THE DISPLACEMENT BOUNDING THE COASTAL PLAIN. 449 



the relations of the actual fault line are mostly exhibited. The displace- 

 ment, as a whole, appears to be continuous throughout, but its amount varies, 

 and in some areas the effects of dislocation become indistinct through dimi- 

 nution in amount, distribution through a zone, or merging into a flexure. 



South of Baltimore, near Relay, the relations of the dislocation are par- 

 ticularly well exhibited, and the amount of displacement is fully 250 feet. 

 The relations in this region are shown in section 2, figure 1, page 435. At 

 the exposures in this vicinity, clay caps the bare steep slope of the crystal- 

 line fault scarp, and at the base, on the downthrown block, a greater or less 

 thickness of clay abuts against it. This relationship is general for some 

 miles south from Baltimore, and west of the city it is exhibited in diminished 

 amount near Loudon Park cemetery, beyond which evidence of the disloca- 

 tion is lost for some distance. 



Northeastward from Baltimore the effects of the dislocation soon become 

 prominent, and its line is marked by a steep scarp in the crystallines, which 

 extends with varying heights and degrees of distinctness through northern 

 Maryland into Delaware and beyond. Usually the Potomac and Appo- 

 mattox materials are either eroded back from the summit of the scarp for 

 some distance or entirely removed, and in the larger depressions the drain- 

 age has cut through a greater or less thickness of Potomac materials on the 

 downthrown side, exposing the crystalline floor for a mile or two eastward. 



At Washington a dislocation traverses the thin outlying feather edges of 

 the Potomac formation and its Appomattox cap just west of Georgetown, 

 and crosses the Potomac river just below the fall line. Thence through 

 northern Virginia the dislocation gives rise to a prominent scarp on each 

 divide, which is more or less continuous and distinct for many miles. At 

 first it dislocates the Appomattox, together with, at some points, a feather 

 edge of the Potomac ; but in the region between Occoquau and Fredericks- 

 burg its amount increases greatly and it traverses a considerable thickness 

 of the Potomac, at one point the western edge of the Pamunkey, and in most 

 cases the Appomattox, with a thrown of from 150 to 300 feet, as shown in 

 sections 3 and 4, figure 1 (page 435). 



This dislocation crosses the Rappahannock a mile above Fredericksburg, 

 and its relations are there well exposed. It has not been definitely traced 

 southward, but there is evidence of displacement near Richmond and Peters- 

 burg, which may be along a continuation of this same dislocation. 



Date. — The date of the displacement is in the main post-Appomattox, but 

 there is some evidence that a series of local movements occurred before the 

 epoch of Appomattox deposition. The greater part of the displacement was 

 effected between Appomattox and Columbia times, apparently just before 

 Columbia deposition. In the gorge of the Potomac river a narrow Columbia 

 terrace extends for some miles above the line of dislocation, and the relative 



