Subtidal shoals composed of Pleistocene and early to middle Holocene deposits, on the north-central South Central inner shelf, have been modified by wave and tidal processes since being flooded during the Holocene sea-level rise. The shoals, which extend seaward up to 20 km, are composed of a wide variety of sediment types, including silt- to gravel-size terrigenous clastic sediments, abundant shell material, and peat. Typically, the shoals terminate on their seaward ends at abrupt dropoffs, with relief commonly exceeding 14 m over a distance of 0.5 km. Three skoals, located off the Santee/Pee Dee Delta, Cape Romain, and the entrance to Bulls Bay, were studied by means of vibracores, scuba observations, box cores, bottom-sediment samples, and extensive bottom fathometer profiles.

The shoal complex seaward of the modern Santee/Pee Dee Delta is the largest of the three studied, most probably because (at a lower sea-level stand) the Santee and Pee Dee Rivers were joined, forming a large deltaic deposit. Shore-parallel scarps on the shoals correlate well with updip stratigraphic units and most probably give evidence of pauses during the most recent sea-level rise.

The shoal seaward of the modern Cape Romain cuspate foreland contains stacked packages of progradational mid- to lower shoreface (barrier-island) deposits. This progradational, downdrift-trending shoreline sequence is backed by a 6- to 7-km-wide back-barrier region and was formed during a pause or a drop of the most recent sea-level rise. All evidence indicates that the present transgressive Cape Romain barrier islands have migrated landward en mass away from the submerged shoal.

The Bulls Bay shoal, which probably originated as an abandoned delta lobe of the ancestral Santee River, shows the highest degree of modification and is characterized by abundant ridge topography and high shell content. The reworked ridges have lengths of 1 to 2 km and elevations of 2 to 4 m.

Shoal surfaces show evidence of significant wave and current modifications, such as abundant bedforms and erosional scarps. Southwesterly-facing scarps were cut presumably by storms (hurricanes). However, bathymetry, sediment-distribution patterns, bed-forms and gross morphology of the shoals indicate thai northeasterly waves dominate normal shelf processes.