A generalized profile of the Bridge-Tunnel route compiled from the 

 borings and published studies is included in Figure 4. Sedimentary- 

 units have been generalized on the basis of gross lithology and age. 

 The letters used to identify sedimentary units on Figure 4 and in the 

 text are the same used on the Bridge Tunnel boring logs to identify 

 these major sediment types. Subscripted numerals used on the logs to 

 identify interunit variations of lithology and soil properties have not 

 been used here. 



Although the Bridge-Tunnel borings show a complex and diverse 

 stratigraphy in detail^ three main sedimentary bodies can be recognized. 

 These bodies are continuous and lie in a vertical sequence. The lower- 

 most body consists of greenish gray compact sand and sandy clay usually 

 containing some silt and shells. The clay (Unit F) is generally upper- 

 most, but also appears below and interbedded with the sand (Unit G) . 

 Standard blow counts (i.e., number of 140 pound hammer drops of 30 

 inches needed to drive a 2-inch outside diameter, split-spoon sampler 

 1-foot) are generally higher in the F-G units than in overlying bodies 

 and usually range from 10 to 25 in the clayey unit (F) and 30 to 100 in 

 the sandy unit (G) . Studies by Harrison (1963) , Harrison et al (1965)' 

 and McLean (1966) show that the F-G sediments are of Miocene age. 



The surface of the F-G sediment body has been deeply eroded, pre- 

 sumably by fluvial processes during a lower relative stand of the sea 

 (Figure 6). Deposited over the old erosion surface is a soft gray 

 silty clay to sandy silt (Units B§C) . The lower part of the soft gray 

 ,sediment is usually silty clay (Unit C) and the upper part a sandy silt 

 (Unit B) . In contrast to the imderlying sediment, this material is 

 characterized by its low bearing strength - standard blow counts rarely 

 exceeded 5 blows per foot anywhere in the unit. 



Units B and C occur along almost all of the Bridge-Tunnel route. 

 At the south end (off Chesapeake Beach) the B-C sediment unit is re- 

 placed by a sand possibly similar to but probably not directly related 

 to the A Unit described below. At the north end of the Bridge-Tunnel 

 route the stratigraphic position of the B-C units is occupied by a 

 complex set of localized lenses of sand, silt, and clay. The B-C sedi- 

 ment pinches out over highs in the underlying greenish-gray sediments 

 and has possibly been eroded by recutting in the deep Channel A under 

 Fisherman Island. Dates on samples within the underlying B-C unit show 

 that these sediments were deposited in shallow marine and fresh water 

 environments during the Holocene transgression. (Harrison et al 1965, 

 Maynard Nichols personal communication and faunal studies of McLean 

 1966, Nelson 1969) 



Fine well-sorted gray sand of very uniform appearance (Unit A) 

 overlies the entire sequence between Fisherman Island and Thimble 

 Shoals Channel. This is the characteristic surface sediment of the 

 Bay Entrance area, and it occurs in the majority of cores obtained for 

 this study. At the Bridge-Tunnel the sand is quite variable in shear 



