seafloor topography (fig. 2), the type and distribution of seafloor 

sediments (fig. 3), and thickness of sediment deposits 

(fig. 4). The imagery derived from seafloor-mapping systems 

was validated by sediment sampling and photography (fig. 5). 

The different data types were integrated as a series of layers 

within a GIS (fig. 6) to produce interpretations of seafloor 

geology. 

The final product, an interpretive geologic map (fig. 7), 

synthesized the complex geology at a regional scale and made 

the information available to both scientific and nontechnical 

audiences. For regional mapping purposes, we defined five map 

units or physiographic zones, which are summarized below. 

The zones are based on seafloor topography, the dominant 

bottom types present, and the thickness of sediment deposits. 

Although considerable variation in these characteristics exists 

within each zone, this method allows efficient mapping of large 

areas and presents geologic information in a readily useable 

format. 

Seafloor Mapping Supports Informed 

Decision Making

As coastal communities expand in population, pressure 

also grows to develop energy, transportation, and other infrastructure in the shallow ocean adjacent to our coasts. Generating accurate maps of seafloor geology in these sensitive areas is 

an important first step toward protecting fish habitat, delineating 

marine reserves, and assessing environmental changes. The 

USGS/CZM/NOAA cooperative project represents part of a 

two-phase, multidisciplinary approach to research on the structure and function of marine habitats. The first phase, described 

in this fact sheet, seeks to gain an understanding of the geologic 

framework in the study area and communicate that information 

to scientists, industry, and the public. The second phase, which 

Figure 7. Geologic map of seafloor for area 1. Five environments, or physiographic zones, are delineated on the basis of the data from seafloor-mapping 

systems and sampling presented in figures 2-5. These geologic environments are described in the table at right. [km2, square kilometers; mi2, square miles]

is just beginning, will use the defined geologic framework to 

guide biological sampling. For example, the Massachusetts 

Department of Marine Fisheries already uses the maps and data 

layers produced for area 1 to monitor the recovery of marine 

habitats disturbed by construction of a submarine pipeline. The 

ultimate goal is to merge the regional geologic mapping with 

site-specific biological assessments, thereby fully characterizing 

benthic habitats and providing guidance for policy decisions that 

affect coastal and nearshore environments.

References

Barnhardt, W.A., Andrews, B.D. and Butman, Bradford, 2005, High-resolution geologic 

mapping of the inner continental shelf—Nahant to Gloucester, Massachusetts: U.S. 

Geological Survey Open-File Report 2005-1293. Available online at http://pubs.usgs.

gov/of/2005/1293/

Butman, Bradford, Valentine, P.C., Danforth, W.W., Hayes, Laura, Serrett, L.A., and 

Middleton, T.J., 2004, Shaded relief, backscatter intensity and sea floor topography of 

Massachusetts Bay and the Stellwagen Bank region, offshore of Boston, Massachusetts: 

U.S. Geological Survey Geologic Investigations Series Map I-2734, scale 1:125,000, 

2 sheets. Available online at http://pubs.usgs.gov/imap/i2734 

National Research Council, 2004, A Geospatial Framework for the Coastal Zone—National 

Needs for Coastal Mapping and Charting, 168 p.

Schlee, J. S., 1973, Atlantic continental shelf and slope of the United States—Sediment 

texture of the northeastern part: U.S. Geological Survey Professional Paper 529-H, 39 p.

U.S. Commission on Ocean Policy, 2004, An Ocean Blueprint for the 21st Century, 676 p. 

Zen, E-an, Goldsmith, R., Ratcliffe, N.M., Robinson, P. and Stanley, R.S., 1983, Bedrock 

geologic map of Massachusetts: U.S. Geological Survey, Washington D.C., scale 

1:250,000, 3 sheets.

For more information contact:

Walter Barnhardt wbarnhardt@usgs.gov or 

Brian Andrews bandrews@usgs.gov

U.S. Geological Survey

Woods Hole Science Center

384 Woods Hole Road

Woods Hole, MA 02543

(508) 548-8700

http://woodshole.er.usgs.gov/project-pages/coastal_mass/

Marblehead

Nahant

Gloucester

Salem

PHYSIOGRAPHIC ZONES

Nearshore Basin

Rocky Zone

Nearshore Ramp

Shelf Valley

Bay Mouth Shoal

70°55' 70°45'

42°

35'

42°

25'

0 2 4 KILOMETERS

0 2 4 MILES

1. Nearshore Basin . smooth, flat-lying areas; thick muddy sediment . sheltered areas within embayments . cover 7.0 km2 (2.7 mi

2, 5.3 percent) of map area

2. Rocky Zone . high-relief rock outcrops, boulders and gravel . exposed areas along open-ocean coast . cover 52.4 km2 (20.2 mi

2, 39.2 percent) of map area

3. Nearshore Ramp . smoothly seaward-sloping areas; generally

sandy sediment . extend offshore from coast to depths

greater than 40 meters . cover 57.3 km2 (22.1 mi

2, 42.9 percent) of map area

4. Shelf Valley . elongate valleys and interconnected depressions

partially filled with sediment . extend offshore, perpendicular to coast . cover 12.5 km2 (4.8 mi

2, 9.3 percent) of map area

5. Bay Mouth Shoal . flat-topped banks of sand and gravel . reworked by waves and tides at mouths of embayments . cover 4.4 km2 (1.7 mi

2, 3.2 percent) of map area
possible for waves and currents to shape wide areas of the inner 

shelf. Erosion of older glacial deposits on the coast and inner 

shelf has produced sand that has built local beaches and muddy 

sediment that has accumulated in small estuaries. Removal 

of the fine-grained sand and mud by ocean processes has left 

behind extensive deposits of bedrock, gravel and boulders in 

exposed areas along and seaward of the outer coast. 

Geologic Characterization of the Seafloor

Mapping the geology in this formerly glaciated region is 

difficult because 1) the rugged, high-relief seafloor exhibits 

abrupt changes in water depth, and 2) different substrates form 

a distinct patchiness on the seafloor that changes over distances 

of only a few meters. For example, high-relief bedrock and 

boulder deposits are commonly exposed on the seafloor in close 

proximity to flat-lying deposits of sand and mud. Rocky areas 

also contain isolated accumulations of shell sediment in small 

cracks and other low-lying areas between outcrops. These 

small-scale variations present a great challenge to marine 

geologists, who need to map complex geologic environments in 

a format that is useful to managers and policy makers. 

The first map for this project was completed in 2005 for 

area 1 (fig. 1), and the resulting data and maps were published 

online (Barnhardt and others, 2005). The map encompassed 

134 square kilometers (52 square miles) of the inner shelf with a 

focus on the nearshore region, which has water depths of 5 to 40 

meters (16 to 132 feet). Three seafloor-mapping systems were 

used to examine the geologic framework in three dimensions: 

bathymetric sonar, sidescan sonar, and subbottom profiling. 

These systems transmit acoustic waves (sound) from a research 

vessel and record the waves reflected from the seafloor back to 

the ship. The acoustic data provided detailed information on 

AREA SHOWN

IN FIGURE 6

Marblehead

Nahant

Gloucester

Salem

DEPTH, IN METERS

0 2 4 KILOMETERS

0 2 4 MILES

70°55' 70°45'

42°

25'

42°

35'

- 4

- 48

Figure 2. Map of seafloor topography. Bathymetric sonar collects soundings 

of water depth in a wide swath beneath the ship. Depth is color-coded with 

reds and oranges indicating shallow water, and blues and greens indicating 

deeper water. These data show the detailed shape of the seafloor, resolving 

high-relief, bouldery deposits such as moraines and drumlins formed by glaciers 

that scoured the region during the last Ice Age. The box indicates the location 

of data layers shown in figure 6.

Marblehead

Nahant

Gloucester

Salem

B

A

C

A

Backscatter, in DN

Photograph location

and identifier

70°55' 70°45'

42°

25'

42°

35'

0

256

0 2 4 KILOMETERS

0 2 4 MILES

Figure 3. Backscatter image of seafloor, produced with sidescan sonar. 

Backscatter values within this image indicate the strength of sound waves 

reflected off the seafloor and are represented by digital numbers (DN) that 

range from 0 to 256. High backscatter (light tones) is associated with generally 

coarse sediment and rock outcrops. Low backscatter (dark tones) is associated 

with generally fine-grained mud and fine sand. Broad areas of dark-toned, finer 

sediment correlate with areas of smooth topography in figure 2; light-toned 

rocky areas correlate with areas of high-relief topography. Yellow circles 

indicate the locations of three photographs shown in figure 5 A-C.

Glacial-marine

Sediment Bedrock

till

Seafloor

multiple

multiple

Bedrock

0 500 1,000 1,500

0

10

20

30

40

50

60

0

10

20

30

40

50

60

DEPTH, IN METERS

DISTANCE, IN METERS

SW NE

Photo 5A

SUBBOTTOM PROFILE

Subbottom Profile. The upper diagram is the original data and the lower 

diagram is an interpreted section showing the geometry and thickness 

of sediment deposits that lie at or beneath the seafloor. The vertical axis 

represents the depth below the sea surface, and the horizontal axis the distance 

traveled while collecting these data. Arrows labeled “multiple” indicate 

artifacts in the data, not actual layers. A subbottom profile is analogous to a 

slice of cake, showing the layered internal structure of materials that compose 

the seafloor. The profile crosses a bedrock-framed valley filled with thick 

glacial-marine sediment (pink), which is locally exposed on the seafloor (photo 

in figure 5A). Surficial sediment is thin to absent on the right side of the profile, 

where bedrock is exposed at the surface. This profile is represented by the 

black line in figure 4. 
possible for waves and currents to shape wide areas of the inner 

shelf. Erosion of older glacial deposits on the coast and inner 

shelf has produced sand that has built local beaches and muddy 

sediment that has accumulated in small estuaries. Removal 

of the fine-grained sand and mud by ocean processes has left 

behind extensive deposits of bedrock, gravel and boulders in 

exposed areas along and seaward of the outer coast. 

Geologic Characterization of the Seafloor

Mapping the geology in this formerly glaciated region is 

difficult because 1) the rugged, high-relief seafloor exhibits 

abrupt changes in water depth, and 2) different substrates form 

a distinct patchiness on the seafloor that changes over distances 

of only a few meters. For example, high-relief bedrock and 

boulder deposits are commonly exposed on the seafloor in close 

proximity to flat-lying deposits of sand and mud. Rocky areas 

also contain isolated accumulations of shell sediment in small 

cracks and other low-lying areas between outcrops. These 

small-scale variations present a great challenge to marine 

geologists, who need to map complex geologic environments in 

a format that is useful to managers and policy makers. 

The first map for this project was completed in 2005 for 

area 1 (fig. 1), and the resulting data and maps were published 

online (Barnhardt and others, 2005). The map encompassed 

134 square kilometers (52 square miles) of the inner shelf with a 

focus on the nearshore region, which has water depths of 5 to 40 

meters (16 to 132 feet). Three seafloor-mapping systems were 

used to examine the geologic framework in three dimensions: 

bathymetric sonar, sidescan sonar, and subbottom profiling. 

These systems transmit acoustic waves (sound) from a research 

vessel and record the waves reflected from the seafloor back to 

the ship. The acoustic data provided detailed information on 

AREA SHOWN

IN FIGURE 6

Marblehead

Nahant

Gloucester

Salem

DEPTH, IN METERS

0 2 4 KILOMETERS

0 2 4 MILES

70°55' 70°45'

42°

25'

42°

35'

- 4

- 48

Figure 2. Map of seafloor topography. Bathymetric sonar collects soundings 

of water depth in a wide swath beneath the ship. Depth is color-coded with 

reds and oranges indicating shallow water, and blues and greens indicating 

deeper water. These data show the detailed shape of the seafloor, resolving 

high-relief, bouldery deposits such as moraines and drumlins formed by glaciers 

that scoured the region during the last Ice Age. The box indicates the location 

of data layers shown in figure 6.

Marblehead

Nahant

Gloucester

Salem

B

A

C

A

Backscatter, in DN

Photograph location

and identifier

70°55' 70°45'

42°

25'

42°

35'

0

256

0 2 4 KILOMETERS

0 2 4 MILES

Figure 3. Backscatter image of seafloor, produced with sidescan sonar. 

Backscatter values within this image indicate the strength of sound waves 

reflected off the seafloor and are represented by digital numbers (DN) that 

range from 0 to 256. High backscatter (light tones) is associated with generally 

coarse sediment and rock outcrops. Low backscatter (dark tones) is associated 

with generally fine-grained mud and fine sand. Broad areas of dark-toned, finer 

sediment correlate with areas of smooth topography in figure 2; light-toned 

rocky areas correlate with areas of high-relief topography. Yellow circles 

indicate the locations of three photographs shown in figure 5 A-C.

Glacial-marine

Sediment Bedrock

till

Seafloor

multiple

multiple

Bedrock

0 500 1,000 1,500

0

10

20

30

40

50

60

0

10

20

30

40

50

60

DEPTH, IN METERS

DISTANCE, IN METERS

SW NE

Photo 5A

SUBBOTTOM PROFILE

Subbottom Profile. The upper diagram is the original data and the lower 

diagram is an interpreted section showing the geometry and thickness 

of sediment deposits that lie at or beneath the seafloor. The vertical axis 

represents the depth below the sea surface, and the horizontal axis the distance 

traveled while collecting these data. Arrows labeled “multiple” indicate 

artifacts in the data, not actual layers. A subbottom profile is analogous to a 

slice of cake, showing the layered internal structure of materials that compose 

the seafloor. The profile crosses a bedrock-framed valley filled with thick 

glacial-marine sediment (pink), which is locally exposed on the seafloor (photo 

in figure 5A). Surficial sediment is thin to absent on the right side of the profile, 

where bedrock is exposed at the surface. This profile is represented by the 

black line in figure 4. 
seafloor topography (fig. 2), the type and distribution of seafloor 

sediments (fig. 3), and thickness of sediment deposits 

(fig. 4). The imagery derived from seafloor-mapping systems 

was validated by sediment sampling and photography (fig. 5). 

The different data types were integrated as a series of layers 

within a GIS (fig. 6) to produce interpretations of seafloor 

geology. 

The final product, an interpretive geologic map (fig. 7), 

synthesized the complex geology at a regional scale and made 

the information available to both scientific and nontechnical 

audiences. For regional mapping purposes, we defined five map 

units or physiographic zones, which are summarized below. 

The zones are based on seafloor topography, the dominant 

bottom types present, and the thickness of sediment deposits. 

Although considerable variation in these characteristics exists 

within each zone, this method allows efficient mapping of large 

areas and presents geologic information in a readily useable 

format. 

Seafloor Mapping Supports Informed 

Decision Making

As coastal communities expand in population, pressure 

also grows to develop energy, transportation, and other infrastructure in the shallow ocean adjacent to our coasts. Generating accurate maps of seafloor geology in these sensitive areas is 

an important first step toward protecting fish habitat, delineating 

marine reserves, and assessing environmental changes. The 

USGS/CZM/NOAA cooperative project represents part of a 

two-phase, multidisciplinary approach to research on the structure and function of marine habitats. The first phase, described 

in this fact sheet, seeks to gain an understanding of the geologic 

framework in the study area and communicate that information 

to scientists, industry, and the public. The second phase, which 

Figure 7. Geologic map of seafloor for area 1. Five environments, or physiographic zones, are delineated on the basis of the data from seafloor-mapping 

systems and sampling presented in figures 2-5. These geologic environments are described in the table at right. [km2, square kilometers; mi2, square miles]

is just beginning, will use the defined geologic framework to 

guide biological sampling. For example, the Massachusetts 

Department of Marine Fisheries already uses the maps and data 

layers produced for area 1 to monitor the recovery of marine 

habitats disturbed by construction of a submarine pipeline. The 

ultimate goal is to merge the regional geologic mapping with 

site-specific biological assessments, thereby fully characterizing 

benthic habitats and providing guidance for policy decisions that 

affect coastal and nearshore environments.

References

Barnhardt, W.A., Andrews, B.D. and Butman, Bradford, 2005, High-resolution geologic 

mapping of the inner continental shelf—Nahant to Gloucester, Massachusetts: U.S. 

Geological Survey Open-File Report 2005-1293. Available online at 

http://pubs.usgs.gov/of/2005/1293/

Butman, Bradford, Valentine, P.C., Danforth, W.W., Hayes, Laura, Serrett, L.A., and 

Middleton, T.J., 2004, Shaded relief, backscatter intensity and sea floor topography of 

Massachusetts Bay and the Stellwagen Bank region, offshore of Boston, Massachusetts: 

U.S. Geological Survey Geologic Investigations Series Map I-2734, scale 1:125,000, 

2 sheets. Available online at http://pubs.usgs.gov/imap/i2734 

National Research Council, 2004, A Geospatial Framework for the Coastal Zone—National 

Needs for Coastal Mapping and Charting, 168 p.

Schlee, J. S., 1973, Atlantic continental shelf and slope of the United States—Sediment 

texture of the northeastern part: U.S. Geological Survey Professional Paper 529-H, 39 p.

U.S. Commission on Ocean Policy, 2004, An Ocean Blueprint for the 21st Century, 676 p. 

Zen, E-an, Goldsmith, R., Ratcliffe, N.M., Robinson, P. and Stanley, R.S., 1983, Bedrock 

geologic map of Massachusetts: U.S. Geological Survey, Washington D.C., scale 

1:250,000, 3 sheets.

For more information contact:

Walter Barnhardt wbarnhardt@usgs.gov or 

Brian Andrews bandrews@usgs.gov

U.S. Geological Survey 

Woods Hole Science Center 

384 Woods Hole Road 

Woods Hole, MA 02543 

(508) 548-8700 

http://woodshole.er.usgs.gov/project-pages/coastal_mass/

Marblehead

Nahant

Gloucester

Salem

PHYSIOGRAPHIC ZONES

Nearshore Basin

Rocky Zone

Nearshore Ramp

Shelf Valley

Bay Mouth Shoal

70°55' 70°45'

42°

35'

42°

25'

0 2 4 KILOMETERS

0 2 4 MILES

1. Nearshore Basin . smooth, flat-lying areas; thick muddy sediment . sheltered areas within embayments . cover 7.0 km2 (2.7 mi

2, 5.3 percent) of map area

2. Rocky Zone . high-relief rock outcrops, boulders and gravel . exposed areas along open-ocean coast . cover 52.4 km2 (20.2 mi

2, 39.2 percent) of map area

3. Nearshore Ramp . smoothly seaward-sloping areas; generally

sandy sediment . extend offshore from coast to depths

greater than 40 meters . cover 57.3 km2 (22.1 mi

2, 42.9 percent) of map area

4. Shelf Valley . elongate valleys and interconnected depressions

partially filled with sediment . extend offshore, perpendicular to coast . cover 12.5 km2 (4.8 mi

2, 9.3 percent) of map area

5. Bay Mouth Shoal . flat-topped banks of sand and gravel . reworked by waves and tides at mouths of embayments . cover 4.4 km2 (1.7 mi

2, 3.2 percent) of map area