Both erosional and constructional processes appear to have formed physiographic features near the shelf break along the southeastern United States, as indicated by extensive echosounder profiles, rock-dredge material, and bottom photographs. Between Cape Hatteras, North Carolina, and Fort Lauderdale, Florida, four distinct physiographic areas are delineated, each having characteristic morphologies and lithologies.
The ridges and well-defined troughs on the outer shelf and upper slope (depths of about 50 to 150 m) between Cape Hatteras and Cape Fear may be related largely to earlier Gulf Stream erosion, and the rocks (algal limestones and sandstones) and sediments dredged from these features probably are mainly Holocene, relict shallow-water deposits forming a thin veneer over this erosional surface of the sea floor. Relatively rapid accumulation of pre-Holocene sediments may account for the general absence of pronounced physiographic features on the outer shelf and upper slope from Cape Fear to Cape Kennedy. Ledges, small terraces, and rises (depths of 50 to 110 m) in this area are probably Holocene features eroded into, or constructed on the pre-Holocene sediments, which are covered by transgressive Holocene algal limestones and sandstones similar to those collected to the north. The lithology, together with radiocarbon dates of rock material, indicate that well-defined ridges in depths of 70 to 90 m between Cape Kennedy and Palm Beach are relict oölitic ridges or “dunes” formed during the Holocene transgression; these features are now covered by modern Oculina sp. coral debris. From Palm Beach to Fort Lauderdale, where the continental shelf is narrow and shallow, a small ridge present at the shelf break (15 to 30 m) is thought to be an “inactive” coral reef.