C EN O ZOIC HISTORY. 573 



extent westward of the Chesapeake and Pamunkey formations is 

 not known, for no shores or shore deposits remain, and the 

 superimposed drainage may be of later age. As they were 

 deposited in moderately deep waters and the shores were probably 

 low, at least in Chesapeake times, it is possible that the deposits 

 may have originally extended far west of their present terminations. 

 In Virginia, both formations abut against steep shores of crystal- 

 line rocks or Potomac sands, but in their original thickness they 

 may have overlapped these local steep shores. About Washing- 

 ton, and west of Baltimore, outliers of probable Chesapeake cap 

 the highest summits under a protecting cap of Lafayette gravels, 

 and as one stands on these outliers and looks westward, there is 

 strong suggestion that the formations may have originally 

 extended far in that direction. 



PLEISTOCENE TERRACES. 



The earlier Pleistocene terrace, or that on which the earlier 

 Columbia deposits lie, is tilted to a high altitude to the northwest- 

 ward, but it dips to the east and south, and finally passes beneath 

 the terrace level of the later Columbia deposits. The latter is 

 also slightly tilted to the eastward. As the southeastward tilt of 

 the Tertiary peneplain is greater than that of the Baltimore 

 terrace, the Lafayette formation passes beneath the deposits of 

 that terrace along a line near the zero line in figure 5. The 

 relative slopes of these terrace levels and the Tertiary peneplain 

 near the latitude of Washington are shown in the following dia- 

 gram : 



Figure 1. Diagram of earlier and later Columbia terrace plains and Tertiary 

 peneplain near the Latitude of Washington. Vertical scale greatly exaggerated. 



In the vicinity of Washington, where the relations are partic- 

 ularly well exhibited, the earlier Columbia terrace has an altitude 

 from 215 to 180 feet, with the Lafayette formation on the Ter- 

 tiary peneplain about 100 feet above. The later Columbian 

 terraces are in a series, of which the highest averages about 100 



