eroded surface of underlying Miocene strata has radio carbon ages 

 placing it in the time frame of the Holocene transgression. (Harrison 

 et al, 1965). 



Since borings do not show any points of overlap between units D 

 and E, their relative age cannot be determined; it is possible that 

 both are of the same age. However the coarse, poorly sorted texture 

 and absence of marine shells in most unit E material suggest a fluvial 

 origin and the heavy iron stains indicate subaerial exposure, which point 

 to a time of origin at least predating the local onset of the Holocene 

 Transgression. 



Sediment units A, B and C which overlie the dated peat horizon D 

 (Harrison et al 1965) are clearly of Holocene age and are judged to 

 include both transgressive and post transgressive facies. Studies of 

 microfossils from the soft silty units B and C have been made by 

 McLean (1966) using bridge tunnel samples and Nelson (1969) and Nelson 

 and Meisburger (1972) using CERC cores. Both studies indicate that the 

 units were formed in fresh to brackish shallow water environments. A 

 carbon-14 age of 11,500 yr BP +1200 yr on organic detritus sampled from 

 9 to 12 foot downhole on CERC Core 37 (Maynard Nichols, personal com- 

 munication) , indicates a transgressive Holocene age for this material 

 judged to be a part of Unit B. 



Based on radioactivity dating of peat in underlying B and D sedi- 

 ment units it is clear that the fine gray sand of unit A is of Holocene 

 age. Evidence suggests that at least part of the unit is post-transgres- 

 sive (i.e., since relative sea level reached its present position). 



Study of microfossils from CERC cores, in unit A showed species 

 adopted to the environment presently existing in the area (Nelson 1969, 

 Nelson and Meisburger 1972). In addition, the top of unit A averages 

 only about -20 feet MLW and locally rises to and slightly above sea 

 level. Reworking or post depositional uplift could also account for 

 the present elevations of the unit, and a Holocene uplift has been 

 postulated by Harrison et al (1965) , but deposition at relative sea 

 level near that of the present time seems the most probable explanation. 



The silt deposit in Lynnhaven Bay may be post-transgressive, thus 

 not directly related to unit B. The Lynnhaven silt appears to have 

 been laid down in deeper water than most of unit B (Nelson 1969) and it 

 lies at a shallower depth. 



The erosion surface plotted from seismic reflection data in Figure 

 6 is judged to correspond with the top of the Miocene in the area to 

 the south and west of the northeast wall of Channel A which cuts diago- 

 nally beneath the Entrance Area. Only a small segment of the Bridge- 

 Tunnel complex lies north of Channel A (Figure 6) thus subbottom lithol- 

 ogies below the penetration range of CERC cores is obscure. According 

 to Bridge-Tunnel borings, the top of the Miocene is truncated at the 

 steep north wall of Channel A about 40 feet below the top of the wall 



33 



