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Figure 13. Hill-shaded multibeam bathymetry obtained during National Oceanic and Atmospheric Administration survey H11445.

Figure 14. Detailed multibeam bathymetry near Plum Gut, the channel between Orient Point and Plum Island.

Figure 15. Interpretation of multibeam bathymetry and sidescan sonar from National Oceanic and Atmospheric Administration survey H11445.

Figure 16. Detailed multibeam bathymetry of sand waves on the shoal north of Plum Island.

Figure 17. Sand-wave profiles of waves on shoal north of Plum Island.

Figure 18. Detailed multibeam bathymetry image of boulders north of Plum Island, which are part of the Harbor Hill-Roanoke Point-Charlestown-Buzzards Bay moraine.

The sea floor in the study area ranges from 1 to 103 m in depth (fig. 13). The deepest area is an isolated depression caused by tidal scour with depths exceeding 100 m. It is located northwest of Plum Island at the northwest corner of the study area. Plum Gut, the channel between Plum Island and Orient Point, is another relatively deep area reaching about 60 m (fig. 14). The sea floor in Plum Gut is hummocky, likely due to currents scouring the submerged moraine in the constricted channel. The shallowest areas occur along the shoreline and north of the eastern arm of Plum Island, on a sand-wave-covered shoal that shallows to about 7 m. In other areas north of Plum Island, the sea floor gradually deepens offshore and is relatively smooth where boulders are not present. Sand waves on the shoal cover an area of 1.4 km² (fig.15). Asymmetries of the sand waves on the western and southern parts of the shoal indicate eastward net sediment transport, while in the northeast they indicate westward net sediment transport (figs. 16 and 17). A boulder on the northeast flank of the shoal exhibits scour marks that indicate westward sediment transport (fig. 16). It is presumed that this counterclockwise movement of sand helps to maintain the shoal. Barchanoid sand waves are located along an east-west transect through the center of the shoal. Four smaller areas (0.1 to 0.2 km²) of sand waves and megaripples surround the basin in the northwest part of the study area (fig. 15). The crests of these sand waves and megaripples tend to be oriented east-west, though those in the southernmost area are oriented northwest-southeast.

Large areas of boulders, covering several square kilometers, flank the shore north of Plum Island and Orient Point (fig. 18). These boulders, some of which reach several meters wide, are a lag deposit of the Roanoke Point-Orient Point-Fishers Island moraine system (Sirkin, 1980). Some of the boulders have scour marks indicating sediment transport directions that are generally eastward. A wreck, a tractor trailer that fell off a ferry in 2002, is visible in the bathymetry imagery in Plum Gut (figs. 14 and 15; National Oceanic and Atmospheric Administration, 2008).