Along the northeast United States, sea level has risen about 90 to 100 m since the end of the 

 Pleistocene epoch, about 12,000 to 15,000 years ago (Nummedal 1983). This Holocene 

 transgression flooded the continental shelves and caused the retreat of barrier islands along much 

 of the eastern seaboard. How do barriers respond to a marine transgression? Two contrasting 

 hypotheses have been proposed: One states that as the sea rises, barriers migrate continuously 

 landward. During this retreat, the breaker zone traverses the entire area that is submerged. 

 Barrier retreat is most likely to occur along shores where there is a large sediment supply and 

 where the rise in sea level is slow. This form of retreat in response to marine transgression has 

 been documented in Rhode Island, where peat exposed on the ocean shoreface demonstrates how 

 former lagoonal sediments are being unearthed seaward of a retreating shoreline (Dillon 1970). 



The second hypothesis suggests that barriers can be drowned in place. As the sea rises, the 

 barrier remains fixed, while the lagoon on its landward side deepens and widens. Eventually, the 

 breaker zone reaches the top of the dunes, the barrier is drowned, and the breakers skip landward 

 a considerable distance to form a new barrier at the landward edge of the former lagoon. Under 

 what circumstances could this "skipping" mechanism occur? A barrier might be drowned if there 

 is limited or decreasing sediment supply. Because of the shallow slope of a typical barrier, a vast 

 sediment supply is needed to accommodate even a minor rise of sea level (this is analogous to 

 breakwater construction: a minor increase in height requires a great quantity of extra rock). 

 Without the copious input of sand, the barrier becomes narrower and narrower and is eventually 

 overtopped. Even with a generous sand supply, a period of exceptionally rapid sea level rise 

 might overwhelm the barrier. In addition, if the barrier is densely vegetated, overwash is 

 impaired, resulting in a steepening of the profile as the sea rises. The barrier is unable to migrate 

 landward and can be drowned in place. Details of these theories and the original papers where 

 they were proposed are reprinted in Schwartz (1973). More discussion on the balance between 

 erosion caused by sea level rise versus accretion dependent on sediment supply is found in 

 Engineeer Manual 1 1 10-2-1810 (Headquarters, U.S. Army Corps of Engineers (HQUSACE) 

 1995) (pp. 2-26). 



Based on examination of cores and seismic records off Fire Island, Sanders and Kumar 

 (1975) proposed the following scenario to describe the Holocene submergence of the barriers off 

 Long Island: 



When sea level stood at -24 m (9,000 years ago), a chain of barriers existed about 7 km 

 offshore parallel to the modern shore. As the sea rose, the barriers remained in place until the sea 

 reached -16 m, at which time it inundated the top of the dunes. The surf zone was then free to 

 jump about 5 km landward to form a new shoreline about 2 km seaward of the present barrier 

 line. New barriers formed at the -16-m shoreline, becoming ancestors of the modern south shore 

 barriers. These barriers have migrated continuously landward as sea level rose from -16 m to its 

 present elevation. Rampino and Sanders (1980) believed that the "skipping" mechanism 

 explained why complete barrier sediment sequences have been preserved on the Long Island 

 shelf, but Panageotou, Leatherman, and Dill (1985) have disputed this interpretation. 



Chapter 2 Geologic Setting and Morphologic Development 



