Sidescan Sonar Imagery, Multibeam Bathymetry, 

and Surficial Geologic Interpretations of the Sea 

Floor in Rhode Island Sound, off Sakonnet Point, 

Rhode Island 

By Katherine Y. McMullen, Lawrence J. Poppe, Erin R. Twomey, William W. Danforth, Todd A. 

Haupt, and James M. Crocker 

U.S. Geological Survey Open-File Report 2007-1150 

U.S. Department of the Interior 

U.S. Geological Survey 
ii

U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2007 

Revised and reprinted: 2007 

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Suggested citation: 

McMullen, K.Y., Poppe, L.J., Twomey, E.R., Danforth, W.W., Haupt, T.A., and Crocker, J.M., 

2007, Sidescan Sonar Imagery, Multibeam Bathymetry, and Surficial Geologic Interpretations of 

the Sea Floor in Rhode Island Sound, off Sakonnet Point, Rhode Island: U.S. Geological Survey 

Open File Report 2007-1150, DVD-ROM. Also available online at 

http://pubs.usgs.gov/of/2007/1150/. 

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endorsement by the U.S. Government. 

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copyright owners to reproduce any copyrighted material contained within this report. 
iii

Contents 

Contents....................................................................................................................................... iii

Introduction ...................................................................................................................................1

Geologic Setting............................................................................................................................1

Data Acquisition and Processing...................................................................................................2

Bathymetry....................................................................................................................................3

Sidescan Sonar Imagery...............................................................................................................3

Rocky and Bouldery Areas.........................................................................................................3

Sand-wave Fields.......................................................................................................................3

Hummocky Areas.......................................................................................................................4

Tabular Erosional Outliers..........................................................................................................4

Small Hills and Scarps ...............................................................................................................5

Trawl Marks................................................................................................................................5

Sedimentary Environments ...........................................................................................................5

Erosion or Nondeposition...........................................................................................................5

Coarse-Grained Bedload Transport ...........................................................................................6

Sorting and Reworking...............................................................................................................6

Discussion.....................................................................................................................................6

Summary.......................................................................................................................................7

GIS Data Catalog ..........................................................................................................................7

Sidescan Sonar Imagery............................................................................................................8

Bathymetric Imagery ..................................................................................................................8

Bathymetric Grids.......................................................................................................................9

Interpretive Data.........................................................................................................................9

Basemap Data ...........................................................................................................................9

Acknowledgements .......................................................................................................................9

Contacts......................................................................................................................................10

References Cited ........................................................................................................................11

Figures 

Figure 1. Location map of NOAA Survey H11320 (red polygon) in Rhode Island Sound. 

Location of stratigraphy profile A-A’ from figure 3 is also shown.................................................13

Figure 2. Location of figures (red polygons), bathymetric profiles (green lines), historic seismicreflection profiles (blue lines, O'Hara and Oldale, 1980), and sand (yellow circles) and rocky (red 

circle) sediment samples (Poppe and others, 2003) within the study area. ................................14

Figure 3. Interpreted stratigraphy of Rhode Island Sound from O'Hara and Oldale (1980). 

Acoustic units include: PzZ - Proterozoic to Paleozoic bedrock, Ku - Cretaceous coastal plain 

sediments, Qdo - Wisconsin glacial drift deposits, Qdm - Buzzards Bay moraine deposits, Qfe - 

Holocene fluvial and estuarine deposits, and Qm - Holocene quiet-water marine deposits. 

Boundaries are dashed where inferred. Vertical exaggeration: 40X. Location of stratigraphy 

profile shown as A-A’ in figure 1..................................................................................................15

Figure 4. Map showing the location of end moraines (solid black polygons) in southern New 

England and New York. The Ronkonkoma-Nantucket moraine shows the maximum extent of the 

Laurentide ice sheet 20 ka, while the Harbor Hill-Roanoke Point-Charlestown-Buzzards Bay 

moraine shows a retreated position of the ice sheet around 18 ka (Uchupi and others, 1996). 
iv

Submarine ridges (dotted lines) mark the underwater extensions of the moraines. H11320 study 

area shown as a gray polygon. Modified from Gustavson and Boothroyd (1987). ......................16

Figure 5. Photo of NOAA Ship RUDE (courtesy of NOAA).........................................................17

Figure 6. Hill-shaded bathymetry of the study area. Four areas outlined in black show multibeam 

bathymetry from H11320, while the rest of the bathymetry is from the NGDC Coastal Relief 

Model. Multibeam bathymetric data have been gridded at 4 m and depths represent mean lower 

low water level in multibeam data and either mean low water or mean lower low water in Coastal 

Relief Model data (Divins and Metzger, n.d.). .............................................................................18

Figure 7. Sidescan sonar imagery of NOAA Survey H11320. Darker regions represent low 

backscatter and generally finer grained sediments; lighter regions represent high backscatter 

and generally coarser grained sediments. ..................................................................................19

Figure 8. Interpretation of sidescan sonar imagery and multibeam bathymetry showing areas of 

rocky and gravelly sediment, sand waves and crest orientations, sediment transport direction, 

hummocky sea floor, tabular erosional outliers, small hills and scarps, and trawl marks. ...........20

Figure 9. Detailed sidescan sonar imagery and bathymetry illustrating the relatively smooth 

central basin floor scattered with rocks and boulders in the south and adjacent ridge to the north. 

Sand waves are located on the ridge and a scarp crosses the ridge in an east-west direction. 

Sand waves have curved crests oriented east-west and wavelengths of 40 to over 100 m in the 

west and about 50 m in the east. High-resolution multibeam bathymetry has been overlaid on 

bathymetry from the NGDC Coastal Relief Model. Figure location is shown in figure 2..............21

Figure 10. Bathymetric profiles of sand waves showing slip faces oriented northward. Profile CC' is of sand waves in the eastern field of east-west oriented crests, which have wavelengths of 

about 50 m. Profile D-D' is of sand waves in the northwestern corner of the study area where 

wavelengths are also about 50 m. Location of profiles shown in figure 2....................................22

Figure 11. Detailed view of sidescan sonar imagery showing mottled backscatter and 

corresponding bathymetry. Mottled backscatter in the sidescan sonar data is caused by the 

hummocky surface of moraines and glacial till inconsistently reflecting backscatter due to the 

variable topography and sediment distribution. Note: black line outlines high-resolution 

multibeam bathymetry which overlies bathymetry from the Coastal Relief Model. Figure location 

is shown in figure 2. ....................................................................................................................23

Figure 12. Sidescan sonar imagery and Boomer seismic-reflection profile (O'Hara and Oldale, 

1980) and interpretation through an area of mottled backscatter. Right side of sidescan sonar 

image shows mottled backscatter (top image). Seismic profile shows Cretaceous coastal plain 

sediments overlain by Wisconsin glacial drift and Holocene fluvial and estuarine sediments. 

Mottling tends to occur in areas where glacial moraine sediments are either exposed or near the 

surface. Vertical exaggeration: 13X. Depths are based on the assumed compressional wave 

velocity of 1500 m/s. Location of seismic line shown in figure 2..................................................25

Figure 13. Detailed area of sidescan sonar image showing low-backscatter targets. These 

features show no relief in the bathymetry data and are thought to be patches of fine-grained 

sediment within larger areas of coarser grained sediment. Figure location shown in figure 2.....26

Figure 14. Detailed view of sidescan sonar imagery and corresponding hill-shaded bathymetry 

showing tabular erosional outliers. Curvilinear shadows in the sidescan sonar imagery 

correspond to the outline of small plateaus in the bathymetry. Boulders are visible in both 

images, on the sidescan sonar imagery as light-toned targets with dark shadows and within the 

bathymetry as dark targets. The area of boulders in the lower right corner of both images shows 

an exposure of till. Lighter backscatter between the erosional outliers is from coarse-grained, 

bouldery sediment. Images are in UTM. Location of figure shown in figure 2. ............................27
v

Figure 15. Sidescan sonar imagery, Boomer seismic-reflection profile (O'Hara and Oldale, 

1980), and interpretation through an area of tabular erosional outliers. Top image shows location 

of the seismic profile on the sidescan sonar data and erosional outliers in the center. Lower 

images of seismic profile and interpretation show stratified sediments, possibly from a small 

glacial lake or pond, below the erosional outliers. Note irregular sea floor and estuarine 

sediments above laminations, likely due to prior erosion of lacustrine sediments. Boulders can 

be seen rising above the sea floor. Vertical exaggeration: 16X. Depths are based on the 

assumed compressional wave velocity of 1500 m/s. Figure location shown in figure 2. .............28

Figure 16. Bathymetric profiles across study area from north to south. Location of profiles shown 

in figure 2. Profile A-A', located in the southwestern part of the study area, crosses a bathymetric 

high with deeper basins to the north and south. Profile B-B', located in the eastern part of the 

study area, crosses a bathymetric high in the north, central basin, and moraine in the south. 

Arrows point to changes in slope interpreted to represent paleoshorelines. ...............................30

Figure 17. Sidescan sonar imagery, Boomer seismic-reflection profile (O'Hara and Oldale, 

1980), and interpretation of an area with small scarps of outcropping strata. Top image shows 

scarp and location of seismic line on sidescan sonar mosaic. Bottom images of seismic line and 

interpretation show Pleistocene glacial drift overlain by Holocene fluvial and estuarine 

sediments. The small scarp displayed within the sidescan sonar mosaic is presumed to 

correspond to the prominent reflector between the fluvial sediments and glacial drift as it nears 

the surface. Vertical exaggeration: 15X. Depths are based on the assumed compressional wave 

velocity of 1500 m/s. Location of seismic line shown in figure 2..................................................31

Figure 18. Detailed view of sidescan sonar imagery showing long lines of low backscatter 

interpreted to be trawl marks from fishing boats. Figure location is shown in figure 2.................33

Figure 19. Map showing sedimentary environments in study area characterized by erosion or 

nondeposition, coarse-grained bedload transport, and sorting and reworking. ...........................34

Conversion Factors 

Multiply By To obtain 

Length 

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi) 

Area 

square kilometer (km2) 247.1 acre 

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). 

Vertical datum, as used in this report, refers to mean lower low water. 
Sidescan Sonar Imagery, Multibeam 

Bathymetry, and Surficial Geologic 

Interpretations of the Sea Floor in Rhode 

Island Sound, off Sakonnet Point, Rhode 

Island 

By Katherine Y. McMullen, Lawrence J. Poppe, Erin R. Twomey, William W. Danforth, 

Todd A. Haupt, and James M. Crocker 

Introduction 

The U.S. Geological Survey (USGS) is working with the National Oceanic and 

Atmospheric Administration (NOAA) to interpret the surficial geology in estuaries and sounds 

along the northeastern coast of the United States. This report interprets the area covered by NOAA 

Survey H11320, about 72 km2 of sea floor in eastern Rhode Island Sound (RIS), located about 8 km 

south of Sakonnet Point, Rhode Island (fig. 1). Previous work in RIS includes studies of both seafloor processes and subsurface geologic framework. McMaster (1960) mapped surficial sediment 

samples in Narragansett Bay and RIS and McMaster and others (1968) conducted a seismicreflection survey in Block Island Sound and RIS. O'Hara and Oldale (1980) collected seismicreflection profiles, sidescan sonar data, and vibracores in eastern RIS (fig. 2). They interpreted the 

geologic history, assessed sand and gravel resources, and evaluated the mining impact of these 

resources. McMaster's (1960) interpretation of the surficial sediment within this study area 

consisted of sand with several isolated areas of gravel. Several other sediment samples were 

previously obtained within the study area: three National Oceanographic Data Center (NODC) 

dredge samples from 1942 consisted of sand and one National Ocean Service (NOS) sample from 

1939 was rocky (fig. 2; Poppe and others, 2003). The purpose of this report is to define the seafloor morphology and sedimentary environments and interpret processes occurring on the sea floor 

using sidescan sonar imagery, multibeam bathymetry, and historic seismic-reflection profiles. 

Geologic Setting 

Rhode Island Sound (RIS) is located offshore of Rhode Island and southeastern 

Massachusetts between Block Island and Martha's Vineyard (fig. 1). It covers an area of over 2500 

km2 to the east of Block Island Sound and reaches depths of about 60 m. 

The stratigraphy underlying RIS is composed of a Proterozoic to Paleozoic basement of 

gneiss and schist with pegmatite and granitic intrusives, overlain by Upper Cretaceous coastal plain 

and continental shelf sediments in the south and Wisconsin glacial deposits throughout the region, 

topped by Holocene estuarine and marine sediments (fig. 3; McMaster and others, 1968; O'Hara 

1
and Oldale, 1980). A cuesta formed from the northern terminus of the Upper Cretaceous sediments 

crosses the southern part of the study area (O'Hara and Oldale, 1980). The uppermost Wisconsin 

glacial deposits and Holocene sediments are further subdivided based on our interpretations of 

seismic-reflection data and those of O'Hara and Oldale (1980). 

The surficial geology in the study area consists of glacial drift, including moraine, outwash 

and glaciolacustrine deposits, and fluvial and estuarine sediments. Two end moraines cross RIS and 

are evident as bands of gravelly sediment and submarine ridges (fig. 4; McMaster, 1960). The 

Ronkonkoma-Nantucket Moraine represents the maximum extent of the Laurentide ice sheet at 

around 20 ka (Uchupi and others, 2001). The Harbor Hill-Roanoke Point-Charlestown-Buzzards 

Bay Moraine represents the retreated ice sheet position at about 18 ka (Uchupi and others, 2001). 

The southwestern segment of the Buzzards Bay Moraine, which crosses the southern part of our 

study area, is thought to be composed of proglacial deltaic and lacustrine sediments capped by a 

thin subglacial till, similar to the Buzzards Bay Moraine (Oldale and O'Hara, 1978; O'Hara and 

Oldale, 1980). Offshore, the moraines in RIS are capped by a lag deposit of sand, gravel, and 

boulders resulting from winnowing of the till by marine processes (O'Hara and Oldale, 1980). The 

northern part of the study area has patches of exposed glacial drift and outwash plain deposits of 

sands and gravels as well as glaciolacustrine silts and clays, thought to be deposited by meltwater 

streams from the Buzzards Bay ice lobe (O'Hara and Oldale, 1980) or from the drainage of Lake 

Cape Cod Bay (Uchupi and others, 2001). Much of our study area was mapped by O'Hara and 

Oldale (1980) as having a surface consisting of Holocene fluvial and estuarine deposits of sands, 

gravels, and muds up to 16 m thick. The fluvial sediments were deposited during subaerial 

exposure of the shelf after glacial retreat and during the drainage of glacial lakes, while estuarine 

sediments were deposited during the northward transgression of the shoreline across the sound. 

Data Acquisition and Processing 

Sidescan sonar and multibeam bathymetric data were acquired by NOAA in 2004, during 

Survey H11320, aboard the NOAA Ship RUDE (fig. 5). A Klein 5500 towfish transmitting at 455-

kHz and Triton-Elics ISIS software were used to acquire sidescan sonar data and to archive the data 

in extended Triton format (XTF). All sidescan sonar data were processed using USGS sidescan 

sonar processing software packages XSonar and ShowImage. A median filtering routine was 

applied to the sidescan sonar data to remove speckle noise, resulting in a 0.06-m pixel size. Timevaried gain was applied to sonar data collected on Julian Day 118 and 133 (year 2004), as the 

dynamic range of these data were low. Sonar data were further processed to correct for slant-range 

and radiometric distortion inherent in the sonar data. All processed data were mosaicked using 

XSonar, resulting in an enhanced, geographically correct, sidescan sonar mosaic with 1 meter/pixel 

resolution. The mosaic was converted to a TIFF image, which was imported into Adobe Photoshop 

CS2 in order to apply a 'stretch' (i.e. increase the dynamic range of the data) to the image. Within 

the sidescan sonar imagery light tones represent strong acoustic reflectivity (generally coarser 

grained sediment) and dark tones represent weak acoustic reflectivity (generally finer grained 

sediment). 

A Reson SeaBat 8125, which operates at 455-kHz, was used to acquire multibeam 

bathymetric data in areas with possible navigational hazards and along several tie-lines. Multibeam 

data were processed using CARIS HIPS/SIPS software. Four large areas of complete multibeam 

bathymetry coverage, totaling 20 km2, were separated from the tie-lines and single-beam 

bathymetric data that were included in the survey's grid, as the single-beam data were vertically 

offset from the multibeam data. The four areas of multibeam data were gridded at 4-m resolution. 

2
Bathymetry from the Coastal Relief Model created by the National Geophysical Data Center 

(NGDC) is used in the study area to provide rough bathymetry on which the higher resolution 

multibeam bathymetry can be overlaid (Divins and Metzger, n.d.). The NGDC Coastal Relief 

Model bathymetry data from around the study area were downloaded as an ASCII raster grid and 

converted to a GeoTIFF and Generic Mapping Tools (GMT) grid using FWTools. The GMT grid 

was resampled at 4 meters and converted back to an ASCII grid and finally a GeoTIFF. Sidescan 

sonar and bathymetry data were converted to geographic coordinates. 

O'Hara and Oldale (1980) collected 670 km of seismic-reflection data in eastern Rhode 

Island Sound and Vineyard Sound with an EG&G Unit Pulse Boomer during 1974 and 1975, about 

30 km of which are within this study area (fig. 2). The transducer was triggered every 0.5 second 

and the sweep rate was 0.25 second. Seismic-reflection data has 1-1.5 m resolution. 

Bathymetry 

The bathymetry in the study area consists of basins and ridges with an overall southerly 

slope (fig. 6). A rounded central basin reaches a depth of about 31 m, while the deepest area is in 

the southwestern corner of the survey and reaches depths of about 36 m. The basins are surrounded 

by ridges, with up to 8 m of relief, which coincide with exposures of glacial drift including 

glaciolacustrine sediments and moraine deposits in the south (O'Hara and Oldale, 1980). Ridges 

tend to be covered with sand waves, a channel-eroded morphology, or composed of tabular 

erosional outliers, while the basins tend to have a smooth sea floor with boulders, which are present 

throughout much of the study area. In the southeast, ridges composed of moraine deposits are 

oriented northeast-southwest. 

Sidescan Sonar Imagery 

Bathymetric, seismic-reflection, and sediment data were used to help interpret the sidescan 

sonar imagery (fig. 7). Six distinctive acoustic backscatter patterns are evident in the sidescan sonar 

imagery (fig. 8). These patterns delineate rocky and bouldery areas, sand-wave fields, hummocky 

areas, tabular erosional outliers, small hills and scarps, and trawl marks. 

Rocky and Bouldery Areas 

Clustered, individual, high-backscatter targets with low-backscatter shadows are widespread 

throughout the region and are interpreted to be rocky and/or bouldery areas (fig. 9). Some boulders 

in the study area exceed 10 m in width. The largest areas of rocks and boulders are located in the 

southeastern, central, and northeastern parts of the study area, and are present both on ridges and in 

basins. 

Sand-wave Fields 

Areas of tiger-striped backscatter occur across much of the northern half of the study area 

and an area near the center of the survey. These areas tend to occur on bathymetric highs in areas of 

relatively high backscatter and are interpreted to be fields of sand waves (fig. 9). Most of the sandwave fields are about 1-2 km2 in area, though one field is over 6 km2. A sand-wave field with 

straight to curved, north-south oriented crests is located in the northeastern corner of the survey. 

The wavelengths are 10-30 m and are nearly symmetrical. The sand waves in the north-central field 

have mostly curved and some straight crests that are east-west oriented and have wavelengths of 40 

3
m to over 100 m. Between these two sand-wave fields are sand waves with curved, east-west 

oriented crests with wavelengths of about 50 m (fig. 9). The slip faces of these waves are oriented 

toward the north, suggesting a northward sediment transport direction (fig. 10). Another field of 

sand waves with curved, east-west oriented crests (though some are north-south oriented) is located 

in the northwest. The bathymetry shows the waves in the northwestern corner have slip faces 

oriented toward the north (fig. 10). In the north-central part of the study area there are two small 

sand-wave fields, each less than 0.1 km2 in area, of sinuous crested waves oriented to the northeast 

with wavelengths of several meters. The sand waves in the center of the study area have curved 

crests oriented to the east-northeast. 

Hummocky Areas 

In the south, north-central, and northeastern corner of the study area, a mottled backscatter 

pattern appears in the sidescan sonar imagery (fig. 11). The mottled areas in the south-central part 

of the study area appear in the bathymetry as mounds up to about 1 m in height and generally less 

than 100 m in width. The mottling covers areas as large as 7 km2 and tends to coincide with 

exposures of a glacial moraine in the south and till deposits in the north (fig. 12). Mottled areas 

around the moraine in the southeast correspond to bathymetry that is cut by channels up to 1 m 

deep (fig. 11). The most prominent channels are oriented to the west, south, and south-southwest. 

The northeastern corner of the sidescan sonar imagery shows mottled backscatter corresponding to 

variable bathymetry. The mottling is interpreted to indicate a hummocky, erosional surface. 

Within an area of mottled backscatter along the southern edge of the study area near the 

southwestern basin is an area with oblong to round, low-backscatter targets (fig. 13). The lowbackscatter targets have sharp boundaries and look like acoustic shadows; however, there are no 

high-backscatter objects to create acoustic shadows. The low-backscatter targets are generally less 

than 10 m in width and around 10-40 m in length. If these features have relief, it is below the 

resolution of the multibeam bathymetry and the seismic-reflection data. Rocks and boulders are 

generally not located within the area of these features, but they exist to the north and east of the 

area. These features are seen to cross multiple track lines. They are inferred to be caused by patches 

of fine-grained sediment located within areas of coarser grained sediment. 

Tabular Erosional Outliers 

The sidescan sonar imagery shows two areas, in the western and eastern parts of the study 

area, with varied high and low backscatter, while the corresponding bathymetry consists of small 

plateaus up to several hundred meters across and about 0.5 m high (fig. 14). The western area 

covers almost 4 km2 and the eastern area covers about 0.5 km2. The plateaus in the eastern part of 

the study area are eroded by a southerly oriented channel complex. Sidescan sonar data show that 

in the western area, the tops of the plateaus have lower backscatter and finer grained sediment, 

whereas the sea floor between the plateaus has higher backscatter and coarser grained, gravelly and 

bouldery sediment. Seismic data show horizontally stratified sediments that underlie the plateaus 

and areas to the northwest, which influence the bathymetry of the estuarine surface sediments (fig. 

15). The plateaus differ from hummocky areas in that the individual plateaus are larger in area, they 

have flatter tops, a more drastic change in slope on their sides, and the plateaus are fine-grained 

with coarse-grained sediment between them. 

The plateaus are interpreted to be tabular erosional outliers composed of stratified glacial 

drift that is inferred to be cohesive glaciolacustrine sediment. The glaciolacustrine sediment onlaps 

till that is exposed to the southeast of the erosional outliers. Till, containing boulders up to 16 m in 

4
width, is also exposed between the erosional outliers where the thin lake sediments have been 

eroded away. 

Small Hills and Scarps 

Discontinuous, curvilinear features of high and low backscatter with sharp boundaries are 

evident in the sidescan sonar imagery around the edges of the bathymetric lows (fig. 9). The 

patterns are oriented mostly east-west, perpendicular to the slope and askew to the ship track. 

Bathymetry in these areas show small hills, up to 0.5 m in height and, in the southwest, a change in 

slope from less than 0.5 degree to 3 degrees where the features are present (fig. 16). These features 

are up to 2.5 km long and occur in water depths of 24-27 m in the northeast and 30-35 m in the 

southwest. The low-backscatter curvilinear features appear to be produced by small, erosional 

scarps and small, elongate hills on the sloped sea floor. Scarps cause a sharp change in backscatter 

where the slope changes. The elongate hills run perpendicular to the general slope and create lowbackscatter acoustic shadows where they are roughly parallel to the ship track, accounting for the 

sharp contrasts in backscatter. 

Because the hills and scarps in the sidescan sonar imagery tend to coincide with fluvial 

deposits and drift deposits that have reflectors terminating near the surface (fig. 17) we infer that 

these paired features were formed by outcropping strata, which are more resistant to erosion than 

the surrounding sediments. Since these features tend to parallel the bathymetry and are located at 

depths similar to those of paleoshorelines described in nearby areas, they may mark 

paleoshorelines. 

Trawl Marks 

Anthropogenic features are also visible in the sidescan sonar imagery. The central, 

northeastern, and southwestern areas of the study area contain several long, thin, straight lines of 

low backscatter, which are interpreted to be trawl marks from fishing boats (fig. 18). They range in 

length from about 100 m to 1 km. 

Sedimentary Environments 

The present day sea floor in the study area is interpreted to contain three sedimentary 

environments characterized by the processes of erosion or nondeposition, coarse-grained bedload 

transport, and sorting and reworking (fig. 19). These environments are delineated using 

interpretations of the sidescan sonar, bathymetric, and seismic-reflection data. The boundaries of 

the sedimentary environments are gradational and borders are inferred. 

Erosion or Nondeposition 

Environments characterized by erosion or nondeposition, which dominate the sea floor in 

the study area, are distinguished by the presence of rocks and gravel, tabular erosional outliers, or a 

hummocky surface. These processes are prevalent on the moraine and flanks of the ridges. These 

high-energy areas are exposed to currents that winnow away finer grained sediments and leave a 

coarser grained lag. 

5
Coarse-Grained Bedload Transport 

Environments characterized by coarse-grained bedload transport are located where sand 

waves are present, which occurs mostly in the northern parts of the study area. The orientations of 

sand wave crests are generally east-west and tend to parallel the bathymetric contours. In two sandwave fields the slip faces are oriented northward, suggesting this is the direction of sediment 

transport. Currents are most likely flowing into and out of the central basin across the northern 

ridge. 

Sorting and Reworking 

Environments characterized by the processes of sorting or reworking are located in the 

central and western basins and on parts of the northern ridges. These areas have moderate 

backscatter and lack sand waves and gravelly sediment as indicated in the interpretations of the 

sidescan sonar imagery. There does not seem to be any fine-grained deposition of sediment in the 

study area as McMaster (1960) interpreted the area to be sandy with some areas of gravel. Thus, 

sediments are interpreted to be undergoing processes of sorting and reworking in areas that lack 

characteristics of erosion and coarse-grained bedload transport. 

Discussion 

Erosional outliers and small hills and scarps are distinct features in the study area that have 

not been previously mapped. They are interpreted to have formed from the deposition of glacial 

lake sediments and from the erosion of paleoshorelines as sea level transgressed northward. 

Glaciolacustrine sediments are thought to have been deposited in the study area during 

recession of the Laurentide ice sheet. Some theories for the origin of glaciolacustrine sediment in 

RIS include transport and deposition of the sediment by meltwater streams (O'Hara and Oldale, 

1980) or by the catastrophic drainage of glacial lakes (Uchupi and others, 2001). However, in this 

area, the presence of up to 15 m of stratified drift associated with erosional outliers of cohesive 

sediments indicates that these are glaciolacustrine sediments deposited in situ. Stratified 

glaciolacustrine sediments are located in what was likely the ice-distal side of the lake where water 

energy was low and fine-grained sediment could settle. Stratification of the lake sediments tends to 

fade to the northwest (fig. 15), suggesting higher energy and ice-proximal environments lay in that 

direction. Erosion of the till and glaciolacustrine sediment probably occurred during lake drainage 

and subaerial exposure prior to marine incursion. 

After glacial recession, when sea level was at a lower stand than at present, much of what is 

now Rhode Island Sound was subaerially exposed. As sea level rose, shorelines moved northward 

across the area, depositing and eroding features in the sea floor. The hills and small scarps in our 

study area are probably erosional features formed as wave-cut terraces in paleoshorelines that have 

been preserved because of the sediment's high cohesiveness. The paleoshoreline features at 24-27 

m probably formed 8.4-8.7 ka and the features at 30-35 m probably formed between 9.0-9.8 ka 

based on a sea-level-rise curve for southeastern Massachusetts (Oldale and O'Hara, 1980). 

Depositional paleoshoreline features have been found in other areas near RIS. Profiles of a 

paleoshoreline located about 24 m below present sea level near Block Island, thought to be from 

8.3-9 ka, consist of barrier spit, lagoon, and headland features as well as terraces (McMaster and 

Garrison, 1967). These features are similar to, although larger than the profiles of the hills and 

scarps in RIS. A drowned spit off Cape Ann, MA with seaward-dipping reflectors marks a sea level 

50 m below present (Oldale, 1985). Though the hills in our study area are similar in profile to the 

6
spits, they are not distinct seismic units with dipping reflectors and they are only about 0.5 m in 

height on the upslope side. If there were spits or other depositional features associated with the 

paleoshorelines in RIS, then they would probably not have been preserved owing to their small 

size. Other previously described paleoshorelines in the region are composed of erosional and 

depositional features. Paleoshorelines in Long Island Sound, lying under about 15 m of estuarine 

deposits, show planar, terraced surfaces with lens-shaped seismic units and erosional steps or 

scarps (Gayes and Bokuniewicz, 1991). The paleoshorelines in Long Island Sound mark shorelines 

at depths including 25-27 m, thought to have formed 8.5-9 ka, 30 m, and 37-38 m, thought to have 

formed 10-10.5 ka (Gayes and Bokuniewicz, 1991). South of our study area, on the continental 

shelf, Garrison and McMaster (1966) found wave cut terraces with scarps and a ridge and 

depression topography interpreted to be barrier beaches and lagoons that occur at depths including 

24 m. Because of their similar depth, we believe that the terraces near Block Island described by 

McMaster and Garrison (1967) and those on the continental shelf described by Garrison and 

McMaster (1966) are coeval with the paleoshoreline features at 24-27 m within the study area. 

Summary 

An analysis of sidescan sonar data together with bathymetric and seismic-reflection data 

indicate six backscatter features in the study area including: rocky and bouldery areas, sand-wave 

fields, hummocky areas, tabular erosional outliers, small hills and scarps, and trawl marks. The 

bathymetry in the study area is composed of basins in the central and southwestern areas that have 

a relatively smooth sea floor aside from boulders, which are ubiquitous. Ridges around the basins 

are composed of glacial drift and have surfaces characterized by sand waves, a hummocky 

morphology, and erosional outliers (fig. 8). Sand wave fields are mostly located on the northern 

ridges and wave crests are roughly perpendicular to the slope. The moraine and areas of exposed 

till have a hummocky erosional surface. Tabular erosional outliers probably consist of cohesive 

glaciolacustrine sediment deposited in situ. Erosional scarps and small elongate hills, which parallel 

bathymetric contours and occur at depths similar to paleoshorelines in the region, were probably 

formed from outcropping strata that are resistant to erosion. Trawl marks are also evident. 

Sedimentary environments characterized by erosion or nondeposition are located on the moraine 

and the flanks of the ridges. Processes associated with coarse-grained bedload transport are 

prevalent along the northern regions; environments characterized by processes associated with 

sorting and reworking occur variously throughout the region where there are no sand waves or 

rocky areas. 

GIS Data Catalog 

This report contains several data layers (or themes) that show bathymetry and sidescan 

sonar imagery of Rhode Island Sound. GeoTIFFs produced from grids of the NOAA sidescan sonar 

and multibeam echo sounder surveys and NGDC Coastal Relief Model are provided in geographic 

and universal transverse mercator projections, along with the multibeam grids. Available vector 

data consists of a polygon data layer, nos80k_84 that provides the coastline for the Rhode Island 

Sound GIS project area. 

Data layers are provided with geographic coordinates to allow the data to be integrated into 

a Geographic Information System (GIS). Data layers archived here do not require additional 

processing to be utilized within the Environmental Systems Research Institute's (ESRI) ArcView 

and ArcGIS software. This does not mean that a user will not wish to do additional processing, 

7
especially if utilizing a different GIS software package or spheroid, but that it is not necessary to do 

additional processing simply to utilize the data in its minimum archive format. 

For those who do not have the ESRI software or a compatible GIS data browser available 

on their computer, a free viewer, ArcExplorer, is available from ESRI. Please note that the 

ArcExplorer software is limited to Microsoft Windows operating systems. The key functionality of 

ArcExplorer is the viewing of spatial data. The user will need to add the selected data layers by 

using ArcExplorer's add data button. 

Each GIS data layer from this publication is cataloged below for easy access. The individual 

data layers are described below and include the GeoTIFF, grid, or shapefile name (e.g. 

h11320_geo_4m) which is linked to a browse graphic showing the data layer extent and coverage. 

Selecting the data layer name will result in the browse graphic being displayed in a separate 

browser window. 

Federal Geographic Data Committee (FGDC) metadata for the individual data layers is 

provided in a text version. Selecting associated metadata files from the table below will open the 

information in a new browser window. 

A 'zip' compressed, downloadable archive file containing the components of the ESRI 

shapefile for each data layer is also provided. Compressed downloadable files were created using 

the Windows program WINZIP v9.0. For those users who do not have software capable of 

uncompressing the archived zip files, they may obtain a free version of the software from Winzip 

Computing, Inc. or Pkware, Inc. 

Sidescan Sonar Imagery 

Data Layer Name and Description Metadata Files

HTML 

FAQ 

txt 

h11320_1m_sss_geo_en - inverted and enhanced sidescan sonar imagery of NOAA survey zip

H11320 (geographic)

h11320_1m_sss_geo - composite sidescan sonar imagery of NOAA survey H11320 (geographic) HTML 

FAQ 

txt 

zip

HTML 

FAQ 

txt 

h11320_1m_sss_utm_en - inverted and enhanced sidescan sonar imagery of NOAA survey zip

H11320 (UTM zone 19, NAD83 spheroid)

HTML 

FAQ 

txt 

h11320_1m_sss_utm19 - composite sidescan sonar imagery of NOAA survey H11320 (UTM zone zip

19, NAD83 spheroid)

Bathymetric Imagery

Data Layer Name and Description Metadata Files

h11320_geo_4m.tif - multibeam echo sounder data from NOAA survey H11320 (geographic) HTML 

FAQ 

txt 

zip

ris_cr_geo_4m.tif - NGDC Coastal Relief Model bathymetric imagery (geographic) HTML 

FAQ 

txt 

zip

8
HTML 

FAQ 

txt 

h11320_utm_4m.tif - multibeam echo sounder data from NOAA survey H11320 (UTM zone 19, zip

NAD83 spheroid)

HTML 

FAQ 

txt 

ris_cr_utm_4m.tif - NGDC Coastal Relief Model bathymetric imagery (UTM zone 19, NAD83 zip

spheroid) 

Bathymetric Grids

Data Layer Name and Description Metadata Files

h11320_geo_4m - multibeam bathymetric grid of NOAA survey H11320 (geographic) HTML 

FAQ 

txt 

zip

HTML 

FAQ 

txt 

h11320_utm_4m - multibeam bathymetric grid of NOAA survey H11320 (UTM zone 19, NAD83 zip

spheroid) 

HTML 

FAQ 

txt 

h11320_1m_UTM19NAD83.txt - space delimited ASCII text file of the bathymetric grid of zip

NOAA survey H11320 (UTM zone 19, NAD83 spheroid) 

Interpretive Data

Data Layer Name and Description Metadata Files

HTML 

FAQ 

txt 

h11320environs - interpretation of the sedimentary environments in NOAA survey H11320 zip

(geographic) 

HTML 

FAQ 

txt 

h11320interp - interpretation of the sidescan sonar and bathymetric imagery from NOAA survey zip

H11320 (geographic) 

Basemap Data

Data Layer Name and Description Metadata Files

HTML 

FAQ 

txt 

nos80k_84 - medium resolution digital vector U.S. shoreline shapefile for the Rhode Island Sound zip

GIS project area (geographic)

h11320outline - outline of sidescan sonar imagery from NOAA Survey H11320 (geographic) HTML 

FAQ 

txt 

zip

Acknowledgements 

The U.S. Geological Survey and National Oceanic and Atmospheric Administration 

provided funding for this project. We would like to thank the crew of the NOAA Ship RUDE for 

9
shipboard support. We appreciate the helpful reviews of Jane Denny, Ben Gutierrez, Valerie 

Paskevich, and Jim Robb (all USGS). 

Contacts 

Lawrence J. Poppe 

U.S. Geological Survey 

Coastal and Marine Geology Program 

384 Woods Hole Road 

Woods Hole, MA 02543-1598 

(508) 548-8700 

lpoppe@usgs.gov 

Katherine Y. McMullen 

U.S. Geological Survey 

Coastal and Marine Geology Program 

384 Woods Hole Road 

Woods Hole, MA 02543-1598 

(508) 548-8700 

kmcmullen@usgs.gov 

Erin R. Twomey 

U.S. Geological Survey 

Coastal and Marine Geology Program 

384 Woods Hole Road 

Woods Hole, MA 02543-1598 

(508) 548-8700 

etwomey@usgs.gov 

William W. Danforth 

U.S. Geological Survey 

Coastal and Marine Geology Program 

384 Woods Hole Road 

Woods Hole, MA 02543-1598 

(508) 548-8700 

bdanforth@usgs.gov 

LCDR Todd A. Haupt 

Southeast Navigation Manager 

Office of Coast Survey, NOAA 

2234 S Hobson Ave. 

Charleston, SC 29405 

(843) 740-1178 

Todd.A.Haupt@noaa.gov 

LCDR James M. Crocker 

NOAA Ship THOMAS JEFFERSON 

10
439 West York Street 

Norfolk, VA 23510 

(757) 441-6322 

James.M.Crocker@noaa.gov 

References Cited 

Divins, D.L. and Metzger, D., n.d., NGDC Coastal Relief Model, Volume 1, 

http://www.ngdc.noaa.gov/mgg/coastal/coastal.html 

Garrison, L.E. and McMaster, R.L., 1966, Sediments and geomorphology of the continental shelf 

off southern New England, Marine Geology, v. 4, p. 273-289. 

Gayes, P.T. and Bokuniewicz, H.J., 1991, Estuarine paleoshorelines in Long Island Sound, New 

York, Journal of Coastal Research, Special Issue 11, p. 39-54. 

Gustavson, T.C. and Boothroyd, J.C., 1987, A depositional model for outwash, sediment sources, 

and hydrologic characteristics, Malaspina Glacier, Alaska; a modern analog of the southeastern 

margin of the Laurentide ice sheet, Geological Society of America Bulletin, v. 99, n. 2, p. 187-

200. 

McMaster, R.L., 1960, Sediments of Narragansett Bay System and Rhode Island Sound, Rhode 

Island, Journal of Sedimentary Petrology, v. 30, n. 2, p. 249-274. 

McMaster, R.L. and Garrison, L.E., 1967, A submerged Holocene shoreline near Block Island, 

Rhode Island, The Journal of Geology, v. 75, p. 335-340. 

McMaster, R.L., LaChance, T.P., and Garrison, L.E., 1968, Seismic-reflection studies in Block 

Island and Rhode Island Sounds, The American Association of Petroleum Geologists Bulletin, v. 

52, n. 3, p. 465-474. 

O'Hara, C.J. and Oldale, R.N., 1980, Maps showing geology and shallow structure of eastern 

Rhode Island Sound and Vineyard Sound, Massachusetts: U.S. Geological Survey Miscellaneous 

Field Studies Map MF-1186, 5 sheets. 

Oldale, R.N., 1985, A drowned Holocene barrier spit off Cape Ann, Massachusetts, Geology, v. 13, 

p. 375-377. 

Oldale, R.N. and O'Hara, C.J., 1978, Thrusted coastal end moraines and a Woodfordian fluctuating 

ice margin: evidence from Massachusetts onshore and offshore areas (abs.): Geological Society 

of America Abstracts with Programs, v. 10, n. 2, p. 78. 

Oldale, R.N. and O'Hara, C.J., 1980, New radiocarbon dates from the inner continental shelf off 

southeastern Massachusetts and a local sea-level-rise curve for the past 12,000 yr, Geology, v. 8, 

n. 2, p. 102-106. 

11
Poppe, L.J., Paskevich, V.F., Williams, S.J., Hastings, M.E., Kelly, J.T., Belknap, D.F., Ward, 

L.G., FitzGerald, D.M., and Larsen, P.F., 2003, Surficial sediment data from the Gulf of Maine, 

Georges Bank and Vicinity: A GIS Compilation, U.S. Geological Survey Open-File Report 03-

001, CD-ROM. 

Uchupi, E., Giese, G.S., Aubrey, D.G., and Kim, D.-J., 1996, The Late Quaternary Construction of 

Cape Cod, Massachusetts: A Reconsideration of the W.M. Davis Model, Special PaperGeological Society of America, v. 309, 69 p. 

Uchupi, E., Dirscoll, N., Ballard, R.D., and Bolmer, S.T., 2001, Drainage of late Wisconsin glacial 

lakes and the morphology and late quaternary stratigraphy of the New Jersey-southern New 

England continental shelf and slope, Marine Geology, v. 172, p. 117-145. 

12
Figure 1. Location map of NOAA Survey H11320 (red polygon) in Rhode Island Sound. 

Location of stratigraphy profile A-A’ from figure 3 is also shown. 

13
Figure 2. Location of figures (red polygons), bathymetric profiles (green lines), historic 

seismic-reflection profiles (blue lines, O'Hara and Oldale, 1980), and sand (yellow circles) 

and rocky (red circle) sediment samples (Poppe and others, 2003) within the study area. 

14
Figure 3. Interpreted stratigraphy of Rhode Island Sound from O'Hara and Oldale (1980). 

Acoustic units include: PzZ - Proterozoic to Paleozoic bedrock, Ku - Cretaceous coastal 

plain sediments, Qdo - Wisconsin glacial drift deposits, Qdm - Buzzards Bay moraine 

deposits, Qfe - Holocene fluvial and estuarine deposits, and Qm - Holocene quiet-water 

marine deposits. Boundaries are dashed where inferred. Vertical exaggeration: 40X. 

Location of stratigraphy profile shown as A-A’ in figure 1. 

15
Figure 4. Map showing the location of end moraines (solid black polygons) in southern 

New England and New York. The Ronkonkoma-Nantucket moraine shows the maximum 

extent of the Laurentide ice sheet 20 ka, while the Harbor Hill-Roanoke PointCharlestown-Buzzards Bay moraine shows a retreated position of the ice sheet around 18 

ka (Uchupi and others, 1996). Submarine ridges (dotted lines) mark the underwater 

extensions of the moraines. H11320 study area shown as a gray polygon. Modified from 

Gustavson and Boothroyd (1987). 

16
Figure 5. Photo of NOAA Ship RUDE (courtesy of NOAA). 

17
Figure 6. Hill-shaded bathymetry of the study area. Four areas outlined in black show 

multibeam bathymetry from H11320, while the rest of the bathymetry is from the NGDC 

Coastal Relief Model. Multibeam bathymetric data have been gridded at 4 m and depths 

represent mean lower low water level in multibeam data and either mean low water or 

mean lower low water in Coastal Relief Model data (Divins and Metzger, n.d.). 

18
Figure 7. Sidescan sonar imagery of NOAA Survey H11320. Darker regions represent low 

backscatter and generally finer grained sediments; lighter regions represent high 

backscatter and generally coarser grained sediments. 

19
Figure 8. Interpretation of sidescan sonar imagery and multibeam bathymetry showing 

areas of rocky and gravelly sediment, sand waves and crest orientations, sediment 

transport direction, hummocky sea floor, tabular erosional outliers, small hills and scarps, 

and trawl marks. 

20
Figure 9. Detailed sidescan sonar imagery and bathymetry illustrating the relatively 

smooth central basin floor scattered with rocks and boulders in the south and adjacent 

ridge to the north. Sand waves are located on the ridge and a scarp crosses the ridge in 

an east-west direction. Sand waves have curved crests oriented east-west and 

wavelengths of 40 to over 100 m in the west and about 50 m in the east. High-resolution 

multibeam bathymetry has been overlaid on bathymetry from the NGDC Coastal Relief 

Model. Figure location is shown in figure 2. 

21
Figure 10. Bathymetric profiles of sand waves showing slip faces oriented northward. 

Profile C-C' is of sand waves in the eastern field of east-west oriented crests, which have 

wavelengths of about 50 m. Profile D-D' is of sand waves in the northwestern corner of the 

study area where wavelengths are also about 50 m. Location of profiles shown in figure 2. 

22
Figure 11. Detailed view of sidescan sonar imagery showing mottled backscatter and 

corresponding bathymetry. Mottled backscatter in the sidescan sonar data is caused by 

the hummocky surface of moraines and glacial till inconsistently reflecting backscatter due 

to the variable topography and sediment distribution. Note: black line outlines high23
resolution multibeam bathymetry which overlies bathymetry from the Coastal Relief Model. 

Figure location is shown in figure 2. 

24
Figure 12. Sidescan sonar imagery and Boomer seismic-reflection profile (O'Hara and 

Oldale, 1980) and interpretation through an area of mottled backscatter. Right side of 

sidescan sonar image shows mottled backscatter (top image). Seismic profile shows 

Cretaceous coastal plain sediments overlain by Wisconsin glacial drift and Holocene 

fluvial and estuarine sediments. Mottling tends to occur in areas where glacial moraine 

sediments are either exposed or near the surface. Vertical exaggeration: 13X. Depths are 

based on the assumed compressional wave velocity of 1500 m/s. Location of seismic line 

shown in figure 2.

25
Figure 13. Detailed area of sidescan sonar image showing low-backscatter targets. These 

features show no relief in the bathymetry data and are thought to be patches of finegrained sediment within larger areas of coarser grained sediment. Figure location shown 

in figure 2. 

26
Figure 14. Detailed view of sidescan sonar imagery and corresponding hill-shaded 

bathymetry showing tabular erosional outliers. Curvilinear shadows in the sidescan sonar 

imagery correspond to the outline of small plateaus in the bathymetry. Boulders are visible 

in both images, on the sidescan sonar imagery as light-toned targets with dark shadows 

and within the bathymetry as dark targets. The area of boulders in the lower right corner of 

both images shows an exposure of till. Lighter backscatter between the erosional outliers 

is from coarse-grained, bouldery sediment. Images are in UTM. Location of figure shown 

in figure 2. 

27
Figure 15. Sidescan sonar imagery, Boomer seismic-reflection profile (O'Hara and Oldale, 

1980), and interpretation through an area of tabular erosional outliers. Top image shows 

location of the seismic profile on the sidescan sonar data and erosional outliers in the 

center. Lower images of seismic profile and interpretation show stratified sediments, 

possibly from a small glacial lake or pond, below the erosional outliers. Note irregular sea 

floor and estuarine sediments above laminations, likely due to prior erosion of lacustrine 

sediments. Boulders can be seen rising above the sea floor. Vertical exaggeration: 16X. 

28
Depths are based on the assumed compressional wave velocity of 1500 m/s. Figure 

location shown in figure 2. 

29
Figure 16. Bathymetric profiles across study area from north to south. Location of profiles 

shown in figure 2. Profile A-A', located in the southwestern part of the study area, crosses 

a bathymetric high with deeper basins to the north and south. Profile B-B', located in the 

eastern part of the study area, crosses a bathymetric high in the north, central basin, and 

moraine in the south. Arrows point to changes in slope interpreted to represent 

paleoshorelines. 

30
Figure 17. Sidescan sonar imagery, Boomer seismic-reflection profile (O'Hara and Oldale, 

1980), and interpretation of an area with small scarps of outcropping strata. Top image 

shows scarp and location of seismic line on sidescan sonar mosaic. Bottom images of 

seismic line and interpretation show Pleistocene glacial drift overlain by Holocene fluvial 

and estuarine sediments. The small scarp displayed within the sidescan sonar mosaic is 

31
presumed to correspond to the prominent reflector between the fluvial sediments and 

glacial drift as it nears the surface. Vertical exaggeration: 15X. Depths are based on the 

assumed compressional wave velocity of 1500 m/s. Location of seismic line shown in 

figure 2. 

32
Figure 18. Detailed view of sidescan sonar imagery showing long lines of low backscatter 

interpreted to be trawl marks from fishing boats. Figure location is shown in figure 2. 

33
Figure 19. Map showing sedimentary environments in study area characterized by 

erosion or nondeposition, coarse-grained bedload transport, and sorting and reworking. 

34