present circulation regime. Unit 1, which characterized the west 

 side (and greater depths) of the area, was a sandy silt marked by 

 numerous surface "mounds". These features appeared approximately 

 10-20 m in diameter and < 2 m in height. Most notably along line 

 4, they were found to surround shallow depressions. Unit 2 

 represented a similar substrate with less frequent occurrences of 

 the moundlike features. Numerous trawl marks, fish pots, and a few 

 gravel patches were also noted (Figure 3-32) . Unit 3 was a 

 featureless, silty sand to sandy silt bottom and occurred in two 

 isolated areas. Unit 4 marked an anomalous gravel deposit in the 

 southwest corner of the site. Results of grain size analyses 

 indicated the area consisted primarily of sand and silt-clay 

 (Figure 3-33 and Table 3-8) ; the silt-clay component predominated 

 at the majority of stations. 



4.0 DISCUSSION 



4.1 Topography of the Brenton Reef Historic Disposal Site 



One of the primary objectives of this study was to 

 determine the areal extent of the historical dredged material 

 deposit at the Brenton Reef disposal site. This was readily 

 delineated by the precision bathymetric and REMOTS® surveys 

 (Figures 3-1 and 3-4) . The only additional evidence of dredged 

 material presence outside the discrete mound and flanks in the 

 western quadrant of the site was a small elliptical area located 

 near the eastern site boundary (Figure 3-4) . Based on the 

 bathymetry, REMOTS®, and benthic community data, the disposal mound 

 showed a physically stable configuration that has not changed in 

 the nine years since the last comprehensive survey. 



4.2 Bottom Topography and Sediment Characteristics 



4.2.1 Geological History 



Another objective of this study was to determine bottom 

 elevations and topography, physical characteristics (including sub- 

 bottom) , and sediment types for the different study areas. 

 Interpretation of the bathymetry, side scan, and sub-bottom results 

 at the Brenton-A, Brenton-B, and Prudence sites is clarified by a 

 review of the geologic history of the area. Upland areas of Rhode 

 Island are composed primarily of Paleozoic crystalline rocks 

 overlain by unconsolidated glacial deposits. The bedrock surface 

 continues beneath Narragansett Bay and offshore, dipping to the 

 south and southeast. In Rhode Island Sound, 15 - 20 km south of 

 the mouth of the Bay, the bedrock is unconformably overlain by a 

 seaward-thickening and seaward-dipping erosional remnant of ancient 

 (Cretaceous) coastal plain strata (McMaster et al., 1968). The 

 landward extent of the coastal plain deposits is marked by a 

 prominent, north-facing, and highly-incised cuesta. This feature 



15 



