basal Calvert and, in ascending order, the Choptank, St. Marys and 

 Yorktown. These formations do not crop out in or near the Bay Entrance 

 area, and consequently are known only from wells and engineering soil 

 borings. Because of apparent lithologic variability within formational 

 boundaries and the sparse data available from wells, lithologic cri- 

 teria for identifying these Miocene formations from well samples are 

 not well established. Difficulties also exist in clearly defining 

 paleontologic criteria (Sinnott and Tibbitts 1957, Harrison and others, 

 1965, McLean, 1966); consequently the Chesapeake group formations are 

 largely undifferentiated in well logs. 



In the eastern shore counties of Virginia (southern end of Delmarva 

 Peninsula) the Miocene and post-Miocene contact has been shown by 

 Sinnott and Tibbitts (1955 § 1957) to lie generally less than 100 feet 

 below sea level. Across the Bay Entrance, on the southeast Virginia 

 coastal plain Miocene sediments have generally been thought to lie no 

 deeper than 100 feet below sea level (Cederstrom 1945, Oaks § Coch 1963, 

 Oaks 1964). Rogers and Spencer (1968), however, believe^that Pleisto- 

 cene deposits extend to 200 feet below sea level under the coastal 

 plain directly south of the study area. 



Pleistocene sediments of the Virginia coastal plain are collec- 

 tively called the Columbia group. Subdivision of the group has until 

 recently been based largely on topographic expression of the deposits 

 which occur in a series of step-like terraces. Columbia group sedi- 

 ments are rarely differentiated in well samples because lithological 

 and paleontological criteria are not defined. 



Recently Oaks and Coch (1963) re-defined the morphologic and 

 stratigraphic units of the Pleistocene Columbia group and pre-Columbia 

 post-Yorktown section of the southeastern coastal plain of Virginia 

 (see also Oaks 1964 and Coch 1965 for detailed studies and revisions) . 



Pleistocene units recognized by Oaks (1964) east of Suffolk Scarp 

 and pertinent to this study are in ascending order: Great Bridge, 

 Norfolk, Kempsville, Londonbridge and Sandbridge formations. All but 

 the Sandbridge formation appear to have been deposited at relative sea 

 levels higher than the present level. 



b. Bay Entrance Study Area 



Strata underlying Chesapeake Bay Entrance are known primarily 

 from a series of exploratory borings along the route of the Chesapeake 

 Bay Bridge Tunnel crossing from Cape Charles to Chesapeake Beach. Logs 

 of these borings (Chesapeake Bay Bridge and Tunnel Commission 1960-1961) 

 and studies of the boring samples and data by Harrison (1963) , Harrison 

 et al (1965) and McLean (1966) have established the characteristics and 

 probable age of sedimentary units underlying the Bridge Tunnel. 



