transgression that were detached by coastal retreat as sea level rose and 

 flooded the shelf. Though nearshore processes were responsible for their 

 formation, isolated shoals are maintained and modified by storm-generated 

 shelf currents and waves. 



Over 200 large shoals have been identified on the Atlantic Shelf, showing 

 modal depths over the crest of 6 to 9 m, 12 to 15 m, and 24 m, mean shoal 

 axis azimuths of 32 deg, and northward-opening angles of 10 to 35 deg 

 between the shoreline and shoal axis (Duane et al. 1972). Seismic reflection 

 profiles indicate that these shoals rest on a near-horizontal surface of the shelf 

 floor. Cores of shoal material are primarily composed of mud-free, well- 

 sorted medium sand or sand and gravel mixtures. In many cases, cores pene- 

 trating the shoal base have recovered mud, peat, and/or shells having a 

 radiocarbon age of early to mid-Holocene. 



Arcuate-shaped shoals, which occur in coastal areas, may be associated 

 with estuaries or capes (Duane et al. 1972). Shoals may form near the 

 mouths of estuaries and sounds where littoral drift material is intercepted and 

 distributed by tidal currents. Cape-associated shoals (Figure 23) occur off 

 cuspate forelands where convergence of littoral drift has resulted in the deposit 

 of low-lying salients which protrude from the coast. Remnants of both 

 estuary- and cape-associated shoals that were formed and abandoned during 

 the Holocene transgression may extend well out on the shelf. These features 

 in aggregate have been called shoal retreat massifs (Swift et al. 1972). They 

 can be traced shoreward to modern arcuate shoals in estuaries and off capes. 



The shapes and sizes of arcuate shoals and shoal retreat massifs are 

 variable because of the influence of waves, and tidal and storm-generated 

 currents. For example, Field and Duane (1974) found evidence that a shoal 

 field off Canaveral Peninsula on the Florida Atlantic coast had been reworked 

 by waves and currents. Sediments as deep as 4 m below the sediment-water 

 interface showed evidence of recent abrasion. In addition, comparison of 

 historical bathymetric surveys revealed that between 1898 and 1965, shoal 

 crests accreted several meters and migrated over 300 m to the southeast. 

 Similarly, studies of bathymetric charts of the past century by Granat and 

 Ludwick (1980) indicate that shoals in Chesapeake Bay entrance changed 

 shape and position during the time covered. Although relict in having origi- 

 nated in past times, most shoal retreat massifs have been influenced by 

 subsequent shelf processes, often leading to radical changes in their form and 

 orientation (Swift et al. 1972). 



Deltas 



Deltas are subaerial and subaqueous accumulations of river-derived sedi- 

 ments deposited at the coast. Sedimentation occurs when streams decelerate 

 by entering and mixing with larger bodies of water (Wright 1982). Deltaic 



60 



Chapter 3 Variable Coastal Features 



