and a net transport of material over the base of the shoal onto the 

 crest and seaward flank. This situation presumably develops when the 

 wave front is from the northeast, the dominant direction of storm waves. 

 The effect of waves from the southeast would be to complement the ridge- 

 building process, as the waves intersect the ridge at a small acute angle 

 and move material northward along the seaward flank. 



The development of a shoreface shoal into a shelf-isolated shoal can 

 be visualized as an overall response to a relative rise in sea level and 

 the concomitant retreat of the coastline. As a shoreface shoal begins 

 to develop by accretion on the northern or distal end, it also begins to 

 lengthen by headward erosion in the trough (Field, 1976). Continued 

 accretion and trough erosion result in a long shoal with several ridges 

 or arms. As the shoreline continues to recede, the shoal will eventually 

 be segmented from the shoreface and isolated on the inner shelf as one 

 or several individual shoals, depending on the final configuration of 

 the shoal before segmentation. 



Because linear shoals lie at depths varying from 10 meters to more 

 than 30 meters, they are probably continuously modified by marine pro- 

 cesses and the effect of these processes presumably differs for shoals 

 of different depths. It is likely the crests of shoals, especially those 

 shoals less than -15 or -18 meters MLW, must be very vulnerable to wave 

 attack. Field (1976) reviewed the current literature on shelf transport 

 and evaluated how wave-induced forces may modify shoals . The net effect 

 of waves is limited to five possibilities: (a) shoals are unaffected; 



(b) shoals are built up; (c) shoals are destroyed; (d) shoals are driven 

 landward; or (e) shoals are driven seaward. Data from seismic reflection 

 profiling, detailed fathometer profiling and historical surveys, and litho- 

 stratigraphic evidence show that possibilities a, b, and c are unlikely, 

 and that d and e are likely. All lines of evidence show that shoals 

 migrate at various rates and in various directions. They can be visual- 

 ized as active features that have a net component of migration that is 

 small compared to its total movement in all directions. 



VI. POTENTIAL SAND RESOURCES 



1 . Depositional Patterns and Processes Along the Coast . 



The origin of barrier islands has been, and remains, a controversial 

 topic among geologists. Although many hybrid theories have been suggested; 

 the three main hypotheses on barrier island origin are: (a) upbuilding 

 of submarine bars; (b) segmentation of elongated spits, by inlets; and 



(c) submergence of mainland coastal ridges. Schwartz (1973) includes many 

 of the original papers, as well as papers which compare and combine indi- 

 vidual hypotheses. 



The Maryland-Virginia barrier island chain has not received much 

 attention relative to other east coast barriers, such as the Georgia sea 

 islands (Hoyt, 1967) and the North Carolina Outer Banks (Pierce and 

 Colqhoun, 1970). Kraft, Biggs, and Halsey (1973) hypothesized that the 



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