strength as indicated by a wide range o£ blow counts both from boring 

 to boring and in places within the same bore hole. In general, however, 

 standard blow counts average 10 to 20 in this unit. 



Two, more restricted, units occur in the Bridge-Tunnel section. 

 Both appear between the greenish-gray sediment (F^G) and the overlying 

 soft, gray sediments. One of these is a peat (Unit D) ; the other is a 

 coarse, iron-stained sand (Unit E) . Except for isolated patches, these 

 units are concentrated in the area south of Chesapeake Channel . There 

 is no clear evidence that one overlies the other at any point; both 

 apparently occupy the same stratigraphic horizon. However, for reasons 

 discussed later, they are not believed to be time equivalents. 



3. Shallow Subbottom Structure and Bedding 



Two distinct patterns of bedding are evident in the 300-foot 

 section of subbottom strata covered by CERC seismic reflection records. 

 In the lower part of the records, the reflector surfaces tend to be 

 continuous, smooth, parallel to sub-parallel and dip very gently in a 

 predominant east to southeast direction. Strata overlying this more or 

 less uniformly bedded section tend to be discontinuous. Truncations 

 and fadeout of reflector surfaces, secondary bedding between primary 

 reflectors, and erosional features commonly occur in this section 

 throughout the study area. 



A buried erosion surface continuously underlies the entire study 

 area. This surface is characterized by a number of deep channels, 

 probably of fluvial origin, crossing the Bay Entrance area in a north- 

 west to southeast direction, and a large channel trending east -west 

 along the southern margin of the area. In places, the erosion surface 

 divides the distinctively bedded upper and lower subbottom sections; 

 elsewhere it lies within the upper sections. 



The impression of complexity afforded by acoustic reflections in 

 the upper section is verified in the western part of the study area by 

 borings for the Chesapeake Bay Bridge Tunnel . These borings show fre- 

 quent discontinuity in the lithologic and physical properties of sedi- 

 ments at similar depths, albeit gross lithology is more regular 

 (Chesapeake Bay Bridge Tunnel Commission (1960-61), also see Harrison 

 (1963), McLean (1966). In general, the complexly bedded section shown 

 in records at and near the Bridge-Tunnel correlates with sediment 

 units A,B,C,D, and E of the Bridge-Tunnel borings (Figure 4), while the 

 lower evenly bedded section correlates with the F and G units. These 

 latter units have been identified as Miocene age sediments by Harrison 

 (1963), Harrison et al (1965), and McLean (1966). 



The general structural trend of strata contained in the evenly bed- 

 ded lower section of the records is illustrated by the map in Figure 5. 

 This map shows contours on a prominent reflecting surface within the 

 lower section. Other reflectors within the evenly bedded section 



15 



