LOCATION OF POTOMAC BEDS. 37 



vicinity of ^yasllillgton the lower Potomac is covered by the same for- 

 mations as at Baltimore. Apparently nowhere north of the Potomac Rivur 

 does the lower Potomac constitnte the surface I'ock; but it is buried more 

 or less deeply under later formations. 



The case is ditferent in Virginia. Here, in a belt extending from 

 Washington to Fredericksburg, it occupies the surface, being in the 

 vicinity of the Potomac River alone, covered l)y (^)nat('rnarv tU-posit.s. 

 This indicates that in going north of the Potomac Kiver the lower 

 member lies at a lower level, or else it has lost more from erosion. 



Passing from the District of Columl)ia into Virginia, we find a con- 

 nected area of lower Potomac rock, which forms the suiface in a nan-ow 

 belt that extends to a point five miles south of Fredericksburg, where 

 the Massaponax Kiver enters the Rappahannock. This ma}- be called the 

 Fredericksburg area. It is the only region in which the lower Potomac 

 comes to the surface over an area of considerable extent. It forms a Ijelt 

 on an average three or four miles wide, having its western margin near 

 the Telegraph road and its eastern near the Potomac River. The Tele- 

 graph road is a dirt road, which runs from Alexandria to Fredericksburg 

 and beyond, crossing the jirincipal streams which flow into the Potomac 

 River near the head of tide in them. The margin of the lower Potomac 

 keeps, for the most part, about a mile west of this road after Alexandria 

 is left. 



Since the u))per or Variegated Clay member of the formation is ab- 

 sent in Virginia, the prefix lower may be omitted in giving an account of 

 the formation in this State ; Potomac formation being here equivalent to 

 lower Potomac. 



Along the Potomac River the principal exposures of the Potomac forma- 

 tion are to be seen on the Virginia shore and in the high points that make 

 down to the river between the creeks. Thus we find extensive exposures 

 at Mount Vernon, White House Point, Freestone Point, Shipping Point, 

 Cockpit Point, and near the mouth of Acxpiia Creek. In many places, 

 however, along the Potomac River, where we would expect to find the for- 

 mation in question, erosion prior to the deposition of the Quaternary has 

 cut it away to below water level, so that we find the banks of the river 



