relief is more subdued. Ridges, where present, exhibit relief of about 

 3 meters (10 feet), which contrasts with the 3.1 to 9.1 meters of relief 

 noted farther north. Swales are more prominent than individual ridges 

 in this area. The dashline in Figure 11 shows the approximate locations 

 of the change from linear-shoal field to the flat-bottom area inshore 

 and subdued topography offshore. Just north of this line and within 

 9.1 kilometers of the shore are a series of well-defined shore-parallel 

 shoals. The presence of these shoals adjacent to the end of Assateague 

 Island, and especially the occurrence of a spit or elbow-shaped shoal 

 offshore, have led some investigators to conclude that the ridges repre- 

 sent drowned spits. 



III. SUBSURFACE STRUCTURE AND BEDDING 



Continuous seismic reflection records obtained from the two field 

 studies show subbottom acoustic reflectors down to nearly 122 meters 

 below sea level. Each seismic source used (3. 5-kilohertz transducer, 

 sparker, Acoustipulse) differs in energy and frequency, hence the re- 

 sulting records differ in penetration, resolution, and record quality. 

 Correlation between the different types of records is possible, but 

 differences in record scales and acoustic response of the substrata 

 occasionally make correlation difficult. The Acoustipulse system pro- 

 vided the best overall data; penetration often exceeded -61 meters (-200 

 feet) MSL and definition of shallow strata was of high quality. Sparker 

 data were most useful for recognizing deeper horizons (30 meters below 

 sea level) whereas strata lying between 12.2 and 30.5 meters (40 and 100 

 feet) were best defined on the records obtained with the 3. 5-kilohertz 

 and Acoustipulse equipment. The 3. 5-kilohertz data provided an extra 

 advantage in allowing qualitative estimates to be made of surface sediment 

 character based on the nature of the return signal. The main difficulty 

 with the 3. 5-kilohertz data, and to some degree with the Acoustipulse 

 data, is lack of penetration (often less than -37 meters (-120 feet) MSL). 

 Difficulties encountered with the sparker data include a general lack of 

 resolution and a masking effect from multiple reflections of the sea 

 floor. Examples of the three types of continuous seismic reflection 

 records are shown in Figure 12. 



The acoustic data show that all strata have a nearly consistent sea- 

 ward dip and a relatively smooth surface. Strata dip generally toward 

 the east and southeast at a slope ranging from 1 on 250 to 1 on 1,600. 

 Those reflectors, which persist and are identified on several profile 

 lines, and especially those which are mappable over a large area (tens 

 of square miles), are called primary reflectors. Secondary reflectors 

 are either local in extent (erosional discontinuities in channels) or 

 occur as internal bedding surfaces between primary reflectors (e.g., 

 foreset bedding) . 



As many as 11 primary reflectors are visible on the reflection 



records. Trend, relative depth, and associated secondary reflections 



of each primary reflector for the central part of the study area are 



schemetized in Figure 13. Detailed cross sections of selected seismic 



36 



