Figure 15 is an isopach map showing sediment thickness over a re- 

 flective horizon called the blue horizon. The isopach reflector surface 

 is more or less level and dips slightly southward. Where it appears, 

 the blue horizon is generally the uppermost clearly definable reflector. 

 The irregular to smooth appearance of the blue horizon suggests that in 

 places it follows an erosional surface and elsewhere lies along a depo- 

 sitional surface. Though acoustically weak or entirely lacking in 

 places, the blue horizon is believed to be present throughout the study 

 area as a zone or interface separating sediments of different physical 

 properties. Cores and borings for the Chesapeake Bridge-Tunnel indicate 

 that the blue horizon is probably near or at the base of the ubiquitous 

 fine gray sand blanket (Type A) covering most of the study area. 



West of Channel A (Fig. 6) data from outcrop, cores, and Bridge- 

 Tunnel borings indicate that the isopach reflector generally overlies 

 finer sediments (Types B,C,D,F,G) or coarse sand (Type E) . East of 

 Channel A, information on underlying sediments is deficient because few 

 cores in this area penetrated the upper sand blanket; however, varia- 

 tion in acoustic contrast along the blue horizon and the partly 

 erosional, partly depositional , appearance of the interface suggest 

 that underlying sediments are variable in physical properties. 



Based on the Figure 15 isopach map, the total volume of sediment 

 above the isopached (blue) reflector within study limits is estimated 

 to be 1.8 X 10^ cubic yards. 



30 



