profile lines, on which Figure 13 was based, are shown in Appendix A. 

 It should be noted that most reflectors are not present everywhere; 

 they tend to "fade in and out" for different reasons (geologic changes, 

 acoustic artifacts on the records). Because of this, primary reflectors 

 are more easily identified on shore-normal (down-dip) lines where slopes 

 may be projected across "fade-out zones" than on shore-parallel (essen- 

 tially along strike) lines where a "fade-out" reflector does not appear 

 at all. 



Reflection units are arbitrarily defined sections of the subsurface 

 bounded by relatively persistent primary reflection horizons. The con- 

 vention established here for units and horizons is numbers and letters 

 increasing downward, which will permit addition of new units by other 

 investigators using deeper penetration seismic sources. 



1. Primary Reflection Horizons and Reflection Units . 



a. Reflection Unit C . The lowermost reflection unit, identified as 

 unit C, is bounded by C^ on the bottom and Cj on the top. Maximum 

 thickness of the unit is about 38 meters (125 feet), but this may be 

 exceeded beyond the seaward limit (10 kilometers or 5.5 miles) in Figure 

 13. Horizons in unit C slope relatively steep compared to overlying 

 strata. Slopes for C l5 C 2 , C 3 , and C k are 1 on 1,000, 250, 350, 

 and 400, respectively. If no change in slope occurs inshore in the 

 deeper strata, then they may pinch out against C 1 ; available data are 

 not adequate for these deeper reflection horizons to determine their 

 overall slope or changes in slope, except for the noted change in C3. 



A small channel 610 meters wide and 7.6 meters deep is incised into 

 horizon C 2 along profile line L (see App. A) at a depth of more than 

 90 meters (300 feet) below sea level. The recording of such a feature at 

 that depth indicates a strong lithologic contrast between the channel fill 

 (probably sand) and the incised unit (probably mud) . The top of reflec- 

 tion unit C is defined by horizon C 1 , a strong and relatively flat 

 reflector ranging in depth from -67 meters (-220 feet) just offshore to 

 about -76 meters, 9.6 kilometers from the shoreline. 



b. Reflection Unit B . The middle reflection unit, unit B, is bounded 

 by two strong, low-relief, nearly parallel reflectors, B-^ and C^ (Fig. 

 13). Within the unit, which is about 38 meters thick, are three other 

 primary reflection horizons. Slopes of the reflectors B 1 , B 2 , B 3 , and 

 B 4 are 1 on 750, 1,600, 550, and 600, respectively. Reflecting horizon 

 B, is both strong and steep relative to other B unit horizons. It lies 

 at about 46 meters below MSL, 1 mile off the beach and drops to over 61 

 meters within 8 kilometers (5 miles) of the beach. The top of unit B is 

 also a strong reflector and ranges from -26 to -38 meters (-85 to -125 

 feet) between 1.6 and 10 kilometers (1 and 6 miles) from northern Assa- 

 teague. Some internal structures, such as scarps and possible channels, 

 are present in reflection unit B. 



c. Reflection Unit A . The uppermost reflection unit identified in 

 this study, unit A, is bounded on the bottom by horizon B, and on the top 



39 



