(Figure 4, App.A, Lines C and 11). The erosion surface mapped on 

 Figure 6 to the north and east o£ Channel A is underlain by about 30 

 feet of post-Miocene sediment deposited prior to the cutting of 

 Channel A and is thus probably older than late Wisconsin. In the area 

 enclosed by the north wall of Channel A, the Bridge-Tunnel and the 

 south Bay shore, this older deposit appears to have been largely if not 

 entirely removed by the various channels and the erosion surface as de- 

 picted in Figure 6 is considered to be essentially at the top of the 

 Miocene. With the exception of Unit E which may possibly be a relic of 

 the post-Miocene but pre-erosion surface deposit, the fill over the 

 Miocene surface south and west of Channel A is entirely Holocene. 



Ryan (1953) estimated that a volume of 46.4 x 10^ cubic yards of sand 

 would be needed in the southern Bay area to fill the old Pleistocene 

 channels to an extent consistent with present bathymetry. He concluded 

 from these estimates that the sediment produced or contributed to the 

 Bay since its invasion by the Holocene seas was not sufficient to 

 account for this fill plus the 14.75 x 10^ cubic yards of fill esti- 

 mated to have been deposited in mid-and upper-Bay locales. A contribu- 

 tion of sediment from the Atlantic Ocean to the lower Bay was thus 

 considered probable. 



Although Ryans' estimate of channel depths in the Bay Entrance did 

 not take into account the possible Holocene uplift later postulated by 

 Harrison et al (1965) which would reduce needed fill in the lower Bay, 

 the volume still needed to fill the uplifted channels coupled with the 

 fact that the old channels in mid- and upper-Bay are only partially 

 filled while they are almost entirely buried in the lower Bay, strongly 

 suggests a sediment influx into the lower Bay from the ocean side. 



The Pleistocene Channels in the Bay Entrance area (Figure 6) have 

 been filled for the most part with fine silty sediment of units B and 

 C deposited in a shallow brackish to fresh water environment (Harrison 

 et al, 1965; McLean 1966, Nelson, 1969, Nelson and Meisburger (1972). 

 This deposition presumably took place during the most recent transgres- 

 sion as the former stream channels were being progressively embayed. 

 It is probable that the bulk of these fine sediments were brought down 

 by the parent streams to come to rest in the embayed esturaries. How- 

 ever the persistent southwest dip of bedding planes in the channel fill 

 detected on seismic reflection profiles (Appendix A) suggests that dep- 

 ositional control and possibly the immediate sediment source may have 

 been to the northeast of the Entrance area. It is possible therefore 

 that the preponderance of unit B and C sediments were swept across the 

 area from the ocean side to aggrade the old channels while the sediments 

 and processes internal to the channels exerted only minor influence on 

 the filling process. 



The fine gray sand (unit A) which comprises the surface and near 

 surface deposits on the terrace surrounding Cape Charles and surmounts 

 the Horseshoe - seems very likely to have originated from sources 



34 



