Summary

Sediment samples and bottom photography, collected by the U.S. Geological Survey at 36 stations on the sea floor, are used to verify multibeam-echosounder and sidescan-sonar data collected by the National Oceanic and Atmospheric Administration in hydrographic survey H12298 south of Fishers Island in Block Island Sound. A number of sea-floor features are found in the study area, including bedrock, drumlins, boulders, cobbles, current-scoured bathymetric depressions, obstacle marks, glaciolacustrine sediments, sand waves and megaripples, and trawl marks. Boulders on the sea floor tend to be covered with sessile flora and fauna. They are found near Fishers Island, on isolated bathymetric highs, and in the bottom of the northern current-scoured bathymetric depression. Boulders near Fishers Island are part of the flank of the Harbor Hill-Roanoke Point-Charlestown-Buzzards Bay end moraine. Rocky, isolated bathymetric highs occur along a northwest-southeast trending line in the eastern part of the study area and individually further to the west and south. These bathymetric highs are likely drumlins where they are aligned, exposed bedrock where they are striated, undifferentiated glacial deposits in the southwest, and coastal plain strata capped by basal till in the southeast.

Just to the west of the drumlins are two deep, current-scoured bathymetric depressions. The steep eastern wall of the Deep Hole, the southern depression, is composed of layered outcrops and rip-up clasts of gray and reddish brown silty clay interpreted to be glaciolacustrine sediment. The northern bathymetric depression has boulders in the bottom and along its eastern wall where it has eroded into the flank of a drumlin and underlying glacial deposits. Sand waves in the study area tend to have 100- to 200-meter wavelengths with north-south-oriented crests, and megaripples with wavelengths of tens of meters are common on their flanks. Sandy areas of the sea floor tend to be rippled with scattered shells and shell hash. Sedimentary environments in the study area are primarily controlled by the strength of the oscillating tidal currents and are characterized by processes of  (1) erosion or nondeposition, where gravel covers the sea floor, in the deep bathymetric depressions, and in east-west-oriented channels; (2) coarse-grained bedload transport, where sand waves and megaripples occur on the sea floor; and (3) sorting and reworking, throughout much of the eastern part of the study area where the sea floor generally lacks bedforms and other current-related features.