U.S. Geological Survey Open-File Report 2010–1007
Sea-Floor Geology and Character Offshore of Rocky Point, New York
The deepest water depths surveyed within the study area lie about 2.4 km northwest of Rocky Point and exceed 36 m (fig. 15). This depression branches both westward and to the southwest where it roughly parallels the coastline and gives the western part of the study area a steeper nearshore to offshore gradient. The shallowest areas are found along the shorelines and on Orient Shoal (fig. 16), an isolated bathymetric high northeast of Rocky Point.
Many small, individual, rounded bathymetric highs are also concentrated in a broad band extending off Rocky Point and along shorelines. These features, which give the sea floor in these areas a rough appearance, are interpreted to be boulders (figs. 16, 17, 18, 19). Although the boulders average 1 to 5 m in width, many exceed 10 m, and the largest exceeds 18 m across. These rocky areas are lag deposits remaining from the winnowed sediments of the Harbor Hill-Roanoke Point-Orient Point moraine. In places, turbulence from the strong tidal currents flowing around boulders has scoured the sea floor on the lee side of the boulders creating features called comet or obstacle marks (fig. 20).
Elsewhere, most of the study area's benthic bathymetric complexity consists of the alternating narrow, elongate bathymetric highs and lows revealing the crests and troughs of adjacent sand waves and megaripples (figs. 19, 21). These fields of bedforms occur (1) in northeast-trending bands in the western part of the study area, where the wave crests trend northwest-southeast, (2) around Orient Shoal, where wave crests are roughly perpendicular to the shoal's flanks, and (3) in small isolated patches adjacent to the exposed boulders off Rocky Point. Both transverse and barchanoid sand waves are present, but transverse waves are more common where sediment supply is abundant, such as near Orient Shoal, and the barchanoid waves are more common in the small isolated patches where sand supply is limited. Amplitudes of the larger transverse bedforms exceed 3 m, and wavelengths can exceed 100 m. Crest to trough amplitudes of the largest barchanoid sand waves exceed 5.6 m.
Asymmetry in the comet marks and sand waves can be used to interpret directions of net sediment transport (figs. 16, 20, 22, 23). For example, the tails of comet marks extend downstream in the transport direction and stoss slopes of sand waves face upstream. Bedform asymmetry indicates that nearshore transport is predominantly eastward, but offshore transport is primarily westward in the northern and western parts of the study area. Sand-wave asymmetry adjacent to Orient Shoal indicates clockwise sediment transport is important to shoal morphology and maintenance.