bedding, dipping southwestward is still apparent. In the deeper parts 

 of Channel A the bedded fill overlies earlier fill of indeterminate 

 thickness. 



A large channel reaching thalweg depths of about -180 feet MLW was 

 crossed by two closely spaced Virginia Beach reconnaissance lines. The 

 approximate position and alignment of the channel is shown on Figure 6 

 by the mid-depth contours of -120 feet MLW and designated Channel E. 

 There is insufficient data presently available to detail the channel 

 and other features of the buried erosion surface off Virginia Beach. 



4. Sediment Characteristics and Distribution 



Much of the data on the character of sediments in Chesapeake Bay 

 Entrance was obtained from 57 cores taken for this study. These 4-inch 

 diameter cores range from 2 to 20 feet long and provide fairly dense 

 coverage of the surveyed area (Fig. 2 and 15). Additional data on sur- 

 face and subbottom sediments within study limits were obtained from 

 studies of grab samples and short cores by Ryan (1953) , engineering bor- 

 ing data reported in Christians and Meisburger (1967) , and chart nota- 

 tions on U.S. Coast and Geodetic Survey charts. Detailed coverage of 

 surface and subbottom sediments along the track of the Chesapeake Bay 

 Bridge Tunnel (at the western border of the study area) are contained 

 in logs of engineering test borings made during foundation studies for 

 that structure (Chesapeake Bay Bridge Tunnel Commission, 1961). A num- 

 ber of cores obtained for a dredging study by the Norfolk District , 

 Corps of Engineers, in 1970 are within this study area and have been 

 made available for study. These cores are plotted on Figure 2. 



The dominant surficial sediment of Chesapeake Bay Entrance is a 

 homogeneous (Figure 7) gray, fine to very fine quartzose sand, usually 

 well sorted and often silty. This fine sand body mantles the bottom 

 almost everywhere within the study limits, (Figs. 8 and 9) except the 

 channels and Lynnhaven Bay where gray silt is the dominant sediment 

 type. Medium and coarse sand is rare; the only sizable concentration 

 at the surface occurs in Thimble Shoals Channel where a light brown to 

 reddish-brown coarse sand with streaks and patches of gravelly sand 

 occur in outcrops (Figures 10 and 11). Smaller concentrations occur 

 in thin patches on the gray sand blanket and on the southwest rim of 

 Chesapeake Channel. 



Of the 57 cores taken for this study and 8 additional cores and 

 borings otherwise available from the study area (Christians and 

 Meisburger, 1967 Norfolk District Dredging Survey, 1970) , only 11 con- 

 tain surface sediments with a mean diameter coarser than fine sand 

 (.250 mm - 2.0 phi). Six of these cores are closely grouped in Thimble 

 Shoals Channel (C33, 34, 42, 45, 48, DH4) within an outcrop area of the 

 type E sand and gravel previously discussed in connection with the 

 Bridge-Tunnel boring data (Figures 2 and 4). Core 51 on the north edge 

 of Tail of the Horseshoe is judged to be in a small outcrop area of the 

 same unit which is continuous under the shoal. 



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