344 UNIVEESITY OF VIRGINIA PUBLICATIONS 



Both normal and thrust faults occur. Where the latter are developed 

 the thrust is commonly along the bedding planes. Thrusts are also 

 found in the igneous rocks indicated by slickensides. The extent of the 

 thrusts cannot be determined but they have probably produced small 

 displacements only. 



Block faulting is now the most conspicuous and interesting structural 

 feature of the area, and the present distribution of the outcrops is 

 largely due to its influence. Normal faults are very common both on a 

 large and small scale. These faults run in all directions and intersect 

 each other at any 'angle. A fine exhibition of block faulting occurs 

 west of the road leading from Warrenton to White Sulphur Springs 

 (fig. 2 geologic section along line A A' of map, fig. 1). Parallel with this 

 road is a series of belts of isolated units of basal Cambrian sandstone 

 (arkosic and quartzitic) which are undoubtedly the remnants of fault 

 blocks. The overlying shale and slate have been entirely removed by 

 erosion and only a small part of the basal deposits remains. These belts 

 are monoclinal in structure. The sandstone and fine conglomerate of 

 which they are composed exhibit only slight metamorphism, except sec- 

 ondary enlargement. Cross-bedding is one of the common character- 

 istics and it is very perfectly preserved. These remnants lie upon the 

 Catoctin schist and dip toward the southeast at angles of from 20 to 80 

 degrees, and the strike varies from north to northwest. Erosion has here 

 removed all the interbedded sandstones and shales which were deposited 

 above the basal sandstone, and the structural difficulties have thereby 

 been reduced to a minimum. In other parts of the district similar blocks 

 of the same material dip and strike in various directions, even at right 

 angles to the axis of the folds of the area. 



It will be seen by consulting the map, figure 1, that these parallel 

 belts of disconnected sandstone outcrops, separated by Catoctin schist, 

 extend from beyond the border of the area in the extreme northern part 

 to within about one mile of Rappahannock River, and suddenly stop. 

 Between this point and the river the strike is generally in a northwesterly 

 direction. This change in strike, as indicated on the map, is attributed 

 to. normal faulting in a transverse direction. It is this that has given 

 rise to the northwesterly trend of one of the slate belts. Because of this ■ 

 structural relation, the slate has not been entirely removed by erosion 

 from this block. 



In a small stream entering Rappahannock River near the eastern edge 

 of the Gaines farm a very good structure section across a part of this 

 fault block is observed. Some interesting relations of bedding and cleav- 



