to have cut through sediment unit E and well into the underlying F-G 

 units. Several cores in Thimble Shoals Channel recovered material at 

 less than -60 feet MLW associated with the E unit and two (C 34 and 

 C 42) penetrated to an underlying silty sediment regarded as probably 

 continuous with the F-G unit. On this basis and support from the 

 Bridge-Tunnel data, a high in the erosion surface near Chesapeake 

 Channel has been extended southward under Tail of the Horseshoe to 

 Thimble Shoals Channel. 



The only reliable geophysical line between Chesapeake Channel and 

 Thimble Shoals Channel is line D-E which shows a continuous reflector 

 at less than -60 feet MLW crossing under Tail of the Horseshoe. This 

 reflector is consistent with the core data. However, a second strong 

 reflector dips southward from a high point near the sediment surface at 

 Chesapeake Channel to a depth of 130 feet just south of Thimble Shoals 

 Channel where subbottom penetration was lost. This second reflector 

 may represent an erosional unconformity in the presumed Miocene sedi- 

 ments below the erosion surface delineated by Harrison et al (1965) 

 and on Figure 6, or it may actually be continuous with the erosion sur- 

 face mapped to the north. If the latter concept is true, the channel 

 as based on the deeper reflector of line D-E and a discontinuous, 

 apparently associated reflector visable on parts of lines 4 and M in 

 the Lynnhaven Grid would be much wider and include the small channel 

 shown on the Bridge-Tunnel section north of the larger Channel D. The 

 probably trend of this larger channel would be southeast rather than 

 east and it would pass under the south Bay Shore between Lynnhaven 

 Inlet and Cape Henry. 



Since firm data on alignment are not available, the interpretation 

 of Channel D as shown on Figure 6 is based on the core and boring data 

 with Channel D trending eastward between the high under Tail of the 

 Horseshoe and the land area to the south where Oaks (1964) interpreted 

 the Miocene surface lying generally at less than -50 feet MLW. This 

 seems the most reasonable explanation based on the meager data at hand. 



Reflections from fill in the various channels is characterized by 

 the common occurrence of high angle bedding surfaces especially in the 

 large A and D Channels. On some records no stratification or bedding 

 is apparent in the valley fill although cross lines run on a, perpen- 

 dicular heading clearly show bedding. Possibly reflectivity is 

 enhanced or diminished by the relative angles between the survey track 

 and the dip of the beds. It may also be that the bedding is not uni- 

 directional but that only certain sets have reflective interfaces. 

 Wherever these bedding surfaces have been detected, they were found to 

 dip southwestward, thus they lie almost normal to the axis of the south- 

 east trending A Channel and dip slightly upstream in the D Channel. 



The fill is thickest where the valleys are deep, but it appears to 

 extend across the low interfluve between the A and B valleys where it 

 thins to only a few feet. Even in this thinning section internal 



19 



