342 UNIVERSITY OF VIRGINIA PUBLICATIONS 



and the arkoses have been converted into gneisses and the finer material 

 into slates and schists. In Fauquier and Culpeper counties and in other 

 adjacent sections, the structure is more simple. Folding has not been so 

 intense, and metamorphism is correspondingly less, hence it is possible to 

 solve the structural problems presented in this portion (northern) of the 

 belt with less difficulty. 



During recent investigations by the Virginia Geological Survey in the 

 Fauquier-Culpeper counties slate district, embracing an area of about 50 

 square miles, it was found that the conditions were especially favorable 

 for deciphering with reasonable accuracy the structure of the Cambrian 

 rocks. After careful detailed study and mapping of the area it was found 

 that the structural conditions were not those of closed folds and thrust 

 faults but dominantly those of open folds and normal faults, and in the 

 latter respect very similar to the Triassic areas of the eastern United States. 



Rocks of both pre-Cambrian and Cambrian age occur in the area. 

 The areal distribution of the different formations is shown on the accom- 

 panying geologic map, figure 1. The pre-Cambrian rocks include 

 granite, gneiss, schist, and extrusive basalt (Catoctin schist), the latter 

 being much the most abundant. Schistosity is especially characteristic 

 of the basalts (Catoctin schist). Overlying the pre-Cambrian rocks 

 are Cambrian beds of the Loudoun group forming a northeast-southwest 

 belt that is more or less continuous throughout the central part of 

 the area. The lithologic character of these sediments is remarkably 

 similar to those found throughout the entire extent of the Cambrian 

 belt in western Piedmont Virginia. They are made up of arkoses, sand- 

 stones, and shales, more or less foliated, but rarely has the metamorphism 

 been so intense as to entirely obliterate the original bedding. Schis- 

 tosity occurs but it is not so highly developed as in the basic igneous rocks 

 and is confined for the most part to certain sheared zones. In many 

 places in the basal sandstone (arkosic and quartzitic members), cross- 

 bedding is perfectly preserved; there is no evidence of foliation (schistos- 

 ity). Small intrusions of basic igneous rocks of the gabbroic type have 

 penetrated the sediments in parts of the district. 



The two dominant types of structure occurring in the area are folding 

 and normal faulting. The folding is mostly of the open type with the 

 axes extending in a general northeast-southwest direction. There seem 

 to be the remnants of two anticlinoria partially included within the area. 

 The structure of these has been greatly complicated by numerous faults 

 which cut the rocks of the area in all directions. Minute structures, such 

 as plications which result from intense folding, are limited in occurrence. 



