low at 40°59.1' N., 68°55.4' W. to approximately 40°58.3' N., 68°58.8' W. These depressions possibly outline the former locations of masses of melting ice. Two other sinuous shallow depressions are at 40°55.8' N., 68°54.0' W. and at 40°55.35' N., 68°50.05' W. Elsewhere in the quadrangle, low ridges and mounds composed of glacial gravel are visible where they have not been obscured by modern sands. An example is the area that extends northward from 40°58' N. (into the southern part of Quadrangle 2) and that is bounded by 68°59' W. and 69°02' W. Similar areas lie between 40°54' N. and 40°58' N. along 68°50' W. Relatively smooth areas of the seabed (that are not dominated by modern sand bedforms) consist of glacial gravel that has been exposed by bottom currents. They are centered at the following locations: 40°59.5' N., 68°50' W.; 40°59.5' N., 68°53.5' W.; 40°59.7' N., 69°0.05' W.; and 40°57.3' N., 68°56.5' W.

Today, strong tidal and storm currents flow dominantly north and south in the region. Transport of sand by these currents has resulted in the construction of large, east-west-trending sand dunes. These large bedforms contrast strongly with, and partly mask, the subdued topography of the older glacial features. The modern bedforms are represented by two major types. One group of features generally is high and steep-sided, has straight axes, and consists of coarse-grained sediment (example at 40°59.15' N., 68°51.3' W.). The second group generally is lower, has sinuous axes, and is composed of fine-grained sediment (example at 40°59.2' N., 68°48.3' W.). The irregular sinuous shapes and fine-grained nature of the bedforms in the second group suggest that they are more mobile than the more symmetrical and coarse-grained bedforms of the first group. The sinuous bedforms are most common along the northeastern margin of the quadrangle. The high, straight-crested bedforms occur throughout the quadrangle, and many are bounded by linear depressions and are separated from neighboring bedforms by gravel seabed (40°56.85' N., 68°58.2' W.). The linear depressions possibly resulted from the scouring of sand from gravelly glacial deposits around the bedform, a process that caused the formation and settling of the gravel seabed. Several of the large isolated bedforms that are located in gravel areas are eroding. In one location (40°57.65' N., 68°57.2' W.), sand eroded from the bedform is represented by linear, en-echelon sand waves that trail away downcurrent (southward) from the bedforms onto the gravel pavement. As erosion continues, the bedforms are removed, depressions are left in the sea floor, and thin linear deposits of rippled sand extend downcurrent onto the surrounding gravel seabed (40°56' N., 68°57.6' W.). Finally, the thin rippled sand deposits are eroded, and the only evidence of the former large bedforms are elongate depressions in the sea floor (40°58.55' N., 69°00.35' W.). The symmetry and the dominant east-west trend of both groups of large bedforms in this quadrangle suggest that storm and daily tidal currents move sand both north and south in the Great South Channel. Sand grains on the flanks of large and small bedforms probably are moved back and forth during each tidal cycle. Although storm currents are less predictable than tidal currents and can transport sand rapidly over longer distances, the positions of the large bedforms probably are relatively stable. However, the orientation of the sand trails from eroding bedforms suggests that, with time, net sediment movement is to the south and southeast.

References Cited

Valentine, P.C., Malczyk, J.T., and Middleton, T.J., 2002a, Sun-illuminated sea floor topography of Quadrangle 2, map D of Valentine, P.C., ed., Maps showing sea floor topography, sun-illuminated sea floor topography, and backscatter intensity of Quadrangles 1 and 2 in the Great South Channel region, western Georges Bank: U.S. Geological Survey Geologic Investigations Series Map I–2698, scale 1:25,000, on a CD-ROM.

Valentine, P.C., Middleton, T.J., and Malczyk, J.T., 2002b, Sea floor topography of Quadrangle 1, map A of Valentine, P.C., ed., Maps showing sea floor topography, sun-illuminated sea floor topography, and backscatter intensity of Quadrangles 1 and 2 in the Great South Channel region, western Georges Bank: U.S. Geological Survey Geologic Investigations Series Map I–2698, scale 1:25,000, on a CD-ROM.

Sheet C—Sea Floor Topography of Quadrangle 2

Introduction

The Georges Bank Mapping Project is a cooperative effort of the U.S. Geological Survey and the National Oceanic and Atmospheric Administration (NOAA), with support from the University of New Brunswick and the Canadian Hydrographic Service. A multibeam echo sounder survey of the Great South Channel region was conducted in November 1998. This map, one in a series of two mapped quadrangles (see location map), presents the results of this survey, in which sea floor topography is depicted at a scale of 1:25,000. For a map showing sun-illuminated sea floor topographic imagery, see the companion map by Valentine and others (2002) on this CD-ROM.

Survey methods

The survey was conducted using a multibeam echo sounder installed aboard the Canadian Hydrographic Service vessel Frederick G. Creed, a SWATH (Small Waterplane Area Twin Hull) ship that surveys at speeds up to 15 knots. During the survey, the ship's position was determined with an accuracy of 10 m or better using a global positioning system (GPS) receiver in conjunction with differential GPS corrections transmitted by U.S. Coast Guard radio beacons. The multibeam echo sounder data were collected by means of a Simrad Subsea EM 1000 Multibeam Echo Sounder (95 kHz) that is permanently installed in the hull of the Creed. In water depths between 5 and 200 m, the EM 1000 generates 60 aimed beams, spaced at intervals of 2.5 degrees, that insonify a swath of sea floor measuring in width approximately 7.5 times the water depth. Horizontal spatial resolution is on the order of 10 percent of the water depth at 15 knots; vertical resolution is approximately 1 percent of the water depth or better. Software developed by the Ocean Mapping Group, University of New Brunswick, was used to process and edit the echo sounder bathymetric and differential GPS navigation data; tidal corrections based on NOAA's Nantucket tide gauge were used to reference depth data to mean lower low water.

Topographic contour mapping

Bathymetric data were contoured using the Arc/Info geographic information system software (Environmental Systems Research Institute, Inc., version 7.0.3). Processed data were formatted into a point file using the Arc/Info "point generate" routine. The point file was transformed to a Mercator projection, with the longitude of the central meridian at 68°55' W. and the latitude of true scale at 40°50' N. The "point grid" routine was used to create a grid from the point file and to assign depth values to individual grid cells. The cell size of the output grid was 9 m. Topographic contours at 5-meter intervals were generated using the "lattice contour" routine. Most of the contour lines are shown here unedited. However, in areas of very smooth sea floor, some contours displayed distortions caused by problems encountered during data acquisition at nadir (directly below the vessel's keel) and by refraction effects at the outermost edge of the swath. These distortions were smoothed by employing a user-defined low-frequency "focal median" filter routine on the grid created by "point grid". A square focal median filter using a 3-cell by 3-cell size was used. The resulting contours were compared with features displayed in shaded-relief seabed imagery of the same data, and were edited manually with "Arc/Edit" to remove small artifacts that remained after filtering. Large east-west-trending bedforms (typically 5 to 10 m high, but ranging up to 20 m) characterize parts of the sea floor in this region. The multibeam echo sounder had difficulty imaging these features because they are oriented at right angles to the ship's tracks and display steep sides and narrow crests. As a result, some of these bedforms are depicted by slightly irregular contours. Each of the quadrangles was contoured independently, and contours that extend into Quadrangle 1 were edited manually to match at the boundary.

Reference Cited

Valentine, P.C., Malczyk, J.T., and Middleton, T.J., 2002, Sun-illuminated sea floor topography of Quadrangle 2, map D of Valentine, P.C., ed., Maps showing sea floor topography, sun-illuminated sea floor topography, and backscatter intensity of Quadrangles 1 and 2 in the Great South Channel region, western Georges Bank: U.S. Geological Survey Geologic Investigations Series Map I–2698, scale 1:25,000, on a CD-ROM.

Sheet D—Sun-Illuminated Sea Floor Topography of Quadrangle 2

Introduction

The Georges Bank Mapping Project is a cooperative effort of the U.S. Geological Survey and the National Oceanic and Atmospheric Administration (NOAA), with support from the University of New Brunswick and the Canadian Hydrographic Service. A multibeam echo sounder survey of the Great South Channel region was conducted in November 1998. This map, one in a series of two mapped quadrangles (see location map), presents the results of this survey, in which sea floor topography is depicted in sun-illuminated (or shaded relief) view at a scale of 1:25,000, with topographic contours overprinted in blue. The image shown here uses a sun elevation angle of 45 degrees above the horizon from an azimuth of 0 degrees and a vertical exaggeration of four times. In effect, topographic relief is enhanced by having the sun illuminate the sea floor from the north, so that shadows are cast on the southern flanks of seabed features. Some features in the image are artifacts of data collection. They are especially noticeable where the seabed is smooth, and they include small highs and lows and unnatural-looking features and patterns that are oriented parallel or perpendicular to survey tracklines, which run north-south. Blank areas (black on the image) represent places where no data exists. Large east-west-trending bedforms (up to 10 m in height) characterize parts of the sea floor in this region. The multibeam echo sounder had difficulty imaging these features because they are oriented at right angles to the ship's tracks and display steep sides and narrow crests. As a result, the crests of some of these bedforms display small gaps or lows that do not exist. For a depiction of the sea floor topography without imagery and for an explanation of survey and topographic data-processing methods, see the companion map by Valentine and others (2002a) on this CD-ROM.

Regional seabed features

The Great South Channel separates the western part of Georges Bank from Nantucket Shoals and is a major conduit for the exchange of water between the Gulf of Maine to the north and the Atlantic Ocean to the south. Water depths range mostly between 55 and 95 m in Quadrangle 2. Minimum depths of 45 to 50 m occur near the southeast corner, and a maximum depth of 100 m occurs in the northwest corner. The major topographic features depicted in the Great South Channel were formed by glacial and postglacial processes. Ice containing rock debris moved from north to south, sculpting the region into a broad shallow depression and depositing sediment to form the irregular depressions and low gravelly mounds and ridges that are visible in parts of the mapped area. Many other smaller glacial features probably have been eroded by waves and currents at work since the time when the region, formerly exposed by lowered sea level or occupied by ice, was invaded by the sea. The low, irregular and somewhat lumpy fabric formed by the glacial deposits is obscured in places by drifting sand and by the linear, sharp fabric formed by modern sand features.

In many respects, the seabed in Quadrangle 2 is similar to that in Quadrangle 1 (Valentine and others (2002b) on this CD-ROM). In both quadrangles the seabed consists of coarse-grained glacial deposits (gravel, sandy gravel, and coarse sand), and, in part, modern sands. In this quadrangle, low ridges and mounds composed of glacial gravel are visible where they have not been obscured by modern sands. An example is the area that extends northward from 41°01' N. to 41°03.5' N. and that is bounded by 68°59' W. and 69°02' W. A similar area lies between 41°01' N. and 41°04' N. and is bounded by 68°55' W. and 68°58' W. Relatively smooth areas of the seabed (that are not dominated by modern sand bedforms) consist of glacial gravel that has been exposed by bottom currents. They are centered at the following locations: 41°02' N., 68°50' W.; 41°01.5' N., 68°52' W.; and 41°02.5' N., 69° 01' W.

Today, strong tidal and storm currents flow dominantly north and south in the region. Transport of sand by these currents has resulted in the construction of large, east-west-trending sand dunes. These large bedforms contrast strongly with, and partly mask, the subdued topography of the older glacial features. The modern bedforms are represented by two major types. One group of features generally is high and steep-sided, has straight axes, and is composed of coarse-grained sediment (example at 41°06.35' N., 68°56.5' W.). The second group generally is lower, has sinuous axes, and consists of fine-grained sediment (example at 41°06.47' N., 68°57.2' W.). The irregular sinuous shapes and fine-grained nature of the bedforms in the second group suggest that they are more mobile than the more symmetrical and coarse-grained bedforms of the first group. The sinuous bedforms are most common in the northern and southeastern parts of the quadrangle. The high, straight-crested bedforms occur throughout the quadrangle, and many are bounded by linear depressions and are separated from neighboring bedforms by gravel seabed (41°01.7' N., 68°54.1' W.). The linear depressions possibly resulted from the scouring of sand from gravelly glacial deposits around the bedform, a process that caused the formation and settling of the gravel seabed. Several of the large isolated bedforms that are located in gravel areas are eroding. Sand eroded from these bedforms is represented by linear, en-echelon sand waves that trail away downcurrent (southeastward) from the bedforms onto the gravel pavement (41°02.9' N., 68°50.6' W.). As erosion continues (see examples in Valentine and others, 2002b), the bedforms are removed, depressions are left in the sea floor, and thin linear deposits of rippled sand extend downcurrent onto the surrounding gravel seabed. Finally, the thin rippled sand deposits are eroded, and the only evidence of the former large bedforms are elongate depressions in the sea floor.

Four wedge-shaped areas of smooth, sandy seabed extend south from the northern edge of the map area, along 69°03' W., 68°57' W., 68°53' W., and 68°49' W. These smooth areas are separated from one another by somewhat shallower areas of seabed that display large bedforms. The shallower areas display a north-to-south progression (as water depth decreases) of features that grade from sinuous fine-grained bedforms to high, straight-crested, coarse-grained bedforms that are separated from one another by gravelly seabed (for example, the area extending south from 41°08' N. along 68°56' W.). The symmetry and the dominant east-west trend of both groups of large bedforms in this quadrangle suggest that storm and daily tidal currents move sand both north and south in the Great South Channel. Sand grains on the flanks of large and small bedforms probably are moved back and forth during each tidal cycle. Although storm currents are less predictable than tidal currents and can transport sand rapidly over longer distances, the positions of the large bedforms probably are relatively stable. However, the orientation of the sand trails from eroding bedforms suggests that, with time, net sediment movement is to the south and southeast.

References Cited

Valentine, P.C., Malczyk, J.T., and Middleton, T.J., 2002a, Sea floor topography of Quadrangle 2, map C of Valentine, P.C., ed., Maps showing sea floor topography, sun-illuminated sea floor topography, and backscatter intensity of Quadrangles 1 and 2 in the Great South Channel region, western Georges Bank: U.S. Geological Survey Geologic Investigations Series Map I–2698, scale 1:25,000, on a CD-ROM.

Valentine, P.C., Middleton, T.J., and Malczyk, J.T., 2002b, Sun-illuminated sea floor topography of Quadrangle 1, map B of Valentine, P.C., ed., Maps showing sea floor topography, sun-illuminated sea floor topography, and backscatter intensity of Quadrangles 1 and 2 in the Great South Channel region, western Georges Bank: U.S. Geological Survey Geologic Investigations Series Map I–2698, scale 1:25,000, on a CD-ROM.

Sheet E—Backscatter Intensity and Sun-Illuminated Sea Floor Topography of
Quadrangles 1 and 2

Introduction

The Georges Bank Mapping Project is a cooperative effort of the U.S. Geological Survey and the National Oceanic and Atmospheric Administration (NOAA), with support from the University of New Brunswick and the Canadian Hydrographic Service. This map shows sea floor imagery of Quadrangles 1 and 2 in the Great South Channel region (see location map) collected during a multibeam echo sounder survey that was conducted in November 1998. Backscatter intensity (or reflectivity) of the seabed is combined here with sun-illuminated sea floor topographic imagery (shaded relief) at a scale of 1:25,000. For maps showing only sea floor topography or sun-illuminated sea floor topographic imagery, and for a description of survey and image processing methods, see the companion maps by Valentine and others on this CD-ROM. Unnatural-looking stripes and patterns oriented parallel or perpendicular to survey tracklines (which run north-south) are artifacts of data collection. Topographic lows are identified by hachured contours (hachures face deeper water). Blank areas (gray on image) represent places where no data exists.

Sea floor character

The Great South Channel separates the western part of Georges Bank from Nantucket Shoals and is a major conduit for the exchange of water between the Gulf of Maine to the north and the Atlantic Ocean to the south. Water depths range mostly between 65 and 80 m in the region. A minimum depth of 45 m occurs in the east-central part of the mapped area, and a maximum depth of 100 m occurs in the northwest corner. The channel region is characterized by strong tidal and storm currents that flow dominantly north and south.

Major topographic features of the seabed were formed by glacial and postglacial processes. Ice containing rock debris moved from north to south, sculpting the region into a broad shallow depression and depositing sediment to form the irregular depressions and low gravelly mounds and ridges that are visible in parts of the mapped area. Many other smaller glacial features probably have been eroded by waves and currents at work since the time when the region, formerly exposed by lowered sea level or occupied by ice, was invaded by the sea. The low, irregular and somewhat lumpy fabric formed by the glacial deposits is obscured in places by drifting sand and by the linear, sharp fabric formed by modern sand features. Today, sand transported by the strong north-south-flowing tidal and storm currents has formed large, east-west-trending dunes. These bedforms (ranging between 5 and 20 m in height) contrast strongly with, and partly mask, the subdued topography of the older glacial features.

Backscatter intensity is a measure of the hardness and roughness of the sea floor as determined by the strength of the sound waves reflected from the seabed during the survey (see backscatter intensity scale bar). In the image shown here, backscatter intensity is represented by a suite of eight colors ranging from blue, which represents low intensity (soft bottom), to red, which represents high intensity (hard bottom). The backscatter intensity data are draped over a shaded relief image created by vertically exaggerating the topography four times and then artificially illuminating the relief by a light source positioned 45 degrees above the horizon from an azimuth of 0 degrees. The resulting image displays light and dark tones within each color band that are determined by a feature's position with respect to the light source. For example, north-facing slopes, receiving strong illumination, show as light tones within a color band, whereas south-facing slopes, being in shadow, show as dark tones within a color band. An interpretation of backscatter intensity based on sediment sampling and video imagery of the sea floor in the mapped area suggests that high backscatter values (6-7-8, yellow-orange-red) represent gravel and sandy gravel. Moderate backscatter values (3-4-5, green-yellow) represent sand and burrowed sand, and low backscatter values (1-2, blue) denote clean, fine- to coarse-grained sand. Some areas display relatively uniform levels of backscatter intensity, implying homogeneity of bottom sediment. However, in other places the backscatter intensity and bottom types are highly variable. For example, prominent east-west-trending sand dunes are separated by troughs where gravel seabed commonly is exposed. This is shown clearly by the backscatter intensity level, which ranges from 1 to 2 (blue) on individual sand dunes and from 5 to 8 (green-yellow-orange-red) on the sand, sandy gravel, and gravel between these features.
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