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Figure 11. Hill-shaded multibeam bathymetry of National Oceanic and Atmospheric Administration survey H12298.

Figure 12. Detailed multibeam-bathymetric image of exposed bedrock on the sea floor of the study area.

Figure 13. Map of the study area showing locations of the detailed multibeam figures, seismic profiles, and interpreted directions of net sediment transport based on sand-wave and obstacle-mark asymmetry.

Figure 14. Detailed multibeam-bathymetric image of the two eastern bathymetric depressions in the study area and the drumlins to their east.

Figure 15. Seismic-reflection profile and interpretation of a section of line a81_218, which crosses the study area to the east of the eastern bathymetric depressions and through glacial sediments.

Figure 16. Detailed multibeam-bathymetric image of the western bathymetric depression and the ridges of cobbles along its eastern and northern sides.

Figure 17. Detailed multibeam-bathymetric image of sand waves and megaripples in the southwestern part of the study area.

Figure 18. Detailed multibeam-bathymetric image of boulders, many with obstacle marks, sand waves and megaripples, and trawl marks, in the southeast part of the study area.

Bathymetry

Water depths in the study area range from 1 m just offshore of Fishers Island to 102 m in a bathymetric depression in the center of the study area (fig. 11). A small shelf borders the southern coast of Fishers Island, where water depths are shallower than 30 m for a distance of about 2 kilometers (km) away from the island. Further offshore, water depths increase to about 40 to 50 m in a couple of east-west-oriented channels. Several isolated bathymetric highs are throughout the study area, and within the channels are three isolated bathymetric depressions. Bedforms and trawl marks are also seen in the bathymetric data.  

Bathymetric Highs

The surface of an isolated bathymetric high in the northwest has numerous, low ridges oriented north-northwest to south-southeast that are clearly visible in the bathymetric data (figs. 12 and 13). These ridges roughly parallel similarly oriented ridges, grooves, and striations described on the bedrock of southwest Rhode Island (Schafer, 1965), southeast Connecticut (Goldsmith, 1962), and offshore of southeast Connecticut (Poppe and others, 2006, 2007).

Rocky, elongate bathymetric highs south of the eastern end of Fishers Island are oriented in a north-northwest to south-southeast direction, roughly parallel to the Laurentide Ice Sheet’s flow direction in this area and the ridges on the bathymetric high in the northwest (fig. 14). Individual bathymetric highs have lengths of about 500 m to 1 km, widths of about 200 m, and heights of several meters. A seismic line over one of these features shows unstratified, acoustically opaque sediment (fig. 15; Needell and Lewis, 1984).

Bathymetric Depressions

Three bathymetric depressions are in the study area (fig. 11); one is along the western edge of the study area, and two are in the center of the study area to the south of the eastern end of Fishers Island—the deepest and largest of which is called the Deep Hole by Bertoni and others (1977). The depressions are about 800 to 1500 m wide and range from 30 to 60 m in depth below the surrounding sea floor.

The two central depressions have steep walls on their eastern (and northern, in the case of the Deep Hole) sides and gradually inclined slopes on their western sides (fig. 14). The bathymetric depression along the western edge of the study area has gradually inclining sides (fig. 16).

Bedforms

Transverse sand waves with wavelengths between 100 and 200 m are in the southern and western parts of the study area (fig. 17). The flanks of these sand waves are composed of megaripples with wavelengths of tens of meters. Megaripples are also along the sides of the east-west-oriented channel in the northern part of the study area. Most of the sand waves have crests that are oriented roughly north-south, and many of the crests bifurcate. Most sand waves and megaripples in the study area are asymmetrical, with westward-oriented, steep, slip faces and eastward-oriented, gently sloping, stoss slopes.

Trawl Marks

Trawl marks are visible in several areas in the eastern part of the study area (fig. 18). These anthropogenic features appear as narrow, linear to curvilinear channels scoured into the sea floor by fishing gear dragging along the bottom.