A total of 68 vibracores and 14 box cores in conjunction with high-resolution seismic records are used to describe the late Quaternary development of a twin-barrier-island complex. Based on the stratigraphy, lithology, radiocarbon dates, and microfossils, both a transgressive outer Holocene and inner Pleistocene barrier-island complex are recognized. The two subaerial subparallel barriers are a result of separate marine transgressions that occurred before and after late Wisconsin glaciation. The present landward migration of the Holocene barrier should put it atop the Pleistocene barrier in approximately 1,400 years.

Conformable and unconformable contacts separate the two barrier-island systems. The outer barrier island, Smith Island, is typical of the Holocene Virginia barrier-island chain; its dimensions are approximately 11 km in length, and width and height mostly in the range of 200 m and 1.5 m above mean sea level, respectively. The Pleistocene barrier, Mockhom Island, lies approximately 7 km landward of Smith Island but within the back-barrier lagoon created by the Holocene marine transgression. Although Pleistocene shorelines are recognized on the mainland, these two chronologically distinct barrier systems are in contact, resulting in a geomorphology and upper Quaternary stratigraphy unique to the middle Atlantic coast.

Holocene sediments deposited in the back-barrier environment show a general shallowing and fining-upward sequence. A significant portion of these back-barrier deposits, at least, but probably greater than, 3 m thick, should be preserved below 75 to 100-cm-thick nearshore sands. However, inlet-fill deposits likely will not be preserved. With present sea-level rise and wave-base conditions continuing into the future, the lower Holocene and uppermost Pleistocene transgressive sequences have a strong preservation potential. The preserved succession of deposits consists of Pleistocene back-barrier mud and shoreface sand below Holocene back-barrier deposits. The stacking of transgressive barrier deposits, albeit those from different transgressions, might serve as a straligraphic petroleum trap if preserved into the geologic record.