area; the cause of the gap is unclear. One distinctive difference 

 between different groups or clusters' of channels is the maximum thalweg 

 depth. There is a well-defined relation between maximum channel depth 

 and distance from shore for 25 channels. The trend appears to be nearly 

 linear, but this may be an artifact of a lack of data very near the shore- 

 line or farther offshore. The gap between the two clusters (3.2 to 5.4 

 kilometers or 1.8 to 3 nautical miles) is apparent in the plot, as are 

 other more narrow gaps (0.9 to 2.2 kilometers or 0.5 to 1.2 nautical 

 miles, 7 to 8.5 kilometers or 3.9 to 4.7 nautical miles, and 9.9 to 11.9 

 kilometers or 5.5 to 6.6 nautical miles). The gap farthest offshore may 

 be an artifact resultant from insufficient data coverage. Figure 21 can 

 be used as a potential application for extrapolating channel depths far- 

 ther offshore and beneath the shoreline. The trend of the plot may not 

 remain linear beyond the area of data coverage in either direction, but 

 there is no valid reason to suspect that it would not, at least for short 

 distances. Thus, to locate or identify remnant channels beneath the 

 Delmarva shoreline, the base of the channel could be assumed to lie 

 between 15.2 and 19.8 meters (50 and 65 feet) below sea level. Weigle 

 (1974) located such a channel --an area beneath 66th Street at Ocean City 

 intruded by saltwater. Borings in this area show medium and coarse sand 

 down to 16.8 meters (55 feet) below MSL; adjacent borings contain sand 

 to only about half that depth (Weigle, personal communication, 1975) . 

 This apparent channel is plotted as an "X" along the shoreline in Figure 

 21. 



4. Acoustic Structure of Shoals . 



Duane, et al. (1972) reported that Atlantic shelf shoals were under- 

 lain by a relatively flat reflector which projected beneath the shoals 

 and often cropped out in intervening swales. Although seismic data were 

 available for over 50 shoals in that study, only 1 shoal, located off 

 Fort Pierce, Florida, showed internal, secondary stratification. Kraft 

 (1971) reported that a shoal off Delaware was not flat on the underside 

 but rather contained a core of Pleistocene beach ridge. 



In the study area, linear shoals characteristically are defined by 

 a strong, relatively flat basal reflector (reflection horizon A 2 ) which 

 intersects the sea floor on either side of the shoal, and little or no 

 internal structure. The sparse indications of internal stratification 

 are restricted to (a) occasional single bedding planes in deeper strata, 



(b) fill-in structures in buried channels (Figs. 17, 19, and 20), and 



(c) occasional bedding planes within the prominent linear shoals. The 

 lack of strong internal bedding planes is probably a result of the random 

 movement of shoals in different directions at different rates (Field, 1976) , 

 For example, in Figure 22, none of the four lines obtained from Great Gull 

 Banks indicate acoustic contrasts within the body of the shoal . They 

 show, however, numerous acoustic contrasts (reflectors) down to 45.7 

 meters (150 feet) below sea level, including the shoal basal reflectors. 



As with the other shoals, some of these reflectors appear to intersect 

 the sea floor (e.g., line L, fix 38-39). 



51 



