raore-or-less parallel the key reflector. Since the key reflector dips 

 below record coverage at the south margin of the entrance area grid 

 lines (Figure 2) , it is too deep to appear on records from the recon- 

 naissance area off Virginia Beach. A hypothetical position of this 

 reflector in the reconnaissance area was constructed by selecting a 

 higher reflector within the evenly bedded section from records near the 

 south margin of the grid, mapping this higher reflector through the 

 reconnaissance area to the south, and applying the depth difference 

 between the higher and lower reflector measured in the area of overlap. 



The major erosion surface is sharply defined in places by a strong 

 reflector; however, in many areas it can only be approximated because 

 of masking by overlying reflectors or deficient acoustic contrast across 

 the boundary. Despite these deficiencies enough information is avail- 

 able for tentative mapping (Fig. 6) . This mapped system may be con- 

 tinuous with the ancient drainage system described by Hack (1957) , 

 Harrison (1963) , and Harrison et al (1965) . 



The largest channel (Channel A, Figure 6) is believed to be con- 

 tinuous with the presumed ancestral Susquehanna, described by Harrison 

 et al (1965) from the region just north and west of the study area. 

 The maximum depth of the thalweg in this valley is not clear because 

 the central portion of the valley is obscured on reflection records. 

 This is due to an acoustically opaque stratum which lies above and 

 apparently follows the thalweg. Unless a gorge exists below the 

 opaque layer the maximum projected depth is probably less than -200 ft. 

 MLW. In this connection, Beckmann, Drake and Sutton (1961) concluded 

 from seismic reflection data at the Bridge-Tunnel crossing that no 

 channels in the subbottom extended deeper than -150 feet; Harrison 

 et al (1965) found channeling to a depth of -160 feet MLW below the 

 Bridge-Tunnel, and McLean (1966) indicates that Pleistocene sediments 

 reach a depth of at least -185 feet MLW off Fisherman Islands. 



Two other channels (B and C) flank and roughly parallel the course 

 of Channel A. Channel B is separated from Channel A by a low divide 

 rising to about -90 feet MLW. Channel C to the east is separated from 

 Channel A by a high broad divide rising to -50 feet MLW. Both Channels 

 B and C have maximum thalweg depths of around 120 to 130 feet within 

 the study limits. 



A fourth channel (D) crosses under the Bridge-Tunnel near its 

 south terminus at Chesapeake Beach. This channel has a thalweg depth 

 of -130 feet MLW and was considered by Harrison et al (1965) as pos- 

 sibly an ancestral channel of the James River. .Because of the sonic 

 attenuation attributed to organic content in a thick silt blanket 

 covering most of Lynnhaven Roads, subbottom reflections were not 

 obtained on most of the seismic profiles covering the study area south 

 of Thimble Shoals Channel. As a consequence alignment of Channel D 

 east of the Bridge-Tunnel has been inferred largely from core and 

 boring evidenence. Channel D appears from the Bridge-Tunnel borings 



17 



