40 THE POTOMAC OR YOUNGER MESOZOIC FLORA. 



House Bluff. Over this lies about twenty feet of laminated sandy gray 

 shale, which appears to correspond to the similarly placed material at 

 Federal Hill, Baltimore, which has been mentioned as containing plant- 

 fossils. 



Between White House and Mount Vernon the Quaternary occupies 

 the river bank for most of the way. Fort Washington, on the Maryland 

 shore, is a little north of east from Mount Vernon and about four miles 

 distant. This place is interesting as showing in the same vertical exposure 

 all the formations associated with the Potomac. For a very limited space 

 the Lower Potomac sand with its included clays may be seen, showing 

 only a few feet in thickness. Resting upon this, in the order ascending, we 

 have the Variegated Clay, the Eocene, and the Quaternary. From this 

 point to Washington, about fourteen miles, the Potomac is mostly cut 

 away, and the Quaternary is along the stream the principal visible rock. 



It will be seen from this account that the most easterly exposures of 

 the formation in question are confined to the immediate vicinity of the 

 Potomac River. It is not to be supposed that the eastern limit of these 

 beds is to be found anywhere near this river. The character of the mate- 

 rial shown in the most eastern localities where the formation is exposed 

 would lead us to suppose that at these places we are still far from the true 

 eastern margin of the formation. Strangeh^ enough, we find in some of 

 the most eastern exposures the maximum thickness of the formation, the 

 greatest amount of coarse material, and the most irregular structure. 

 Indeed, if we were to judge from the lithologic and structural features 

 alone, we would be inclined to think that the material came from the east 

 and not from the west. 



In this area, as in all others, no strikes and dips can be made out for 

 the Potomac beds. It is clear that the formation as a whole declines to 

 the southeast. As from Washington to the mouth of Acquia Creek the 

 general course of the Potomac River is southwest, the declination of the 

 formation is nearly at right angles to its direction, and on the Maryland 

 side it would be soon carried beneath the plane of erosion. Near the 

 mouth of Ac(piia Creek the Potomac River bends due east and enters 

 more and more deeply the Tertiary terrane. Hence the last exposures of 

 the Potomac formation on it occur near the mouth of that creek. 



